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The identification of meteorite parent bodies provides the context for understanding planetesimal formation and evolution as well as the key Solar System events they have witnessed. However, identifying such links has proven challenging and some appear ambiguous. Here, we identify that the family of asteroid fragments whose largest member is (161) Athor is the unique source of the rare EL enstatite chondrite meteorites, the closest meteorites to Earth in terms of their isotopic ratios. The Athor family was created by the collisional fragmentation of a parent body 3 Gyr ago in the inner main belt. We calculate that the diameter of the Athor family progenitor was 64 km in diameter, much smaller than the putative size of the EL original planetesimal. Therefore, we deduce that the EL planetesimal that accreted in the terrestrial planet region underwent a first catastrophic collision in that region, and one of its fragments suffered a more recent catastrophic collision in the main belt, generating the current source of the EL meteorites.

International audience; On asteroids, fractures develop due to stresses driven by diurnal temperature variations at spatial scales ranging from sub-millimetres to metres. However, the timescales of such rock fracturing by thermal fatigue are poorly constrained by observations. Here we analyse images of the asteroid (101955) Bennu obtained by the Origins, Spectral Interpretation, Resource Identification and Security-Regolith Explorer (OSIRIS-REx) mission and show that metre-scale fractures on the boulders exposed at the surface have a preferential meridional orientation, consistent with cracking induced by diurnal temperature variations. Using an analytical model of fracture propagation, we suggest that fractures the length of those on Bennu’s boulders can be produced in 104–105 years. This is a comparable or shorter timescale than mass movement processes that act to expose fresh surfaces and reorient boulders and any preferential direction signature. We propose that boulder surface fracturing happens rapidly compared with the lifetime in near-Earth space of Bennu and other carbonaceous asteroids. The damage due to this space-weathering process has consequences for the material properties of these asteroids, with implications for the preservation of the primordial signature acquired during the accretional phases in the protoplanetary disk of our solar system.

Spacecraft missions have observed regolith blankets of unconsolidated subcentimetre particles on stony asteroids1–3. Telescopic data have suggested the presence of regolith blankets also on carbonaceous asteroids, including (101955) Bennu4 and (162173) Ryugu5. However, despite observations of processes that are capable of comminuting boulders into unconsolidated materials, such as meteoroid bombardment6,7 and thermal cracking8, Bennu and Ryugu lack extensive areas covered in subcentimetre particles7,9. Here we report an inverse correlation between the local abundance of subcentimetre particles and the porosity of rocks on Bennu. We interpret this finding to mean that accumulation of unconsolidated subcentimetre particles is frustrated where the rocks are highly porous, which appears to be most of the surface10. The highly porous rocks are compressed rather than fragmented by meteoroid impacts, consistent with laboratory experiments11,12, and thermal cracking proceeds more slowly than in denser rocks. We infer that regolith blankets are uncommon on carbonaceous asteroids, which are the most numerous type of asteroid13. By contrast, these terrains should be common on stony asteroids, which have less porous rocks and are the second-most populous group by composition13. The higher porosity of carbonaceous asteroid materials may have aided in their compaction and cementation to form breccias, which dominate the carbonaceous chondrite meteorites14. The absence of fine regolith on the asteroid Bennu is due to the high porosity of its rocks, which compress rather than fragment after impacts and exhibit slow thermal cracking.

IntroductionA lunar impact flash (LIF) is a rapid burst of light caused by a hypervelocity impactor hitting the moon, releasing a fraction of its energy as light. They are observable though moderately sized (0.4m) telescopes, and are therefore observable by both amateur and professional astronomers.LIF observations are typically performed in visible or NIR, and can only take place when the lunar phase is between 10-50%, during local night, and are only observable against the lunar night-side; the most commonly observed LIFs are faint, approximately MagI = 9, MagR = 10 [1], making them indistinguishable against the illuminated lunar background. This limits potential observations of LIFs to ~30h per month on average, with weather limiting this further.In order to increase the average number of observable hours that LIF observations can be made, and thereby increase the number of observed LIFs, we have developed new techniques to perform LIF observations at all local times, and all lunar illuminations. This technique theoretically only requires clear weather and a visible moon.TheoryLIFs have been shown to behave as black bodies, releasing energy according to Planck's law [2]. The temperature of LIFs observed by NELIOTA is between 1300-5800 k, with average temperature, Tavg = 2800k [3]. This average has peak wavelength of λpeak = 1.035 μm. The illuminated portion of the lunar surface is reflecting light from the sun (T = 5778 k, λpeak = 0.5 μm). In R-band and I-band, the ratio of energy radiated by the sun is greater by a factor of 57 in R-band, and 28 in I-band. My observing in wavelengths closer to the λpeak of the flash, we can improve this ratio greatly.Figure 1:The Planck curve for the sun (5778k) and the average LIF (2750k), R, I, and J-band regions are highlighted in orange, red, and grey respectively.As illustrated in Fig. 1, by observing in J-band the irradiance of the sun is lowered, being only 10 times greater than the 2800 k LIF. This effective dimming of the background leads to an increase in the signal-to-noise ratio (S/N) of the LIF against the sun-illuminated background.The S/N ratio is a measure of how detectable a signal is above the background, and can be defined as:S/N = S" / (σ × √Apix)Where S" is the total count of the signal, σB is the standard deviation of the background, and Apix is the area of the background measurement. A S/N ratio above ≈10 is generally required for a signal to be considered detectable.ObservationsObservations were performed on 2022-02-23 from the Observatoire de la Côte d'Azur, using a 0.5 m diameter, 2 m focal length telescope with Ninox 640 II camera and J-band filter attached. For the first set of observations, high gain mode was used, for the second set of observations low gain mode was used. A 30 μm exposure time was used for both sets of observations.The star HD137463 was observed, which has MagJ = 5.423, obtained by Simbad [4] Observations began during the night, and continued until after sunrise in order to measure the brightness of the sky during daytime. By performing A-B subtraction to remove the dark current and background of the observed frames, as shown in Fig. 2, a pixel count for the clean star signal, S", can be obtained. As it is constant, an average for the star signal can be used as ground truth in order to calculate the S/N ratio for the daytime sky brightness, as well as the illuminated lunar surface.Figure 2: The A-B subtraction of two observations of HD137463. The subtraction removes any dark current or background to leave only a clean positive and negative signal.ResultsThe S/N ratio for the a MagJ = 5.423 star are shown in Fig. 3. A sharp decrease in S/N ratio is observed as air-glow begins around solar altitude = -5. The S/N continues to decrease, slowly levelling off around S/N = 15 as the sun reaches solar altitude = 5. This puts the magJ = 5.423 star well above the detectable threshold, and would correspond to a MagR = 6.7 LIF. Figure 3: Left: The S/N ratio in high gain mode, of HD137463 against the sky background between solar altitudes -10 & -1. Right: The S/N ratio in low gain mode, of HD137463 against the sky background between solar altitudes -1 & -5.By taking measurements of the lunar background brightness, we can verify the S/N ratio we would expect from these LIF when observed on the lunar surface. Table 1 contains the results from count-averaged observations of the background, and the S/N of the MagJ star if observed against the background, emulating a flash. Again, even against the illuminated bright limb, the S/N ratio is above the detectable threshold, and shows that with this system, we would be able to detect LIFs of MagJ ≥ 5.423, with the possibility of even fainter magnitudes under more favourable conditions.Table 1: The S/N ratio of the reference star used against different regions on the lunar surface, from darkest to brightest.Region S/N Lunar Night Side 147 Terminator 42 Lunar Day Side 21 Lunar Bright Limb 19 References[1] Liakos et al (2019) A&A, 633, 29 pp.[2] Eichhorn (1975) P&SS, 23, 11, 1519-1525[3] Avdellidou & Vaubaillon (2019) MNRAS, 484, 5212-5222[4] Wegner et al (2000) A&A, 143, p.9-22

Introduction: Linking a meteorite type to a specific parent asteroid allows us to gain insight into the composition of the latter as well as the time, and indirectly the heliocentric distance of its formation. Up to now there have been established solid links between the HEDs and the inner main belt asteroid family of (4) Vesta [1] as well as between the ordinary chondrites and asteroids belonging to the so-called spectroscopic S-complex [2]. Here we report on our search for the enstatite chondrites parent body. We base our analysis on two facts: that (i) inner main belt asteroid collisional families are the most favoured to deliver meteorites to Earth, and (ii) enstatite chondrites (divided in EH and EL groups) have reflectance spectra that are within the broad asteroid spectroscopic X-complex [3]. The newly discovered asteroid families of Athor and Zita [4] are the only two families of the inner main belt that belong to the spectroscopic X-complex and thus are promising candidates. Methods and Results: In order to investigate the potential link between the enstatite chondrite meteorites and the aforementioned X-complex asteroid families, we performed near-infrared observations of a statistically significant number of members of Athor and Zita. These were combined with the visible data from the literature and finally each asteroid spectrum was classified using the most common asteroid spectral taxonomy. We showed that the Athor and Zita families are spectroscopically distinct from each other and homogenous among their respective members. Moreover, both families have distinct geometric albedo values, with Athor family being brighter. Focusing on the Athor family, we performed curve matching and absolute reflectance comparison with all the available laboratory meteorite spectra in NASA Reflectance Experiment Laboratory and Planetary Spectrophotometer Facility databases. We will report on our matching and provide a number of further evidence that inner main belt families could indeed deliver enstatite chondrites to Earth. Acknowledgments: We acknowledge support from the ANR ORIGINS (ANR-18-CE31-13-0014). CA was supported by the project “Investissements d’Avenir” UCA-JEDI (ANR-15-IDEX-01) and the European Space Agency, AM acknowledges support from the ERC advanced grant HolyEarth N. 101019380. KJW acknowledges support from the Project ESPRESSO, a NASA SSERVI program at SwRI. This work is based on data provided by the Minor Planet Physical Properties Catalogue (MP3C, mp3c.oca.eu) of the Observatoire de la Côte d'Azur. This research utilises spectra acquired at the NASA RELAB facility at Brown University and at Planetary Spectrophotometer Facility (PSF) at University of Winnipeg.References: [1] Russell C.T. et al. 2012, Science 336, 6082, 684.[2] Reddy V. et al. 2015, Asteroids IV, University of Arizona Press, Tucson, 895, 43-63[3] Vernazza P. et al. 2009. Icarus 202, 2, 477-486.[4] Delbo M. et al. 2019. Astronomy & Astrophysics 624, A69.

