Post by Tim Goudge, Department of Geological Sciences, Brown University, Providence, RI
There is much morphologic evidence that there was flowing water on the surface of Mars early in its history. Such evidence includes fluvial channels and valleys, often termed valley networks, (e.g., Pieri, 1980; Irwin, 2005a; Fassett and Head, 2008a) as well as paleolake basins that are fed by these valley networks (e.g., Goldspiel and Squyres, 1991; Cabrol and Grin, 1999, 2001; Irwin et al., 2005b; Fassett and Head, 2005, 2008b).
The best evidence for these basins actually having contained lakes is the presence of outlet channels (making the basins hydrologically open), as this requires ponding of water within the basin to at least the elevation of the outlet valley head before breaching to form an outlet channel (Fassett and Head, 2005, 2008b).
While there is clear morphologic evidence for paleo open-basin lakes based on observed inlet and outlet valley networks, the geologic evidence for deposition of sediment associated with these paleolakes is not as obvious. Of approximately 230 documented open-basin lakes, only about one-third show evidence for sedimentary deposits of probable lacustrine origin, such as deltaic deposits or exposed layered deposits (Image 1) (Goudge et al., 2012). Additionally, very few of these exposed sedimentary deposits show evidence for the presence of aqueous alteration minerals based on analysis of spectral reflectance data, which is likely to reflect the lack of such alteration phases within the watersheds of these paleolakes. Exposed lacustrine sedimentary deposits that do appear to contain aqueous alteration minerals are likely to have sourced these minerals from their watershed as opposed to having formed them in situ (e.g., Ehlmann et al., 2008; Dehouck et al., 2010; Ansan et al., 2011; Goudge et al., 2012).
The reason so few of these paleo open-basin lakes contain exposed sedimentary deposits is due to their age. It is widely thought that the fluvial activity that fed these paleolakes ceased at approximately the boundary between the Noachian and Hesperian periods (~3.6-3.7 Ga) (Irwin et al., 2005b; Fassett and Head, 2008a). Therefore, these paleolakes have been subject to a variety of geologic processes over the past ~3.6-3.7 Gyr that have acted to resurface and modify their interiors. All of the identified open-basin lakes appear to be resurfaced or modified to some degree by such processes as volcanism (Image 2), glacial activity, or aeolian infilling. Of these resurfacing processes, volcanism appears to be the most widespread, having affected ~40% of identified open-basin lakes (Goudge et al., 2012).
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