Long-term evolution of the subsurface water environment on Mars over the past million years

Mars is an extremely cold and dry planet today, but it is thought to have been a water-rich planet in the past. Most of the water reservoir could represent hydrated crust and/or ground ice interbedded within sediments. Unlike Earth, Mars does not have a large satellite, so its obliquity varies greatly, and atmospheric circulation, water circulation, and subsurface water distribution are expected to change significantly over time. Currently, water ice is unstable at the pressure-temperature conditions found at the surface or subsurface of low/mid-latitude Mars, but recent observations by SHARAD revealed that large amounts of water remain beneath Utopia Planitia, which is thought to have formed during periods of high obliquity. Here, we have newly developed a fully coupled global water circulation model for the atmosphere, hydrosphere, and cryosphere down to a depth of 1 km in the subsurface, and we used an iterative time integration scheme. We performed a series of simulations with changing Martian obliquity and eccentricity over the last few million years, and north polar layer deposit as an initial water reservoir. Our model implemented a water exchange scheme between the atmosphere and the regolith/crust for different porosities and grain sizes. We found that in the recent Milankovitch cycle, during the smaller obliquity periods, subsurface ice was mainly distributed around higher latitudes, but during the larger obliquity periods, the distribution of subsurface ice extended to lower latitudes of around 40° N. It is possible that water ice with a volume content of more than 10% may remain at high latitudes above 60° N. The abundance of water at such high latitudes could be an important indicator in the search for possible life on Mars, or a valuable water resource in future manned Mars missions.