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Martian moons exploration, MMX, is the new sample return mission planned by the Japan Aerospace Exploration Agency (JAXA) targeting the two Martian moons with the scheduled launch in 2024 and return to the Earth in 2029. The major scientific objectives of this mission are to determine the origin of Phobos and Deimos, to elucidate the early Solar System evolution in terms of volatile delivery across the snow line to the terrestrial planets having habitable surface environments, and to explore the evolutionary processes of both moons and Mars surface environment. To achieve these objectives, during a stay in circum-Martian space over about 3 years MMX will collect samples from Phobos along with close-up observations of this inner moon and carry out multiple flybys of Deimos to make comparative observations of this outer moon. Simultaneously, successive observations of the Martian atmosphere will also be made by utilizing the advantage of quasi-equatorial spacecraft orbits along the moons’ orbits. Graphical Abstract

The science operations of the spacecraft and remote sensing instruments for the Martian Moon eXploration (MMX) mission are discussed by the mission operation working team. In this paper, we describe the Phobos observations during the first 1.5 years of the spacecraft’s stay around Mars, and the Deimos observations before leaving the Martian system. In the Phobos observation, the spacecraft will be placed in low-altitude quasi-satellite orbits on the equatorial plane of Phobos and will make high-resolution topographic and spectroscopic observations of the Phobos surface from five different altitudes orbits. The spacecraft will also attempt to observe polar regions of Phobos from a three-dimensional quasi-satellite orbit moving out of the equatorial plane of Phobos. From these observations, we will constrain the origin of Phobos and Deimos and select places for landing site candidates for sample collection. For the Deimos observations, the spacecraft will be injected into two resonant orbits and will perform many flybys to observe the surface of Deimos over as large an area as possible. Graphical Abstract

The investigation of asteroids near the Sun is important for understanding the final evolutionary stage of primitive solar system objects. A near-Sun asteroid, (155140) 2005 UD, has orbital elements similar to those of (3200) Phaethon (the target asteroid for the JAXA's $DESTINY^+$ mission). We conducted photometric and polarimetric observations of 2005 UD and found that this asteroid exhibits a polarization phase curve similar to that of Phaethon over a wide range of observed solar phase angles ($ \alpha = 20 - 105^\circ $) but different from those of (101955) Bennu and (162173) Ryugu (asteroids composed of hydrated carbonaceous materials). At a low phase angle ($\alpha \lesssim 30^\circ$), the polarimetric properties of these near-Sun asteroids (2005 UD and Phaethon) are consistent with anhydrous carbonaceous chondrites, while the properties of Bennu are consistent with hydrous carbonaceous chondrites. We derived the geometric albedo, $ p_\mathrm{V} \sim 0.1 $ (in the range of 0.088-0.109); mean $ V $-band absolute magnitude, $ H_\mathrm{V} = 17.54 \pm 0.02 $; synodic rotational period, $ T_\mathrm{rot} = 5.2388 \pm 0.0022 $ hours (the two-peaked solution is assumed); and effective mean diameter, $ D_\mathrm{eff} = 1.32 \pm 0.06 $ km. At large phase angles ($ \alpha \gtrsim 80^\circ$), the polarization phase curve are likely explained by the dominance of large grains and the paucity of small micron-sized grains. We conclude that the polarimetric similarity of these near-Sun asteroids can be attributed to the intense solar heating of carbonaceous materials around their perihelia, where large anhydrous particles with small porosity could be produced by sintering., Comment: 15 pages, 9 figures, accepted for publication in Monthly Notices of Royal Astronomical Society (MNRAS)

Recent cosmochemical studies have shown that most of Earth’s building blocks were close to enstatite meteorites in isotopic compositions. This implies the formation of an impact-induced proto-atmosphere enriched in H2 and CH4 on accreting Earth. Such a reduced proto-atmosphere would have been largely lost by hydrodynamic escape, but its flux and time-scale for hydrogen depletion remain highly uncertain. Here we carry out 1D hydrodynamic escape simulations for such an H2–CH4 proto-atmosphere by incorporating expanded chemical networks and radiative cooling processes for estimation of the duration of the H2-rich surface environment on early Earth. In the escape outflow, CH4 is dissociated effectively by direct photolysis and chemical reactions with photochemically produced ion species. On the other hand, radiative cooling by photochemical products such as H$_{3}^{+}$, CH, and CH3 significantly suppresses atmospheric escape. Even though CH4 and their concentrations are small, the heating efficiency decreases to $\sim 5\, {{\ \rm per\ cent}}$ when CH4/H2 = 0.007 in the lower atmosphere and CH4 would suffer negligible escape when CH4/H2≳ 0.01. The time-scale for H2 escape consistent with the constraints of the isotopic compositions and the amount of C and N on the present Earth is possibly more than several hundred million years. Our results suggest that a long-lived hydrogen-rich reduced environment played important roles in climate warming and the generation of organic matters linked to the emergence of living organisms during the first several hundred million years of Earth.

Mt. Shinmoedake, a part of the Mt. Kirishima cluster of volcanoes in Kyushu, Japan, erupted on 10 March 2018. Our infrasound sensor network located at a distance of more than 200 km from the source detected signals emitted by an explosive eruption of Mt. Shinmoedake. The arrival time of the signals is divided into three time intervals. To reveal how the observed infrasound signals propagated from the source to the sensors, we carry out three-dimensional ray tracing on the basis of the Hamilton equations including the vertical profiles of the temperature and wind around the ray path. We present formulas for calculating travel time and distance of infrasound from a source to an observation site and its turning altitude in the atmosphere. We have identified four kinds of signals, namely, the waves propagated in the troposphere undergoing multiple refraction and those refracting from the stratosphere, the mesosphere, and the lower thermosphere. Brief discussion is devoted to some of the unidentified signals.

