MARS NORTHERN PLAINS OCEAN

A fundamental aspect of the Mars ocean hypothesis is trying to understand how, when, and where the water that might have formed oceans came from, and what has happened to it. Was it accreted along with the planet and outgassed at the end of accretion, or was it delivered later as part of the Late Heavy Bombardment, or some combination of the two? Recent measurements of the atmospheric mass loss rate put constraints on the integrated amount of water lost to space, with a Global Equivalent Layer between outgassing and cometary contribution as high as 2,700 meters being possible. Can the geomorphology associated with the proposed shorelines help to determine whether a Mars ocean was primordial and regressed through the Hesperian, or whether it transgressed to a maximum elevation due to impact contribution to the planet’s water inventory during Late Heavy Bombardment before regressing?

Our starting point for this work is the global map of the proposed paleoshorelines that is being improved since acquisition of high-quality global image and topography data since the late 1990s. By carefully studying the morphology of the proposed shorelines, we can assess how they were most likely formed (e.g., wave action in a warm environment, ice push driven by wind or thermal expansion of an ice cover in a cold environment, or low-viscosity volcanic plains emplacement at the end of a sequence of depositional events into the basin.

Shorelines follow an equipotential surface with respect to the planetary geoid, where the water surface intersects the subjacent terrain. On Earth, abandoned paleoshorelines of oceans and large lakes deviate from equipotential surfaces, because the water load depresses the crust and mantle by significant amounts (up to 1/3 of the water depth). When the water is removed, the crust rebounds such that shorelines around islands can often be elevated with respect to those along the basin margin. By determining geopotential shoreline elevations, we can look for evidence of rebound or subsidence, which has implications for the fate of the water and the rheology of the crust. Our preliminary examination of the topography suggests that the proposed shorelines are depressed toward the plains interior relative to the margin, suggesting the crustal load represented by the water and sediment may still be present but frozen.