UNANSWERED QUESTIONS ABOUT THE ORIGIN OF GYPSUM IN THE NORTH POLE ON MARS: HOW CAN THESE BE ADDRESSED?

The sulfate mineral gypsum is distributed widely within the circumpolar dunes around the North Polar ice cap on Mars. Various geochemical models have been proposed for the origin of gypsum including atmospheric deposition of volcanic aerosols, sulfide weathering, and erosion of gypsum-rich sand layers underlying the ice cap. One of the difficulties addressing the origin of sulfate in the North Polar dunes is that no place exists on Earth to directly compare with this unusual martian phenomenon. An additional difficulty is that previous geochemical models for Mars are mainly based on a single-sulfate-source approach, overlooking the fact that multiple sources of aqueous sulfate are usually activated by the water cycle on the planet’s surface. In this study, we used morphological features of the paleo-erg within the basal unit underlying the North Polar ice cap and geochemical proxies for the sulfur cycle in terrestrial environments to unravel the plausible formation processes of polar gypsum on Mars. We have determined that the bright albedo surfaces of the basal unit exposed in the interdunes of Olympia Undae are usually accompanied by planar cross-bedding surfaces with preserved shapes of past parabolic dunes and polygonal surfaces. These features are consistent with gypsum crusts forming in the interdunes at White Sands due to groundwater raise, carrying aqueous sulfate from bedrock weathering and atmospheric deposition. Therefore, we propose that the North Polar gypsum might have formed as a result of past groundwater activity within the basal unit prior to formation of the ice cap when liquid water was stable on the surface. Water interaction with basaltic sand would have enhanced leaching of weathering products from sulfide oxidation and atmospheric deposition and resulted in formation of secondary gypsum salts. Our initial calculations of total gypsum mass suggest that the North Polar dunes may contain ~16 times more gypsum than the White Sands dune field. However, the latter comprises only 0.1 % of the surface area covered by the North Polar dunes. This implies a relatively small, geologic sulfur reservoir (e.g., bedrock, atmosphere) for the secondary gypsum detected in the circumpolar dunes on Mars, which is in good agreement with the expected low sulfate flux from basaltic bedrock during groundwater activity.