Fast numerical method for growth and retreat of subsurface ice on Mars

Subsurface water ice on Mars evolves due to exchange of vapor with the atmosphere, in the form of loss of ice to the atmosphere or in the form of the growth of interstitial ice. Described here is an accelerated numerical method for the long-term evolution of subsurface ice. This accelerated method is five orders of magnitude faster than explicit vapor transport calculations, enabling fundamentally new types of climate models. Its speed matches that of purely thermal models. The speedup is achieved primarily by solving time-averaged equations for vapor transport and ice volume change. Processes incorporated are growth of interstitial pore ice, retreat of pore ice, retreat of an ice sheet, and retreat of pore ice due to geothermal heating from below. Two example applications illustrate this numerical method’s capabilities. Near the permafrost margin at 55° latitude, ice is periodically depleted and slowly recharged, leading to a pore ice layer estimated to be currently no more than a few meters thick. At the Phoenix Landing Site, it shows the formation of a three layered structure, whereby the layer of pore ice can be very thin.