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New Mars Global Surveyor Image - October 22, 1999

October 22, 1999

The following new image taken by the Mars Global Surveyor spacecraft
is now available:

   o Possible Rootless Cones or Pseudocraters on Mar

The image resides on the Mars Global Surveyor website:


The image caption is appended below.

Mars Global Surveyor was launched in November 1996 and has been
in Mars orbit since September 1997.   It began its primary
mapping mission on March 8, 1999.  Mars Global Surveyor is the 
first mission in a long-term program of Mars exploration known as 
the Mars Surveyor Program that is managed by JPL for NASA's Office
of Space Science, Washington, DC.  Malin Space Science Systems (MSSS)
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.

Ron Baalke


                            Mars Global Surveyor
                             Mars Orbiter Camera

              Possible Rootless Cones or Pseudocraters on Mars

                MGS MOC Release No. MOC2-186, 22 October 1999

High-resolution images from the Mars Global Surveyor (MGS) Mars Orbiter
Camera (MOC) have revealed small cone-shaped structures on lava flows in
southern Elysium Planitia, Marte Valles, and northwestern Amazonis Planitia
in the northern hemisphere of the red planet. The most likely interpretation
of these cones is that they may be volcanic features known as
"pseudocraters" or "rootless cones". They share several key characteristics
with pseudocraters on Earth: they are distributed in small clusters
independent of structural patterns, are superimposed on fresh lava flows,
and they do not appear to have erupted lavas themselves.

The white box in the picture above left shows the location of one of the MOC
images of possible psuedocraters on Mars. The white box is drawn upon a MOC
red wide angle context image acquired at the same time as the high
resolution view, shown on the right above. Located in northwestern Amazonis
Planitia near 24.8N, 171.3W, both the context image and high-resolution
view are illuminated from the lower left. The high resolution view shows
several possible psuedocraters (cone-shaped features with holes or pits at
their summits) that occur on top of a rough-textured lava plain. The context
frame covers an area 115 km (71 mi) across, the high-resolution view is 3 km
(1.9 mi) across.

Pseudocraters form by explosions due to the interaction of molten lava with
a water-rich surface. Possible martian pseudocraters are of interest because
they may mark the locations of shallow water or ice at the time the lava was

Viking Orbiter images have shown structures in other regions of Mars that
were interpreted to be pseudocraters, but the interpretations were uncertain
because the morphology was poorly resolved, it was unclear if they occurred
on volcanic surfaces, and they have diameters as much as a factor of 3
larger than terrestrial pseudocraters. The cone-shaped morphology is well
resolved in the cones imaged by MOC, and they have basal diameters of less
than 250 m (273 yards), consistent with terrestrial examples. The cones rest
on a surface with a distinctive morphology consisting of ridged plates that
have rafted apart, which MOC team members have interpreted as the surface of
voluminous lava flows.

The surface shown here (above right) looks relatively fresh and has very few
impact craters on it, which suggests that the lava flows and the cones are
both geologically young. However, MOC images in other areas reveal such
apparently young surfaces being exhumed (presumably by wind erosion) from
beneath a blanket of overlying material. Impact processes may harden the
blanket, or cover it with materials that cannot be removed by wind, so the
wind erosion leaves behind elevated "pedestal craters". The cones shown here
are not typical of pedestal craters, but it is important to consider this
alternative interpretation.

MGS MOC first began taking pictures of Mars in mid-September 1997. The
planet that has been revealed by this camera is often strange, new, and
exciting. The possibility that lava and water or ice have interacted to
create features like psuedocraters indicates that Mars has had a diverse and
complex past that researchers are only just begining to understand.

The pictures shown here are the subject of a talk on MOC views of volcanism
on Mars being presented in an invited talk at the Geological Society of
America (GSA) Annual Meeting in Denver, Colorado on Monday, October 25,
1999, by MOC scientist Alfred McEwen of the University of Arizona, Tucson.
For a full-resolution (approximately 3.7 m/pixel--12 ft/pixel) view of the
MOC narrow angle image (1.8 Mbytes), CLICK HERE

Image credit: NASA/JPL/Malin Space Science Systems and University of Arizona


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