The present atmosphere of Venus contains almost no water, but recent measurements indicate that in its early history Venus had an Earth-like ocean. Understanding how the Venusian atmosphere evolved is important not only for Venus itself, but also for understanding the evolution of other planetary atmospheres. In this study, we quantify the escape rates of oxygen ions from the present Venus to infer the past of the Venusian atmosphere. We show that an extrapolation of the current escape rates back in time leads to the total escape of 0.02-0.6 m of a global equivalent layer of water. This implies that the loss of ions to space, inferred from the present state, cannot account for the loss of an historical Earth-like ocean. We find that the O+ escape rate increases with solar wind energy flux, where more energy available leads to a higher escape rate. Oppositely, the escape rate decrease slightly with increased EUV flux, though the small variation of EUV flux over the measured solar cycle may explain the weak dependency. These results indicate that there isn’t enough energy transferred from the solar wind to Venus’ upper atmosphere that can lead to the escape of the atmosphere over the past 3.9 billion years. This means that the Venusian atmosphere didn’t have as much water in its atmosphere as previously assumed or the present-day escape rates don’t represent the historical escape rates at Venus. Otherwise, some other mechanisms have acted to more effectively remove the water from the Venusian atmosphere.

Due to the lower ionospheric thermal pressure and existence of the crustal magnetism at Mars, the Martian ionopause is expected to behave differently from the ionopause at Venus. We study the solar wind interaction and pressure balance at the ionopause of Mars using both in situ and remote sounding measurements from the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on the Mars Express orbiter. We show that the magnetic pressure usually dominates the thermal pressure to hold off the solar wind in the ionopause at Mars, with only 13% unmagnetized ionopauses observed over a 11-year period. We also find that the ionopause altitude decreases as the normal component of the solar wind dynamic pressure increases. Moreover, our results show that the ionopause thickness at Mars is mainly determined by the ion gyromotion and equivalent to about 5.7 ion gyroradii.

Using over 6 years of magnetic field data (2014.10-2020.12) collected by the Mars Atmosphere and Volatile EvolutioN (MAVEN), we conduct a statistical study on the three-dimensional average magnetic field structure around Mars. We find that this magnetic field structure conforms to the pattern typical of an induced magnetosphere, that is, the interplanetary magnetic field (IMF) which is carried by the solar wind and which drapes, piles up, slips around the planet, and eventually forms a tail in the wake. The draped field lines from both hemispheres along the direction of the solar wind electric field (E) are directed towards the nightside magnetic equatorial plane, which looks like they are “sinking” toward the wake. These “sinking” field lines from the +E-hemisphere (E pointing away from the plane) are more flared and dominant in the tail, while the field lines from the –E-hemisphere (E pointing towards) are more stretched and “pinched” towards the plasma sheet. Such highly “pinched” field lines even form a loop over the pole of the –E-hemisphere. The tail current sheet also shows an E-asymmetry: the sheet is thicker with a stronger tailward J×B force at +E-flank, but much thinner and with a weaker J×B (even turns sunward) at –E-flank. Additionally, we find that IMF Bx can induce a kink-like field structure at the boundary layer; the field strength is globally enhanced and the field lines flare less during high dynamic pressure; however, the rotation of the planet, against expectations, modulate the configuration of the tail current sheet insignificantly.