Chapter 15 - Thermokarst-like depressions on Mars: age constraints on ice degradation in Utopia Planitia

Abstract

On Earth, thermokarst features found at higher latitudes result from the melting of excess ground ice. On Mars, similar-looking morphologic features (such as scalloped depressions and expanded craters) have been observed in the mid-latitudes and are interpreted to result from surface collapse due to the loss of massive/clean ice. It is, however, challenging to parse whether the ground ice associated with the Martian morphologies is (1) excess ice wedges/lenses (similar to terrestrial thermokarst) or (2) buried ice sheets/ice-rich mantle deposits. To help one to clarify this question, this chapter will constrain the timing of ice deposition and the development of scalloped depressions in a Utopia Planitia study area. The observations suggest that intact mantle deposits are 1.8 ± 0.3 Myr old, consistent with deposition during a recent high-obliquity climate cycle. The age estimates of the units containing scalloped depressions are poorly constrained (with high error bars), which may indicate episodic or ongoing collapse—but they do appear to be younger than the mantled units, consistent with the removal of mantle ice, although contributions from excess ice cannot be definitively ruled out.