Sliding Terrains and Dislocated Plates in Amazonis Planitia (Mars) as the Result of Instability of a Frozen Martian Ocean

Some peculiar landforms over a large area (about 400,000 km2) of Amazonis Planitia on subequatorial Mars include groups of parallel erosion marks ("grooves") that are some kilometers long and 2 km wide, bright on darker terrain, and associated with light areas interpreted as open fractures hundreds of kilometers wide ("sliding terrain"), mounds located at one end of the grooves, sinuous folds in the rugged terrain, and kilometer-wide blocks that have been roto-translated from their initial position. Similar features in other Martian areas (in particular Athabasca-Cerberus) have been earlier attributed either to volcanic action or to displacement of ice plates. Here the combined analysis of the landforms with mathematical estimates is consistent with the model of floating ice extending over the northern part of Amazonis Planitia. An initial phase, with ice floating on water (likely an ocean), was followed by a drop in water level. As the ice floes came into contact with the gently sloping floor, they no longer floated on an equipotential surface. Tension increased and ice repeatedly broke into smaller pieces that moved for some kilometers, pushed by the weak gradients. The large, bright areas are the product of such fractures. Grooves were formed by ice sliding on the ground, and collisions between floes produced pressure ridges. The depth of the grooves and of sliding terrain, on the order of a few meters, indicates the thickness of the ice sheet, comparable to that of the present-day Arctic. Dating of the ice sliding events and other features are found to be incompatible with volcanic processes.