Terrestrial planets currently in the habitable zones around M dwarfs likely experienced a long-term runaway greenhouse condition because of a slow decline in host-stellar luminosity in its pre-main sequence phase. Accordingly, they might have lost significant portions of their atmospheres including water vapor at high concentration by hydrodynamic escape induced by the strong stellar XUV irradiation. However, the atmospheric escape rates remain highly uncertain due partly to a lack of understanding of the effect of radiative cooling in the escape outflows. Here we carry out 1-D hydrodynamic escape simulations for an H$_{2}$-H$_{2}$O atmosphere on a planet with mass of $1M_{\oplus}$ considering radiative and chemical processes to estimate the atmospheric escape rate and follow the atmospheric evolution during the early runaway greenhouse phase. We find that the atmospheric escape rate decreases with the basal H$_{2}$O/H$_{2}$ ratio due to the energy loss by the radiative cooling of H$_{2}$O and chemical products such as OH and H$_{3}^{+}$: the escape rate of H$_{2}$ becomes one order of magnitude smaller when the basal H$_{2}$O/H$_{2}=0.1$ than that of the pure hydrogen atmosphere. The timescale for H$_{2}$ escape exceeds the duration of the early runaway greenhouse phase, depending on the initial atmospheric amount and composition, indicating that H$_{2}$ and H$_{2}$O could be left behind after the end of the runaway greenhouse phase. Our results suggest that temperate and reducing environments with oceans could be formed on some terrestrial planets around M dwarfs., Comment: 15 pages, 7 figures

Context. Asteroids in comet-like orbits (ACOs) consist of asteroids and dormant comets. Due to their similar appearance, it is challenging to distinguish dormant comets from ACOs via general telescopic observations. Surveys for discriminating dormant comets from the ACO population have been conducted via spectroscopy or optical and mid-infrared photometry. However, they have not been conducted through polarimetry. Aims. We conducted the first polarimetric research of ACOs. Methods. We conducted a linear polarimetric pilot survey for three ACOs: (944) Hidalgo, (3552) Don Quixote, and (331471) 1984 QY1. These objects are unambiguously classified into ACOs in terms of their orbital elements (i.e., the Tisserand parameters with respect to Jupiter $T_\mathrm{J}$ significantly less than 3). Three ACOs were observed by the 1.6 m Pirka Telescope from UT 2016 May 25 to UT 2019 July 22 (13 nights). Results. We found that Don Quixote and Hidalgo have polarimetric properties similar to comet nuclei and D-type asteroids (optical analogs of comet nuclei). However, 1984 QY1 exhibited a polarimetric property consistent with S-type asteroids. We conducted a backward orbital integration to determine the origin of 1984 QY1, and found that this object was transported from the main belt into the current comet-like orbit via the 3:1 mean motion resonance with Jupiter. Conclusions. We conclude that the origins of ACOs can be more reliably identified by adding polarimetric data to the color and spectral information. This study would be valuable for investigating how the ice-bearing small bodies distribute in the inner Solar System., 12 pages, 6 figures, accepted for publication in Astronomy and Astrophysics (A&A)

Many icy Solar System bodies possess subsurface oceans. On Pluto, Sputnik Planitia’s location near the equator suggests the presence of a subsurface ocean and a locally thinned ice shell. To maintain an ocean, Pluto needs to retain heat inside. On the other hand, to maintain large variations in its thickness, Pluto’s ice shell needs to be cold. Here we show, by thermal evolution and viscous relaxation calculations, that the presence of a thin layer of clathrate hydrates (gas hydrates) at the base of the ice shell can explain both the long-term survival of the ocean and the maintenance of shell thickness contrasts. Clathrate hydrates act as a thermal insulator, preventing the ocean from completely freezing while keeping the ice shell cold and immobile. The most likely clathrate guest gas is methane, derived from precursor bodies and/or cracking of organic materials in the hot rocky core. Nitrogen molecules initially contained and/or produced later in the core would probably not be trapped as clathrate hydrates, instead supplying the nitrogen-rich surface and atmosphere. The formation of a thin clathrate hydrate layer cap to a subsurface ocean may be an important generic mechanism to maintain long-lived subsurface oceans in relatively large but minimally heated icy satellites and Kuiper belt objects. Pluto’s subsurface ocean and thickness variation in its ice shell may be maintained by a layer of methane clathrates forming an insulating cap to the ocean, according to calculations of thermal evolution and viscous relaxation.

The Japanese MMX sample return mission to Phobos by JAXA will carry a Rover developed by CNES and DLR that will be deployed on Phobos to perform in-situ analysis of the Martian moon's surface properties. Past images of the surface of Phobos show that it is covered by a layer of regolith. However, the mechanical and compositional properties of this regolith are poorly constrained. In particular nothing is known regarding the particle sizes, their chemical composition, the packing density of the regolith as well as other frictional parameters and surface dynamics from current remote images. Understanding the properties and dynamics of the regolith in the low-gravity environment of Phobos is important to trace back its history and surface evolution. Moreover, this information is also important to support the interpretation of data obtained by instruments onboard the main spacecraft and to minimize the risks involved in the sampling by the spacecraft. The instruments onboard the Rover are an infrared radiometer (miniRad), a Raman spectrometer (RAX), two cameras looking forwards for navigation and science purposes (NavCams), and two cameras observing the flow of regolith around the rover wheels (WheelCams). The Rover will be deployed before the sampling of Phobos' surface by MMX spacecraft and will be the first rover driving on a Martian moon and in a low-gravity environment.

