Hidenori Genda

The habitability of terrestrial exoplanets orbiting M dwarfs is a key topic in the search for extraterrestrial life. The climates of these planets differ significantly from the Earth’s due to their likely tidal locking, resulting in hotter daysides and colder nightsides caused by uneven stellar irradiation. On tidally locked planets around the outer edge of the habitable zone (HZ), although the definition of the classical HZ requires a thick CO ₂ atmosphere, CO ₂ can condense onto the surface, leading to the reduction of the greenhouse effect. However, the dayside permanent stellar irradiation could maintain a surface liquid water area. The onset of atmospheric collapse and the persistence of surface liquid water are governed by global heat redistribution, which is influenced by factors such as atmospheric mass, stellar irradiation, and greenhouse effects. In this study, we used a 3D global climate model to investigate the impact of atmospheric collapse on the presence of dayside surface liquid water. Our results indicate that surface liquid water could counterintuitively persist despite atmospheric collapse. This is because the loss of atmospheric CO ₂ weakens not only the greenhouse effect but also day–night heat transport, leading to less redistribution of the energy of dayside insolation to the nightside. While atmospheric collapse is typically seen as an obstacle to maintaining a habitable climate, our findings suggest that it could play a positive role in sustaining surface liquid water on tidally locked planets. Our work provides new light into the relationship between atmospheric collapse and planetary habitability.

The climates of terrestrial planets are largely determined by the composition of their atmospheres and the spectral types of their host stars. Previous studies suggest that a wide range of carbon species abundances (CO₂, CO, and CH₄) can result from variations in reducing fluxes and stellar spectral types that influence photochemistry. However, a systematic investigation of how varying carbon species, particularly CO, affect planetary climates across wide parameter spaces remains limited. Here, we employ a 1D radiative–convective equilibrium model to examine the dependence of planetary climate on the abundances of carbon species and host star type. We find that CO, due to weak absorption of stellar radiation, induces only moderate changes in stratospheric temperature, while its effect on surface temperature is negligible. Under Earth-like p N 2 (where p _i is the partial pressure on the surface of species i), for cases with fixed p CO 2 , an increase in CO leads to surface cooling on planets orbiting Sun-like stars unless the sum of p CO 2 and p CH 4 exceeds ∼1 bar, whereas it results in surface warming for planets around M-type stars. When the total pressure of carbon species is fixed, converting CO₂ or CH₄ into CO always induces cooling. These effects arise from a combination of CO Rayleigh scattering, pressure broadening of greenhouse gas absorption lines, and varying water vapor levels. We further discuss how CO- and CH₄-driven cooling (warming) can trigger positive (negative) climate-photochemistry feedback, influencing atmospheric evolution. Additionally, we suggest that CO-rich planets may be less susceptible to water loss and atmospheric oxidation due to lower stratospheric water vapor content.

Habitable terrestrial exoplanets around M dwarfs are remarkable targets due to the ease to detect and characterize. These planets are thought to have a different climate from solar system planets because these planets are expected to be in a tidally-locked state due to closer orbits and strong tidal forces. According to the studies of planet formation, terrestrial exoplanets around M dwarfs could sustain a large amount of surface water (tens or hundreds of Earth ocean mass). Such planets with large amount water (deep ocean) would be globally ocean covered, resulting in no continents. Earth’s ocean circulation largely affects the climate (e.g. meridional heat transport, water cycle). Ocean circulation is roughly classified into wind-driven circulation and density-driven circulation. In particular, density-driven circulation, which passes deep ocean, is affected by sea water temperature and salinity, resulting from heat exchange to atmosphere and salinity change by precipitation or sea ice formation. Although salinity has large influences on ocean behavior such as strength of ocean circulation and condition of sea ice formation, this effect on tidally-locked ocean planet has been still unknown. To investigate the influence on climate by salinity, first, we developed an atmospheric and oceanic global climate model (AOGCM) for exoplanets based on MIROC4m. Both atmospheric part and oceanic part solve 3-dimensional hydrodynamics, thermodynamics, and tracer transports (such as water vapor in the atmosphere and salinity in the ocean), and a coupler exchanges the information about the sea surface between both parts for example radiative heating and evaporation. By using the AOGCM, we simulated the TRAPPIST-1e’s climate assuming an ocean covered planet. We set 1 Earth ocean mass (3200 m depth because of its planetary radius) with Earth-like salinity (35.4 psu) or pure water (0 psu). In addition to the AOGCM simulations, we also run AGCM simulation which includes slab ocean instead of ocean GCM part. We integrated over 2000 years for AOGCM simulations and 50 years for AGCM simulation. An eyeball-shape open sea appears on the dayside according to the AGCM simulation, however, AOGCM simulations result in slushball-shape open sea due to the equatorial ocean currents. Comparing the results of AOGCM results, we found that salinity expands open sea distribution due to freezing-point depression and affects intensity of surface ocean current and vertical circulation. In this presentation, we will show the physical mechanisms of these differences by salinity.

