Outcrops of Martian Sedimentary Rocks and Polar Strata as Viewed from Orbit at Aerial Photograph Scales

Edgett, Kenneth S.¹ (1) Malin Space Science Systems, San Diego, CA

Mars Global Surveyor has been acquiring airphoto-scale images of Mars since 1997. More recently, in 2006, the Mars Reconnaissance Orbiter joined the effort to document martian geology and geomorphology at high spatial resolution. Outcrops of light-toned, layered rock seen from orbit have erosional expressions, physical properties, and bedding styles suggestive of clastic and perhaps chemical sedimentary rocks. Owing to the inability to distinguish clearly from orbit, some of these outcrops may include tephra. Observations from the Mars Exploration Rover, Opportunity, in Meridiani Planum have shown that at least this location presents an example of eolian sandstone with a complex depositional and diagenetic history. Orbiter images of other outcrops reveal unconformities, differences in bedding thickness, repetition, erosional expression, and bedding features in the rock. Bedding features include a few locations with ripples large enough to be seen from orbit. The spacecraft images permit the interpretations that different rock units (formations) occur on Mars; stratigraphy can be mapped (at least to the rudimentary extent that one can accomplish with air photos and no field work); and different depositional settings are represented. A few cases exhibit clear examples of depositional fans or deltas, and some locations include interbedded, filled, buried, and later exhumed and inverted channels. One thing that differs between martian and terrestrial stratigraphic sequences is the abundance of interbedded, filled, buried, and in some cases exhumed, impact craters. Boulder-forming, layered outcrops in the north polar region, which are essentially rock although the cementing agent might be ice, include spectacular erosional unconformities and clearly distinct geologic formations, including one sand-rich unit that is the source for modern windblown dune sediment.