The asteroid exploration project "Hayabusa2" has successfully returned samples from the asteroid (162173) Ryugu. In this study, we measured the linear polarization degrees of Ryugu using four ground-based telescopes from 2020 September 27 to December 25, covering a wide-phase angle (Sun-target-observer's angle) range from 28$^\circ$ to 104$^\circ$. We found that the polarization degree of Ryugu reached 53$\%$ around a phase angle of 100$^\circ$, the highest value among all asteroids and comets thus far reported. The high polarization degree of Ryugu can be attributed to the scattering properties of its surface layers, in particular the relatively small contribution of multiply-scattered light. Our polarimetric results indicate that Ryugu's surface is covered with large grains. On the basis of a comparison with polarimetric measurements of pulverized meteorites, we can infer the presence of submillimeter-sized grains on the surface layer of Ryugu. We also conjecture that this size boundary represents the grains that compose the aggregate. It is likely that a very brittle structure has been lost in the recovered samples, although they may hold a record of its evolution. Our data will be invaluable for future experiments aimed at reproducing the surface structure of Ryugu., Comment: 8 pages, 3 figures, 2 tables, Accepted for publication in ApJ Letters

Martian Moon eXploration (MMX) mission by Japan Aerospace Exploration Agency (JAXA) is the third Japanese sample return mission. One of the main mission goals is to decipher the origin of these moons, which will provide important clues on planetary formation and how water is delivered to inner planets. MMX will be launched in September 2024 to Martian system to bring back samples from Phobos conducting detailed observations of Phobos and Deimos, and monitoring Mars’s climate. The mission is five-year round trip with return sample on Earth on July 2029. The spacecraft will arrive to Mars system on August 2025, stay three years, and have QSO (Quasi Satellite Orbits) around Phobos at different altitudes to select the landing sampling sites. The spacecraft will land for several hours on the Phobos surface to collect at least 10g of Phobos regolith using a corer going down to a depth of at least 2cm. MMX may collect Phobos samples in two different sites. After three years the spacecraft will leave the Martian system and return the samples to Earth, completing the first round-trip to the Martian system. The principal objectives of the mission are: To settle the controversy on the origin of the Martian moons by close-up observations and return sample analysis To constrain processes for planetary formation and material transport in the region connecting the inner and outer solar systems To reveal evolutionary processes of the Martian system in the circum-Martian environments A set of mission instruments are defined and under development to achieve the major mission goals. The included instruments are: the wide-angle camera OROCHI (Optical RadiOmeter composed of CHromatic Imagers), the telescopic (narrow-angle) camera TENGOO (TElescopic Nadir imager for GeOmOrphology), the laser altimeter LIDAR (Light Detection and Ranging), CMDM (Circum-Martian Dust Monitor), MSA (Mass Spectrum Analyzer), MEGANE (Gamma rays and Neutrons Spectrometer) provided by NASA, a near-infrared spectrometer MIRS (MMX InfraRed Spectrometer) provided by CNES, SMP sampling device and the sample return capsule. A small Rover (total weight less than 30kg) developed by CNES and DLR is also a part of the mission. The Rover payload includes four scientific instruments: a IR radiometer (miniRAD), a Raman spectrometer (RAX), a stereo pair of cameras looking forward (NavCAM) and two cameras looking at the interface wheel-surface (WheelCAM) and consequent Phobos’ regolith mechanical properties. ESA will participate to the mission assisting with deep space communication equipment. 1. MIRS MIRS instrument is built at LESIA-Paris Observatory in collaboration with four other French laboratories (LAB, LATMOS, LAM, IRAP-OMP), with collaboration and financial support of CNES and close collaboration with JAXA and MELCO. MIRS is an imaging spectrometer in the 0.9 - 3.6 microns spectral band with spectral resolution better than 20 nm. The IFOV is 0.35 mrad and FOV of +/-1.65°. The SNR is higher than 100 up to 3.2 µm in a maximum integration time less than 2s. The detector has dimension of 256 x 256 pixels with pixel pitch of 30 microns. The total mass is lower than 11.9 kg (including 20% margin), and the volume of complete instrument is about 320 x 150 x 400 mm3. 2. MIRS science objectives MIRS is expected to characterize Phobos and Deimos surfaces and Mars atmospheric composition by remotely identifying diagnostic features in the near-infrared range. MIRS is used to achieve some of the mission-requirements, in particular: 1: To grasp the surface distribution of the constituent materials of Phobos. Hydrous minerals and other related minerals should be identified and characterized spectroscopically for main parts of the full body in correspondence with its topography (at horizontal spatial resolutions of 20 m or better) and in a radius of 50 m or more around the sampling point (at spatial resolutions of 1 m or better). For the global areas, MIRS is expected to spectroscopically measure water (ice) (absorption band at 3.0-3.2 μm) and hydrous silicate minerals (features at 2.7-2.8 μm) at the wavelength resolutions (wavelength width) of 20 nm, S/N ratios > 100 and a spatial resolution of 20 m (for +/-30° latitude). The spectral radiometric absolute accuracy is expected to be of 10%, and the relative accuracy of 1%. If possible, also to measure organic matter (3.3.-3.5 μm) and hydrous/anhydrous silicate minerals (1.0 μm). An area within 50 m from a selected sampling spot at 1 m resolution will be also mapped. 2: To grasp the distribution of constituent materials of Deimos, from spectroscopic information, clarify the surface distribution of hydrous minerals and other related minerals corresponding to its topography at characteristic parts of the moon with a horizontal spatial resolution of 100 m or better. MIRS is expected to spectroscopically map major regions of Deimos at a spatial resolution better than 100 m for major absorption bands as observed in Phobos. 3: To constrain transport processes for dust and water near the Martian surface, continuous observations of the mid- to low-latitude distributions of dust storms, ice clouds, and water vapor in the Martian atmosphere are performed from high altitudes equatorial orbit in different seasons to within 1-hour time resolutions. MIRS is expected to perform observations of distributions of total amount of water vapor columns at 10 km spatial resolutions and spectral radiometric absolute accuracy of 10%, and spectral radiometric relative accuracy of 1% with temporal resolution less than 1-hour for the mid- to low-latitude selected areas. These observations are expected to be performed over several successive days in different seasons. 3. Conclusions MIRS will allow compositional characterization of Phobos, Deimos and temporal characterization of particular phenomena of Mars atmosphere. It will be also a fundamental instrument to evaluate sampling site candidates and support the selection of the two sampling sites on the Phobos surface. The mission will be able to clarify the origin of the Martian moons and may also be able to elucidate the process of the evolution of the Mars environment. Acknowledgements MMX is under developed and built by JAXA, with contributions from CNES, DLR and NASA. We thank the MMX JAXA teams for their efforts in defining and building the mission. The MIRS team thanks CNES for the financial support and collaboration to build MIRS instrument.

