Comparison of Global Models for the Escape of Martian Atmospheric Plasma

The atmospheres of planets lacking global magnetic fields are directly exposed to the plasma in which they are embedded, such as the solar wind. Atmospheric particles ionized at high altitudes (near the exobase region) can be accelerated away from the planet by the flowing plasma, resulting in atmospheric escape. This process is observed to occur at Venus, Mars, and Titan. However, the role this process plays in altering the atmospheres of these planets over solar system history is still unclear. One method of assessing the importance of solar wind stripping processes, especially at past epochs, is to model them. At present there are many global models for the Martian plasma interaction, spanning a variety of physical assumptions and implementation decisions. But it is difficult to determine whether and how the results of the various models compare in light of their differences, since they are seldom run for similar input conditions.

We present the results of a community-wide effort to compare the results of global models for the Martian plasma interaction and atmospheric ion escape. Nine different models were run for identical input conditions, and the results compared in different ways. Each model was run three times in a Venus-like limit (lacking crustal magnetic fields), representing cases for Mars at solar minimum with and without an exosphere and solar maximum with an exosphere. The global escape rate of ions was extracted from each model, as well as 2D slices of various modeled quantities, and modeled quantities along a Mars Express spacecraft orbit trajectory. We will show how the models compare with each other and with observations, and discuss what we can learn from such models about the evolution of the Martian atmosphere.