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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.03.016 Byline: P. Vernazza (a), R. Brunetto (b), R.P. Binzel (c), C. Perron (d), D. Fulvio (e), G. Strazzulla (e), M. Fulchignoni (f) Keywords: Asteroid; surfaces; Spectroscopy; Experimental techniques; Meteorites Abstract: We present new irradiation experiments performed on the enstatite chondrite Eagle (EL6) and the mesosiderite Vaca Muerta. These experiments were performed with the aims of (a) quantifying the spectral effect of the solar wind on their parent asteroid surfaces and (b) identifying their parent bodies within the asteroid belt. For Vaca Muerta we observe a reddening and darkening of the reflectance spectrum with progressive irradiation, consistent with what is observed in the cases of silicates and silicate-rich meteorites such as OCs and HEDs. For Eagle we observe little spectral variation, and therefore we do not expect to observe a significant spectral difference between EC meteorites and their parent bodies. We evaluated possible parent bodies for both meteorites by comparing their VNIR spectra (before and after irradiation) with those of [approximately equal to]400 main-belt asteroids. We found that 21 Lutetia (Rosetta's forthcoming fly-by target) and 97 Klotho (both Xc types in the new Bus-DeMeo taxonomy) have physical properties compatible with those of enstatite chondrite meteorites while 201 Penelope, 250 Bettina and 337 Devosa (all three are Xk types in the Bus-DeMeo taxonomy) are compatible with the properties of mesosiderites. Author Affiliation: (a) Research and Scientific Support Department, European Space Agency, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands (b) Institut d'Astrophysique Spatiale, CNRS, UMR-8617, Universite Paris-Sud, bAcentstiment 121, F-91405 Orsay Cedex, France (c) Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA (d) Laboratoire d'Etude de la Matiere Extraterrestre, Museum National d'Histoire Naturelle, Paris, France (e) INAF-Osservatorio Astrofisico di Catania, via S. Sofia 78, I-95123 Catania, Italy (f) Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique, Observatoire de Paris, 5 Place Jules Janssen, Meudon, F-92195, France Article History: Received 6 November 2008; Revised 11 March 2009; Accepted 13 March 2009

Observations of emissivity features of 10 Hygiea have been made for the first time in the relatively unexplored thermal-infrared wavelength region with the ISO (Infrared Space Observatory) satellite. Spectrophotomer (PHT-S) and short wavelength spectrometer (SWS) spectra of 10 Hygiea, obtained at 5.8-11.6 and 7-45 [micro]m, respectively, are presented. In order to remove the thermal emission continuum, an advanced thermo-physical model has been applied to the observational data. To better interpret the spectral features above the thermal emission continuum, we compared the ISO observations with laboratory spectra available in the literature. Several laboratory experiments on minerals and meteorites have been performed to complete the analysis and to study the spectral behavior at various grain sizes. A possible spectral similarity with CO carbonaceous chondrites at small grain size is demonstrated. Key Words: asteroids; 10 Hygiea; infrared observations; spectroscopy; ISO.

The Edgeworth-Kuiper Belt (EKB) objects are fossil remnants of the formation of the Solar System and they represent an important reservoir of primordial, thermally unprocessed material. The study of these objects has rapidly evolved in the last few years. New observational work allowed planetary scientists to investigate evolutionary and dynamical processes in the outer solar system, putting some constraints on, for example, the origin of comets and the accretion of planetesimals. The chemical and physical information on this population is still very poor: intensive observational studies are needed. For this reason we started a spectrophotometric observation program in 1997 at ESO with the NTT telescope to investigate the EKB. We present the obtained results for six objects (1994 JR(sub 1), 1994 TB, 1995 QY(sub 9), 1996 TL(sub 66), 1996 TO(sub 66), and 1996 TP(sub 66)). We analyzed all the B, V, R, and I data available in the literature to investigate some bulk properties. We found indications of a complex and inhomogeneous population. Key Words: trans-neptunian objects; Kuiper Belt objects; photometry; spectrophotometry.