Phobos and Deimos occupy unique positions both scientifically and programmatically on the road to the exploration of the solar system. Japan Aerospace Exploration Agency (JAXA) plans a Phobos sample return mission (MMX: Martian Moons eXploration). The MMX spacecraft is scheduled to be launched in 2024, orbit both Phobos and Deimos (multiple flybys), and retrieve and return >10 g of Phobos regolith back to Earth in 2029. The Phobos regolith represents a mixture of endogenous Phobos building blocks and exogenous materials that contain solar system projectiles (e.g., interplanetary dust particles and coarser materials) and ejecta from Mars and Deimos. Under the condition that the representativeness of the sampling site(s) is guaranteed by remote sensing observations in the geologic context of Phobos, laboratory analysis (e.g., mineralogy, bulk composition, O-Cr-Ti isotopic systematics, and radiometric dating) of the returned sample will provide crucial information about the moon’s origin: capture of an asteroid or in-situ formation by a giant impact. If Phobos proves to be a captured object, isotopic compositions of volatile elements (e.g., D/H,13C/12C,15N/14N) in inorganic and organic materials will shed light on both organic-mineral-water/ice interactions in a primitive rocky body originally formed in the outer solar system and the delivery process of water and organics into the inner rocky planets.

The existence of liquid water within an oxidized environment on early Mars has been inferred by the Mn-rich rocks found during recent explorations on Mars. The oxidized atmosphere implied by the Mn-rich rocks would basically be comprised of CO$_2$ and H$_2$O without any reduced greenhouse gases such as H$_2$ and CH$_4$. So far, however, it has been thought that early Mars could not have been warm enough to sustain water in liquid form without the presence of reduced greenhouse gases. Here, we propose that H$_2$O$_2$ could have been the gas responsible for warming the surface of the oxidized early Mars. Our one-dimensional atmospheric model shows that only 1 ppm of H$_2$O$_2$ is enough to warm the planetary surface because of its strong absorption at far-infrared wavelengths, in which the surface temperature could have reached over 273~K for a CO$_2$ atmosphere with a pressure of 3~bar. A wet and oxidized atmosphere is expected to maintain sufficient quantities of H$_2$O$_2$ gas in its upper atmosphere due to its rapid photochemical production in slow condensation conditions. Our results demonstrate that a warm and wet environment could have been maintained on an oxidized early Mars, thereby suggesting that there may be connections between its ancient atmospheric redox state and possible aqueous environment., Comment: 8 pages, 7 figures, published in ApJ

In order to investigate a possible variety of atmospheric states realized on a synchronously rotating aquaplanet, an experiment studying the impact of planetary rotation rate is performed using an atmospheric general circulation model (GCM) with simplified hydrological and radiative processes. The entire planetary surface is covered with a swamp ocean. The value of planetary rotation rate is varied from zero to the Earth’s, while other parameters such as planetary radius, mean molecular weight and total mass of atmospheric dry components, and solar constant are set to the present Earth’s values. The integration results show that the atmosphere reaches statistically equilibrium states for all runs; none of the calculated cases exemplifies the runaway greenhouse state. The circulation patterns obtained are classified into four types: Type-I characterized by the dominance of a day-night thermally direct circulation, Type-II characterized by a zonal wave number one resonant Rossby wave over a meridionally broad westerly jet on the equator, Type-III characterized by a long time scale north-south asymmetric variation, and Type-IV characterized by a pair of mid-latitude westerly jets. With the increase of planetary rotation rate, the circulation evolves from Type-I to Type-II and then to Type-III gradually and smoothly, whereas the change from Type-III to Type-IV is abrupt and discontinuous. Over a finite range of planetary rotation rate, both Types-III and -IV emerge as statistically steady states, constituting multiple equilibria. In spite of the substantial changes in circulation, the net energy transport from the day side to the night side remains almost insensitive to planetary rotation rate, although the partition into dry static energy and latent heat energy transports changes. The reason for this notable insensitivity is that the outgoing longwave radiation over the broad area of the day side is constrained by the radiation limit of a moist atmosphere, so that the transport to the night side, which is determined as the difference between the incoming solar radiation and the radiation limit, cannot change greatly.

