New evidence for sedimentary volcanism on Chryse Planitia, Mars

Kilometre-sized flows (KSFs) have been observed in many regions on Mars and have been typically interpreted as lava flows. However, sedimentary volcanism has been proposed as an alternative origin for some KSFs. Remarkable examples of such hypothesized sedimentary KSFs are located at the southern margin of Chryse Planitia. There, the flows are associated with conical and dome-shaped edifices; however their formation mechanism remains enigmatic due to the absence of ground truth. Previous studies revealed that these KSFs consist of three morphological elements: a central depression, leveed central channels, and a distal portion of the fading channel(s). Here, we present new morphological results obtained on these KSFs using seven newly available Digital Elevation Models computed from HiRISE stereo pairs. Our investigation confirms that these features are aggradational and formed by the transport of a liquid. This material emerged from identified depressions and the presence of subtle mounds inside them is interpreted to mark the position of feeder vents. We also observe that the margins surrounding the central large channels are not continuous. They are cut by meter-sized troughs linking the central channels to units which have distinctive albedo and roughness compared to their surroundings. These bright units do not have a clear topographical expression, suggesting that the effused material originally flowing away from the central channel was easily removed after its emplacement. Such surface features are unlikely to be related to igneous deposits, since once lava is released from a main channel, it would rapidly solidify due to the heat loss and hence result in topographically distinct features. In contrast, such morphological expressions are more likely related to sedimentary volcanism and the emplacement of low viscosity water-rich mud. Sublimation, evaporation, infiltration or a combination of these processes should lead to water loss from the flows without leaving a detectable topographic expression but changing the roughness and hence albedo of the surface. The southern part of Chryse Planitia is a region on Mars where subsurface sediment mobilization could have operated in the past and hence represents a promising site for future exploration where deeper-sourced sedimentary deposits are exposed at the surface.