Introduction The small bodies (also known as minor planets) in our solar system are usually discovered by telescopic surveys. The results of these observations are organised by the Minor Planet Center (MPC), which determines orbits from the astrometric observations of the surveys. As of May 15, 2022, the MPC has an orbit database of 1,193,635 minor planets. However, the MPC data do not tell us what a minor body actually is, as they not contain physical properties such as albedo, diameter, volume, shape, composition, mass, bulk density, period of rotation and direction in the sky of the axis of rotation. Asteroids have very diverse values of these physical properties, whose knowledge is important for scientific studies of our solar system and its origin, planetary formation and evolution, space exploration, and planetary defence. The physical properties of small bodies are spread across many publications, several websites (e.g. the NASA planetary data service) and archives. Many compositionally diagnostic spectra are also presented in a series of publications, but their source files are not necessarily public. This poses a fundamental problem to our ability to massively exploit the physical properties of minor bodies. Moreover, the values ​​of these physical parameters are extremely heterogeneous, obtained by several different groups, project teams and telescopic surveys, and even individual researchers, using different techniques. For example, thermal models are used to interpret observations in the mid-infrared (between 3.5 and 100 μm) from space and from the ground for the measurement of diameters and albedos. However, other techniques such as radar or the measurement of stellar occultation times provide measurements of diameters. A centre of minor planets’ physical properties The Minor Planet Physical Properties Catalogue, or MP3C for short, collects and organises in a single place and makes available to the community values of physical properties of asteroids and other small bodies of our solar system. The MP3C is designed for very heterogeneous and big amount data: as of March 2022 it contains 3,710,587 measured properties, all of which are fully referenced to their published sources. The MP3C offers two main interfaces to explore the data (Figs. 1 and 2), namely (i) a web portal (mp3c.oca.eu) that allows to perform extraction of physical properties values on the basis of a single or list of minor bodies identifiers (names, numbers), but also on ranges of physical and dynamical properties; (ii) a data server offering a Table Access Protocol (TAP), which is defined and compatible with the standards of the Virtual Observatory (VO). The data server is accessible at https://dachs.oca.eu. MP3C is registered as service of the Virtual Observatory and therefore searchable and accessible using Virtual Observatory tools such as TOPCAT (Fig. 2). Using TOPCAT and/or the TAP, a human or robotic user can perform queries in a standardised database language (ADQL). MP3C contains sizes, albedos, absolute magnitudes, masses and rotational period, along with osculating and proper orbital elements of 1,169,632 minor bodies. At the time of writing we are also implementing in the MP3C table of spectral classes, colours, rotation vector orientation, and later reflectance spectra of minor planets. Interface with the Virtual European Solar and Planetary Access of EUROPLANET In addition to the general TAP interface, the best current determinations of all parameters and objects are accessible via the EPN-TAP protocol. EPN-TAP, designed in Europlanet/VESPA, uses a specific metadata vocabulary to uniformly describe Solar System data, which facilitates cross-searches in various data services - for instance, several services providing spectra of small bodies are also accessible in this format. All EPN-TAP services can be queried from the VESPA portal (http://vespa.obspm.fr) or from scripts in various languages, e.g. for mass processing. The future big data challenge In the future, the problem of the spreading of small-body physical properties will become very important: Soon, the Near-Earth Object Surveyor Telescope, decided to be implemented by NASA's Planetary Defense Coordination Office in the fall of 2019, with a launch in 2026 will determine the sizes and albedos of nearly 8 million asteroids from observations of their thermal emission. At the end of 2022 LSST will be commissioned and will begin operations. About two years later, LSST will start publishing the data. In a single visit, LSST detects up to 5,000 solar system objects. Over its 10 year lifespan, LSST could catalog over 5 million Main Belt asteroids, almost 300,000 Jupiter Trojans, over 100,000 near-Earth objects, and over 40,000 trans-neptunian objects. Many of these objects will receive 100s of observations in multiple bandpasses. This amounts to an increase of at least 10× the known population, with similar increases in the number of objects with enough data to generate light-curves and colours. We therefore imagine a very significant increase in the physical properties of asteroids and comets. Clearly the latter is a major big-data problem, which will necessitate scientific and technical implementation skills. Acknowledgments M. Delbo and C. Avdellidou acknowledge support from the ANR Origins (ANR-18-CE31-13-0014). The work of C. Avdellidou was also partially funded by he French National Research Agency under the project “Investissements d’Avenir” UCAJEDI with the reference number ANR-15-IDEX-01 (2018-2020). The work of S. Erard for linking MP3C to EUROPLANET VESPA was supported by the the Europlanet-2024 Research Infrastructure project that received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 871149. Figure 1: (Left) HTML output of all properties listed by MP3C for Pallas. (Right) Search form by list of object identifiers and/or range of their physical and orbital properties. Figure 2: The MP3C service is accessible via the TAP protocol by the applications of the Virtual Observatory (here, TOPCAT). (Left) List of tables with their description, including the EPNCore table available in particular for the VESPA service. (Right) Graphs and tables obtained by submitting ADQL queries.

Asteroids, along with other small bodies, are what is left over of the original planetesimal disk from the planet-formation era. Therefore, these objects are considered the best tracers for the processes that occurred during the earliest history of our Solar System. However, the majority of asteroids are fragments generated by the collisional breakup of the planetesimals, the first ~100-km sized bodies (Morbidelli et al., 2008, Delbo' et al., 2019) that accreted in the protoplanetary disk of our Sun. Nevertheless, a small fraction of the planetesimal population survived the collisional evolution. In order to study these objects, the first step is to identify these surviving planetesimals among all the other (fragment) asteroids. To do so, we “cleaned” the inner part of the asteroid main belt (2,1 < a < 2,5) from all asteroid collisional family members (using the method of Bolin et al., 2017, Delbo' et al., 2017, 2019), thus revealing those asteroids that are not fragments that formed in the main belt. Thanks to this method, we revealed 64 surviving planetesimals in the inner main belt.We carried out a spectroscopic survey of these identified IMB planetesimals, in order to constrain their composition and mineralogy. In particular, we performed visible and near-infrared spectroscopy using several telescopes such as the 1.82m Copernico Telescopio (Asiago, Italy) for the visible spectroscopy and the 4.2 Lowell Discovery Telescope (Flagstaff, USA); the 3.2 NASA Infrared Telescope Facility (Hawaii, USA) and the Telescopio Nazionale Galileo (La Palma, Spain) for the near-infrared part. To complete our survey, we also used spectra in the visible and near-infrared published in the literature, as well as size and albedo information that we obtained from the Minor Planet Physical Properties Catalog (https://mp3c.oca.eu/). We performed the taxonomic classification following the Bus-DeMeo taxonomy (Bus et al., 2002; DeMeo et al., 2009), and using the M4AST tool (http://m4ast.imcce.fr) (Popescu et al., 2012). A visual inspection to identify the presence of absorption bands characteristic of some classes was performed to check the robustness of the automatic taxonomic classification. In addition, we compute for each planetesimal several spectral parameters, such as spectral slopes, and center, depth and area of absorption bands, when these are present. We also performed calculation of their mass, based on the method of (Carry, 2012). Finally, we used the RELAB database (Pieters, 1983), to look for meteorite analogues of each planetesimal. We found that planetesimals of the Inner Main Belt (IMB) belong mainly to the S-complex (~45%), followed by C-complex (~25%) and X-complex (~17%). Further investigations on S-complex planetesimals showed that, for a majority of them, they are best matched by spectra of ordinary chondrites meteorites. We did not find any correlation with diameters, semimajor axis and the ratio of olivine/pyroxene. Almost 60% of the C-complex planetesimals belong to the Ch/Cgh types, showing spectroscopic features associated with hydrated minerals, and consequently indicating the presence of liquid water in the early formation phases of these objects. We also found that almost 5% of the IMB planetesimals belong to the D/T types with a diameter greater than 25 km. As this taxonomical class, as well as Ch/Cgh types, are likely to have formed in the outer part of the Solar System (at 3-7 au), their presence in the IMB (2,1-2,5 au) can be explained by dynamical models invoking large semimajor axis migration of these objects (e.g., Grand Tack of Walsh et al., 2011; low-mass asteroid belt of Raymond and Izidoro, 2017b for the C-complex and Vokrouhlický et al., 2016 for the D/T types).Here, we will present the spectroscopic, physical and compositional results of our IMB planetesimals survey as well as the implications for planetary formation models.Acknowledgements: We acknowledge support from the ANR ORIGINS (ANR-18-CE31-13-0014).References : Morbidelli A. et al, Physica Scripta, Volume 130, Issue, id 014028 (2008) ; Delbo M. et al., A&A, Volume 624, id. A69 (2019) ; Bolin B. T. et al., Icarus, Volume 282, 2017, Pages 290-312 ; Delbo M. et al., Science, Volume 357, Issue 6355, pages 1026-1029, (2017) ; Bus S. J. et al., Icarus, Volume 158, Issue 1, Pages 146-177 (2002) ; DeMeo F. E. et al., Icarus, Volume 202, Issue 1, Pages 160-180 (2009); Carry, B. Planetary and Space Science, Volume 73, Issue 1, p. 98-118. (2012) ; Popescu M. et al., A&A, Volume 544, id. A130 (2012) ; Pieters, C. M., J. Geophys. Res., Volume 88, pages 9534– 9544 (1983) ; Walsh K. J. et al., Nature, Volume 475, Issue 7355, Pages 206-209 (2011) ; Raymond S. N. et al., Science Advances, Volume 3, Issue 9, Pages e1701138 (2017) ; Vokrouhlický D. et al., The Astronomical Journal, Volume 152, Issue 2, id. 39 (2016).