UX Orionis stars (UXors) are Herbig Ae/Be or T Tauri stars exhibiting sporadic occultation of stellar light by circumstellar dust. GM\,Cephei is such a UXor in the young ($\sim4$~Myr) open cluster Trumpler\,37, showing prominent infrared excess, emission-line spectra, and flare activity. Our photometric monitoring (2008--2018) detects (1)~an $\sim$3.43~day period, likely arising from rotational modulation by surface starspots, (2)~sporadic brightening on time scales of days due to accretion, (3)~irregular minor flux drops due to circumstellar dust extinction, and (4)~major flux drops, each lasting for a couple of months with a recurrence time, though not exactly periodic, of about two years. The star experiences normal reddening by large grains, i.e., redder when dimmer, but exhibits an unusual "blueing" phenomenon in that the star turns blue near brightness minima. The maximum extinction during relatively short (lasting $\leq 50$~days) events, is proportional to the duration, a consequence of varying clump sizes. For longer events, the extinction is independent of duration, suggestive of a transverse string distribution of clumps. Polarization monitoring indicates an optical polarization varying $\sim3\%$--8$\%$, with the level anticorrelated with the slow brightness change. Temporal variation of the unpolarized and polarized light sets constraints on the size and orbital distance of the circumstellar clumps in the interplay with the young star and scattering envelope. These transiting clumps are edge-on manifestations of the ring- or spiral-like structures found recently in young stars with imaging in infrared of scattered light, or in submillimeter of thermalized dust emission., 20 pages, 9 figures

Context.To investigate the physical properties of the surface layers of small Solar System objects, we take advantage of the phase-angle dependence of the linear polarization degree, which varies with albedo, composition, and other factors. In particular, the angle at which the positive and negative are reversed is a polarimetric parameter known as inversion angle. A group with large inversion angles (the so-called Barbarians) is unusual and is also noteworthy because of their association with meteorites.Aims.We identified an object with such a large inversion angle in the near-Earth asteroids and derived previously unknown parameters, including the maximum polarization degree. By comparing the polarimetric parameters and spectral properties of meteorites, we inferred the surface conditions based on the degree of polarization of the asteroid.Methods.We carried out multiband polarization imaging observations of an L-type near-Earth asteroid, (85989) 1999 JD6, at the Hokkaido University Observatory in 2015 and 2019, covering a wide range of phase angles 30°–105°. Of the polarimetric parameters, we derived the maximum value, inversion angle, and the slope at that angle from the phase angle-polarization degree curve.Results.We found that the inversion angle and the maximum polarization degree of 1999 JD6are 27° and 13%. The inversion angle is significantly larger than those of the majority of all asteroids observed before, but is consistent with that of Barbarian asteroids. 1999 JD6is the first example belonging to this group that is also among the near-Earth asteroids.Conclusions.We claim that 1999 JD6is a member of the Barbarians and has rougher surface particles than lunar regolith.

Tidal deformation of icy satellites provides crucial information on their subsurface structures. In this study, we investigate the parameter dependence of the tidal displacement and potential Love numbers (i.e., h2 and k2, respectively) of Ganymede. Our results indicate that Love numbers for Ganymede models without a subsurface ocean are not necessarily smaller than those with a subsurface ocean. The phase lag, however, depends primarily on the presence/absence of a subsurface ocean. Thus, the determination of the phase lag would be of importance to infer whether Ganymede possesses a subsurface ocean or not based only on geodetic measurements. Our results also indicate that the major control on Love numbers is the thickness of the ice shell if Ganymede possesses a subsurface ocean. This result, however, does not necessarily indicate that measurement of either of h2 or k2 alone is sufficient to estimate the shell thickness; while a thin shell leads to large h2 and k2 independent of parameters, a thick shell does not necessarily lead to small h2 and k2. We found that to reduce the uncertainty in the shell thickness, constraining k2 in addition to h2 is necessary, highlighting the importance of collaborative analyses of topography and gravity field data.

Direct-imaging techniques of exoplanets have made significant progress recently, and will eventually enable to monitor photometric and spectroscopic signals of earth-like habitable planets in the future. The presence of clouds, however, would remain as one of the most uncertain components in deciphering such direct-imaged signals of planets. We attempt to examine how the planetary obliquity produce different cloud patterns by performing a series of GCM (General Circulation Model) simulation runs using a set of parameters relevant for our Earth. Then we use the simulated photometric lightcurves to compute their frequency modulation due to the planetary spin-orbit coupling over an entire orbital period, and attempt to see to what extent one can estimate the obliquity of an Earth-twin. We find that it is possible to estimate the obliquity of an Earth-twin within the uncertainty of several degrees with a dedicated 4 m space telescope at 10 pc away from the system if the stellar flux is completely blocked. While our conclusion is based on several idealized assumptions, a frequency modulation of a directly-imaged earth-like planet offers a unique methodology to determine its obliquity., Comment: 29 pages, 18 figures, accepted for publication in ApJ

Mars may have obtained a proto-atmosphere enriched in H2, CH4, and CO during accretion. Such a reduced proto-atmosphere would have been largely lost by hydrodynamic escape, but its flux is highly uncertain. To estimate the evolution of the proto-atmosphere of Mars correctly, an exact escape modeling including exact radiative balance and chemical processes is required partly because those reduced species and their photochemical products may act as an effective coolant that suppresses the escape of atmosphere. Here we develop a one-dimensional hydrodynamic escape model that includes radiative processes and photochemical processes for a multi-component atmosphere and apply to the reduced proto-atmosphere on Mars. Under the enhanced XUV flux suggested for young Sun, the escape flux decreases by more than one order of magnitude with increasing the mixing fraction of CH4 and CO from zero to > 10 % mainly because of the energy loss by radiative cooling by these infrared active chemical molecules. Concurrently, the mass fractionation between H2 and other heavier species becomes to be enhanced. Given that the proto-Mars initially obtained > 10 bar of H2 and carbon species equivalent to 1 bar of CO2 was then left behind after the end of the hydrodynamic escape of H2, the total amount of carbon species lost by hydrodynamic escape is estimated to be equivalent to 20 bar of CO2 or more. Such a severe loss of carbon species may explain the paucity of CO2 on Mars compared to Earth and Venus. If the proto-Mars obtained > 100 bar of H2, the timescale for H2 escape exceeds ~100 Myr. This implies that an atmosphere with reduced chemical compositions allowing the production of organic matter deposits may have been kept on early Mars traceable by geologic records.