The analysis of visible and near-infrared reflectance spectra of C-class objects has revealed the presence of features that appear to indicate a history of aqueous alteration on their surfaces. This result can provide constraints on our understanding of the early Solar System. We have recorded spectra covering 0.48-0.92/[[micro]meter] for 29 C-class objects with orbital semimajor axes between 2.3 and 3.6 AU. About 70% of the observed objects show features produced as the result of aqueous alteration processes.

The Eos family detected by Hirayama in 1918 has been always considered to be compositionally homogeneous. To investigate the composition and the homogeneity of the members of this family, we started a spectroscopic survey at the European Southern Observatory (ESO) with wavelength coverage ranging from 4800 to 9200 [Angstrom]. We observed 45 Eos asteroid members, which constitutes the first large survey of this family. Our results reveal the Eos objects have spectral signature characterizing the whole family: a maximum at [Lambda] [similar to] 8000-8500 [Angstrom] and a reflectivity gradient spanning a continuous range. Only two of the 45 investigated objects seem to be interlopers. While the lower range of this spectral distribution has been easily connected with CO-CV chondrites, we have found no satisfactory meteorite counterpart to the upper range. We have interpreted the spread out of Eos spectra to be the results of compositional variation among the Eos members, implying that the Eos parent body was partially differentiated. Moreover, a space weathering effect has been proven to be present, but with a minor role played in the diversity of Eos family, the major role being the compositional variation. Key Words: asteroids composition; spectroscopy; meteorites.

We present the results of photometric observations of 12 asteroids, performed from 1985 through 1989 at the Astronomical Observatory of Teramo (Italy), Astronomical Observatory of Turin (Italy), and European Southern Observatory (ESO, La Silla, Chile). Using the EA pole determination method described in G. De Angelis (Planet. Space Sci., 41, 285-290, 1993), we have obtained the first spin and shape solution of 85 Io and 360 Carlova and an improvement of the previous determinations for 12 Victoria, 192 Nausikaa and 423 Diotima, for which we determined for the first time the sidereal period and the sense of rotation. We have checked the determinations obtained by other authors for the asteroids 3 Juno, 6 Hebe, 20 Massalia, 43 Ariadne, and 52 Europa, and suggested a very rough estimate of the spin and shape parameters of 115 Thyra.

The near-infrared spectra of nine dark asteroids were obtained to determine the presence of the cyanide (CN) overtone in the 2.24 micrometer wavelength region. The spectra were obtained using 2.2 m telescope at the Mauna Kea Observatory with the K-band spectrograph. The presence of CN could not be detected as the spectra of asteroids did not exhibit any characteristic features in these regions.

Photoelectric studies of 16 asteroids at Chile's European Southern Observatory with the 1-m telescope reveals that the 56 single-night light curves of the asteroids vary with the rotational period. Fourier analysis helps compute the synodic rotational period for 11 asteroids. The other five asteroids do not exhibit a complete rotational period. Rotational period values help evaluate reliability code, lightcurve amplitude and the diameters of color bands.

Data from the European Southern Observatory in Chile from 1984 to 1992 help determine rotational behavior of small asteroids and derive bias-free photoelectric observations of small asteroids. A computer program that includes the extinction difference between asteroid and comparison star helps reduce the data that determines complete light curves and reliable rotation periods for various asteroids.

Spatial resolving power of ground based optical telescopes is reduced by the turbulence in the Earth's atmosphere, causing 'seeing' effects in the images. These defects in optical telescopes can be rectified using adaptive optics in the near infrared regions. This new technique is useful for observing planets and other smaller bodies of the solar system which are resolved by bigger telescopes nearing the diffraction limits. The satellites of Saturn, Pallas and Titan, asteroids and other smaller planets are observed by this technique.

Photometric observations of asteroid 951 Gaspra taken prior tothe Galileo deep-space probe flyby were analyzed to determine the characteristics of the minor planet. The results showed that Gaspra had a solarphase angle dependence which exhibited a G value of around 0.29 for the lightcurve maxima. In combination with data obtained by the Galileo space probe, the information gathered from these images allowed a groundbased model of Gaspra to be formulated.

