Transient subsurface features in Mars Express radar data: an explanation based on ionospheric holes

This study was motivated by the discovery of semi-circular subsurface craters, or basins, at multiple locations on Mars by the MARSIS (Mars Advanced Radar for Subsurface and Ionospheric Sounding) radar sounder on board the Mars Express spacecraft. The nature of these subsurface structures was called into question when it was realized that some of the radar observations were not repeatable on subsequent passes over the same region. If they were true geological structures, such as ancient craters buried by dust, one would expect to always see them when the spacecraft passes over these regions. The transient nature of the observations led to the suggestion that these structures were actually of ionospheric origin.

In this paper we will provide evidence, including a proof-of-concept result, that these features are produced by holes in the ionosphere, and not by subsurface structures. We discuss the possibility that the ionospheric holes are caused by an interaction of the ionosphere with local crustal magnetic fields. We introduce the ionospheric model which we used to simulate the MARSIS sounder moving and pulsing radio waves through the Martian ionosphere, and show that the results of ray tracing through this density profile are consistent with data seen in the MARSIS radargrams.