Finding a link between meteorites and asteroids thanks to spectroscopy gives a great insight of the composition of the bodies in the asteroid population. Meteorites such as the Howardite-Ensatites-Diogenites (HEDs) have been successfully linked to the asteroid Vesta (McCord et al. (1970), summary by McSween Jr. et al. (2013)), and the ordinary chondrite meteorite population has been connected to a subtype of S-type asteroids, after having taken into account space weathering processes (Binzel et al. 2001). These processes alter the surface of asteroids and modify their reflectance spectra with respect to that of the fresher meteorites, hence they need to be taken into account prior to comparing asteroids with meteorites spectra (Hapke 2001). Other meteorite types have been studied and linked to asteroid types, but some objects still do not show convincing connections (DeMeo et al. 2022). Linking meteorites to the primordial population of objects would give information about the formation and evolution of bodies in the Solar System. Indeed, the planetesimals that formed earliest in the Solar System are thought to have been fully differentiated, due to the melting induced by the heating produced by the radioactive decay of 26Al isotopes. This should have resulted in planetesimals presenting a basaltic crust, an olivine-dominated mantle and an iron-rich core (DeMeo et al. 2019). However most of these primitive objects have evolved, have been disrupted or integrated to other bodies (such as planets) and their formation processes and the composition of their primordial crusts is not well known.We will report about searching for potential parents bodies of meteorites coming from differentiated planetesimals, using literature data and the new dataset of reflectance spectra that will be released in the Gaia Data Release 3 (DR3) in June 2022. This dataset will present low resolution mean reflectance spectrum of 60 518 Solar System objects, acquired by Gaia’s blue and red spectrophotometers (BP and RP) between August 2014 and May 2017. Each spectrum will consist of 16 discrete wavelength bands that spans the visible wavelength range from 374 to 1034 nm (Gaia collaboration, Galluccio, Delbo et al. 2022). These data should allow to look inside asteroid collisional families and search for hints of differentiation, looking for members corresponding to the crust, mantle or core of a disrupted planetesimal. We intend to use these data to try to link meteorites that are known to have been produced by differentiation (e.g. metallic or andesitic bodies) to the primordial population of asteroids. Finding the parent body of differentiated meteorites would allow to understand better the formation and differentiation of planetesimals in our Solar System.To compare the reflectance spectra of meteorites with Gaia’s asteroids, we will first use a curve matching method. Curve matching consists in determining whether any given spectrum is similar to a reference spectrum, by the means of a coefficient that has to be minimized (such as the χ2) or maximized (correlation coefficient for example) (Popescu et al. 2012). We will as well study the features of the spectra in order to better characterize the asteroids and validate the matches (Gaffey 2011). Space weathering processes will be taken into account. AcknowledgementsThe authors acknowledge the financial support of CNES and OCA, which make this study possible. M. Delbo thanks financial support from the ANR ORIGINS (ANR-18-CE31-13-0014).ReferencesBinzel, R. P., Rivkin, A. S., Bus, S. J., Sunshine, J. M., & Burbine, T. H. 2001, Meteoritics & Planetary Science, 36, 36DeMeo, F. E., Burt, B. J., Marsset, M., et al. 2022, Icarus, 380, 114971DeMeo, F. E., Polishook, D., Carry, B., et al. 2019, Icarus, 322, 13Gaffey, M. J. 2011, AIP Conference Proceedings, 1386, 1386Gaia collaboration, Galluccio, Delbo et al. 2022 Gaia Data Release 3: Reflectance spectra of Solar System Small Bodies. A&A under review.Hapke, B. 2001, J. Geophys. Res., 106, 10039McCord, T. B., Adams, J. B., & Johnson, T. V. 1970, Science, 168, 1445McSween Jr., H. Y., Binzel, R. P., De Sanctis, M. C., et al. 2013, Meteoritics & Planetary Science, 48, 48Popescu, M., Birlan, M., & Nedelcu, D. M. 2012, A&A, 544, 544

The thermal conductivity of granular planetary regolith is strongly dependent on the porosity, or packing density, of the regolith particles. However, existing models for regolith thermal conductivity predict different dependencies on porosity. Here, we use a full-field model of planetary regolith to study the relationship between regolith radiative thermal conductivity, porosity, and the particle non-isothermality. The model approximates regolith as regular and random packings of spherical particles in a 3D finite element mesh framework. Our model results, which are in good agreement with previous numerical and experimental datasets, show that random packings have a consistently higher radiative thermal conductivity than ordered packings. From our random packing results, we present a new empirical model relating regolith thermal conductivity, porosity, temperature, particle size, and the thermal conductivity of individual particles. This model shows that regolith particle size predictions from thermal inertia are largely independent of assumptions of regolith porosity, except for when the non-isothermality effect is large, as is the case when the regolith is particularly coarse and/or is composed of low thermal conductivity material.Plain Language SummaryThe temperature of a planetary surface is strongly controlled by the thermal inertia of the surface materials. Specifically, if the surface is covered in a granular regolith, then the size, thermal conductivity, and packing density of the regolith particles strongly affects the surface thermal inertia, which in turn controls surface temperatures. In this work, we use 3D numerical simulations of heat transfer through beds of spherical particles, representing a planetary regolith, to investigate how thermal conductivity and thermal inertia are controlled by the packing density and thermal conductivity of the spheres. Our results are presented in the form of a new empirical model, which could be used to calculate regolith thermal conductivity from knowledge of particle size, composition, and packing density. The use of this model is demonstrated in the typical reverse fashion, where an observed planetary thermal inertia is converted into a predicted regolith particle size. Our model shows that the predicted particle size is largely independent of regolith particle packing density, in contrast to other common regolith models.

The Gaia mission of the European Space Agency (ESA) was launched in December 2013 and began it scientific operations in July 2014. Gaia is essentially devoted to the measurement of positions, parallaxes, proper motions, brightnesses, and colours of stars. However, Gaia has also obtained astrometric, photometric, and spectroscopic measurements for several hundreds of thousands of asteroids. The Gaia Data Release 3 (DR3) is the first to contain low resolution reflectance spectra of 60,518 solar system small bodies, the large majority of which are asteroids of the main belt (Gaia collaboration, Galluccio, Delbo et al. 2022). The global survey properties, the methods of data production and validation are detailed in the aforementioned paper published by the journal Astronomy and Astrophysics, which accompanies the DR3, and also summarised in the presentation of Galluccio et al. (2022) at this conference (EPSC 2022). Here we present preliminary results about the Gaia view of asteroid collisional families. We identified asteroids belonging to families during the validation of Gaia asteroid spectra. Based on the family identification of Nesvorny et al. (2015), with the addition of the New Polana and the Eulalia families from Walsh et al. (2013), we found that in the Gaia DR3 there are 25,088 asteroids that belong to 116 collisional asteroid families of the 121 listed in Tab.1. This table also reports the number of family members and the number of members with a Gaia DR3 reflectance spectrum. However, the family identification of current catalogues is conservative by construction in order to keep good separation between the families. Hence, it is very likely that (i) some of the known families are more extended than what is available in current catalogues, namely, that catalogues based on dynamical criteria do not capture the whole extent of a family, and (ii) some unknown families are yet to be identified. This implies that the aforementioned number of asteroid family members with Gaia DR3 reflectance spectra is likely an estimation lower than the true number. We will present some important features that Gaia observed in families. One of these is the correlation between some spectral parameters, such as the slope and the depth of the 1-micron absorption band of the reflectance spectrum, and the age of members of asteroid families belonging to the so called spectroscopic S-complex (DeMeo et al. 2009). These correlations can be explained in terms of a space weathering process that reddens and reduces the 1-band depth as a function of time on S-complex asteroids. For the brightest asteroids, the extension of Gaia reflectance spectra at wavelengths shorter than 0.45 micron is very useful for distinguishing the composition of primitive, carbonaceous asteroid families, which are difficult to be discriminated with the ground based spectroscopic data currently present in the literature. References: - DeMeo, F. E., Binzel, R. P., Slivan, S.~M., Bus, S. J. 2009. Icarus 202, 160–180. - Gaia collaboration, Galluccio, Delbo et al. 2022. A&A under review. - Nesvorný, D., Brož, M., & Carruba, V. 2015. in Asteroids IV (P. Michel, et al. eds.) University of Arizona Press, Tucson., - Walsh, K. J., Delbo, M., et al. 2013. Icarus 225, 283–297.

The Gaia mission of the European Space Agency (ESA) published its third release (DR3) on June 13, 2022, including for the first time a survey of 60 518 mean reflectance spectra of Solar System objects (SSOs). Each reflectance spectrum was derived from calibrated measurements obtained by the onboard Blue and Red low-resolution spectro-photometers (BP/RP) and consists of sixteen discrete wavelength bands, along with additional information about the data quality for each band. We will describe the processing of the Gaia spectral data of SSOs and the steps of our internal validation procedures. We present the adopted approach for external validation against SSO reflectance spectra available in the literature, obtained from ground-based and space-borne telescopes, to assess the quality of Gaia SSO reflectance spectra. This work summarizes a more detailed description by Galluccio et al. (2022) [1]. Introduction Gaia DR3 contains information for a sample of more than 150,000 SSOs, including astrometry, photometry, osculating elements, and mean reflectance spectra [2]. The selection of these objects is described by Tanga et al. (2022) [2]. Based on the quality of the data and some criteria developed hereafter, DR3 contains 60518 mean reflectance spectra of Near-Earth Asteroids (NEAs), main-belt asteroids, Jupiter Trojans, transneptunian objects (TNOs), and other classes (Table 1) observed at relatively large phase angles (~20°). Table 1: No. of SSOs with Gaia DR3 spectra for each of the dynamical classes listed on the NASA JPL website. The classes are defined according to the criteria based on the orbital semi-major axis a, the perihelion distance q, and the aphelion distance Q. Dynamical Class No. of SSO Criterion (values in au) NEA Aten 6 a < 1.0 & Q > 0.983 NEA Apollo 52 a > 1.0 & q < 1.017 NEA Amor 47 1.017 < q < 1.3 Mars-Crosser 729 1.3 < q < 1.666 Inner Main Belt 1 221 a < 2.0 & q > 1.666 Main Belt 55 976 2.0 < a < 3.2 & q > 1.666 Outer Main Belt 1 995 3.2 < a < 4.6 Jupiter Trojan 477 4.6 < a < 5.5 Centaur 5 4.6 < a < 5.5 TNO 7 a > 30.1 Other 2 none of the above Production of SSO reflectance spectra The first step in computing the SSO mean reflectance spectra consisted of a calibration of the BP/RP spectra and removal of all instrumental and astrophysical effects to produce internally calibrated epoch spectra [3]. Each of them is an array of 60 internal flux measurements and corresponding uncertainties, computed for each one of the 60 pixel-long XP (BP/RP). The BP operates in the wavelength range λ between 330 and 680 nm, and RP between 640 and 1050 nm. Next, BP and RP epoch reflectance R(λi)t were determined by dividing the flux ft(λi) of each SSO spectrum at the epoch t by the reference solar analogue spectrum F(λ). The latter has been computed by averaging Gaia DR3 mean spectra of several known solar analogue stars widely used in asteroid ground-based spectroscopy because their spectra closely mimic the one of our Sun. Epoch reflectance was then normalized at λ = 550 nm. To compute the mean reflectance spectrum, we first defined a set of fixed wavelength bands, each one having a width of 44 nm, in the interval from 374 nm to 1034 nm. Inside each wavelength band, we computed a weighted average reflectance using all epoch reflectance present in each bin after applying a sigma-clipping procedure (see Figure 1). Figure 1: Example of the mean computed reflectance R(λi) (black circles) for the asteroid (61) Danae. The grey points are the R(λi)t corresponding to each epoch reflectance. The R(λi) t values accepted by our filtering method are coloured in blue (BP) and red (RP). Some filters based on SNR and on quality criteria were finally applied to the sample of computed mean reflectance spectra. Along with the mean reflectance spectra, the Gaia DR3 catalogue also contains for each source a reflectance_spectrum_flag field, indicating the overall quality of the resulting reflectance spectrum. Validation The validation of the Gaia DR3 mean reflectance spectra was obtained by studying and comparing spectral parameters including the spectral slope and the absorption band depth around 900 nm (equivalent to SDSS z-i colour) with those available in the literature, including the SMASSII survey [4] (Figure 2), ground-based observations at large phase angle [5] or space-borne telescope spectra. In general, Gaia DR3 mean reflectance spectra are in good agreement with the literature data. No significant spectral reddening was detected during the validation step. Figure 2: z colour vs. spectral slope of the asteroids of DR3 (grey dots). Over-plotted with circles of different colours are the same spectral parameters computed by us (see Section 4.2) for the asteroids of SMASSII [4]. The letters C, S, and X represent taxonomic complexes, the other letters spectral classes Conclusions Gaia DR3 includes the largest space-based survey of reflectance spectra of Solar System small bodies observed at visible wavelengths with excellent quality. In the presentation of Delbo et al. (2022) at this conference some preliminary results about the Gaia view of asteroid collisional families will be described. Acknowledgements This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. References [1] Gaia collaboration, Galluccio, Delbo et al. 2022 Gaia Data Release 3: Reflectance spectra of Solar System Small Bodies. A&A under review. [2] Gaia collaboration, Tanga et al. 2022 Gaia Data Release 3: the Solar System survey. A&A under review. [3] De Angeli et al. 2022, Gaia Data Release 3: Processing and validation of BP/RP low-resolution spectral data A&A under review.[4] Bus and Binzel. 2002. Icarus 158, 146-177.[5] Cellino, Bendjoya, Delbo’ et al. 2020. A&A, 642, A80