Abstract: On its course to the comet 67P/Churyumov–Gerasimenko, Rosetta will fly past two main belt asteroids: 2867 Steins and 21 Lutetia. In order to explore the possible risks of impact between the Rosetta probe and particles orbiting 21 Lutetia, the asteroid’s dynamical environment is studied. Stable retrograde, nearly equatorial orbits, for small particles (1mm) can reach about 180 asteroidal radii on the X − Y directions. Stable inclined orbits, for small particles (1mm) reach 50 asteroidal radii on the Z direction. The importance of the solar radiation pressure and of the solar gravitational perturbations in affecting the behavior of distant orbits is shown. In particular, the stability zone for 1cm particles are much larger than for 1mm-sized particles, due to the reduced relative importance of the solar radiation pressure, while particles smaller than 1mm should be almost completely swept away by this perturbation. [Copyright &y& Elsevier]

In the framework of the activities going on in preparation for the mission of the Huygens probe in Titan''s atmosphere (January 2005), the Huygens Atmospheric Structure Instrument (HASI) team scheduled and performed several balloon campaigns to test the HASI sensors’ performance in flight conditions in the Earth''s atmosphere. In particular, pressure conditions reached during each test are similar to those expected in Titan''s lower atmosphere. A 1:1 scaled mock-up of the Huygens probe was launched with a stratospheric balloon in 2001 (Br. Assoc. Adv. Sci. 33 (2001) 1109) and in 2002 (Br. Assoc. Adv. Sci. 34 (2002) 911; Adv. Space. Sci. (2003)) from the G. Broglio base of the Italian Space Agency, located in Trapani Milo (Sicily). In both cases the mock-up was dropped from an altitude higher than 27 and 32 km, respectively, and recovered on the ground after a parachuted descent. In this paper, we describe the results obtained in reconstructing (i) the probe descent trajectory and (ii) the profiles of the physical quantities characterizing the Earth''s atmosphere, on the basis of a complete analysis of the data obtained during the HASI 2002 balloon flight experiment. Using temperature and pressure measurements, we are able to reach an accuracy of the order of 0.5% on the altitude reconstruction during the descent. We validate both the models used for trajectory reconstruction and to check the sensors’ performance. We describe the problems faced in determining the Huygens probe descent trajectory in Titan''s atmosphere focusing our discussion on the critical aspects of the descent reconstruction (such as the uncertainties due to measurement errors, limited knowledge of the atmospheric composition, etc.) and the validity of the adopted assumptions. [Copyright &y& Elsevier]

We developed a series of balloon experiments parachuting a 1:1 scale mock-up of the Huygens probe from an altitude just over 30 km to simulate at planetary scale the final part of the descent of the probe through Titan''s lower atmosphere. The terrestrial atmosphere represents a natural laboratory where most of the physical parameters meet quite well the bulk condition of Titan''s environment, in terms of atmosphere composition, pressure and mean density ranges, though the temperature range will be far higher.The probe mock-up consists of spares of the HASI sensor packages, housekeeping sensors and other dedicated sensors, and also incorporates the Huygens Surface Science Package (SSP) Tilt sensor and a modified version of the Beagle 2 UV sensor, for a total of 77 acquired sensor channels, sampled during ascent, drift and descent phase.An integrated data acquisition and instrument control system, simulating the HASI data-processing unit (DPU), has been developed, based on PC architecture and soft-real-time application. Sensor channels were sampled at the nominal HASI data rates, with a maximum rate of 1 kHz. Software has been developed for data acquisition, onboard storage and telemetry transmission satisfying all requests for real-time monitoring, diagnostic and redundancy.The mock-up of the Huygens probe mission was successfully launched for the second time (first launch in summer 2001, see Gaborit et al., 2001) with a stratospheric balloon from the Italian Space Agency Base “Luigi Broglio” in Sicily on May 30, 2002, and recovered with all sensors still operational. The probe was lifted to an altitude of 32 km and released to perform a parachuted descent lasting 53 min, to simulate the Huygens mission at Titan. Preliminary aerodynamic study of the probe has focused upon the achievement of a descent velocity profile reproducing the expected profile of Huygens probe descent into Titan.We present here the results of this experiment discussing their relevance in the analysis of the data which will be obtained during the Huygens mission at Titan. [Copyright &y& Elsevier]

