Space Weather at Mercury and Mars: A Comparative Approach between an Unlikely Pair (Invited Presentation)

The solar wind’s influence on a planet is highly dependent on the properties of the local space environment. The magnetospheres of Mercury and Mars provide two drastically different scenarios from which it is possible to compare the impact of space weather activity. As the closest planet to sun, Mercury experiences the largest degree of solar wind forcing compared to any other planet in our solar system. Mercury’s intrinsic magnetic field is able to stand off the solar wind under nominal conditions to shield the thin, collisionless atmosphere (classified as a surface‐bounded exosphere) and the planetary surface. In contrast, Mars lacks a global magnetic field but has localized crustal magnetic fields that rotate with the planet. This variable magnetic environment allows the solar wind to directly impinge upon the Martian upper atmosphere. Given the stark differences between these two planetary environments and solar wind characteristics at varying heliocentric distances, a variety of processes are observed during space weather events. We will review these processes using in situ measurements from the MESSENGER mission at Mercury, along with in situ and remoting sensing observations from the MAVEN mission at Mars. Both missions have directly observed the planetary response to Coronal Mass Ejections (CMEs) and Stream Interaction Regions (SIRs). At Mercury, increased rates of magnetic reconnection have been observed with the dayside magnetosphere disappearing to expose the planetary surface. At Mars, space weather events influence atmospheric escape processes and can generate global aurora events. Comparing the impact of extreme space weather activity at Mercury and Mars considers a wide range of scenarios that can serve to enhance our understanding of space weather at exoplanetary bodies.