Asteroids reflectance spectra in the visible light will be one of the novel products of the Gaia Data Release 3 (DR3). These spectra are produced from Gaia observations obtained by means of the blue and red photometers — the so-called BP and RP, respectively. We will review the strategy adopted to produce asteroid reflectance spectra from BP-RP data, focusing on the choice of spectro-photometric calibrations computed taking into account solar system object astrometry and suitable lists of solar-analog stars. Our preliminary investigation shows that we will be able to obtain reflectance spectra for asteroids as small as some km in the main belt, by exploiting the fact that each object has been observed multiple times by Gaia. We will show the capability of Gaia to probe the detailed compositional gradient of the main belt down to small sizes and to study correlations between spectral classes and other asteroid physical parameters, such as albedo and size. Concerning the brightest asteroids, we expect to have substantial signal at wavelengths shorter than 450 nm, allowing Gaia to examine this region of the spectrum that has been poorly investigated by ground-based asteroid spectroscopic surveys. This region is characterised by the presence of a reflectance downturn that is diagnostic for the composition of classes of primitive asteroids, for instance those including the parent bodies of carbonaceous chondrites. These asteroids may have played an important role for the delivery of prebiotic compounds to Earth during the early phases of solar system' s history and, as such, are at the center of attention of the planetary science community.

Context. Carbon is one of the most abundant components in the Universe. While silicates have been the main focus of solid phase studies in protoplanetary discs (PPDs), little is known about the solid carbon content especially in the planet-forming regions (~0.1–10 au). Fortunately, several refractory carbonaceous species present C-H bonds (such as hydrogenated nano-diamond and amorphous carbon as well as polycyclic aromatic hydrocarbons), which generate infrared (IR) features that can be used to trace the solid carbon reservoirs. The new mid-IR instrument MATISSE, installed at the Very Large Telescope Interferometer (VLTI), can spatially resolve the inner regions (~1–10 au) of PPDs and locate, down to the au-scale, the emission coming from carbon grains. Aims. Our aim is to provide a consistent view on the radial structure, down to the au-scale, as well as basic physical properties and the nature of the material responsible for the IR continuum emission in the inner disk region around HD 179218. Methods. We implemented a temperature-gradient model to interpret the disk IR continuum emission, based on a multiwavelength dataset comprising a broadband spectral energy distribution and VLTI H-, L-, and N-bands interferometric data obtained in low spectral resolution. Then, we added a ring-like component, representing the carbonaceous L-band features-emitting region, to assess its detectability in future higher spectral resolution observations employing mid-IR interferometry. Results. Our temperature-gradient model can consistently reproduce our dataset. We confirmed a spatially extended inner 10 au emission in H- and L-bands, with a homogeneously high temperature (~1700 K), which we associate with the presence of stochastically heated nano-grains. On the other hand, the N-band emitting region presents a ring-like geometry that starts at about 10 au with a temperature of 400 K. Moreover, the existing low resolution MATISSE data exclude the presence of aromatic carbon grains (i.e., producing the 3.3 μm feature) in close proximity tothe star (≲1 au). Future medium spectral resolution MATISSE data will confirm their presence at larger distances. Conclusions. Our best-fit model demonstrates the presence of two separated dust populations: nano-grains that dominate the near- to mid-IR emission in the inner 10 au region and larger grains that dominate the emission outward. The presence of such nano-grains in the highly irradiated inner 10 au region of HD 179218 requires a replenishment process. Considering the expected lifetime of carbon nano-grains from The Heterogeneous dust Evolution Model for Interstellar Solids (THEMIS model), the estimated disk accretion inflow of HD 179218 could significantly contribute to feed the inner 10 au region in nano-grains.Moreover, we also expect a local regeneration of those nano-grains by the photo-fragmentation of larger aggregates.

On 2017-09-27 at 18:56:12 UT a large meteoroid impacted the lunar surface, causing an impact flash with a duration of 1.12 seconds. Using Lunar Reconnaissance Orbiter Camera images, and the PyNAPLE (Python NAC Automatic Pair Lunar Evaluator) software pipeline, the resultant crater was identified. The crater is located at latitude φ= 8.0288, longitude λ = -76.546, approximately 14km east of the crater Glushko. The asymmetrical morphology of the ejecta blanket indicates the direction of impact was from the south, and impacting incidence angle was o. Introduction Lunar impacts are monitored on a regular basis, by both amateur and professional groups [1,2,3]. Since the first confirmed recording of a lunar impact flash in 1999, at least 600 confirmed impact flashes have been observed. This large volume of data has let to several studies on both the properties of the impactor, and the thermal evolution of the impact itself [4,5,6]. These studies make use of the observed brightness of the flash, and the most likely parent meteoroid stream to calculate the total energy and temperature of the impact, and estimate the resultant crater size. Very few of these impact craters have located [7] however, as finding metre scale craters and linking them to an observed impact with certainty is difficult. By identifying the resultant crater from an observed impact flash, valuable ground truth data can be obtained to help constrain the energy of the impact through use of crater scaling laws. The ejecta pattern of the impact can also tell us important information about the trajectory of the impactor. Impact Flash At 18:56:10 UT on 2017-09-27, a meteoroid impacted the lunar surface. The impact was observed by SdR UAI Luna members Bruno Cantarella and Luigi Zanatta, using a dual telescope system capturing with ZWO ASI120MM cameras, running at 25fps in one telescope, and 30fps in the other. It was also confirmed to be observed by Stefano Sposetti in Gnosca, Switzerland, ruling out the possibility the flash being a false detection. The flash lasted 29 frames in the 25fps camera, giving it a duration of 1.12s. The first 22 frames are shown in Fig. 1. Its peak luminosity is reached in the second frame after only 0.04s, with the peak quantum efficiency of the camera at 590nm. Figure 1: The first 22 frames of the 2017-09-27 impact flash. As there were no meteor streams active at the time of the impact, it is concluded that the impactor belonged to the sporadic meteor background. Crater Detection The PyNAPLE software [8] was utilised in order to search for the resultant impact crater in Lunar Reconnaissance Orbiter Narrow Angle Camera images (LRO NAC). PyNAPLE works by searching for all before-after LRO NAC image pairs for a given time and location on the lunar surface, and running them through several operations which calibrate, map project, register, and align the images in order for pixel by pixel division to occur. This produces a final image in which similarities have cancelled out, and changes between the images are highlighted. The crater, Fig. 2, was discovered in both LRO NAC images M1315871095R and M1344064055L, taken in 2019 and 2020 respectively, and and is absent from any images of the location from pre-2017. Figure 2: The crater formed during the 2017-09-27 lunar impact. Left: The ‘before’ image M1180620010R. Centre: The ‘after’ image M1315871095R. Right: The resultant image from dividing M1315871095R by M1180620010R. Crater Analysis The ejecta blanket and newly exposed regolith are higher albedo than the surrounding undisturbed material. Consequently, in the current LROC images the fresh regolith is overexposed, and no measurements as the craters diameter can be made, although the extent of the ejecta can be measured up to 600m away from the point of impact. This prevents the calculation of impact energy by employing crater scaling laws at this time. As the LRO is still collecting images, however, once new images of the site get released, these calculations will be possible. The ejecta blanket of the impact crater tells us some information on the impact. The asymmetrical morphology of the ejecta blanket, shown in Fig. 3, is formed by the meteoroid impacting with an angle below ~30o incidence, according to Shuvalov’s findings for projectiles below 100m in size [9]. The exclusion zone towards the bottom of the image containing no ejecta is indicative of the up-range direction of the impactor being towards the bottom of the image. Knowing the direction of the impact is important as incidence angle plays a large part in a number of crater scaling laws [10], but also as a way to differentiate potential parent meteoroid streams if there were multiple possible candidates. Figure 3: The cropped ratio image, with a threshold applied to highlight only the ejecta blanket. The presence of smaller exclusion zones within the ejecta blanket are likely due to effects caused by the local terrain. Future Work Once new images of the crater are collected and released by the Lunar Reconnaissance Orbiter, a size measurement for the impact crater can be obtained. This will allow for the application of crater scaling laws to calculate the impacting kinetic energy, and to solve backwards through current impact flash methodologies. Bibliography [1] Madiedo et al (2010) Advances in Astronomy, 2010:167494 [2] Xilouris et al (2018) A&A , 619, A141 [3] Suggs et al (2008) Earth Moon and Planets, 102:293298 [4] Avdellidou and Vaubaillon (2019) MNRAS, 484, 5212-5222 [5] Madiedo et al (2018) MNRAS, 480, 5010-5016 [6] Bonanos et al (2018) A&A, 612, A76 [7] Suggs et al (2014) Icarus, 10.1016 [8] Sheward et al (2019) EPSC-DPS2019, Abstract 1032-1 [9] Shuvalov (2011) Meteoritics & Planetary Science, 11, 1713-1718 [10] Horedt and Neukum (1984) Earth moon and Planets, 31, 265-269

International audience; Chondrites are leftover solids from the early evolution of the solar protoplanetary disk that never experienced melting since their formation. They comprise unequilibrated assemblages of low-and high-temperature components, including volatile-rich, fine-grained matrices, Fe-Ni metal, sulfides, refractory inclusions, and chondrules. Consequently, chondrites are commonly described as pristine, primitive, or primordial rocks of the solar system. However, impact-generated secondary features are abundant in chondrites, suggesting that collisions among early-formed planetesimals and their fragmentation and reassembly have been effective throughout the evolution of the solar system. In this report, we review evidence of the major role of impacts in generating the current mineralogical and petrographic characteristics of chondrites. We provide perspective to these meteoritic features by discussing recent analyses of large-scale structures of the main asteroid belt and remotesensing observations of asteroids. Observations at various spatial scales all attest that the 'primitive' materials formed during the evolution of the solar system have largely been 2 reprocessed, confirming previous studies that primitivity is relative, not absolute. This implies that (i) chondrites (and some differentiated meteorites) should systematically be envisioned as reprocessed and heterogeneous materials and (ii) brecciated meteorites should be considered the norm and unbrecciated meteorites the exception.