A mock-up of the probe descending in the Titan atmosphere as part of the Huygens Cassini Mission was successfully launched and recovered on 30th May 2002 after a stratospheric balloon launch from the Italian Space Agency Base “Luigi Broglio” in Trapani, Sicily. To simulate the Huygens mission at Titan, the probe was lifted to an altitude of 32 km and then released to perform a 45 min parachute decelerated descent. The probe was hosting spares of HASI instruments, housekeeping sensors and other dedicated sensors, Beagle II UV Sensors and Huygens Tilt Sensor, for a total of 76 acquired sensor channels and sampled during the ascent, drift and descent phase. An integrated data acquisition and instrument control system was developed, based on PC architecture and soft-real-time application. Sensors channels were sampled at the nominal HASI data rates, with a max rate of 1 kHz. Software was developed for data acquisition, onboard storage and telemetry transmission satisfying all requests for real-time monitoring, diagnostic and redundancy. The main goal of this flight was to verify sensor performance and perform a realistic functional test for HASI hardware in dynamical and environmental conditions similar to those of the Titan atmosphere as well as the impact detection sequence of HASI accelerometer and HASI during ground landing. Aerodynamic study of the probe has contributed in achieving descent velocity and spin rate profiles close to the ones envisioned for the Huygens Titan descent. Profiles have been calculated by solving a system of ODE describing the translational and rotational motion of the probe through the Earth’s atmosphere during parachute aided descent. Results of these calculations have driven the choice of an appropriate angle of attack of the blades in the bottom of the probe and ballast weight during flight.Passive thermal control of the probe has also been designed and implemented in order to guarantee proper temperature ranges on critical components and instrument during all mission phases. Preliminary results of main instruments are also presented in this work. [Copyright &y& Elsevier]

We present the results of the first-ever visible spectroscopic survey fully dedicated to the small (absolute magnitude H ≥ 20) near-Earth asteroid (NEA) population. Observations have been performed at the New Technology Telescope (NTT) of the European Southern Observatory (ESO), during a 30-night Guaranteed Time Observations programme, in the framework of the European Commission financed NEOShield-2 project. The visible spectra of 147 objects have been obtained and taxonomically classified. They show a peculiar taxonomic distribution, with respect to larger NEAs. In particular, olivine-rich A-types and organic-rich D-types are more abundant than what could be expected by extrapolating the taxonomic distribution of larger NEAs. Such results have implications for the investigation of the first phases of solar system history, including the delivery of prebiotic material on the early Earth. Having been obtained over a large range of solar phase angles, our data allowed us to evidence peculiar phase reddening behaviours for asteroids belonging to different taxonomic types. Low-albedo asteroids display no or limited phase reddening, compared to moderate- and high-albedo objects. This result suggests a promising novel way to distinguish primitive asteroids in the X-complex. In agreement with previous laboratory experiments, olivine-rich surfaces are the most affected by phase reddening. [ABSTRACT FROM AUTHOR]

Besides being of great scientific interest, near-Earth objects represent a well-founded threat to life on our planet. Nonetheless, up to now there has been no concerted international plan on how to deal with the impact threat, and how to prepare and implement mitigation measures. The NEOShield project is funded by the European Commission to address such issues, to investigate the feasibility of techniques to prevent a potentially catastrophic impact on Earth by an asteroid or a comet, and to develop detailed designs of appropriate space missions to test deflection techniques. In this work we present and discuss the scientific and strategic aspects of the asteroid impact threat, highlighting the necessary steps so as to be ready to react to future hazardous objects. [ABSTRACT FROM AUTHOR]

