Comparing environmental changes and habitability settings in the geological history of Sahara and Mars

Abstract:

Sahara has experienced during its long geological history a large number of climatic changes from humid conditions (with savanna-type environments) to dry conditions (with hot desert environments). Therefore, since the late Miocene (?), Sahara experienced periods with rivers, lakes, deltas, and swamps alternated with dry periods with strong aeolian activity and the formation of deflation surface and sand seas. The fluvio-lacustrine deposits formed during humid periods have been strongly reworked by wind processes during the dry periods leaving only the coarse-grained portion of the sediments (conglomerate to rudite). This highly efficient reworking eeolian mechanism affected both of the fluvial and deltaic channel deposits and the large inland lakes, flood plains and other waterlogged areas. The former occurs, at present day, as morphological features and coarse-grained remnants of large fluvial systems whereas the latter are mostly buried below sand seas such as Grand Erg Oriental, Erg Chech, and Azawad. While the sand to silt material accumulated (mostly by saltation) in the sand seas and sand sheets, the finer portion (able to enter the wind as suspended material) was probably swallowed in the large- scale atmospheric circulation redistributed in Sahara itself, in adjacent continents (mostly Europe and South America), and oceans. This geological setting is similar to the one of Mars where fluvial deposits and morphologies abound as largely eroded discontinuous remains. Large-scale alluvial basins and terminal lakes or waterlogged areas (such as the allucial plain in Zephyria) have been largely present in the Martian Noachian-Hesperian time (and possibly even later) but relatively scanty sedimentary deposits testify their existence. When deposits are present they are basically coarse-grained (e.g. the meandering channels of the Eberswalde deltaic plain) due to the long lasting aeolian erosion similarly than the Sahara example. The wind, that dominated the long depositional history, accumulated sand to silt sediment in sand seas while the finer portion as draping dust. The analogy with the Sahara is compelling and provides a clue on the environmental variability of Mars and clearfy that alternating cliamte chsnges modify the environments and the habitability potential.