International audience; Thermal stress weathering is now recognized to be an active and significant geomorphological process on airless bodies. This study aims to understand the key factors governing thermal stresses in rocks on airless bodies through extensive numerical calculations and analytic analyses. Some of the key factors governing thermal stresses, are found to be the diurnal surface temperature variation, the second-order spatial gradient of the temperature field, the thermal skin depth, and the rock size of interest. Microscopic (grain-scale) thermal stresses are driven primarily by the amplitude of the magnitude of the maximum diurnal temperature variation at said depth. Macroscopic (rock-scale) thermal stresses are more complex, and their nature fundamentally depends on two length scales: the thermal skin depth and the rock size of interests. For rock sizes larger than the thermal skin depth, macroscopic thermal stresses are driven primarily by second (and higher) order spatial gradients of temperature. For rock sizes smaller than the thermal skin depth, macroscopic thermal stresses are primarily driven by the ratio of rock size to thermal skin depth with macroscopic thermal stresses being greatest when this ratio is 1/2. Additionally, scaling relations for diurnal surface temperature variation, time-rateof-change of surface temperature, as well as peak microscopic (grain-scale) and macroscopic (rock-scale) thermal stresses are derived to provide a more accessible modeling tool. These scaling relations are remarkably accurate when compared to both the numerical calculations as well as three-dimensional finite element calculations. The model formulation, results, and scaling relations provided here allow the estimation of diurnal temperatures and thermal stresses on rocks of various size and materials on airless bodies at any orbital distance with a broad spectrum of spin rates. Lastly, we postulate and confirm that there is a critical spin rate where macroscopic thermal stresses will be greatest.

We present a new method for the determination of the surface properties of airless bodies from measurements of the emitted infrared flux. Our approach uses machine learning techniques to train, validate, and test a neural network representation of the thermophysical behavior of the atmosphereless body given shape model, illumination and observational geometry of the remote sensors. The networks are trained on a dataset of thermal simulations of the emitted infrared flux for different values of surface rock abundance, roughness, and values of the thermal inertia of the regolith and of the rock components. These surrogate models are then employed to retrieve the surface thermal properties by Markov Chain Monte Carlo Bayesian inversion of observed infrared fluxes. We apply the method to the inversion of simulated infrared fluxes of asteroid (101195) Bennu -- according to a geometry of observations similar to those planned for NASA's OSIRIS-REx mission -- and infrared observations of asteroid (25143) Itokawa. In both cases, the surface properties of the asteroid -- such as surface roughness, thermal inertia of the regolith and rock component, and relative rock abundance -- are retrieved; the contribution from the regolith and rock components are well separated. For the case of Itokawa, we retrieve a rock abundance of about 85% for pebbles larger than the diurnal skin depth, which is about 2 cm. The thermal inertia of the rock is found to be lower than the expected value for LL chondrites, indicating that the rocks on Itokawa could be fractured. The average thermal inertia of the surface is around 750 $J s^{-1/2} K^{-1} m^{-2}$ and the measurement of thermal inertia of the regolith corresponds to an average regolith particle diameter of about 10 mm, consistently with in situ measurements as well as results from previous studies., Comment: 20 pages, 8 figures, 6 tables. Accepted for publication in Icarus

We are preparing a telescope system to carry out a survey of detection and analysis of lunar impact flashes. In the framework of this project, here we present all necessary methods to automatically identify these luminous events, their lunar impact coordinates, the origin of the impacting meteoroids, as well as the estimation of their physical properties such as mass and size. We tested our methods against confirmed impact events and constructed the meteoroid size frequency distribution of impactors using literature data of the last 20 years. In addition, we present the first lunar impact event observed from the Observatoire de la Cote d’Azur that was detected during the testing phase of our project.

It has been shown that temperature cycles on airless bodies of our Solar System can cause damaging of surface materials. Nevertheless, propagation mechanisms in the case of space objects are still poorly understood. Present work combines a thermoelasticity model together with linear elastic fracture mechanics theory to predict fracture propagation in the presence of thermal gradients generated by diurnal temperature cycling and under conditions similar to those existing on the asteroid Bennu. The crack direction is computed using the maximal strain energy release rate criterion, which is implemented using finite elements and the so-called Gθ method (Uribe-Suarez et al. 2020. Eng. Fracture Mech. 227:106918). Using the implemented methodology, crack propagation direction for an initial crack tip in different positions and for different orientations is computed. It is found that cracks preferentially propagate in the North to South (N-S), in the North-East to South-West (NE-SW) and in the North-West to South-East (NW-SE) directions. Finally, thermal fatigue analysis was performed in order to estimate the crack growth rate. Computed value is in good agreement with available experimental evidence.

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Context. The inner few au region of planet-forming disks is a complex environment. High angular resolution observations have a key role in understanding the disk structure and the dynamical processes at work. Aims. In this study we aim to characterize the mid-infrared brightness distribution of the inner disk of the young intermediate-mass star HD 163296, from VLTI/MATISSE observations. Methods. We use geometric models to fit the data. Our models include a smoothed ring, a flat disk with inner cavity, and a 2D Gaussian. The models can account for disk inclination and for azimuthal asymmetries as well. We also perform numerical hydro-dynamical simulations of the inner edge of the disk. Results. Our modeling reveals a significant brightness asymmetry in the L-band disk emission. The brightness maximum of the asymmetry is located at the NW part of the disk image, nearly at the position angle of the semimajor axis. The surface brightness ratio in the azimuthal variation is $3.5 \pm 0.2$. Comparing our result on the location of the asymmetry with other interferometric measurements, we confirm that the morphology of the $r, accepted for publication in A&A

The nature of circumstellar envelopes (CSE) around Cepheids is still a matter of debate. The physical origin of their infrared (IR) excess could be either a shell of ionized gas, or a dust envelope, or both. This study aims at constraining the geometry and the IR excess of the environment of the long-period Cepheid $\ell$ Car (P=35.5 days) at mid-IR wavelengths to understand its physical nature. We first use photometric observations in various bands and Spitzer Space Telescope spectroscopy to constrain the IR excess of $\ell$ Car. Then, we analyze the VLTI/MATISSE measurements at a specific phase of observation, in order to determine the flux contribution, the size and shape of the environment of the star in the L band. We finally test the hypothesis of a shell of ionized gas in order to model the IR excess. We report the first detection in the L band of a centro-symmetric extended emission around l Car, of about 1.7$R_\star$ in FWHM, producing an excess of about 7.0\% in this band. In the N band, there is no clear evidence for dust emission from VLTI/MATISSE correlated flux and Spitzer data. On the other side, the modeled shell of ionized gas implies a more compact CSE ($1.13\pm0.02\,R_\star$) and fainter (IR excess of 1\% in the L band). We provide new evidences for a compact CSE of $\ell$ Car and we demonstrate the capabilities of VLTI/MATISSE for determining common properties of CSEs. While the compact CSE of $\ell$ Car is probably of gaseous nature, the tested model of a shell of ionized gas is not able to simultaneously reproduce the IR excess and the interferometric observations. Further Galactic Cepheids observations with VLTI/MATISSE are necessary for determining the properties of CSEs, which may also depend on both the pulsation period and the evolutionary state of the stars., Comment: 13 pages, 8 figures, accepted in Astronomy and Astrophysics

We present the early installment of the third Gaia data release, Gaia EDR3, consisting of astrometry and photometry for 1.8 billion sources brighter than magnitude 21, complemented with the list of radial velocities from Gaia DR2. Gaia EDR3 contains celestial positions and the apparent brightness in G for approximately 1.8 billion sources. For 1.5 billion of those sources, parallaxes, proper motions, and the (G_BP-G_RP) colour are also available. The passbands for G, G_BP, and G_RP are provided as part of the release. For ease of use, the 7 million radial velocities from Gaia DR2 are included in this release, after the removal of a small number of spurious values. New radial velocities will appear as part of Gaia DR3. Finally, Gaia EDR3 represents an updated materialisation of the celestial reference frame (CRF) in the optical, the Gaia-CRF3, which is based solely on extragalactic sources. The creation of the source list for Gaia EDR3 includes enhancements that make it more robust with respect to high proper motion stars, and the disturbing effects of spurious and partially resolved sources. The source list is largely the same as that for Gaia DR2, but it does feature new sources and there are some notable changes. The source list will not change for Gaia DR3. Gaia EDR3 represents a significant advance over Gaia DR2, with parallax precisions increased by 30 percent, proper motion precisions increased by a factor of 2, and the systematic errors in the astrometry suppressed by 30--40 percent for the parallaxes and by a factor ~2.5 for the proper motions. The photometry also features increased precision, but above all much better homogeneity across colour, magnitude, and celestial position. A single passband for G, G_BP, and G_RP is valid over the entire magnitude and colour range, with no systematics above the 1 percent level.

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An asteroid’s history is determined in large part by its strength against collisions with other objects1,2 (impact strength). Laboratory experiments on centimetre-scale meteorites3 have been extrapolated and buttressed with numerical simulations to derive the impact strength at the asteroid scale4,5. In situ evidence of impacts on boulders on airless planetary bodies has come from Apollo lunar samples6 and images of the asteroid (25143) Itokawa7. It has not yet been possible, however, to assess directly the impact strength, and thus the absolute surface age, of the boulders that constitute the building blocks of a rubble-pile asteroid. Here we report an analysis of the size and depth of craters observed on boulders on the asteroid (101955) Bennu. We show that the impact strength of metre-sized boulders is 0.44 to 1.7 megapascals, which is low compared to that of solid terrestrial materials. We infer that Bennu’s metre-sized boulders record its history of impact by millimetre- to centimetre-scale objects in near-Earth space. We conclude that this population of near-Earth impactors has a size frequency distribution similar to that of metre-scale bolides and originates from the asteroidal population. Our results indicate that Bennu has been dynamically decoupled from the main asteroid belt for 1.75 ± 0.75 million years. Analysis of the size and depth of craters on boulders on the asteroid (101955) Bennu indicates that Bennu has been in near-Earth space for 1.75 ± 0.75 million years.

