Kiyoshi Kuramoto

Giant impacts during Earth’s final accretion likely generated global magma oceans, where the disproportionation of Fe ²⁺ at lower mantle pressures and segregation of the resulting metallic iron into the core could have oxidized the magma ocean. Earth may nevertheless have had an H ₂ -rich proto-atmosphere in chemical disequilibrium with the magma ocean, but the chemical coupling between this atmosphere and an oxidized magma ocean remains poorly constrained. Here, we develop a coupled interior-atmosphere evolution model that includes diffusive transport in the boundary layer between the magma ocean and atmosphere, redox reactions in the melt, partitioning of oxide components and H ₂ O among coexisting phases, and planetary cooling through a proto-atmosphere, with metallic iron produced by Fe ²⁺ disproportionation removed into the core as it forms. Atmospheric H ₂ transport into the magma ocean and subsequent H ₂ O production become more efficient with increasingly frequent boundary layer renewal driven by buoyant overturn of dense melt cooled at the surface, allowing the atmosphere and magma ocean to chemically equilibrate within ∼10 ⁴ yr when the magma ocean is sufficiently deep and hot to extend into the lower mantle. Redox exchange with the atmosphere buffers the solidification-driven redox evolution of the magma ocean, preventing extreme oxidation or reduction of the coupled system. These results suggest that the coupled evolution could have contributed to early Earth’s water inventory while maintaining a reducing H ₂ -rich atmosphere.

The thermal evolution of magma oceans formed by giant impacts is strongly influenced by a proto-atmosphere through its blanketing effect, which suppresses outgoing planetary radiation. While both radiative absorption and Rayleigh scattering by atmospheric species can contribute to this effect, the role of the scattering in suppressing thermal radiation from magma oceans remains unclear. In this study, we develop a 1D radiative transfer model for planetary and solar radiation in a proto-atmosphere composed of H₂O and H₂, and a coupled thermal evolution model of a planetary interior and proto-atmosphere, to investigate the scattering blanketing effect on planetary radiation and magma ocean cooling. Our results show that Rayleigh scattering significantly reduces the outgoing planetary radiation at wavelengths below ∼1 μm, particularly in hot, thick atmospheres where scattering is highly effective. Consequently, the planetary outgoing radiation flux decreases by up to about 1 to 2 orders of magnitude and the magma ocean lifetime is prolonged by up to about three times due to the scattering blanketing effect when the total amounts of H₂O and H₂ are equivalent to or greater than the present-day terrestrial seawater. These findings suggest that the prolonged magma ocean phase facilitated efficient differentiation between compatible and incompatible elements, even in the lower mantle. Furthermore, they imply that sustained magma oceans likely persisted throughout much of the giant impact phase, supporting a magma ocean origin of the Moon, consistent with its observed chemical characteristics.

Radiative cooling by molecules is a crucial process for hydrodynamic escape, as it can efficiently remove the thermal energy driving the outflow, acquired through X-ray and extreme UV absorption. Carbon oxides, such as CO and CO2, and their photochemical products are anticipated to serve as vital radiative cooling sources not only in atmospheres dominated by carbon oxides but also in H2-rich atmospheres. However, their specific effects on the hydrodynamic escape, especially in H2-rich atmospheres, have been inadequately investigated. In this study, we conduct 1-D hydrodynamic escape simulations for H2-rich atmospheres incorporating CO, CO2, and their chemical products on an Earth-mass planet. We consider detailed radiative cooling processes and chemical networks related to carbon oxides to elucidate their impacts on the hydrodynamic escape. In the escape outflow, CO2 undergoes rapid photolysis, producing CO and atomic oxygen, while CO exhibits photochemical stability compared to CO2. The H2 oxidation by atomic oxygen results in the production of OH and H2O. Consequently, the hydrodynamic escape is significantly suppressed by the radiative cooling effects of CO, H2O, OH, and H3+ even when the basal mixing fraction of CO and CO2 is lower than ~0.01. These mechanisms extend the lifetime of H2-rich atmospheres by about one order of magnitude compared to the case of pure hydrogen atmospheres on early Earth, which also results in negligible escape of heavier carbon- and nitrogen-bearing molecules and noble gases.

Context. Ch-type asteroids are distinctive among other dark asteroids in that they exhibit deep negative polarization branches (NPBs). Nevertheless, the physical and compositional properties that cause their polarimetric distinctiveness are less investigated. Aims. We aim to investigate the polarimetric uniqueness of Ch-type asteroids by making databases of various observational quantities (i.e., spectroscopic and photometric properties as well as polarimetric ones) of dark asteroids. Methods. We conducted an intensive polarimetric survey of 52 dark asteroids (including 31 Ch-type asteroids) in the R_C band to increase the size of polarimetric samples. The observed data were compiled with previous polarimetric, spectroscopic, and photometric archival data to find their correlations. Results. We find remarkable correlations between these observed quantities, particularly the depth of NPBs and their spectroscopic features associated with the hydrated minerals. The amplitude of the opposition effect in photometric properties also shows correlations with polarimetric and spectral properties. However, these observed quantities do not show noticeable correlations with the geometric albedo, thermal inertia, and diameter of asteroids. Conclusions. Based on the observational evidence, we arrive at our conclusion that the submicrometer-sized structures (fibrous or flaky puff pastry-like structures in phyllosilicates) in the regolith particles could contribute to the distinctive NPBs of hydrated asteroids.

