Thickness of Martian Ground Ice: Implication from Multi-water-reservoir Model

Introduction: Martian surface ice is currently observed only as polar layered deposits (PLDs), whereas Mars Odyssey Gamma Ray Spectrometer [1,2] and Mars Express’s radar sounder observations [3] propose the presence of much larger amount of ground ice in the midto high-latidutes. Such a ground-ice region is expected to spread over a few tenths of percent of the total Martian surface, yet the thickness (i.e. volume) is poorly constrained [3]. The thickness of the ground ice is related to the evolution history of the Martian water reservoirs. After ancient oceans became extinct, the oceanic water would become “surface ice", which currently occurs as PLDs, and “ground ice" which would extend from high latitude to midor low-latitude. Atmospheric escape of hydrogen and oxygen through the Martian history causes decrease of the amount of the ice. The signature of the evolution history is recorded by hydrogen isotope ratio (D/H). Martin atmosphere and soil have D/H ratio of ~ 6 (relative to SMOW) [4,5], which is distinctly higher than the Martian primitive D/H ratio of ~ 1.3 [6]. The escape rate is estimated by using the D/H ratio and the amount of PLDs, which is relatively well constrained [7,8]. Then the amount of the ground ice can be constrained by the estimated escape rate and the D/H ratio. This study constrains the thickness of the ground ice by using an evolution model of D/H ratio of multi-water-reservoirs during the ice age.