Thermophysical analyses of planetary bodies such as the Moon, Mars, and numerous asteroids have allowed for remote estimates of regolith physical properties, such as particle size and packing density, as well as the relative spatial abundance of boulders. Here we define “regolith” as a particulate assemblage where most particles are comparable to or smaller than the length scale of the diurnal skin depth (the e-folding depth of the diurnal thermal wave). Until recently, regolith and boulders were believed to be thermally quite distinct; regolith on the Moon, Mars, and most asteroids was usually known or suspected to be fine (i.e., ranging from fine dust to sand), meaning that it was known or assumed to have a thermal inertia much lower than that of boulders and bedrock. Upon the arrival of NASA OSIRIS-REx at asteroid Bennu and JAXA Hayabusa2 at asteroid Ryugu and the subsequent thermophysical analyses of the respective asteroid surfaces, this preconceived notion of thermophysically distinct regolith and boulders/bedrock was found to be flawed (DellaGiustina and Emery et al., 2019; Sugita et al., 2019). Boulders cover the vast majority of both asteroids’ surfaces, yet the thermal inertia values determined for these boulder-rich surfaces fall within a range that was previously believed to exclusively represent coarse, sand-to-pebble–sized regolith. Recent work has been devoted to the analysis of the boulders’ thermal inertia and the physical interpretation thereof (Grott et al., 2019; Rozitis et al., in revision); the general conclusion so far is that the boulders have very low thermal conductivity and density owing to the presence of numerous pores and fractures, more so than most (or perhaps all) carbonaceous chondrite meteorites in Earth’s collections. As such, these boulders are likely to be structurally distinct from all known meteorite specimens. Although fine-particulate regolith is rare on the surface of Bennu, it is present in some locations. It is still of great interest to probe the physical properties of this regolith by means of thermal analysis so as to make predictions about the properties of the samples that will be returned by OSIRIS-REx and to learn about the evolution of the asteroid surface and the mechanisms by which regolith is produced and lost or destroyed. There remains the enigmatic question: although we suspect that the boulders on Bennu are distinct from the meteorite collection, will the returned samples of particulate material share these distinct properties? At what scales can we define the physical thermally relevant properties of boulders and regolith particles on Bennu? In other words, if the boulders are indeed highly fractured and porous, are the regolith particles also fractured and porous or are their dimensions below the relevant length scales? It is very challenging to estimate the physical properties of the regolith on Bennu using thermal data, even in regions where it appears to be abundant, due to the coarseness of the regolith. Commonly used planetary thermophysical models rely on the assumption that the material on the surface, be it regolith or rock, can be approximated as a continuous, non-discretized material with physical properties that are either constant with depth or are allowed to vary with depth in some well-defined way (e.g. an exponential density increase, or up to a few layers of physically distinct material, such as regolith on bedrock or dust coatings). This assumption of material continuity is valid when regolith particles are smaller, or perhaps even much smaller, than the diurnal skin depth of the thermal wave. However, high-resolution images of the Nightingale Crater on Bennu, which is the OSIRIS-REx mission’s primary sample collection site, revealed a particle size frequency distribution (SFD) that crosses this threshold; i.e., there are particles present that are smaller than, comparable to, and larger than the diurnal skin depth (~1­–5 cm) present within a single, meters-wide observation footprint. The thermophysical behavior of such a regolith configuration has never been comprehensively studied and likely cannot be properly approximated with standard 1D thermal modeling methods. We will present preliminary results using a 3D regolith model where we render hundreds of regolith particles and rocks, approximated as spheres, in a finite element mesh framework. The model is heated diurnally with a solar source to study the thermal response of such skin depth–crossing SFDs under Bennu surface-like conditions. The SFD of the particles is informed by particle size counts in Nightingale Crater from high-resolution visible images. With the SFD partially constrained, we are able to focus our efforts on exploring the material property parameter space, namely to estimate the thermal conductivity and density of individual regolith particles. Given the coarseness of the regolith on Bennu, we find that the model is more sensitive than one might expect to the thermal conductivity of the individual regolith particles owing to the effects of particle non-isothermality (Ryan et al., 2020). Although this present modeling work is focused on analyzing thermal emission data of Bennu obtained by the OSIRIS-REx Thermal Emission Spectrometer (OTES, Christensen et al., 2018), we aim to expand our efforts to study the more general thermal behavior of coarse regoliths and regoliths with wide SFDs under a range of solar heating conditions. Acknowledgements This material is based upon work supported by NASA under Contract NNM10AA11C issued through the New Frontiers Program. We are grateful to the entire OSIRIS-REx Team for making the encounter with Bennu possible. References: Christensen, P.R. et al. (2018). The OSIRIS-REx Thermal Emission Spectrometer (OTES) Instrument. Space Science Reviews 214, 87. DellaGiustina, D.N., Emery, J.P., et al. (2019) Properties of rubble-pile asteroid (101955) Bennu from OSIRIS-REx imaging and thermal analysis, Nature Astronomy, DOI:10.1038/s41550-019-0731-1. Grott, M., Knollenberg, J., et al. (2019) Low thermal conductivity boulder with high porosity identified on C-type asteroid (162173) Ryugu, Nature Astronomy, DOI:10.1038/s41550-019-0832-x. Rozitis, B., Ryan, A.J., Emery, J.P., et al. (in revision) Asteroid (101955) Bennu’s Weak Boulders and Thermally Anomalous Equator, Science Advances, submitted March 2020. Ryan, A., Pino Muñoz, D., Bernacki, M., Delbo, M. (2020) Full-Field Modeling of Heat Transfer in Asteroid Regolith: Radiative Thermal Conductivity of Polydisperse Particulates, JGR:Planets. DOI:10.1029/2019JE006100. Sugita, S., Honda, R., et al. (2019) The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes, Science. DOI:10.1126/science.aaw0422

Lunar impact flashes: analysis methods Munaibari(1), R. Larson(2), C. Avdellidou(1), M. Delbo(1), J. Vaubaillon(3), P. Hayne(2), D. Sheward(4), A. Cook(4) (1)Laboratoire Lagrange, Observatoire de la Côte d’Azur, UCA, France (2)University of Colorado, USA (3)IMCCE, Observatoire de Paris, France (4)Aberystwyth University, UK contact: chrysa.avdellidou@oca.eu Abstract We present our complete method to analyse lunar impact flashes; from the identification of the selenographic coordinates to the estimation of masses and sizes of the meteoroids. For this work we used archival data from the ESA-funded NELIOTA survey and we report an updated catalogue of the impact coordinates and link to meteoroid streams. This project is in the framework of our project Flash!, which aims to detect impacts in real time during the observations, identify the lunar coordinates and attempt the discovery of the fresh craters using LRO data. The scientific problem is to establish a link between the diameter of the meteoroid impactor and the diameter of the impact crater quantifying this scaling with live impact observations. Selenographic coordinates and link to meteor streams In this work, we exploit the publicly available datafrom NELIOTA (1,2), an ESA-funded survey at the National Observatory ofAthens (NOA). The first step is to identify the impact coordinates on the Moon. Using the method described by Larson et al. EPSC 2019 we have discovered that several impact locations given by the NELIOTA team are inaccurate and in some cases are off by 10s of degrees. The importance ofaccurately locating the position of an impact flashcomes for the fact that it is needed to perform theprocess of linking the impactor to its source and this is crucial to identify the originof the impactor. In this work we evolve from our previous studies (3) and we investigate also the possibility an impactor to originate from the sporadic population and not only from meteoroid streams. Masses and Sizes of the meteoroids According to previous studies (2,3), an impact flashis treated as a black body whose spectral energy distribution is described by Planck’s law. Using the flash magnitudes in R and I bands we estimate temperature values as described with details in (3). The mass of an impactor can be derived from its kinetic energy (KE). To calculate the impact KE it is necessary to measure the luminous energy ELum from the telescopic observations, which is just asmall fraction, η, of the KE. In order to be able to estimate the kinetic energyof an impactor, it is essential to know the velocityat which it impacted onto the lunar surface. Thisvelocity is obtained by linking the impactor to its source, either a known meteoroid stream or the sporadic background population. In the case of streams, the velocity of their meteoroids is known.Once the link is established, the velocity of an impactor is easily computed by finding the differencebetween the velocity of the meteor shower’s particles and that of the Moon at the time of the impact. Assuming a spherical shape for the impactors andwith the estimation of their masses we can now estimate their sizes. In this work, we utilised th e bulk densities that have been reported (4) for the fragments of some ofthe shower streams by assigning impactors with thebulk density of the meteor showers we found to betheir plausible sources. Finally, we construct the size frequency distribution with a slope of -2.48 as shown in Fig.1 and is comparable with the NASA survey (5). Acknowledgements This work was supported by the ProgrammeNational de Planetologie (PNP), France of CNRS/INSU, co-funded by CNES, France and bythe program "Flash!" supported by Crédits Scientifiques Incitatifs (CSI), France of the UniversitéNice Sophia Antipolis. This work has made use ofdata from the European Space Agency (ESA) NELIOTA project. We thank the EUR Spectrum for supporting Mr. Munaibari with a 3-month UCA Master Scholarship to perform this masterthesis. References [1] E. Xirouris, et al. A&A,619, A141 (2018). [2] A. Z. Bonanos, et al. A&A, 612 (2018) [3] C. Avdellidou & J. Vaubaillon. MNRAS, 484(4):5212–5222 (2019). [4] P. B. Babadzhanov & G. I. Kokhirova., A&A, 495:353–358(2009). [5] R. Suggs et al. Icarus, 238, 23 (2014).

