Crater morphology and surface age of asteroid (162173) Ryugu are characterized using the high-resolution images obtained by the Hayabusa2 spacecraft. Our observations reveal that the abundant boulders on and under the surface of the rubble-pile asteroid affect crater morphology. Most of the craters on Ryugu exhibit well-defined circular depressions, unlike those observed on asteroid Itokawa. The craters are typically outlined by boulders remaining on the rim. Large craters (diameter >100 m) host abundant and sometimes unproportionally large boulders on their floors. Small craters (<20 m) are characterized by smooth circular floors distinguishable from the boulder-rich exterior. Such small craters tend to have dark centers of unclear origin. The correlation between crater size and boulder number density suggests that some processes sort the size of boulders in the shallow (<30 m) subsurface. Furthermore, the crater size-frequency distributions (CSFDs) of different regions on Ryugu record multiple geologic events, revealing the diverse geologic history on this 1-km asteroid. Our crater counting analyses indicate that the equatorial ridge is the oldest structure of Ryugu and was formed 23-29 Myr ago. Then, Ryugu was partially resurfaced, possibly by the impact that formed the Urashima crater 5-12 Myr ago. Subsequently, a large-scale resurfacing event formed the western bulge and the fossae 2-9 Myr ago. Following this process, the spin of Ryugu slowed down plausibly due to the YORP effect. The transition of isochrons in a CSFD suggests that Ryugu was decoupled from the main belt and transferred to a near-Earth orbit 0.2-7 Myr ago.

Mercury is valuable to us because we can see the interaction between the planet and its space environment. This research aims to clarify how Mercury’s neutral Na exosphere was produced. Data from MESSENGER/MASCS and model calculations that examine possible generation, transportation and dissipation processes will be compared. First, seasonal variability of the amount of Na exosphere is analyzed for each local time (LT) using MASCS data. Previous research has shown that the amount of Na above LT12 reaches its maximum at aphelion, and it is found that this maximum is recorded only at LT12. Following this result, we construct 3-D Na exosphere model to understand the key seasonal variability processes occurring around LT12. The numerical calculation produced results that are consistent with the MASCS observations regarding the vertical profile and the seasonal variability at LT06 and LT18. However, the peak that occurs around aphelion at LT12 could not be reproduced. Yet the model produced results suggesting that less than 10 kg particles of comet stream dust per Mercury year could be the local and short-term source of Na.

In celestial bodies with 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 distribution of Mg, Ca, and Na, the primary exospheric components on Mercury, have exosphere-surface correlation (ESC) by analyzing the observation data of the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) onboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. As a result, it was found that Mg has strong ESC, Ca has weak ESC and Na has little ESC. The Monte Carlo simulations of trajectory in the exosphere show that the weak ESC of Ca is due to the relatively large solar radiation acceleration. Na has ESC only in high-temperature regions around 0°E. This can be explained well 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 ESC in celestial bodies with thin atmospheres include low volatility and little solar radiation acceleration.

Quasi-periodic variations of a few to several days are observed in the energetic plasma and magnetic dipolarization in Jupiter’s magnetosphere. Variation in the plasma mass flux related to Io’s volcanic activity is proposed as a candidate of the variety of the period. Using a long-term monitoring of Jupiter by the Earth-orbiting space telescope Hisaki, we analyzed the quasi-periodic variation seen in the auroral power integrated over the northern pole for 2014–2016, which included monitoring Io’s volcanically active period in 2015 and the solar wind near Jupiter during Juno’s approach in 2016. Quasi-periodic variation with periods of 0.8–8 days was detected. The difference between the periodicities during volcanically active and quiet periods is not significant. Our dataset suggests that a difference of period between this volcanically active and quiet conditions is below 1.25 days. This is consistent with the expected difference estimated from a proposed relationship based on a theoretical model applied to the plasma variation of this volcanic event. The periodicity does not show a clear correlation with the auroral power, central meridional longitude, or Io phase angle. The periodic variation is continuously observed in addition to the auroral modulation due to solar wind variation. Furthermore, Hisaki auroral data sometimes shows particularly intense auroral bursts of emissions lasting <10h. We find that these bursts coincide with peaks of the periodic variations. Moreover, the occurrence of these bursts increases during the volcanically active period. This auroral observation links parts of previous observations to give a global view of Jupiter’s magnetospheric dynamics.