Asteroids, and more globally small bodies, are keys to unravel the origin and the evolution of our Solar system. Remnants of the material which formed planets early on, they captured the composition and the conditions of formation of the latter at that time, since they did not undergo too harsh internal alteration during their lifetimes. The Hayabusa2 spacecraft explored the Cb-type asteroid (162173) Ryugu between June 2018 and November 2019, notably collecting data with its Near-InfraRed Spectrometer (NIRS3) in the 1.8–3.2 ​μm range. We selected five NIRS3 datasets among the ones having the highest spatial resolution on which we applied the G-mode multivariate statistical analysis, in order to spot small heterogeneities at the surface of Ryugu. Both global scale and local scale cases were investigated. With a confidence level of 3 σ , we obtained two classes at global and local scales, while decreasing to 2 σ results in the identification of more classes (five at global scale, four at local scale). Overall, our results are consistent among them and with previous studies. In particular, terrains spectrally redder as well as terrains spectrally bluer than the average surface are newly identified by our analysis. The floors of all main craters at the surface of Ryugu appear redder than their surroundings, and are associated with a probable smaller grain size material. A large depression of the Northern hemisphere may contain bluer heterogeneities likely due to a more packed or/and fresher, subsequently deposited material. Finally, a comparison with data from the Hayabusa2 optical camera ONC-T shows no evident correlation between the spectral properties in the near-infrared and in the visible. • We selected high-resolution near-infrared spectra of asteroid Ryugu. • We applied a multivariate statistical analysis at both global and local scale. • Small heterogeneities of composition/alteration are identified. • No clear spatial correlation was found with multispectral images of Ryugu. [ABSTRACT FROM AUTHOR]

Orbiting asteroid (4) Vesta from July 2011 to August 2012, the Framing Camera on board the Dawn spacecraft has acquired several tens of thousand images of the asteroid surface, revealing a complex landscape. The topography is dominated by craters of all sizes and shapes, from fresh, simple, bowl-shaped craters to giant basins, as seen in the southern hemisphere. Craters of different ages or states of degradation can be seen all over the surface; some have very sharp rims and simple morphology, whereas others are highly eroded and have sometimes been filled by landslides and ejecta from nearby craters. The general depth/Diameter (d/D) distribution on Vesta is similar to what has been observed on other small rocky objects in the Solar System with a distribution peaking at 0.168±0.01 in the range 0.05–0.35. However, the global map of d/D reveals important geographic variations across the surface, unlike any other asteroid. The northern most regions of Vesta show d/D values comparable to other asteroid surfaces, with a mean d/D of 0.15±0.01, and a steep cumulative distribution. Craters in the regions affected by the giant southern impacts are deeper (mean d/D=0.19±0.01) and show less erosion. It can be interpreted as the southern surface being younger than the rest of the asteroid, or made of a material which either allows the formation of deeper features or prevents their erosion. This picture is consistent with the idea of a southern Vestan hemisphere resurfaced relatively recently by the giant impact that created the Rheasilvia basin. The analysis of depth-to-Diameter variations over the whole surface also brings some insight into the transition regions between different cratering regimes: about 20 km for the strength-to-gravity dominated regime, and 38 km for the beginning of the simple-to-complex transition. [ABSTRACT FROM AUTHOR]

Abstract: Although discussions are underway within the Action Team 14 of the United Nations COPUOS, there is currently no concerted international plan addressing the impact threat from near-Earth objects (NEOs) and how to organize, prepare and implement mitigation measures. We report on a new international project to address impact hazard mitigation issues, being the subject of a proposal submitted to the European Commission in response to the 2011 FP7 Call “Prevention of impacts from near-Earth objects on our planet”. Our consortium consists of 13 research institutes, universities, and industrial partners from 6 countries and includes leading US and Russian space organizations. The primary aim of the project, NEOShield, is to investigate in detail the three most promising mitigation techniques: the kinetic impactor, blast deflection, and the gravity tractor, and devise feasible demonstration missions. Furthermore, we will investigate options for an international strategy for implementation when an actual impact threat arises. The NEOShield project was formally accepted by the European Commission on 17 November 2011 and funded with a total of 5.8 million Euros for a period of 3.5 years. The kick-off meeting took place at the DLR Institute of Planetary Research, Berlin, in January 2012. In this paper we present a brief overview of the planned scope of the project. [Copyright &y& Elsevier]

