<p>Duricrust. While it sounds like industrial product, “duricrust” is a hard mineral crust formed at or near the surface of soil in semiarid regions by the evaporation of groundwater. Investigating this for past water is a good rationale for future exploration. 
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NASA/JPL/University of Arizona (283 km above the surface.) <a rel="nofollow" class="external text" href="https://www.uahirise.org/PSP_007073_2030">www.uahirise.org/PSP_007073_2030</a>
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/71688597@N05">UAHiRISE (NASA)</a>
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/uahirise-mars/38301927295/">PSP_007073_2030</a>