The near-Earth asteroid (3200) Phaethon is the parent body of the Geminid meteor stream. Phaethon is also an active asteroid with a very blue spectrum. We conducted polarimetric observations of this asteroid over a wide range of solar phase angles α during its close approach to the Earth in autumn 2016. Our observation revealed that Phaethon exhibits extremely large linear polarization: P = 50.0 ± 1.1% at α = 106.5°, and its maximum is even larger. The strong polarization implies that Phaethon’s geometric albedo is lower than the current estimate obtained through radiometric observation. This possibility stems from the potential uncertainty in Phaethon’s absolute magnitude. An alternative possibility is that relatively large grains (~300 μm in diameter, presumably due to extensive heating near its perihelion) dominate this asteroid’s surface. In addition, the asteroid’s surface porosity, if it is substantially large, can also be an effective cause of this polarization., (3200) Phaethon is a near-Earth asteroid discovered in 1983 that has large inclination and eccentricity. Here, the authors perform polarimetric observation of Phaethon over a wide range of solar phase angle and report that the asteroid exhibits a very strong linear polarization.

Laser altimetry is a powerful tool for addressing the major objectives of planetary physics and geodesy, and have been applied in planetary explorations of the Moon, Mars, Mercury, and the asteroids Eros, and Itokawa. The JUpiter Icy Moons Explorer (JUICE), led by European Space Agency (ESA), has started development to explore the emergence of habitable worlds around gas giants. The Ganymede Laser Altimeter (GALA) will be the first laser altimeter for icy bodies, and will measure the shape and topography of the large icy moons of Jupiter, (globally for Ganymede, and using flyby ground-tracks for Europa and Callisto). Such information is crucial for understanding the formation of surface features and can tremendously improve our understanding of the icy tectonics. In addition, the GALA will infer the presence or absence of a subsurface ocean by measuring the tidal and rotational responses. Furthermore, it also improves the accuracy of gravity field measurements reflecting the interior structure, collaborating with the radio science experiment. In addition to range measurements, the signal strength and the waveform of the laser pulses reflected from the moon's surface contain information about surface reflectance at the laser wavelength and small scale roughness. Therefore we can infer the degrees of chemical and physical alterations, e.g., erosion, space weathering, compaction and deposition of exogenous materials, through GALA measurements without being affected by illumination conditions. JUICE spacecraft carries ten science payloads including GALA. They work closely together in a synergistic way with GALA being one of the key instruments for understanding the evolution of the icy satellites Ganymede, Europa, and Callisto., 10 pages, 6 figures, accepted for publication in Trans. JSASS Aerospace Tech. Japan

We present a unique and significant polarimetric result regarding the near-Earth asteroid (152679) 1998 KU$_\mathrm{2}$ , which has a very low geometric albedo. From our observations, we find that the linear polarization degrees of 1998 KU$_\mathrm{2}$ are 44.6 $\pm$ 0.5\% in the R$_\mathrm{C}$ band and 44.0 $\pm$ 0.6\% in the V band at a solar phase angle of 81.0\degr. These values are the highest of any known airless body in the solar system (i.e., high-polarization comets, asteroids, and planetary satellites) at similar phase angles. This polarimetric observation is not only the first for primitive asteroids at large phase angles, but also for low-albedo (< 0.1) airless bodies. Based on spectroscopic similarities and polarimetric measurements of materials that have been sorted by size in previous studies, we conjecture that 1998 KU$_\mathrm{2}$ has a highly microporous regolith structure comprising nano-sized carbon grains on the surface., 9 pages, 5 figures, and 3 tables, accepted for publication in A&A

著者人数: 14名, Accepted: 2014-12-21, 資料番号: SA1004983000

We measure the spectrum of cosmic rays with energies greater than 10 18.2 eV with the fluorescence detectors (FDs) and the surface detectors (SDs) of the Telescope Array Experiment using the data taken in our first 2.3-year observation from May 27, 2008 to September 7, 2010. A hybrid air shower reconstruction technique is employed to improve accuracies in determination of arrival directions and primary energies of cosmic rays using both FD and SD data. The energy spectrum presented here is in agreement with our previously published spectra and the HiRes results.

A series of long-term numerical simulations of moist convection in Jupiter’s atmosphere is performed in order to investigate the idealized characteristics of the vertical structure of multi-composition clouds and the convective motions associated with them, varying the deep abundances of condensable gases and the autoconversion time scale, the latter being one of the most questionable parameters in cloud microphysical parameterization. The simulations are conducted using a two-dimensional cloud resolving model that explicitly represents the convective motion and microphysics of the three cloud components, H 2 O, NH 3 , and NH 4 SH imposing a body cooling that substitutes the net radiative cooling. The results are qualitatively similar to those reported in Sugiyama et al. (Sugiyama, K. et al. [2011]. Intermittent cumulonimbus activity breaking the three-layer cloud structure of Jupiter. Geophys. Res. Lett. 38, L13201. doi:10.1029/2011GL047878): stable layers associated with condensation and chemical reaction act as effective dynamical and compositional boundaries, intense cumulonimbus clouds develop with distinct temporal intermittency, and the active transport associated with these clouds results in the establishment of mean vertical profiles of condensates and condensable gases that are distinctly different from the hitherto accepted three-layered structure (e.g., Atreya, S.K., Romani, P.N. [1985]. Photochemistry and clouds of Jupiter, Saturn and Uranus. In: Recent Advances in Planetary Meteorology. Cambridge Univ. Press, London, pp. 17–68). Our results also demonstrate that the period of intermittent cloud activity is roughly proportional to the deep abundance of H 2 O gas. The autoconversion time scale does not strongly affect the results, except for the vertical profiles of the condensates. Changing the autoconversion time scale by a factor of 100 changes the intermittency period by a factor of less than two, although it causes a dramatic increase in the amount of condensates in the upper troposphere. The moist convection layer becomes potentially unstable with respect to an air parcel rising from below the H 2 O lifting condensation level (LCL) well before the development of cumulonimbus clouds. The instability accumulates until an appropriate trigger is provided by the H 2 O condensate that falls down through the H 2 O LCL; the H 2 O condensate drives a downward flow below the H 2 O LCL as a result of the latent cooling associated with the re-evaporation of the condensate, and the returning updrafts carry moist air from below to the moist convection layer. Active cloud development is terminated when the instability is completely exhausted. The period of intermittency is roughly equal to the time obtained by dividing the mean temperature increase, which is caused by active cumulonimbus development, by the body cooling rate.