Satellites play a crucial role in understanding the formation and evolution of trans‐Neptunian objects (TNOs). The spin–orbit evolution of satellite systems depends on their thermal histories, allowing us to constrain the rock mass fraction within TNOs based on their current spin–orbit states. In this study, we perform coupled thermal–orbital evolution calculations for two satellite systems around undifferentiated TNOs: Orcus–Vanth and Salacia–Actaea. Our results demonstrate that the current spin–orbit states of these systems are consistent with a rock mass fraction of approximately 20%–30%. Additionally, we estimate the organic mass fraction within the TNOs and find that it is comparable to the rock mass fraction. These findings suggest that the chemical composition of TNOs closely resembles that of comets.

Saturn’s rings have been estimated to be as young as about 100 to 400 million years old according to the hypothesis that non-icy micrometeoroid bombardment acts to darken the rings over time and the Cassini observation indicated that the ring particles appear to be relatively clean. These young age estimates assume that the rings formed out of pure water ice particles with a high accretion efficiency of impacting non-icy micrometeoroid material (η ≳ 10%). Here we show, using numerical simulations of hypervelocity micrometeoroid impacts on a ring particle, that non-icy material may not be as readily accreted as previously thought. We found that the complete vaporization and expansion of non-icy impactor material on energetic collision with a ring particle leads to the formation of charged nanoparticles and ions that are subsequently removed from the rings through collision with Saturn, gravitational escape or electromagnetic drag into Saturn’s atmosphere. Despite uncertainties in our models that assume no porosity, strength or ring particle granularity, we suggest minimal accretion of non-icy materials would occur following micrometeoroid impact. This pollution resistance mechanism implies a low accretion efficiency (η ≲ 1%). Thus we suggest that the apparent youth of Saturn’s rings could be due to pollution resistance, rather than indicative of young formation age.

Habitable planets around M dwarfs are remarkable targets as they are among the least difficult terrestrial-size planets to detect and characterize. However, the climate characteristics on these planets are thought to be different from solar system terrestrial planets in particular because these planets are expected to be in a tidally-locked state, with a permanent irradiated hemisphere (dayside) and opposite one (nightside). If a planet orbits far from its host star, nightside temperatures can become extremely low, which leads volatile species such as CO2 and CH4 in the atmosphere to condense onto the surface. This phenomenon, known as atmospheric collapse, is thought to prevent habitable environment since removal from the atmosphere (especially CO2) decreases the stabilizing greenhouse effect.We investigated the relationship between atmospheric collapse and habitability using the Generic Planetary Climate Model (Generic PCM), a 3-D global climate model historically developed at the Laboratoire de Météorologie Dynamique (LMD), by changing stellar insolation and CO2 partial pressure. The onset of atmospheric collapse for each case is decided by the surface amount of condensed CO2. As a result, we found cases where locally habitable environment on the dayside remains during/after the atmospheric collapse event in spite of the decrease in greenhouse effect. This is because the decrease in atmospheric mass and thus in atmospheric pressure makes day-night atmospheric heat transport less efficient, resulting in less of energy by insolation on the dayside. In addition, lower background gas cases were more habitable (i.e., more likely to have surface liquid water) over a wide range of CO2 partial pressure. While high background gas pressure is usually considered to enhance the greenhouse effect due to pressure broadening, on tidally-locked planets less background gas contributes to an increase in dayside temperatures.To conclude, our results provide a new picture of the relationship between atmospheric collapse and habitability on tidally-locked, cool planets.

