OphirChasma

This picture shows materials that fill most of Ophir Chasma, one of
the canyons of the central Valles Marineris. It was acquired on
January 4, 1998 by the Mars Orbiter Camera (MOC), during the 82nd
orbit around Mars of the Mars Global Surveyor spacecraft. At the top
of the image is the floor of the chasm; the bottom of the image is as
much as 4 km (2.5 miles) higher. Much of the relief between these two
elevations occurs in the deeply channeled section in the upper
one-half of the image.

The picture covers an area of 9.3 x 28 kilometers (5.8 x 17.4 miles),
and features as small as 10 meters (about 30 feet) can be seen. The
image is centered at 4.1°S, 72.2°W. At the bottom of the image, the
surface displays many small, parallel ridges that are mostly likely
sand dunes or large ripples. The interior of the chasm-fill material
(exposed in the upper half of the image) appears to be of uniform
brightness and resistance to erosion. Layering is only weakly
expressed. Although dark layers and more competent layers are found
elsewhere in the interior layered deposits, these deposits appear much
more uniform. The erosional form of the channels suggests zones of
slightly differing material strength substantially etched by the wind.

The origin of these deposits is not known. One theory proposes that
they formed gradually in a set of giant lakes that filled the central
canyons of Valles Marineris sometime in the past. Another hypothesis
is that they are the result of enormous explosive volcanic
eruptions. In either case, they were subsequently substantially
eroded, although those processes are also subject to debate. Some
argue that large amounts of water were needed to scour the materials
away, while others contend that wind action and landslides are
responsible for the features we see today. Detailed study of these MOC
images may shed additional light on these competing theories, or we
may need to wait for the images to be acquired by MOC during the
mapping phase of the mission, which will be a factor of three higher
in resolution.




Malin Space Science Systems and the California Institute of Technology
built the MOC using spare hardware from the Mars Observer
mission. MSSS operates the camera from its facilities in San Diego,
CA. The Jet Propulsion Laboratory's Mars Surveyor Operations Project
operates the Mars Global Surveyor spacecraft with its industrial
partner, Lockheed Martin Astronautics, from facilities in Pasadena, CA
and Denver, CO.