Densification of Water Ice Deposits on the Residual North Polar Cap of Mars

Understanding the density evolution of any ice deposits on the martian north polar residual cap is key to understanding whether the cap contains a climate record in the form of occluded samples of past atmospheres. We present a physical model for densification of water ice deposits on the cap due to gravity-driven sintering and temperature-gradient-driven vapor transport. The model predicts the steady-state profile of porosity versus depth in the ice cap for specified, time-independent mass accumulation rate and surface deposition density and for surface temperatures with specified diurnal and seasonal variations that comprise an invariant annual cycle. We find that, in contrast to the analogous situation on Earth, vapor transport dominates densification in the current martian climate. This mechanism can produce very low-porosity near-surface ice, even when accumulation rates are small but positive, comparable to those presently thought to occur. Thus recent inferences of high thermal inertia on the cap may not necessarily imply that the cap is ablating. The model predicts a sharp transition between temperature and accumulation conditions under which a deep firn column results and those characterized by very low surface porosity. Finally, we derive a temporal-frequency-dependent thermal inertia for a vertically inhomogeneous medium. Observations of this quantity are, at least in principle, useful for remote estimation of the ice cap density variations within about 10 m of the surface.