Ancient playa in Glenelg Region, Gale Crater, Mars: evidence derived from the polygonal flagstone network in mudstone imaged by Curiosity

Abstract

Polygonal patterned surfaces (PPS) are common on Mars, and found in the Glenelg region of Gale Crater that was extensively studied with the Curiosity rover in the first 200 sols of the Mars Science Laboratory (MSL) mission. PPS can form by various mechanisms that lead to volumetric contraction of the regolith, sediments, or bedrock. It is hypothesized that a network of contraction fractures formed at Glenelg as the fine-grained clay-rich sediment lost moisture, and produced the distinct array of widely spaced flagstones approaching one meter in width evident in images from Curiosity. This study suggests that Glenelg PPS is not the product of thermal contraction of permafrost as noted elsewhere on Mars. The uniform distributions of fracture orientation and spacing between flagstones, large volumetric contraction, fine-grained texture of the sediments, and presence of swelling clays are all consistent with the PPS forming in a drying playa. These findings are in agreement with diverse, recent studies suggesting that Glenelg is a former lake bed at the base of an alluvial fan. This would imply that Glenelg PPS formed when the surface temperature and air pressure supported liquid water, and allowed temporary playa conditions of sufficient duration to permit ponding, sediment accumulation, and desiccation.