  The distribution of subsurface water ice on Mars is a key constraint on past
climate, while the volumetric concentration of buried ice (pore-filling versus
excess) provides information about the process that led to its deposition. We
investigate the subsurface of Arcadia Planitia by measuring the depth of
terraces in simple impact craters and mapping a widespread subsurface
reflection in radar sounding data. Assuming that the contrast in material
strengths responsible for the terracing is the same dielectric interface that
causes the radar reflection, we can combine these data to estimate the
dielectric constant of the overlying material. We compare these results to a
three-component dielectric mixing model to constrain composition. Our results
indicate a widespread, decameters-thick layer that is excess water ice ~10^4
km^3 in volume. The accumulation and long-term preservation of this ice is a
challenge for current Martian climate models.
