Late Noachian to Early Hesperian Climate Change on Mars: Evidence from Crater Lakes and Thermokarst Terrain near Ares Vallis

A significant transition in the climate of Mars may have occurred near 3.8 Ga to 3.5 Ga at the boundary of the Noachian and Hesperian periods. This period of martian geologic time is marked by a change in landform morphology and mineralogy, with a diminishing occurrence of valley networks and a disappearance of phyllosilicates within layered units. Here, we use high-resolution CTX images, CTX derived topography data, HRSC images and HRSC topography data to characterise morphologic indicators of this climate change in the Ares Vallis region. In this area we have identified several putative crater lakes that suggest the presence of past ponded water and thermokarst terrain which indicates regional permafrost degradation. The previously undescribed crater basins exhibit small outlet channels. Importantly, these crater basins lack inlets and suggest that groundwater ponded, overspilled, and/or catastrophically drained through breaches in the crater rims. Crater statistics reveal that the craters were degraded and infilled by sediment during the Late Noachian (3.7 Ga). This period of intense crater modification and infilling was followed by lake drainage and outlet channel formation in the Hesperian (3.5 Ga - 2.9 Ga). Little to no evidence for crater degradation was identified on craters in the region that are < 3.6 Ga in age. The crater data indicate a delay between the time of crater modification/infilling and outlet channel formation suggesting that water ponded within the basins during the Late Noachian, likely froze to form ice, and was subsequently partially to completely melted during transient warm periods in the Hesperian. As related features, the thermokarst terrains in Ares Vallis comprise sub-circular to lobate, flat-floored rimless topographic depressions that occur at varying elevations (Fig. 1). From CTX images we report the discovery of narrow channels connecting the depressions that provide evidence for the previous presence of ponded liquid water. Crater counts on these surfaces and relative relationships with Ares Vallis flood channels indicate that the thermokarst features formed between 3.6 Ga and 3.0 Ga corresponding with drainage of the nearby crater lake systems. We infer that thermokarst lakes formed by thawing of ice within a near surface ice-rich unit during warm periods in the Hesperian. Fig. 1: CTX derived topography showing thermokarst depressions with connecting channels in Ares Vallis.