Polyphase Mid-Latitude Glaciation on Mars: Chronology of the Formation of Superposed Glacier-Like Forms from Crater-Count Dating

Abstract

Reconstructing Mars's glacial history informs understanding of its physical environment and past climate. The known distribution of viscous flow features (VFFs) containing water ice suggests that its mid-latitudes were glaciated during the Late Amazonian period (the last several hundred million years). The identification of a subgroup of VFFs—called superposed glacier like forms (SGLFs)—flowing onto other VFFs, indicates multiple glacial phases may have occurred during this time. To explore the history and spatial extent of these glaciations, we record the distribution of SGLFs globally and use impact-crater counting to date the SGLFs and the VFFs onto which they flow. Our inventory expands the handful of SGLFs reported in earlier literature to include 320 located throughout the mid-latitudes. Our dating reveals these SGLFs to be much younger than their underlying VFFs, which implies a spatially-asynchronous glaciation. SGLFs have been forming since ∼65 Ma, and their ages are clustered in two distinct groups around 2–20 and 45–65 Ma, whereas the ages of their underlying VFFs span the last ∼300 Ma diffusely. We discuss these results in the light of well-known uncertainties with the crater-dating method and infer that while ice sheets decayed over the Late Amazonian period, alpine glaciers waxed and waned in at least two major cycles before their final demise approximately two million years ago.