This image of the northern wall of Coprates Chasma, in Valles Marineris,
was taken by the Compact Reconnaissance Imaging Spectrometer for Mars
(CRISM) at 1227 UTC (8:27 a.m. EDT) on June 16, 2007, near 13.99 degrees
south latitude, 303.09 degrees east longitude. CRISM's image was taken in
544 colors covering 0.36-3.92 micrometers, and shows features as small as
20 meters (66 feet) across. The region covered is just over 10 kilometers
(6.2 miles) wide at its narrowest point.
Valles Marineris is a large canyon system straddling Mars' equator, with a
total size approximating the Mediterranean Sea emptied of water. It is
subdivided into several interconnected "chasmata" each hundreds of
kilometers wide and, in some cases, thousands of kilometers long. The
walls of several of the chasmata, including Coprates Chasma, expose a
section of Mars' upper crust about 5 kilometers (3 miles) in depth.
Exposures like these show the layers of rock that record the formation of
Mars' crust over geologic time, much as the walls of the Grand Canyon on
Earth show part of our planet's history.
The upper panel of this montage shows the location of the CRISM image on a
mosaic from the Mars Odyssey spacecraft's Thermal Emission Imaging System
(THEMIS), taken in longer infrared wavelengths than measured by CRISM. The
CRISM image samples the base of Coprates Chasma's wall, including a
conspicuous horizontal band that continues along the wall for tens of
kilometers to the east and west, and a topographic shelf just above that.
The middle two panels show the CRISM image in visible and infrared light.
In the middle left panel, the red, green, and blue image planes show
brightness at 0.59, 0.53, and 0.48 microns, similar to what the human eye
would see. Color variations are subdued by the presence of dust on all
exposed surfaces. In the middle right panel, the red, green, and blue
image planes show brightness at 2.53, 1.51, and 1.08 microns. These three
infrared wavelengths are the "usual" set that the CRISM team uses to
provide an overview of infrared data, because dust has a less obscuring
effect, and because they are sensitive to a wide variety of minerals.
Layering is clearly evident in the wall rocks. The conspicuous band
running along the base of the chasma wall appears slightly yellowish, and
the scarp at the edge of the topographic bench appears slightly green.
The bottom two panels use combinations of wavelengths to show the
strengths of absorptions that provide "fingerprints" of different
minerals. In the lower left panel, red shows strength of a 0.53-micron
absorption due to oxidized iron in dust, green shows strength of an
inflection in the spectrum at 0.6 microns that may be related to rock
coatings, and blue shows strength of a 1-micron absorption due to the
igneous minerals olivine and pyroxene. The conspicuous horizontal band
appears slightly blue, indicating a stronger signature of olivine and/or
pyroxene. In the lower right panel, red is a measure of an absorption
particular to olivine, green is a measure of a 2.3-micron absorption due
to phyllosilicates (clay-like minerals formed when rock was subjected to
liquid water), and blue is a measure of absorptions particular to
pyroxene. The conspicuous horizontal band is now resolved into an upper
portion richer in pyroxene, underlain by material richer in olivine than
the rest of the wall rock. Also, erosion-resistant material forming the
topographic bench is underlain by phyllosilicate-containing material
exposed on the scarp.
Taken together, these data reveal a layer cake-like composition of the
crustal material exposed in Coprates Chasma's wall. Most of the rock is
rich in pyroxene, which is expected because much of Mars' crust consists
of volcanic basaltic rock. However discrete layers are richer in olivine,
and in some layers the presence of phyllosilicates indicates interaction
of rock with liquid water. Because the phyllosilicate-containing layer is
low on the walls and deeply buried, it likely represents an early period
of Mars' history that was exposed when the canyon system formed.
The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) is one of
six science instruments on NASA's Mars Reconnaissance Orbiter. Led by The
Johns Hopkins University Applied Physics Laboratory, the CRISM team
includes expertise from universities, government agencies and small
businesses in the United States and abroad.