Numerous small bodies inevitably lead to cratering impacts on large planetary bodies during planet formation and evolution. As a consequence of these small impacts, a fraction of the target material escapes from the gravity of the large body, and a fraction of the impactor material accretes onto the target surface, depending on the impact velocities and angles. Here, we study the mass of the high-speed ejecta that escapes from the target gravity by cratering impacts when material strength is neglected. We perform a large number of cratering impact simulations onto a planar rocky and icy targets using the smoothed particle hydrodynamics method. We show that the escape mass of the target material obtained from our numerical simulations agrees with the prediction of a scaling law under a point-source assumption when vimp > ~ 10 vesc, where vimp is the impact velocity and vesc is the escape velocity of the target. However, we find that the point-source scaling law overestimates the escape mass up to a factor of ~ 70, depending on the impact angle, when vimp < ~ 10 vesc (Figure 1). Using data obtained from numerical simulations, we derive a new scaling law for the escape mass of the target material􏰁. We also derive a scaling law that predicts the accretion mass of the impactor material onto the target surface upon cratering impacts by numerically evaluating the escape mass of the impactor material. Our newly derived scaling laws would be useful for predicting the erosion of the target body and the accretion of the impactor for a variety of cratering impacts that would occur on large rocky and icy planetary bodies during planet formation and collisional evolution from ancient times to today (Figure 2). Figure 1. Schematic illustration of cratering impacts (left) and velocity distribution of the ejecta (right). High- and low-speed ejecta are launched closer to and farther from the impact point, respectively; that is, the escape mass is launched from the vicinity of the impact point (left). Irregular-shaped objects in the left panel indicate ejecta of target and impactor materials (the total amount and their fractions depend on the impact condition). The point-source scaling law is characterized by a unique slope in the velocity distribution, while direct SPH simulations of cratering impacts demonstrated that high-speed ejecta would have a steeper slope for the velocity distribution (the gray line in the right panel). This discrepancy leads to an overestimation in the escape mass when using the point-source scaling law, depending on the considered escape velocity of the target vesc. Figure 2. Impact angle-averaged scaling laws for rocky (red lines) and icy (blue lines) bodies as a function of vimp/vesc. Left: the escape mass of the target material (Mesc,tar) normalized by impactor mass (mimp). Middle: the accretion mass of the impactor material onto the target surface (Macc,imp/mimp). Right: total escape mass (Mesc,tot = Mesc,tar + Mesc,imp in a unit of mimp; Mesc,tot > 1 indicates a net erosion and vice versa for a net accretion). Examples of typical outcomes of cratering impacts on different solar system bodies under different contexts are indicated by the labels in each panels.

Japan Aerospace Exploration Agency's Martian Moons eXploration (MMX) mission will launch a spacecraft in 2024 to return samples from Phobos in 2029. Curatorial work for the returned Phobos samples is critical for the sample allocation without degrading the sample integrity and subsequent sample analysis that will provide new constraints on the origin of Phobos and the evolution of the circum‐Mars environment. The Sample Analysis Working Team of the MMX is designing the sample curation protocol. The curation protocol consists of three phases: (1) quick analysis (extraction and mass spectrometry for gases), (2) pre‐basic characterization (bulk‐scale observation), and (3) basic characterization (grain‐by‐grain observation and allocation of the sample aliquots). Nondestructive analyses within the clean chamber (e.g., visible and near‐infrared spectral imaging) and outside the chamber (e.g., gas mass spectrometry) are incorporated into the curation flow in coordination with the MMX mission instrument teams for ground‐truthing the remote‐sensing data sets. The MMX curation/sample analysis flow enables the seamless integration between the sample and remote‐sensing data sets to maximize the scientific value of the collected Phobos samples.