IMAGING UNDERGROUND: ADVANCEMENTS IN MAPPING THE SUBSURFACE STRATIGRAPHY OF PLANETS

The upper tens- to hundreds- of-meters of planetary surfaces provide a record of the past processes such as regolith development, volcanism, impact cratering, and fluvial and cryospheric activity. Sounding and synthetic aperture radar data, combined with high-resolution optical imaging of the surface and topographic measurements, have provided a new capability to map the stratigraphy of this part of the subsurface. Comparisons of radar and optical observations have revealed how ejecta and melts are emplaced during impacts, including the important role of pre-existing topography. Shallow Radar (SHARAD) and High Resolution Imaging Science Experiment (HiRISE) data from Mars Reconnaissance Orbiter have been used to map subsurface ice deposits and have revealed large CO₂ deposits at the south pole. Radar observations of lava flows have shown emplacement structures and textures that are buried just beneath the surface, and have provided context to features seen in optical images such as ridges, dark mantling deposits, and layering observed in cliff faces and crater walls. In all these cases, the clearest information about the geology of the region has come from merging imaging products from instruments operating at very different wavelengths. Future technological advances in geophysical remote sensing instruments, cameras, communications and downlink, and data processing systems will provide an unprecedented ability to reproduce the 3D structure of the surface and subsurface from remote observations, providing a deeper window into the geologic history of rocky and icy worlds.