Yudai Suzuki

Abstract Phobos always keeps the same side facing its host planet like Earth’s Moon, making it a key comparative target for studying the coevolution of planet–satellite systems. The heterogeneity of satellite surface evolution under a host planet’s gravity is crucial for understanding the evolution of such systems. This study examines the crater size–frequency distribution (CSFD) across four equatorial regions of Phobos—leading, near, trailing, and far sides—to investigate surface evolution heterogeneities linked to resurfacing and orbital dynamics following the formation of its largest crater, Stickney. We focus on the crater size indicating the number of craters lower than expected from the production function (PF) model. We also estimate the crater erasure scale due to the ejecta blanket and assess deviations in the size–frequency distribution (SFD) of impactors from the PF model. This study shows three main conclusions about surface heterogeneity in Phobos’ CSFD and ejecta blanket thickness. (1) The leading side experienced significant crater erasure, likely due to the thickest ejecta blanket from the Stickney impact. (2) The density of craters larger than $$\sim$$ 1.5 km reflects pre-Stickney conditions, whereas the densities of craters measuring 0.8–1.5 km and 400–800 m in diameter reflect post-Stickney surface evolution. (3) Crater number density on the near side was consistently lower than on the far side, likely due to planetary screening. This reduction in crater formation on the near side is particularly attributed to its proximity to Mars. Our results suggest that the leading side experienced the greatest deposition of the ejecta blanket on Phobos. This deposition likely occurred immediately after Stickney’s formation, followed by global resurfacing facilitated by Phobos’ post-Stickney impact spin. One possible explanation for the scarcity of craters smaller than 800 m is that later-arriving, low-energy impacts erased smaller preexisting craters while leaving larger ones comparatively well preserved, leading to a net reduction in their population.

Abstract <p>Temporal variation in the cloud-top temperature of Venus is an essential observable for understanding its atmospheric dynamics and related phenomena, such as thermal tides and planetary-scale waves. While multiband monitoring of both phenomena over years could hint at ongoing dynamics, spaceborne observations of Venus over the last decade are limited to single-band imagery or short timeframe. As a complementary data for the lack of decadal multiband infrared measurements of Venus, the Japanese meteorological satellites Himawari-8/9 may be utilized because they have been coincidentally imaging Venus in space adjacent to the Earth’s rim. These images can serve as a new dataset for both Venus science and instrument calibrations in planetary missions, though they have never been utilized for such purposes. This study first archived all the Venus images taken by Himawari-8/9 from 2015 to 2024 and succeeded in retrieving disk-normalized brightness temperatures and their temporal variation on day to year scales. The archived data were compared with other observations from the Akatsuki and BepiColombo missions. Our comparison shows that the long-wave infrared camera (LIR) on Akatsuki has underestimated the infrared radiance by 15–17%, which needs to be considered in future LIR data analyses. From comparisons of the observed temperatures at each local time on Venus, we also found that the retrieved temporal variations contain changes in the patterns of thermal tides. Particularly at sunrise, the observed brightness temperatures were not constant between 2015 and 2024, implying variations in the amplitude of diurnal thermal tides. Furthermore, the amplitude of the 5-day Rossby waves decreased at altitudes of 68 km or higher, as suggested by previous numerical circulation models. Although retrieval of the Rossby-wave amplitude was successful only in two observation periods, a variation in altitude dependence was confirmed between 2015 and 2024. These observed temporal variations may be caused by several factors, including a change in static stability observed in the Himawari-8/9 measurements. These results demonstrate that meteorological satellites can serve as additional eyes to access the Venusian atmosphere from space and complement future observations from planetary missions and ground-based telescopes.</p>

Abstract In celestial bodies with tenuous collisionless atmospheres, such as Mercury, the spatial distribution of the exosphere is expected to reflect the surface composition. In this study, we discuss whether the distributions of Mg, Ca, and Na, the primary exospheric components of Mercury, have a local exosphere–surface correlation by analyzing the observation data of the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) and X-ray spectrometer (XRS) onboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. It was found that Mg has a strong local exosphere–surface correlation and Ca has a weak correlation. The Monte Carlo simulations of trajectories in the exosphere show that the weak correlation of Ca is due to the relatively large solar radiation acceleration. In addition, Na production rate in high-temperature regions is longitudinally dependent. This can be explained by considering that the weakly physisorbed Na layer on the surface is depleted under high temperature and that the distribution of strongly chemisorbed Na atoms is reflected in the exosphere. Based on these results, the conditions for components with a correlation in celestial bodies with thin atmospheres may include low volatility and low solar radiation acceleration. Graphical Abstract

Abstract Dust storms on Mars play a role in transporting water from its lower to upper atmosphere, seasonally enhancing hydrogen escape. However, it remains unclear how water is diurnally transported during a dust storm and how its elements, hydrogen and oxygen, are subsequently influenced in the upper atmosphere. Here, we use multi-spacecraft and space telescope observations obtained during a major dust storm in Mars Year 33 to show that hydrogen abundance in the upper atmosphere gradually increases because of water supply above an altitude of 60 km, while oxygen abundance temporarily decreases via water ice absorption, catalytic loss, or downward transportation. Additionally, atmospheric waves modulate dust and water transportations, causing alternate oscillations of hydrogen and oxygen abundances in the upper atmosphere. If dust- and wave-driven couplings of the Martian lower and upper atmospheres are common in dust storms, with increasing escape of hydrogen, oxygen will less efficiently escape from the upper atmosphere, leading to a more oxidized atmosphere. These findings provide insights regarding Mars’ water loss history and its redox state, which are crucial for understanding the Martian habitable environment.

Abstract Mercury is valuable to us because we can see the interactions between the planet and its space environment. This research aims to clarify how Mercury's neutral Na exosphere was produced. Data from MErcury Surface, Space ENvironment, GEochemistry, and Ranging/Mercury Atmospheric and Surface Composition Spectrometer (MESSENGER/MASCS) and model calculations that examine possible generation, transportation, and dissipation processes were compared. First, the seasonal variability in the amount of Na exosphere was analyzed for each local time (LT) using MASCS data. Previous research has shown that the amount of Na above LT12 reaches its maximum at the aphelion and that this maximum is recorded only at LT12. Following this result, we constructed a 3‐D Na exosphere model to understand the key seasonal variability processes occurring around LT12. The numerical calculation produced results that were consistent with the MASCS observations regarding the vertical profile and the seasonal variability at LT06 and LT18. However, the peak that occurs around the aphelion at LT12 could not be reproduced. However, the model produced results suggesting that less than 10⁸ kg of comet stream dust particles per Mercury year could be the local and short‐term source of Na.