Hypervelocity impacts on carbonaceous asteroid simulants: comparison with observations C. Avdellidou(1,2), Marco Delbo(1), Cody Schultz(3), Mark Price(2), Alice DiDonna(4), Bart Harthong(4), Mike Cole(2),Daniel Britt(3), Robert Peyroux(4) Laboratoire Lagrange, Observatoire de la Côte d’Azur, UCA, France CAPS, University of Kent, UK Exolith Lab, University of Central Florida, US 3SR Lab, Grenoble, France contact: chrysa.avdellidou@oca.eu Introduction Two on-going sample return space missions, Hayabusa2 and OSIRIS-REx are orbiting and characterising two near-Earth asteroids, (162173) Ryugu and (101955) Bennu respectively. Initial ground-based observations and mission data indicate that the composition of these small objects is similar to the CM or CI meteorites (1-4). However, their thermo-mechanical properties appear to be different (5,6). This is expected since Earth's atmosphere filters out the weakest materials and thus we do not have them in our meteoritic collections. How does this weak materials respond to impacts? What type of regolith is produced during micrometeoroid bombardment? Will we expect to find exogenous materials embedded on the weak surfaces of Bennu and Ryugu? Materials In our work we used as targets to hypervelocity impact experiments, asteroid analogue materials with mineralogy similar to the CM meteorites. Specifically, the CM2 regolith simulant is a close mineralogical match to the Murchison CM2 carbonaceous chondrite meteorite Since one of the aims of the project was to examine qualitatively the produced regolith after impact events, we used as inclusions glass beads for an easier examination. The samples were produced and casted at the Exolith Lab of the University of central Florida. Samples were also mechanically tested at the 3SR Lab in Grenoble, where the compressive stress and tensile strength were measured. In addition we measured the thermal conductivity of the samples. Experiments In order to study the response of the CM-like asteroid analogue material to collisions with small projectiles, at typical impact speeds occurring in the asteroid Main Belt, we performed a series of laboratory hypervelocity impact experiments. We used the facilities of the Impact Lab of the University of Kent in the UK. main instrument used here is a 2-stage light-gas gun (LGG), which can achieve speeds up to 7.5 km/s. Targets were the squared blocks of CM analogues with dimensions 9.5 cm x 9.5 cm and 4.4 cm thickness, while as projectile we used stainless steel of different diameters. In these experiments we measured the depth and diameter of the craters, the quality of the ejecta and the state of the inclusions. In particular we wanted to test a part of the hypothesis that on materials with inclusions, impacts produce mainly ``multimineralic" fragments, whereas thermal cracking, as a slower process can produce fragments of single composition. After the impact experiments the roughness of the impacted samples was also calculated. Comparison with spacecraft observations First of all we compare the materials we produced with the inferred data from the space mission observations. Furthermore, we compare our laboratory impact results with impact observations of the same scale on the boulders of asteroid Bennu (mini-craters,7). Combining the mechanical properties of the asteroid simulants and the impact results, we predict that the impact shockwave dissipates very rapidly in such a soft material, therefore one would expect to see also fragments with one unique composition. Fig.1 Comparison between the surface of Ryugu (left) and our post-impact asteroid simulant (right). Acknowledgements This work was supported by the BONUS QUALITE RECHERCHE Lagrange (BQR) 2017; by the French National Research Agency under the project ``Investissements d’Avenir" UCAJEDI with the reference number ANR-15-IDEX-01; by the Programme National de Planetologie (PNP) of CNRS/INSU, co-funded by CNES. References [1] Grott et al. Nature Astronomy, 3, p. 971-976, 2019 [2] DellaGiustina, Emery et al., Nature Astronomy, 3, 341, 2019 [3] Kitazato et al., Science, 364, 272, 2019 [4] Moskovitz et al., Icarus, 224, 24, 2013 [5] Perna et al., A&A, 599, L1, 2017 [6] LeCorre et al., MNRAS, 475, 614, 2018 [7] Ballouz et al. Nature (under revision) 2020

The complex history of Bennu's surface The near-Earth asteroid (101955) Bennu is a carbon-rich body with a rubble pile structure, formed from debris ejected by an impact on a larger parent asteroid. The Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) spacecraft is designed to collect a sample of Bennu's surface and return it to Earth. After arriving at Bennu, OSIRIS-REx performed a detailed survey of the asteroid and reconnaissance of potential sites for sample collection. Three papers present results from those mission phases. DellaGiustina et al. mapped the optical color and albedo of Bennu's surface and established how they relate to boulders and impact craters, finding complex evolution caused by space weathering processes. Simon et al. analyzed near-infrared spectra, finding evidence for organic and carbonate materials that are widely distributed across the surface but are most concentrated on individual boulders. Kaplan et al. examined more detailed data collected on the primary sample site, called Nightingale. They identified bright veins with a distinct infrared spectrum in some boulders, which they interpreted as being carbonates formed by aqueous alteration on the parent asteroid. Together, these results constrain Bennu's evolution and provide context for the sample collected in October 2020. Science , this issue p. eabc3660 , p. eabc3522 , p. eabc3557

Introduction: The OSIRIS-REx mission’s observation campaigns [1] using the PolyCam instrument, part of the OSIRIS-REx Camera Suite (OCAMS) [2 3], have returned images of the surface of near-Earth asteroid (NEA) (101955) Bennu. These unprecedented-resolution images resolved cavities on Bennu’s boulders (Fig. 1) that are near-circular in shape and have diameters ranging from 5 cm to 5 m. We made hundreds of measurements of these cavities in image and laser altimeter data and found more than 100 boulders that exhibit at least one on their surface (Fig. 1). The most likely mechanism for the creation of these cavities is impacts. However, it is unclear whether these “mini-craters” were formed during Bennu’s residence in the main asteroid belt, or if they were formed more recently, after Bennu became a near-Earth asteroid (NEA). We use our observations of mini-craters to derive the strength of solid C-type objects against impacts. Our results have implications for Bennu’s history in the main-belt and in near-Earth space. The Strength of Monolithic C-type Objects: The strength of asteroids against collisions is crucial for understanding the surface evolution of airless planetary bodes, the dynamical evolution of asteroids throughout Solar System history, and the incorporation of planetesimals into planets [4,5]. Laboratory data on centimeter-scale meteorites have been extrapolated and buttressed with numerical simulations and analytic formalisms to derive the cratering threshold at the asteroid scale [6–8]. However, thus far it has not been possible to directly assess the strength of the boulders that constitute the building blocks of a rubble-pile asteroid. Apollo lunar rocks and spacecraft missions to near-Earth asteroids indicate only two modes of impact-induced breakdown of boulders: 1) abrasion by micro-meteorites (sand-blasting), and 2) catastrophic rupture by a single large impact [9–11]. Widespread cratering on boulders has not been observed heretofore. Figure 1: Centimeter-scale impact features on the sur-face of a boulder (image 20190703T044506S720_pol, taken July 3, 2019, by the OCAMS PolyCam imager; 1 cm/pixel). We developed a new method to derive the cratering efficiency and the disruption threshold of C-type objects by combining scaling laws and observations of craters on C-type boulders and asteroids [12­–14]. We postulate that the largest crater on a boulder of a given size signifies an impact energy close to that required for disrupting that boulder. This type of analysis has been previously done for the study of the largest craters on planetary bodies larger than tens of kilometers using scaling laws [6] and laboratory experiments [15]. Here, we extend that analysis to objects of smaller size. We find that the crater to impactor size ratio on C-type objects is ~ 15, and that the collisional disruption of 1-m radius boulders on the surface of Bennu is efficient in the main belt (~ 1 Myr) but effectively ceases in near-Earth space as their collisional lifetime (~50 Myr) becomes greater than the dynamical lifetime of NEAs ( Towards a mini-crater clock for NEA surfaces: Detailed analysis of the surface density of mini-craters on Bennu’s boulders may provide a new way to determine relative ages of different regions of the surface. This can be used as a basis of comparison or calibration for alternative approaches to chronology, such as assessments of the small crater population on Bennu’s surface [12] or space-weathering on Bennu’s surface [17]. Our findings may be validated by analyzing the cosmic ray exposure ages of the returned sample. Acknowledgements This work was supported in part by NASA grant NNX15AH90G awarded by the Solar System Workings program. S.R.S. acknowledges support from NASA Grant no. 80NSSC18K0226 as part of the OSIRIS-REx Participating Scientist Program. This material is based upon work supported by NASA under Contract NNM10AA11C issued through the New Frontiers Program. P.M. acknowledges support from the Centre National d’Études Spatiales and from the Academies of Excellence on Complex Systems and Space, Environment, Risk and Resilience of the Initiative d’EXcellence “Joint, Excellent, and Dynamic Initiative” (IDEX JEDI) of the Université Côte d’Azur. We are grateful to the entire OSIRIS-REx team for making the encounter with Bennu possible. References [1] Lauretta et al. (2017) Space Science Reviews 212, 925.[2] Rizk, B. et al. (2018) Space Science Reviews, 214, 26. [3] Golish, D.R. et al. (2020) Space Sci. Rev. 216, 12. [4] Michel, P. et al. (2001) Science, 294, 1696. [5] Bottke, W.F. et al. (2005) Icarus, 179, 63. [6] Holsapple, K.A. (1993) AREAP, 21, 333. [7] Benz, W., & Asphaug, E. (1994). Icarus, 107, 98. [8] Jutzi, M. et al. (2010) Icarus, 207, 54. [9] Hörz, F., et al. (1975) The Moon, 235. [10] Cheng, A.F. (2002) Asteroids III, University of Arizona Press, 351-366. [11] Nakamura, A.M. et al. (2008) Earth, Planets, and Space, 60, 7. [12] Walsh, K.J. et al. (2019) Nature Geoscience, 12, 242. [13] Sugita, S. et al. (2019) Science, 364, 252. [14] Veverka, J. et al. (1997) Science, 278, 2109. [15] Leliwa-Kopystyński, J. et al. (2008) Icarus, 195, 817. [16] Michel, P., & Delbo, M. (2011) Icarus, 209, 520. [17] Trang, D. et al. (2019) LPSC L, Abstract #2172.[1] Lauretta et al. (2017) Space Science Reviews 212, 925