Abstract: All available data regarding compositionally diagnostic measurements of asteroid (21) Lutetia are assembled. We review ground-based observations together with recent spacecraft mission (Rosetta, Herschel and Spitzer) measurements. The aim of this paper is to gather all the pieces of the Lutetia puzzle, in order to derive the most probable surface composition interpretation. During the flyby Rosetta observed only the northern hemisphere where OSIRIS imaging system found albedo variations of up to 30%. From our analyses of all the available data, taken at different aspect angles, variations on Lutetia''s surface are inferred to be due to compositional and morphological differences (at macro and micro scales). We conclude that Lutetia has a chondritic surface, possibly composed of mixtures of “incompatible” types of materials: carbonaceous and enstatite chondrites. Finding these materials juxtaposed could be the consequence of large impacts that created the present surface. [Copyright &y& Elsevier]

Abstract: The Huygens Probe measured the electrical conductivity of Titan atmosphere from about 140km down to the surface, employing relaxation and mutual impedance techniques. Previous analyses have shown some differences on the conductivity measurements obtained with two independent sensors—relaxation probe (RP) and mutual impedance probe (MIP). A 20-fold maximum discrepancy occurred around the conductivity peak at 60–70km. To understand the nature of such discrepancy, we reassess the RP data by taking into account a geometrical factor related to the electrode finite size and the Debye length of the ionized medium. The present analysis implies replacing the standard Laplace field distribution by a more elaborated model considering the Poisson equation and the resistance between the RP electrodes and the medium. Although a complete understanding of the conductivity profile is still missing, this work brings RP and MIP data to a much better agreement. The conductivity maximum difference derived from the two sensors is now lower a factor of 2. This reassessment is also useful for future instruments and missions. [ABSTRACT FROM AUTHOR]

Abstract: We investigate the possible presence of heterogeneous surface features on asteroid (2867) Steins, using the G-mode multivariate statistical method () applied to Rosetta/OSIRIS images. We analyze both NAC and WAC images obtained near around the closest approach that occurred on September 5th, 2008, through different filters centered on wavelengths ranging from 295 to 986nm. The shape of Steins is modeled as a polyhedron of almost 58000 facets. Photometric corrections were performed using model to compensate for the variable illuminations conditions at the surface. The G-mode classification method was performed on all visible and illuminated facets, i.e. in a region limited to [−50°,+60°] in latitude and [−40°,+90°] in longitude, that represents almost 30% of the total surface. The analyzed set of facets does not show any significant difference in the reflected light content, suggesting no surface inhomogeneities larger than 4% at the 95% confidence level. [Copyright &y& Elsevier]

Abstract: On 5 September 2008, the Rosetta spacecraft encountered the asteroid 2867 Steins on its way to the comet 67P/Churyumov-Gerasimenko. This was the first of two planned asteroid fly-bys performed by the probe, the second fly-by being with the much larger asteroid 21 Lutetia in July 2010. The VIRTIS imaging spectrometer (IFOV 0.250mrad, overall spectral range 0.25–5.1μm) onboard Rosetta acquired data of Steins already before the closest approach phase, when the target was spatially unresolved, in order to obtain a light curve of the asteroid in the infrared spectral range extending up to 5μm, that was never explored before. The VIRTIS light curve campaign started at 11:30UTC onboard time, when the spacecraft was about 221,377km away from the target, and ended at 17:58UTC, at a distance of 20,741km away from Steins. During this timeframe, the solar phase angle of the asteroid was roughly constant, ranging from 38.2° to 36.3°. Assuming the most recent value derived for the rotational period of Steins (), the VIRTIS observations covered slightly more than one rotation of the asteroid. In this interval, VIRTIS collected 8 hyperspectral cubes where Steins was captured 119 times, both in the visual and in the infrared range. Given the low signal and the unresolved appearance of the source, for which the instrument was not designed, only a small subset of wavelengths turned out to be suitable to sample the light curve. Nevertheless, in both the VIS and NIR ranges we find a similar trend, with two different maxima and minima during one rotational period, and amplitudes consistent with the results in the visual range obtained in previous works, including the data set acquired by the OSIRIS camera onboard Rosetta. We also report the presence of a new broad feature centered at approximately 0.81–0.82μm, which is seen in the visual data throughout the rotation of the asteroid. [Copyright &y& Elsevier]