Hydrodynamic escape of hydrogen driven by solar extreme ultraviolet (EUV) radiation heating is numerically simulated by using the constrained interpolation profile scheme, a high-accuracy scheme for solving the one-dimensional advection equation. For a wide range of hydrogen number densities at the lower boundary and solar EUV fluxes, more than half of EUV heating energy is converted to mechanical energy of the escaping hydrogen. Less energy is lost by downward thermal conduction even giving low temperature for the atmospheric base. This result differs from a previous numerical simulation study that yielded much lower escape rates by employing another scheme in which relatively strong numerical diffusion is implemented. Because the solar EUV heating effectively induces hydrogen escape, the hydrogen mixing ratio was likely to have remained lower than 1 vol% in the anoxic Earth atmosphere during the Archean era.

Numerical experiments are performed in order to investigate an MHD dynamo in a rotating spherical shell with stress-free top and no-slip bottom boundaries. The Ekman number, the Prandtl number, and the ratio of inner to outer radii are fixed as 10 −3 , 1, and 0.35, respectively. The magnetic Prandtl number is varied from 5 to 50, and the modified Rayleigh number is increased from 1.5 to 10 times the critical Rayleigh number. The initial imposed magnetic field is either a weak or strong magnetic field, where the magnetic energy of the initial field is approximately two orders of magnitude smaller or larger than the kinetic energy of the quasi-steady state of non-magnetic thermal convection. For cases involving a weak initial magnetic field, self-sustained dynamo solutions are established when the magnetic Prandtl number is larger than or equal to 5, and the modified Rayleigh number is larger than or equal to 5 times the critical Rayleigh number. The solutions are categorized as a weak field-dynamo, where the mean magnetic energy is one order of magnitude smaller than the mean kinetic energy. The dynamo solutions are characterized by a radially two-layer spatial structure. The upper layer is dominated by a strong prograde zonal mean zonal flow with large-scale prograde propagating spiral vortices having a longitudinal wavenumber of 3. Toroidal kinetic energy is converted to toroidal magnetic energy through stretching of the field lines by large-scale prograde propagating spiral vortices. On the other hand, the lower layer contains small-scale retrograde propagating columnar convective vortices having a longitudinal wavenumber of 8. The magnetic field lines are not concentrated in the vortices, but rather wind around the vortices in each layer. Poloidal kinetic energy is converted to poloidal magnetic energy through winding of the field lines around small-scale retrograde propagating columnar convective vortices. For cases involving a strong initial magnetic field, self-sustained dynamo solutions are established when the magnetic Prandtl number is greater than or equal to 4 and the modified Rayleigh number is greater than or equal to 3 times the critical Rayleigh number. In contrast with the cases involving a weak initial magnetic field, all of the dynamo solutions are strong-field solutions. Bistability is observed when the magnetic Prandtl number is 5, and the modified Rayleigh number is approximately equal to 10 times the critical Rayleigh number, where weak-field and strong-field dynamo solutions coexist. However, transition between the weak- and strong-field solutions does not occur in this case.

Accepted: 2011-06-09, 資料番号: SA1003064000

Meteorites and their components have anomalous oxygen isotopic compositions characterized by large variations in 18 O/ 16 O and 17 O/ 16 O ratios. On the basis of recent observations of star-forming regions and models of accreting protoplanetary disks, we suggest that these variations may originate in a parent molecular cloud by ultraviolet photodissociation processes. Materials with anomalous isotopic compositions were then transported into the solar nebula by icy dust grains during the collapse of the cloud. The icy dust grains drifted toward the Sun in the disk, and their subsequent evaporation resulted in the 17 O- and 18 O-enrichment of the inner disk gas.

In order to study the stability of martian climate, we constructed a two-dimensional (horizontal–vertical) energy balance model. The long-term CO2 mass exchange process between the atmosphere and CO2 ice caps is investigated with particular attention to the effect of planetary ice distribution on the climate stability. Our model calculation suggests that high atmospheric pressure presumed for past Mars would be unstabilized if H2O ice widely prevailed. As a result, a cold climate state might have been achieved by the condensation of atmospheric CO2 onto ice caps. On the other hand, the low atmospheric pressure, which is buffered by the CO2 ice cap and likely close to the present pressure, would be unstabilized if the CO2 ice albedo decreased. This may have led the climate into a warm state with high atmospheric pressure owing to complete evaporation of CO2 ice cap. Through the albedo feedback mechanisms of H2O and CO2 ices in the atmosphere-ice cap system, Mars may have experienced warm and cold climates episodically in its history.