Lunar impact flashes: first detection from the Observatory of Nice Avdellidou(1),M. Delbo(1), E. Munaibari(1), R. Larson(2), J. Vaubaillon(3), P. Hayne(2), D. Sheward(3), A. Cook(3) (1)Laboratoire Lagrange, Observatoire de la Côte d’Azur, UCA, France (2)University of Colorado, USA (3)IMCCE, Observatoire de Paris, France (4)Aberystwyth University, UK contact: chrysa.avdellidou@oca.eu Abstract We report the first lunar flash due to meteoroid impact observed by our team at Observatoire de la Côte d’Azur (OCA) in south France. Introduction Meteoroids impacting onto the lunar surface can produce very short bursts of light–commonly calledimpact flashes. Such flashes have been the subject of several lunar monitoring surveys over the last 20 years (1,2,3) for the purpose of determining the size frequency distribution of near-Earth objects in the cm–dm size range. The goal of our international team is to build a network of moderate telescopes that will survey the lunar surface for impact flashes and subsequently locate the produced impact craters. We have developed all the necessary algorithms in order to detect the flash events in real time during the observations, identify the selenographic coordinates, link the meteoroid to a parent meteoroid stream, measure the mass and size of the meteoroid (4) and discover the potential fresh lunar crater. Updates on the methodology is presented by the accompanying EPSC 2020 abstract of Munaibari & Larson et al. The description of the crater identification is presented by the accompanying EPSC 2020 abstract of Sheward et al. First confirmed impact flash from OCA On the night of May 27th, 2020 at 20:48:49.420 UTC, we detected our first impact flash from the Observatoire de la Côte d’Azur (site of Mt. Gros). This is the first live impact observed for the project "Flash!", the first from the Observatoire de la Côte d’Azur and the first from all France by professional sites. The telescope used is a 16" MEADE coupled with a CMOS ASI ZWO 183mono camera. The frame rate was 20 fps and the frame integration time was 0.05 sec. The telescope was guiding on the lunar crescent using the lunar autoguider that we developed in the framework of the master course of MAUCA (University of Côte d’Azur). Acknowledgements This work was supported by the ProgrammeNational de Planetologie (PNP), France ofCNRS/INSU, co-funded by CNES, France and bythe program "Flash!" supported by Crédits Scientifiques Incitatifs (CSI), France of the UniversitéNice Sophia Antipolis. This work has made use ofdata from the European Space Agency (ESA) NELIOTA project. We thank the EUR Spectrum for supporting Mr. Munaibari with a 3-monthUCA Master Scholarship to perform this masterthesis. References [1] J. L. Ortiz, et al.A&A,343: L57–L60, (1999). [2] J. L. Ortiz, et al. Nature, 405:921–923, (2000). [3] R. M. Suggs, et al.Icarus,238:23–36, (2014). [4] C. Avdellidou & J. Vaubaillon.MNRAS, 484(4):5212–5222, (2019).

When rubble-pile asteroid 2008 TC3 impacted Earth on 7 October 2008, the recovered rock fragments indicated that such asteroids can contain exogenic material1,2. However, spacecraft missions to date have only observed exogenous contamination on large, monolithic asteroids that are impervious to collisional disruption3,4. Here, we report the presence of metre-scale exogenic boulders on the surface of near-Earth asteroid (101955) Bennu—the 0.5-km-diameter, rubble-pile target of the OSIRIS-REx mission5 that has been spectroscopically linked to the CM carbonaceous chondrite meteorites6. Hyperspectral data indicate that the exogenic boulders have the same distinctive pyroxene composition as the howardite–eucrite–diogenite (HED) meteorites that come from (4) Vesta, a 525-km-diameter asteroid that has undergone differentiation and extensive igneous processing7–9. Delivery scenarios include the infall of Vesta fragments directly onto Bennu or indirectly onto Bennu’s parent body, where the latter’s disruption created Bennu from a mixture of endogenous and exogenic debris. Our findings demonstrate that rubble-pile asteroids can preserve evidence of inter-asteroid mixing that took place at macroscopic scales well after planetesimal formation ended. Accordingly, the presence of HED-like material on the surface of Bennu provides previously unrecognized constraints on the collisional and dynamical evolution of the inner main belt. Six bright boulders of exotic material on near-Earth asteroid (101955) Bennu stand out from the average asteroidal surface. This unexpected record of impactors offers clues to the formation history of Bennu.

Context.TheGaiamission of the European Space Agency is measuring reflectance spectra of a number to the order of 105small Solar System objects. A first sample will be published in theGaiaData Release scheduled for 2021.Aims.The aim of our work was to test the procedure developed to obtain taxonomic classifications for asteroids based only onGaiaspectroscopic data.Methods.We used asteroid spectra obtained using the DOLORES (Device Optimised for the LOw RESolution) instrument, a low-resolution spectrograph and camera installed at the Nasmyth B focus of the Telescopio NazionaleGalileo. Because these spectra have a higher spectral resolution than that typical of theGaiaspectra, we resampled them to more closely match the expectedGaiaspectral resolution. We then developed a cloning algorithm to build a database of asteroid spectra belonging to a variety of taxonomic classes, starting from a set of 33 prototypes chosen from the 50 asteroids in our observing campaign. We used them to generate a simulated population of 10 000 representative asteroid spectra and employed them as the input to the algorithm for taxonomic classification developed to analyzeGaiaasteroid spectra.Results.Using the simulated population of 10 000 representative asteroid spectra in the algorithm to be used to produce theGaiaasteroid taxonomy at the end of the mission, we found 12 distinct taxonomic classes. Two of them, with 53% of the sample, are dominant. At the other extreme are three classes each with Conclusions.We demonstrated the robustness of our algorithm for taxonomic classification by using a sample of simulated asteroid spectra fully representative of what is expected to be in theGaiaspectroscopic data catalogue for asteroids. Increasingly larger data sets will become available as soon as they are published in the futureGaiadata releases, with the next one coming in 2021. This will be exploited to develop a correspondingly improved taxonomy, likely with minor tweaks to the algorithm described here, as suggested by the results of this preliminary analysis.

One of the most challenging issues in computational fracture mechanics is the propagation of a crack through a finite element mesh for arbitrary crack paths. In this work, this problem is approached by means of an advanced remeshing technique that propagates a crack using cohesive elements. The crack direction is computed using the maximal energy release rate criterion which is implemented using finite elements and the G θ method. The remeshing procedure used here is composed of two stages. In the first step, a conforming mesh is obtained in the computed crack direction, ensuring that edges are placed over the sought direction. In the second stage, cohesive elements are dynamically inserted at the conforming edges previously remeshed. The combination of this remeshing technique with dynamic insertion of cohesive elements, leads to a mesh-independent crack propagation method. The effects of different numerical and physical parameters regarding the crack path and fracture energy is investigated.

Characterizing the surface material of an asteroid is important for understanding its geology and for informing mission decisions, such as the selection of a sample site. Diurnal surface temperatur...

Characterizing the surface material of an asteroid is important for understanding its geology and for informing mission decisions, such as the selection of a sample site. Diurnal surface temperature amplitudes are directly related to the thermal properties of the materials on the surface. We describe a numerical model for studying the thermal conductivity of particulate regolith in vacuum. Heat diffusion and surface-to-surface radiation calculations are performed using the finite element (FE) method in three-dimensional meshed geometries of randomly packed spherical particles. We validate the model for test cases where the total solid and radiative conductivity values of particulates with monodisperse particle size frequency distributions (SFDs) are determined at steady-state thermal conditions. Then, we use the model to study the bulk radiative thermal conductivity of particulates with polydisperse, cumulative power-law particle SFDs. We show that for each polydisperse particulate geometry tested, there is a corresponding monodisperse geometry with some effective particle diameter that has an identical radiative thermal conductivity. These effective diameters are found to correspond very well to the Sauter mean particle diameter, which is essentially the surface-area-weighted mean. Next, we show that the thermal conductivity of the particle material can have an important effect on the radiative component of the thermal conductivity of particulates, especially if the particle material conductivity is very low or the spheres are relatively large, owing to non-isothermality in each particle. We provide an empirical correlation to predict the effects of non-isothermality on radiative thermal conductivity in both monodisperse and polydisperse particulates., Comment: Accepted by Journal of Geophysical Research: Planets

The distributions of size and chemical composition in the regolith on airless bodies provides clues to the evolution of the solar system. Recently, the regolith on asteroid (25143) Itokawa, visited by the JAXA Hayabusa spacecraft, was observed to contain millimeter to centimeter sized particles. Itokawa boulders commonly display well-rounded profiles and surface textures that appear inconsistent with mechanical fragmentation during meteorite impact; the rounded profiles have been hypothesized to arise from rolling and movement on the surface as a consequence of seismic shaking. We provide a possible explanation of these observations by exploring the primary crack propagation mechanisms during thermal fatigue of a chondrite. We present the in situ evolution of the full-field strains on the surface as a function of temperature and microstructure, and observe and quantify the crack growth during thermal cycling. We observe that the primary fatigue crack path preferentially follows the interfaces between monominerals, leaving them intact after fragmentation. These observations are explained through a microstructure-based finite element model that is quantitatively compared with our experimental results. These results on the interactions of thermal fatigue cracking with the microstructure may ultimately allow us to distinguish between thermally induced fragments and impact products., 23 pages, 7 figures

著者人数: 37名 (所属. 宇宙航空研究開発機構宇宙科学研究所(JAXA)(ISAS): 岡田, 達明; 坂谷, 尚哉; 田中, 智; 矢野, 創), Number of authors: 37 (Affiliation. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency(JAXA)(ISAS): Okada, Tatsuaki; Sakatani, Naoya; Tanaka, Satoshi; Yano, Hajime）, Accepted: 2019-06-04, 資料番号: SA1190164000

International audience; Carbonaceous (C-type) asteroids1 are relics of the early Solar System that have preserved primitive materials since their formation approximately 4.6 billion years ago. They are probably analogues of carbonaceous chondrites2,3 and are essential for understanding planetary formation processes. However, their physical properties remain poorly known because carbonaceous chondrite meteoroids tend not to survive entry to Earth’s atmosphere. Here we report on global one-rotation thermographic images of the C-type asteroid 162173 Ryugu, taken by the thermal infrared imager (TIR)4 onboard the spacecraft Hayabusa25, indicating that the asteroid’s boulders and their surroundings have similar temperatures, with a derived thermal inertia of about 300 J m−2 s−0.5 K−1 (300 tiu). Contrary to predictions that the surface consists of regolith and dense boulders, this low thermal inertia suggests that the boulders are more porous than typical carbonaceous chondrites6 and that their surroundings are covered with porous fragments more than 10 centimetres in diameter. Close-up thermal images confirm the presence of such porous fragments and the flat diurnal temperature profiles suggest a strong surface roughness effect7,8. We also observed in the close-up thermal images boulders that are colder during the day, with thermal inertia exceeding 600 tiu, corresponding to dense boulders similar to typical carbonaceous chondrites6. These results constrain the formation history of Ryugu: the asteroid must be a rubble pile formed from impact fragments of a parent body with microporosity9 of approximately 30 to 50 per cent that experienced a low degree of consolidation. The dense boulders might have originated from the consolidated innermost region or they may have an exogenic origin. This high-porosity asteroid may link cosmic fluffy dust to dense celestial bodies1

International audience; In 2021, the Gaia mission will release its third data set of measurements including for the first time thousands of reflectance spectra of asteroids. For the final release of Gaia, this number is expected to increase to 100 000. The reflectance spectra are derived from spectro-photometric detectors covering wavelengths from blue to red (from 325 to 1100 nm). The official full Gaia Taxonomy of asteroids will be delivered during the final data release. In this presentation, we will describe the density-based clustering methods used for deriving this taxonomy but also other modern approaches that could be considered in the future.