Naoki Terada

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.

Aims . We use carbon monoxide (CO), a long-lived species in the Martian atmosphere, as a tracer to constrain atmospheric transport by characterizing its vertical distribution from the near-surface to an altitude of about 120 km. Methods . We retrieved CO vertical profiles from solar occultation (SO) spectra acquired by the SO channel of the Nadir and Occultation for MArs Discovery (NOMAD) instrument on board the ExoMars Trace Gas Orbiter (TGO). The dataset covers L _s = 160° in Mars year (MY) 34 to L _s = 120° in MY 36, enabling analyses over broader altitude and seasonal ranges than in previous works. We validated the retrievals using co-located measurements from the from Atmospheric Chemistry Suite (ACS) instrument on board TGO. Results . The retrieved CO volume mixing ratio (VMR) is enhanced above 40 km at both poles during seasonal spring and autumn. In summer and winter, the enhancement is largely confined to the winter polar region. These variations are in quantitative agreement with earlier observations and are consistent with seasonal changes in the meridional circulation. We also find larger CO number densities near the morning terminator during local winter in the 80–100 km altitude range, consistent with a modulation of the transport by a semidiurnal tide. During the MY 34 planet-encircling dust event, enhanced CO VMRs are confirmed at 80–110 km in altitude, but no corresponding enhancement in the CO number density is seen, suggesting that the apparent VMR variation is driven primarily by changes in atmospheric dynamics, rather than by photochemistry.

On early Mars, transient warm periods during the late Noachian and early Hesperian were likely triggered by episodic inputs of reducing greenhouse gases, such as H ₂ . In such a weakly reducing atmosphere, formaldehyde (H ₂ CO), a precursor for the abiotic formation of sugars via the formose reaction, can be effectively produced. While this pathway is a candidate source for Martian organic matter, previous 1D photochemical modeling could not predict the spatial distribution of organic synthesis. In this study, we developed a lookup table of H ₂ CO production based on key atmospheric variables using a photochemical model and coupled it with a global climate model. We calculated the global distribution of the atmospheric deposition fluxes of H ₂ CO under a 2 bar CO ₂ atmosphere containing 6% H ₂ with obliquity of 40°. Our results reveal a strong positive correlation between atmospheric H ₂ CO deposition and H ₂ O vapor abundance, driven by both the enhanced photochemical production of H ₂ CO through H ₂ O photolysis and the efficient scavenging by precipitation in humid regions. Significant atmospheric deposition of H ₂ CO is observed over water bodies, including the northern ocean. Furthermore, atmospheric deposition hotspots emerge around topographic highs, such as the Tharsis and Elysium mons, due to orographic precipitation. We suggest that basins adjacent to persistently humid regions, which likely experienced H ₂ CO accumulation and subsequent organic synthesis, could be potential landing sites for future missions to investigate ancient chemical evolution and whether it led to the origin of life.

Water plays an important role in the Martian climate. Orbital observations of the Martian atmosphere show that the water vapor column density corrected for topographic effects anti‐correlates with surface pressure, which has not been well reproduced in models. In this study, we investigate the effects of water exchange between the regolith and the atmosphere via adsorption‐desorption on the scaled water vapor density, which has not been considered previously. Our simulations show that the regolith releases 10 ⁻⁹ –10 ⁻⁸ kg m ⁻² s ⁻¹ of water during the day, corresponding to 0.086–0.346 pr‐µm over a 6‐hr integration period, creating a daytime enrichment near the surface. Enrichment also occurs at altitudes, where terrain‐induced daytime updrafts transport water vapor to the upper atmosphere. The water vapor is then trapped in clouds at night. Overall, the model captures enhanced water vapor in regions consistent with observations, but the simulated longitudinal contrasts of the scaled water vapor column density are smaller than those retrieved from observations. Nevertheless, our results suggest that regolith‐atmosphere interactions and anabatic winds play non‐negligible roles in modulating near‐surface water vapor variations and in reproducing the retrieved pattern of the scaled water vapor column density.