Abstract: The Huygens Probe data provided a direct measurement of CH4, temperature, and pressure in Titan''s atmosphere. This data can be used to compare to the Voyager data in which the effects of CH4, temperature, and pressure were mixed together. Comparison with Huygens data indicates that values of the surface relative humidity of CH4 at the Voyager ingress and egress were between 20% and 45%, and values above 60% are inconsistent with this comparison. The most parsimonious explanation for the Voyager data is that the temperature and CH4 surface humidity at the Voyager ingress and egress profiles were identical to the Huygens values; a surface temperature of 93.65±0.25K, and a surface relative humidity of 43%. Thus, it is likely that these values have characterized the equatorial region of Titan from 1980 until 2005. The small reduction of 1K, between the tropopause temperatures of the Voyager profiles and the tropopause temperature of the Huygens profile is explainable by a change in the antigreenhouse flux from the stratosphere from 0.13 to 0.1 of the total average solar flux. This could result from a small seasonal change in the optical properties of the stratospheric haze. [Copyright &y& Elsevier]

Abstract: The Huygens probe returned scientific measurements from the atmosphere and surface of Titan on 14 January 2005. Knowledge of the trajectory of Huygens is necessary for scientific analysis of those measurements. We use measurements from the Huygens Atmospheric Structure Instrument (HASI) to reconstruct the trajectory of Huygens during its mission. The HASI Accelerometer subsystem measured the axial acceleration of the probe with errors of 3E−6ms−2, the most accurate measurements ever made by an atmospheric structure instrument on another planetary body. The atmosphere was detected at an altitude of 1498km. Measurements of the normal acceleration of the probe, which are important for determining the probe''s attitude during hypersonic entry, were significantly less accurate and limited by transverse sensitivity of the piezo sensors. Peak acceleration of 121.2ms−2 occurred at 234.9km altitude. The parachute deployment sequence started at 157.1km and a speed of 342.1ms−1. Direct measurements of pressure and temperature began shortly afterwards. The measured accelerations and equations of motion have been used to reconstruct the trajectory prior to parachute deployment. Measured pressures and temperatures, together with the equation of hydrostatic equilibrium and the equation of state, have been used to reconstruct the trajectory after parachute deployment. Uncertainties in the entry state of Huygens at the top of the atmosphere are significant, but can be reduced by requiring that the trajectory and atmospheric properties be continuous at parachute deployment. [Copyright &y& Elsevier]

Abstract: We have calculated the condensation and evaporation of ternary CH4–N2–C2H6 liquid drops and solid CH4 hail as they fall through Titan''s lower atmosphere to determine the likelihood that precipitation reaches the ground. Assuming the humidity profile determined by the Huygens probe, binary liquid CH4/N2 condensate grows in the region from ∼8 to 15km in Titan''s atmosphere because the combined humidity of CH4 and N2 exceeds saturation. These drops evaporate below ∼8km. We determine the fate of 10μm seeds composed of ethane, which is expected to provide condensation sites. In addition, we study the fate of already formed raindrops with radii of 1–4.75mm falling out of the growth region. High (50%) and low (0%) ethane relative humidities (RH) are considered in the calculation. We find that drops with radii ∼3mm and smaller dropping from 8km reach the ground in compositional equilibrium with the atmosphere in the high ethane RH case as a result of the stabilizing influence of the ethane, and evaporate in the atmosphere in the low ethane RH case. Large drops (>∼3mm) reach the surface large and cold because the latent heat loss due to the evaporation of methane cools the drop and slows the evaporation rate. Pure methane hail hits the ground if its radius is initially more than 4mm at 16km above the surface and sublimates in the atmosphere if its radius is smaller. [Copyright &y& Elsevier]

