Carbon Dioxide Ice Sublimation: an Agent of Contemporary Martian Surface Feature Formation

Introduction: Carbon dioxide is Mars’ primary atmospheric constituent [1] and is an active and dynamic driver of Martian surface evolution. Kieffer’s hypothesis that gas venting from beneath seasonally deposited south polar CO2 slabs modifies the Martian landscape [2], spearheaded an advent of CO2 modelling. In a climate in which it is difficult to sustain liquid water, the agency of sublimation in the form of the CO2 block hypothesis [3], cryoventing [4] and basal sublimation-driven debris flows [5] is now recognized as a plausible architect of many contemporary features. These include the dendritic araneiform terrain of the south polar cryptic region, sand furrows and linear gullies. However, there is a dearth of empirical evidence to support CO2 conceptual models. Here we present the results of a suite of laboratory experiments undertaken to understand the interaction between subliming CO2 ice blocks and porous substrate. We test the CO2 block hypothesis, which asserts that subliming CO2 ice blocks may form linear gully pits [3]. We find that subliming CO2 can mobilize grains to form features which may be analogous in morphology to those on Martian dunes. We present (1) pits and (2) furrows, formed via CO2 sublimation, providing compelling evidence that CO2 may be responsible for Martian features previously linked with water [7]. Based on a survey of Proctor and Russell Crater dunes, we present a new hypothesis for detached pit formation at linear gully termini. Methods: Laboratory Experiments. A low humidity chamber was constructed in a constant temperature (ΔT ≈ 1 K) environment. An experimental chamber with a trap door was filled with silica beads and ‘dessicating’ CO2 ice blocks were used for dehumidification purposes. This avoided water ice deposition onto the ~ 80oC CO2 ice block which could hamper heat transfer and erase surface microtopography. Guyson Honite Glass Beads of four grain diameter ranges (4-45 μm, 45-90 μm, 75-150 μm and 160-212 μm) were poured into the chamber for four separate experimental trials. Grain sizes finer than aeolian dune grains were chosen to account for the disparity in gravity and atmospheric pressure between Earth and Mars. We propose that coarser grains would be mobilised in a similar manner under Martian conditions. Once relative humidity decreased below 10%, the trap door was opened and a CO2 ice block of average mass 790g was either placed or slid onto the bed. The chamber was immediately re-sealed and the block was allowed to sublime during each experimental trial. The resultant geomorphic feature in each case was imaged at overlapping angles. Images were then fed into Agisoft Photoscan, using Structure from Motion to generate a DEM and corresponding orthophoto of each feature (0.1 mm/pixel). Feature dimensions were then measured in ArcGIS using the DEM data. HiRISE DTM Analysis. Linear gully pits in Proctor and Russell Crater DTM’s were surveyed. The dimensions of terminal pits and their surrounding detached pits were recorded along with slopes and corresponding channel widths using ArcGIS. Results: Pit Formation. Pits (Fig. 1a) analogous in morphology to Martian linear gully pits (Fig. 1d,e) were observed following each experimental trial. These were characterized by depressions in the substrate which increased in depth with decreasing grain size. Levées and vent apertures were identified in most cases, which were also observed on levées bounding Russell crater linear gully channels. The Leidenfrost Effect [8], whereby a liquid or solid makes thermal contact with a surface at a temperature far beyond its boiling or sublimation point, caused each block to levitate on a cushion of vapor. The force of this escaping gas pushed grains from beneath the block to the side forming levées. These increased in height with decreasing grain size.