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.