The enrichment of heavy isotopes of volatile elements in the Martian atmosphere indicates that Mars lost a large portion of its atmosphere through escape to space. Recent atmospheric measurements by the ExoMars Trace Gas Orbiter (TGO) have suggested that the vertical profiles of oxygen isotopic compositions are influenced by chemical reactions involving isotopic fractionation. However, their quantitative impacts have not yet been fully evaluated. In this study, we develop a 1D photochemical model that incorporates oxygen isotopic fractionation associated with CO ₂ photolysis and O ₃ formation to investigate the vertical profiles of oxygen isotopic compositions. Our calculations show that CO is depleted in heavy oxygen isotopes relative to CO ₂ , reaching δ ¹⁸ O ∼ −25‰ and δ ¹⁷ O ∼ −15‰, primarily due to isotopic fractionation during CO ₂ photolysis. The vertical profiles of oxygen and carbon isotopic compositions are in good agreement between our model and the TGO measurements. O ₃ is strongly enriched in ¹⁸ O and ¹⁷ O, reaching δ ¹⁸ O ∼ 100‰ and δ ¹⁷ O ∼ 50‰ as a consequence of the isotopic fractionation during its formation, whereas atomic oxygen is highly depleted in the heavy oxygen isotopes with δ ¹⁸ O ≲ −100‰ and δ ¹⁷ O ≲ −50‰ so as to compensate for their enrichment in O ₃ . These chemical fractionation processes can deplete the heavy oxygen isotopes in species that escape from the upper atmosphere and thereby enhance the isotopic fractionation associated with oxygen escape to space. Such fractionated isotopic compositions of escaping oxygen may be detectable by the Martian Moons eXploration (MMX) mission.

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.

This study explores the subsurface water distribution near the surface throughout Mars, using a Mars Global Climate Model (MGCM). We coupled a regolith model that solves the equations of water transport, adsorption, and condensation in the regolith with our MGCM to investigate the effects of inhomogeneous regolith properties on the subsurface water distribution. This regolith model includes a regolith property model that calculates inhomogeneous regolith properties (grain size, porosity, and adsorption coefficient) based on the results of laboratory experiments. The simulations were performed with five different regolith situations over 500 Martian years, aiming for a quasi‐steady state of adsorbed water concentration within the top meters of the regolith. Our results revealed a high adsorbed water concentration near the equator (2.0–4.0 kg m⁻² for depths within 2 m, with a high adsorption coefficient of ∼200 kg m⁻³), which was caused by the highly adsorbed regolith, which has a large specific surface area. This indicates the nonnegligible effects of the spatial inhomogeneity of the adsorption coefficient on the adsorbed water distribution. Highly adsorptive regolith also promotes pore ice accumulation in shallower layers at high latitudes because in the winter the freezing of adsorbed water is faster than the water transport in the regolith. In the determination of long‐term water retention and runoff on Mars, highly adsorptive regolith acts to retain subsurface water, decelerating water transport within the regolith because the water vapor concentration at equilibrium decreases with increasing adsorption coefficient. This study highlights the significant effects of regolith adsorbability on the subsurface water distribution.

C⁺ emission is generated by electron impact, dissociative ionization, photoionization, and resonant scattering with carbon-related atoms, molecules, and ions in the Martian ionosphere and thermosphere. The contribution of each mechanism to the emission, however, has not been elucidated due to the difficulty of observation and the fact that a part of the emission cross section is unclear. The current paper isolates the C⁺ emission mechanism using remote-sensing and in situ observations on board Mars Atmosphere and Volatile EvolutioN. Both electron impact and dissociative ionization/photoionization contribute to C⁺ emission below 150 km altitude when the CO density is high, but only dissociative ionization/photoionization contributes to the emission for the low CO density case, while only dissociative ionization/photoionization dominates the emission at altitudes between 150 and 165 km for both CO density cases. It is difficult to estimate the total flux of suprathermal electrons in the ionosphere from remote-sensing observations of C⁺ emission because the contribution of electron impact to C⁺ emission is small. In contrast, C-atom remote-sensing observations might provide a better understanding of the total flux of suprathermal electrons in the ionosphere than C⁺ emission, and global ultraviolet observations could be utilized as a tool for monitoring the ionosphere. The total flux of suprathermal electrons estimated from C-atom emission may be utilized to isolate the contribution of each C⁺ emission process to the brightness more accurately. This suggests that the C⁺ and C-atom emissions might be tracers of spatiotemporal variations in the Martian ionosphere and thermosphere.