Mars Global Surveyor (MGS)
has been operating in Mars orbit longer than any other spacecraft. The
Mars Orbiter Camera (MOC) began taking pictures in September 1997,
shortly after MGS arrived. Since that time, it has only imaged about
3% of the martian surface with its high resolution (1.5 to 12 meters,
5 to 40 feet, per pixel), narrow angle (NA) camera system. Thus, an
important discovery from MOC can--and does--come at any time, even
five and six years into the mission.
This week, the journal
Science has
published online (in
Science Express)
the most recent MOC discovery: an ancient,
eroded, and exhumed sedimentary distributary fan located in
a crater at 24.3°S, 33.5°W. A distributary fan is
a generic term used by geologists to describe a family of deposits
that includes river deltas and alluvial fans. Sometime in the distant
past, when it was still possible for liquid water to flow across the
martian surface, sediments transported through valleys by water formed
a fan-shaped deposit in a 64-kilometer (40 miles) -diameter crater
northeast of Holden Crater.
What is important about this discovery? First, it provides clear,
unequivocal evidence that some valleys on Mars experienced the same
type of on-going, or, persistent, flow over long periods of
time as rivers do on Earth. Second, because the fan is today a deposit
of sedimentary rock, it demonstrates that some sedimentary rocks on
Mars were, as has been suspected but never clearly demonstrated,
deposited in a liquid (probably water) environment. Third, the general
shape, pattern of its channels, and low topographic slopes provide
circumstantial evidence that the feature was actually a delta--that
is, a deposit made when a river or stream enters a body of water. In
other words, the landform discovered by MOC may be the strongest
indicator yet that some craters and other depressions on Mars once
held lakes. Although hundreds of other locations on Mars where valleys
enter craters and basins have been imaged by MOC, this is the first to
show landforms like those presented here.
The first picture, MOC2-543a, is a mosaic of MOC high resolution
images acquired between August 2000 and September 2003. The area
covered is 14 km (8.7 mi) wide and 19.3 km (12 mi) high; north is up
and the scene is illuminated by sunlight from the left. The MOC NA camera
takes grayscale images; the color added to this and the other images
was derived from data collected nearby using the
Thermal Emission Imaging System (THEMIS) Visible Imaging Subsystem
(built by Malin Space Science Systems for Arizona State University)
on-board the Mars
Odyssey spacecraft. The picture shows the entire distributary fan. The
fan is a fossil landform--that is, it is an eroded remnant of a
somewhat larger, somewhat thicker deposit. The originally loose
sediment has been turned to rock and then eroded over time to present
the features seen today. The channels through which sediment was
transported are no longer present; instead, only their floors have
remained, and these have been elevated by erosion so that former
channels now stand as ridges. The floors of former channels became
inverted because they were more resistant to the forces of
erosion---either they were more strongly cemented than the surrounding
materials, or they have more coarse grains (which are harder to
remove), or both.
The second picture, MOC2-543b, shows the same mosaic, with the locations
of MOC2-543c and MOC2-543d indicated by white boxes.
The third image, MOC2-543c, shows a critical observation: the loop at
the center of the picture is the inverted form of a former meandering
stream that was cut off as the channel adjusted its course. Meanders
and cut-off meanders are the prime evidence for on-going, persistent
flow of water through this area some time early in martian
history.
The fourth image, MOC2-543d, shows another critical observation, that
of inverted channels at different levels within the sedimentary
deposit, also indicating a long and complex history of water-related
processes. One ridge crosses over another just left of the center of
this frame. The top of the lower ridge is the former floor of a
channel that was transporting water and sediment toward the lower
right. The top of the upper ridge was once the floor of a channel that
moved material toward the right/upper right. The lower ridge is the
older channel; it was completely filled and buried beneath the surface
when the upper channel formed.
The fifth picture is a map showing the location of the distributary
fan. It is at the west end of a ~64 km (~40 mi) wide crater ("Holden
NE" Crater) northeast of the much larger, Holden Crater. The white box
indicates the location of the fan. To the west of the fan are several
valleys that fed water and sediment to the "Holden NE" Crater. This
map is a mosaic of Mars Odyssey daytime thermal infrared images
released by the
THEMIS team. The contours are topography, relative
to the martian datum (the elevation 0 contour) derived from observations
acquired by the MGS
Mars Orbiter Laser Altimeter (MOLA) experiment.
Sunlight illuminates the mosaic
from the upper left, and the 10 kilometers scale bar is about 6.2
miles across.
The sixth picture (MOC2-543f) is a short
"movie" (.mov file) simulating a fly-around,
followed by a birds-eye view, of the fan in the crater northeast of Holden
Crater. The fly-over combines all of the spacecraft data sets described
previously: MGS MOC images, Mars Odyssey THEMIS images, and MGS MOLA topography.
The MGS MOC continues to return new pictures every day, and will
provide critical support to the
Mars Exploration Rover (MER)
mission beginning in January 2004. Together with the MGS Mars Relay (MR)
system, MOC will provide near-realtime coverage of engineering telemetry
during the critical entry, descent, and landing (EDL) of both MER-A
("Spirit") and MER-B ("Opportunity"). After landing, MOC/MR will
relay rover science data from the surface of Mars back to Earth, while
simultaneously continuing its on-going science investigations, in
particular searching for further evidence of the environmental
conditions both in early martian history and more recently. Given the
results to date, exciting, unexpected discoveries are certain to
follow.
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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, California.
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,
California and Denver, Colorado.
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