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Figure 1: White boxes indicate location high resolution views | Figure 2: Context in THEMIS IR mosaic of Eberswalde Crater; North is Down> | Figure 3: Sub-meter-per-pixel cPROTO image S01-00795 |
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Figure 4: Context View of PIA04293> | Figure 5: Inverted channels |
Scientifically, perhaps the most important result from use of the Mars
Orbiter Camera on NASA's Mars Global Surveyor during that spacecraft's
extended mission has been the discovery and documentation of a fossil
delta. The feature is located in a crater northeast of Holden Crater,
near 24.0 degrees south latitude, 33.7 degrees west longitude. Since the
announcement of the discovery of the delta in November 2003, the
International Astronomical Union has provided a provisional name (pending
final approval) for the crater in which the landforms occur. The crater
has been named Eberswalde, for a town in Germany.
This image offers a higher-resolution view of a portion of the fossil
delta than any seen earlier. North is up. At the bottom of the frame, the
image includes the north end of a looping, inverted, meandering channel.
The image covers an area of about 3 by 3 kilometers (1.9 x 1.9 miles). It
was produced using a technique called "compensated pitch and roll targeted
observation," in which the rotation rate of the spacecraft is adjusted to
match the ground speed under the camera. At full resolution, this
map-projected image is at 50 centimeters (20 inches) per pixel.
Additional images from Mars Orbiter Camera provide some context (figure 4)
and show a nearby portion of the fossil delta's inverted channels (figure 5)
at a spatial scale of 1.5 meters (about 5 feet) per pixel. The relative
positions of these three images are indicated in a mosaic image of the
entire delta (figure 1), for which the unmarked version was released
in November 2003 (PIA04869).
The first Mars Orbiter Camera narrow angle images of some of the landforms
in the delta were acquired in 2000, during the Mars Global Surveyor
primary mission, but those pictures did not show very well the unambiguous
inverted channel forms. Not until the second Earth year of the orbiter's
extended mission were the deltaic features recognized in Mars Orbiter
Camera images obtained in March and June of 2002.
Following the initial observations in 2002, the Mars Orbiter Camera team
began a systematic effort to map the entire Eberswalde Crater delta. Most
of this imaging required slewing the whole spacecraft in a technique
called "roll only targeted observation" so that it pointed the camera
toward the feature. In this way, the camera team was able to build up a
mosaic of the delta much more quickly than would have been the case if
the team had simply relied upon chance crossing of the delta by the
orbiter's usual ground track. This technique was not employed during Mars
Global Surveyor's primary mission, except in the search for Mars Polar
Lander, but became a routine part of the tool kit during the extended
mission.
Even with the "roll only targeted observation" technique, it took more
than one Earth year to build up a complete mosaic of images of the delta.
In the meantime, the first data showing the deltaic landforms were
archived and released to the public and scientific community, long before
the Mars Orbiter Camera team's analysis and mosaic were complete. Some
scientists began independent analyses of the landform at that time. The
initial analysis and announcement of the feature was finally published in
November 2003.
The Eberswalde delta provides the first clear, "smoking gun" evidence
that some valleys on Mars experienced persistent flow of a liquid with
the physical properties of water over an extended period of time, as do
rivers on Earth. In addition, because the delta today is lithified -- that
is, hardened to form rock -- it provided the first unambiguous evidence
that some martian sedimentary rocks were deposited in a liquid
(presumably, water) environment. The presence of meandering channels, a
cut-off meander, and crisscrossing channels at different elevations (one
above the other), provided the clear geologic evidence for these
interpretations.
After the sediments were deposited to form the delta, the material was
further buried by other materials -- probably sediments -- that are no
longer present. The entire package of buried material became cemented and
hardened to form rock. Later, erosive processes such as wind stripped away
the overlying rock, re-exposing the delta. Now preserved essentially as a
fossil, the former floors of channels in the delta became inverted, to
form ridges, by erosion. Channels can be inverted by erosion on both Earth
and Mars. Usually this happens when the channel floor, or the material
filling the channel, is harder to erode than the surrounding material into
which the channel was cut. In some cases, the channels on Earth and Mars
have been filled by lava to make them more resistant to erosion. In the
case of Eberswalde, there are no lava flows; instead, the channel floors
may have been rendered resistant to erosion either by being
better-cemented than the surrounding material, or composed of
coarser-grained sediment (such as sand and gravel as opposed to silt), or
both.
The Mars Orbiter Camera was built and is operated by Malin Space Science
Systems, San Diego, Calif. Mars Global Surveyor left Earth on Nov. 7,
1996, and began orbiting Mars on Sept. 12, 1997. JPL, a division of the
California Institute of Technology, Pasadena, manages Mars Global Surveyor
for NASA's Science Mission Directorate, Washington.