Stokes Crater, pictured here, is one of at least nine craters in the northern lowlands of Mars with exposures of hydrated minerals detected from orbit, according to a June 25, 2010, report.
These minerals, including phyllosilicates, have previously been found in thousands of small outcrops in the southern highlands of Mars, but had not previously been identified in the northern lowlands, which cover nearly half of the planet. The numerous outcrops in the south have been interpreted as evidence that early Mars -- about 4 billion years ago -- had wet conditions necessary for producing phyllosilicates and possibly conducive to life.
The exposures in some northern craters indicate these minerals are in an older layer underneath the younger surface of northern Mars and are made visible where crater-forming impacts have exposed the underlying material. The new report in the journal Science by John Carter of the University of Paris and his co-authors says that the northern finds suggest the ancient, wet conditions extended globally. Their report draws upon observations by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) aboard NASA's Mars Reconnaissance Orbiter and the OMEGA spectrometer orbiting aboard the European Space Agency's Mars Express.
Stokes spans 66 kilometers (41 miles) in diameter, centered at 55.6 degrees north latitude, 171.2 degrees east longitude.
The above image shows an area near the center of the crater, with color coding [see below] for where CRISM observations have shown exposures of three types of hydrated minerals and nearby exposures of two volcanic minerals unaltered by water: pyroxene and olivine. The scale bar is 2 kilometers (1.2 miles).
Image credit: NASA/ESA/JPL-Caltech/JHU-APL/MSSS/FU-Berlin
Note: The color key for the above image can be found here. Unfortunately, this is a tif file, which I'm unable to upload. I'm also typing in the color key here: red = Iron-magnesium phyllosilicate or chlorite; light blue (cyan) = Aluminum phyllosilicate "montmorillonite"; dark blue = Aluminum phyllosilicate "kaolinite"; orange = pyroxene; and light green = olivine.