Accumulation of sedimentary debris in the south polar region of Mars and implications for climate history

Abstract

Stratigraphic units of the south polar region of Mars were mapped, relative chronology determined, and detailed modeling of the observed crater populations was used to set absolute constraints on the age of emplacement of materials. Significant secular variation in the net debris accumulation rate over history is evident. An episode of enhanced crater obliteration at about 3.7 Ga ago, lasting a few hundred Ma, is inferred for south polar cratered terrains. A similar peak in low latitude obliteration rates suggests that the event may have been global in scale. Whether the debris is volcanic or aeolian in origin, the event suggests a possible enhancement in atmospheric density at the time. Modeling results imply that cratered terrains poleward of 65° south latitude have subsequently experienced steady state net accumulation of material at a rate of about 0.1 km/Ga, while equatorial cratered terrains have been retained in relatively pristine form. Fifteen craters of impact origin were discovered on the south polar layered terrain, formerly thought to be almost devoid of craters. Their presence implies that the surface is at least a few 100 Ma old, and that the net accumulation rate is no more than 10 km/Ga. If layer formation is modulated by periodic climatic effects, either the period of oscillation is a few Ma or longer, or the layer deposition mechanism ceased operating at least several 100 Ma ago.