著者人数: 15名, Accepted: 2014-05-20, 資料番号: SA1140086000

Oxygen isotopic composition of our solar system is believed to have resulted from mixing of two isotopically distinct nebular reservoirs, 16 O-rich and 17,18 O-rich relative to Earth. The nature and composition of the 17,18 O-rich reservoir are poorly constrained. We report an in situ discovery of a chemically and isotopically unique material distributed ubiquitously in fine-grained matrix of a primitive carbonaceous chondrite Acfer 094. This material formed by oxidation of Fe,Ni-metal and sulfides by water either in the solar nebula or on a planetesimal. Oxygen isotopic composition of this material indicates that the water was highly enriched in 17 O and 18 O (δ 17,18 O SMOW = +180‰ per mil), providing the first evidence for an extremely 17,18 O-rich reservoir in the early solar system.

Recent understandings of planetary accretion have suggested that accumulation of a small number of large planetesimals dominates intermediate to final growth stages of the terrestrial planets, with impact velocity high enough to induce extensive melting of the planetesimal and target materials, resulting in formation of a large molten region in which gravitational segregation of silicate and metal, that is, core formation proceeds. In case of homogeneous accretion, volatiles contained in each planetesimal are likely subjected to partitioning among gas, silicate melt, and molten metallic iron at significantly high temperatures and pressures in such a massive molten region. Each phase would subsequently form the proto-atmosphere, -mantle, or-core, respectively. Such chemical reprocessing of H and C associated with core formation, which is followed by both degassing from mantle and atmospheric escape, may result in a diverse range of H 2 O CO 2 in planetary surface environments, which mainly depends on the H and C content relative to metallic iron in planetary building stones. This may explain inferred difference in volatile distribution between the Earth's (relatively H 2 O-rich, CO 2 -poor) and the martian (H 2 O-poor, CO 2 -rich) surface environments. Such volatile redistribution may be systematically described by using the retentivity of H 2 O, ξ, defined as follows: ξ = 1 − ([ CO ] 0 + 2[ CH 4 ] 0 + 2[ C(gr) ] 0 ) [ H 2 O] 0 , where [ i ] 0 represents mol number of species i partitioned into non-metallic phases, that is, gas and silicate melt in impact-induced molten region. When ξ > 0.5, relatively H 2 O-rich and CO 2 -poor surface environment may eventually evolve, although a small portion of H 2 O partitioned into the non=metallic phases are possibly consumed by subsequent chemical reactions with reduced C-species with producing CO 2 and H 2 . When ξ 2 O consumption by the above reactions and selective loss of H 2 to space may result in relative H 2 O-depleted and CO 2 -rich surface environment. Given the building stone composition by the two-component model by Ringwood (1977) and Wanke (1981), ξ is found to decrease with increasing the mixing fraction of the volatile-rich component: ξ > 0.5 for the mixing fraction smaller than about 15–20% and ξ H C ratio in the building stone. The estimated mixing fraction of the volatile-rich component, about 10% for the Earth and 35% for Mars, is consistent with the observed difference in volatile distribution between the surfaces of both planets.

Partitioning of H and C among fluid, silicate melt, and molten metallic iron within a growing Earth at temperatures 2000–2500 K and pressures 0.2–5 GPa is estimated by using a thermodynamic model based on the recent knowledge on gas solubility into silicate melts and molten metallic iron. The repulsive interactions among H, C, and S dissolved in molten metallic iron are taken into account. It is shown that partition coefficient of H between molten metallic iron and silicate melt increases with pressure and temperature. Under the presence of Fe-rich metal, CO2 content in silicate melt is suggested to be very low because of low oxygen fugacity under such condition. Assuming a homogeneous accretion of planetesimals with the composition given by the two-component model slightly modified from Ringwood [1977] and Waanke [1981], it is shown that C is preferentially partitioned to molten metallic iron and quite less to silicate melt, whereas a substantial proportion of H is partitioned to silicate melt as H2O and also to molten metallic iron as interstitial atoms. For such concentrations of H, C, and S, the effect of thermodynamic interaction among them in molten metallic iron is not strong enough to cause the oversaturation of graphite. H and C partitioned to molten metallic iron may account for a significant portion of the density deficit in Earth's core. The estimated amount of H2O partitioned to silicate melt is possibly large enough to explain the sources for (1) H2O in the hydrosphere and mantle, (2) oxygen which partially oxidizes ferrous iron to ferric iron in the mantle, and (3) oxidant for metals which may fail to segregate to the core and act as the source of the highly siderophile elements in the mantle of the present Earth. The higher H/C and the lower C/36Ar ratios in the silicate Earth including the hydrosphere compared to various classes of meteorites are possibly explained if these elements are derived from the early mantle material after the H and C partitioning to molten metallic iron and core segregation. Accretion of the late veneer material such as the highly oxidized, CI chondrite-like material seems difficult to explain such elemental abundance pattern without invoking unknown large volatile reservoir in the mantle.

The Telescope Array's Middle Drum fluorescence detector was instrumented with telescopes refurbished from the High Resolution Fly's Eye's HiRes-1 site. The data observed by Middle Drum in monocular mode was analyzed via the HiRes-1 profile-constrained geometry reconstruction technique and utilized the same calibration techniques enabling a direct comparison of the energy spectra and energy scales between the two experiments. The spectrum measured using the Middle Drum telescopes is based on a three-year exposure collected between December 16, 2007 and December 16, 2010. The calculated difference between the spectrum of the Middle Drum observations and the published spectrum obtained by the data collected by the HiRes-1 site allows the HiRes-1 energy scale to be transferred to Middle Drum. The HiRes energy scale is applied to the entire Telescope Array by making a comparison between Middle Drum monocular events and hybrid events that triggered both Middle Drum and the Telescope Array's scintillator Ground Array., Comment: 27 pages, 21 figures