Constraining the Movement of Groundwater Within Playa Environments on Mars Through Subsurface Imaging of the Makgadikgadi Salt Pans of Botswana

Across the surface of Mars there is evidence of past lacustrine and evaporitic environments found within basins and craters, where often layered sedimentary deposits and hydrated minerals are observed. However, the intensity, duration and precise phases of water cycle activity during their deposition remain unresolved. Although several geological processes and locations on Earth have been previously proposed as examples to describe these deposits on Mars, we lack a strong visualization of what water activity might have looked like during evaporitic stages within basins and craters. The Makgadikgadi Salt Pans of Botswana, where once the Makgadikgadi Lake existed, is a present evaporitic environment rich in hydrated minerals and water activity. It is a depression located at the southwestern end of a northeast-southwest set of graben. Faults have been previously proposed to have been pathways for groundwater to enter basins and craters on Mars, which then contributed to both the deposition and alteration of the sedimentary deposits. Thus, imaging the subsurface of a similar environment on Earth can help us to better understand how water processes on Mars might have continued as the Martian global climate became drier.

Through remote sensing techniques, we located areas within the pans where several regional faults occurred then conducted four electrical resistivity surveys perpendicular to the faults using an IRIS Syscal Pro imaging resistivity meter. Fault locations were determined by using a combination of topographic and aeromagnetic data. Fault scarps were observed terminating at the shorelines of the pans and their azimuths were used to trace the best locations of the faults underneath the sediment within the pans. These locations were then constrained further by using the aeromagnetic data which showed regional dikes that had been laterally offset in areas associated with the fault scarps, as well as anomalies that ran parallel and adjacent to the fault scarps. We successfully laid one 840 m and three 1,200 m long survey lines. The four survey lines intersected where these faults were determined to occur and were able to image the subsurface up to a depth of approx. 92 m.

In this way, we can detect low electrical resistance in void space produced by any faults and associated fractures in the overlaying water saturated sediment. Specific craters noted for their similarity to the study area include several in Arabia Terra (e.g. Oyama, Kotido, Firsoff and Jiji), and also Gale crater. The analog concept could also potentially be connected to layered deposits in Meridiani Planum and Valles Marineris. This work has wide implications for determining how putative water table elevations could have interacted within sediment filled craters on Mars by resolving areas of low resistivity and identifying faults that water could have used as pathways, which is not possible with the current instrumentation present on Mars. Results can also allow us to better infer what the underlying lithology of layered deposits within craters might look like. Furthermore, it demonstrates the scientific importance of future missions to employ subsurface imaging techniques on Mars.