Abstract: The propagation of extremely low frequency (ELF) electromagnetic waves in the Earth''s ionospheric cavity and the associated resonance phenomena have been extensively studied, in relation with lightning activity. We perform a similar investigation for Titan, the largest moon of Saturn. There are important differences between Earth and Titan, as far as the cavity geometry, the atmospheric electron density profile, and the surface conductivity are concerned. We present an improved 3D finite element model that provides an estimate of the lowest eigenfrequencies, associated quality factors (Q-factors), and ELF electric field spectra. The data collected by the electric antenna of the Permittivity, Waves, and Altimetry (PWA) instrument reveals the existence of a narrow-band signal at about 36Hz during the entire descent of Huygens upon Titan. We assess the significance of these measurements against the model predictions, with due consideration to the experimental uncertainties. [Copyright &y& Elsevier]

Abstract: During the descent of the Huygens probe through the atmosphere of Titan, on January 14th, 2005, the permittivity, waves and altimetry (PWA) subsystem, a component of the Huygens atmospheric structure instrument (HASI), detected an ionized layer at altitudes around 63km with two different instruments, the relaxation probe (RP) and the mutual impedance probe (MIP). A very detailed analysis of both data sets is required, in order to correct for environmental effects and compare the two independent estimates of the electrical conductivity. The present work is dedicated to the MIP data analysis. New laboratory tests have been performed to validate or improve the available calibration results. Temperature effects have been included and numerical models of the MIP sensors and electric circuitry have been developed to take into account the proximity of the Huygens probe body. The effect of the vertical motion of the vessel in the ionized atmosphere is estimated in both analytical and numerical ways. The peculiar performance of the instrument in the altitude range 100–140km is scrutinized. The existence of a prominent ionized layer, and of enhancements in the conductivity and electron density profiles at 63km, are discussed in the light of previous theoretical predictions. [Copyright &y& Elsevier]

Abstract: The permittivity, waves and altimetry (PWA) instrument was designed for the investigation of the electric properties and other related physical characteristics of the atmosphere of Titan, from an altitude around 140km down to the surface. PWA carried sensors to measure the atmospheric conductivity, and record electromagnetic and acoustic waves up to frequencies of 11.5 and 6.7kHz, respectively. PWA also measured the relief roughness during the descent and the permittivity of the surface after touchdown. The measurements and the results of the preliminary analysis are presented. An ionized layer is detected at altitudes above 50km, using two independent techniques, and the presence of free electrons in the upper atmosphere is confirmed. An electric signal at around 36Hz is observed throughout the descent, but it is not yet confirmed that this emission is unambiguously related to a resonance of the ionospheric cavity. The relative dielectric constant of Titan''s surface material is nearly 2 and the electric conductivity 4×10−10 Sm−1. The electric properties of the surface seem to evolve after touch-down, possibly due to a local warming of the landing site by the Huygens Probe body. [Copyright &y& Elsevier]

In the framework of the Cassini-Huygens mission towards the Saturnian system, the Huygens Atmospheric Structure Instrument (HASI) is a multisensor package which has been designed to measure the physical quantities characterizing the atmosphere of Titan, Saturn''s largest satellite. HASI sensors are devoted to the study of Titan''s atmospheric structure and electric properties, and also to provide information on the surface, whatever its phase, solid or liquid.A description of the experiment, its subsystems and sensors package is reported together with their performances. [Copyright &y& Elsevier]