A "WATER STAIN" covers part of Meridiani Planum: a layer rich in the iron-oxide mineral hematite. Discovered by the Thermal Emission Spectrometer (TES) on the Mars Global Surveyor orbiter, the hematite - which requires liquid water to form - led NASA to choose Meridiani as the landing site for the Mars Exploration Rover Opportunity. Colors map percentages of hematite in the surface materials. Opportunity landed within the black oval line and found rocks that had once been drenched in water. Click on the image to download a high-resolution version (5.8 MB). NASA/JPL/Arizona State University
References:
Christensen, P.R., and S.W. Ruff, The formation of the hematite-bearing unit in Meridiani Planum: Evidence for deposition in standing water, J. Geophys. Res., 109, E08003, doi:10.1029/2003JE002233, 2004.
Christensen, P.R., S.W. Ruff, R.L. Fergason, T.D. Glotch, N. Gorelick,
B.M. Jakosky, M.D. Lane, A.S. McEwen, H.Y. McSween, Jr., G.L. Mehall,
K. Milam, J.E. Moersch, S.M. Pelkey, A.D. Rogers, and W.B. Wyatt, Mars
Exploration Rover candidate landing sites as viewed by THEMIS, Icarus, 187, 12-43, 2005.