We analyze the distribution of volatiles such as water, hydrogen, and carbon within the atmosphere, mantle, and core of Mars during its accretion stages. The partitioning of these volatiles is crucial as it influences the generation of intrinsic magnetic fields, mantle convection, and magmatic activity. The study suggests that the accretion process is responsible for the presence of water and light elements within the Martian mantle and core. We employ a hybrid-type proto-atmosphere model, which considers an upper layer composed of nebula components and a lower layer made up of degassed components from the impact degassing of the simplified building blocks based on the two-component model. Our numerical analysis implies that the formation of a magma ocean at this high temperature and pressure after around the half-size of the present Mars triggers the migration of water vapor, carbon, and hydrogen from the primordial atmosphere into the planetary interior. Ultimately, several 10²¹ kg of water could have been distributed to the mantle and, if degassed again by the formation of flood basalts on the Tharsis Plateau, could have supplied Mars’ ancient oceans. Meanwhile, several 10²⁰ kg of carbon and 10¹⁹ kg of hydrogen (equivalent to 10²⁰ kg H₂O) were found to be distributed in the core. Although the distribution of sulfur was simplified so that all of the sulfur is in the core, a result of sulfur accounting for 14 wt% to 19 wt.% of the core mass would be consistent with the core density suggested by InSight.

MMX (Martian Moons eXploration) is the 3rd sample return mission of JAXA/ISAS following Hayabusa and Hayabusa2. The MMX spacecraft will be launched in 2024 by an H-III rocket and make a round trip to the Martian system ~5 years. In the proximity of the Martian moons for 3 years, MMX will observe them along with the Martian atmosphere and surrounding space and conduct multiple landings on Phobos to collect Phoboss-indigenous materials. Owing to the lack of definitive evidence, the origin of Phobos and Deimos is under debate between the two leading hypotheses: the capture of volatile-rich primordial asteroid(s) and the in-situ formation from a debris disk that generated by a giant impact onto early Mars. Whichever theory is correct, the Martian moons likely preserve key records on the evolution of the early solar system and the formation of Mars. Through close-up observations of both moons and sample return from Phobos, MMX will settle the controversy of their origin, reveal their evolution, and elucidate the early solar system evolution around the region near the snow line. Global circulation and escape of the Martian atmosphere will also be monitored to reveal basic processes that have shaped and altered the Martian surface environment. The MMX spacecraft consists of three modules with chemical propulsion systems. By releasing used modules at appropriate timings, the spacecraft mass is reduced to allow orbital tuning to quasi-satellite orbits around Phobos, landings on Phobos surface, and the escape from the Martian gravity to return to the Earth. MMX will arrive at the Martian system in 2025 and start close-up observations of Phobos from quasi-satellite orbits. Among the total of 7 mounted instruments for scientific observations, TENGOO (telescope camera) and LIDAR will conduct high-resolution topography mapping and OROCHI (multi-band visible camera), MIRS (infra-red spectrometer provided by CNES), MEGANE (gamma-ray and neutron spectrometer provided by NASA), and MSA (ion mass spectrum analyzer) will survey surface composition and its heterogeneity. Hydrous minerals and interior ice are important observational targets because they, if identified, strongly support the capture hypothesis. Data taken by these instruments will be also useful for the landing site selection and characterization. Before the first landing, a rover (provided by CNES/DLR) will be released near the sampling site to collect data on surface regolith properties to be referred for the mothership landing operation. The rover will carry cameras, miniRAD (thermal mapper), and RAX (laser Raman spectrometer) to collect data on the physical and mineralogical characteristics of the Phobos surface around the sampling site. In early 2027, Mars will come to its closest approach to the Earth which minimizes the communication delay between the spacecraft and the Earth station. Together with the timing relatively far from Sun-Mars conjunctions and the Martian equinoxes, this period is the most favorable for landing operations that need real-time communication with the ground station and solar illumination undisturbed by eclipses. MMX will use two sampling systems, the C-sampler using a coring mechanism equipped on the tip of a manipulator and the P-sampler (provided by NASA) using a pneumatic mechanism equipped on a landing leg. After the stay near Phobos, the MMX spacecraft will be transferred to Deimos-flyby orbits to conduct Deimos observations, and then the return module will depart the Martian system in 2028. During the stay in the Martian system, MMX will also conduct wide-area observations of the Martian atmosphere using imagers (OROCHI, MIRS, and TENGOO) to study the atmospheric dynamics and the water vapor and dust transport. Simultanenousely, MSA will survey ions not only released and sputtered from Phobos's surface but also escaped from the Martian upper atmosphere. CMDM (dust monitor) will continuously survey the dust flux around the moons to assess the processes of space weathering by micrometeoroid bombardments and the possible formation of dust rings along the moons’ orbits. The sample capsule will come back to the Earth in 2029. Complimentarily with remote sensing studies, returned samples will provide us strong cosmo-chemical constraints for the origin of Phobos as well as those for early solar system processes.

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_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.

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