Status Report

Mars Odyssey THEMIS Image: Ulysses Patera

By SpaceRef Editor

July 17, 2002

Filed under Daily Themis Pictures, Mars Odyssey THEMIS, US

Mars Odyssey THEMIS Image: Ulysses Patera

Image Context:

Context image credit: NASA/Mars Orbiter Laser Altimeter (MOLA) Team

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It is helpful to look at the context for this THEMIS image, which covers a
large area over the summit of Ulysses Patera. Ulysses Patera is one of the
many volcanoes that make up the giant Tharsis volcanic province, although
Ulysses itself is fairly small in comparison to the other volcanoes in this
area. In the context image, there are 3 circular features near the top of the
volcano. The large, central feature is called a “caldera”, and is the result
of volcanic activity at Ulysses. The other two circular features are impact
craters. The THEMIS image primarily spans across the central caldera, but also
covers a portion of the northernmost impact crater. We know that the large
central caldera must have formed earlier than the two craters, because its
circular form has been cut by the smaller crater rims.

In the THEMIS image, there are stair-stepping plateaus in the northern portion
of the image. These are part of the rim of the northern crater, and are
caused by collapse or subsidence after the impact event. Just to the south of
this crater, “rayed” patterns can be seen on part of the caldera floor. The
rayed pattern is most likely due to a landslide of material down the crater
rim slope. Another possibility is that the impact that formed the northern
crater caused material to be ejected radially, and then parts of the ejecta
have either been buried or eroded away. Other signs of mass movement events
in this image are dark streaks, caused by dust avalanches, visible in the
caldera’s northern wall. In the central portion of the image, there are two
lobe-shaped features-one overlaps the other-that appear to have flowed
westward. It is likely that these features are ejecta lobes, because they are
located adjacent to the southeastern crater (see context image). The
fluidized appearance of these ejecta lobes is probably due to a significant
amount of ice or water being present in the soil at the time of impact. We
know that the southeastern crater must have formed after the northern crater,
because the fluidized ejecta lobe overlies the rayed pattern. A close-up look
at the fluidized ejecta lobes reveals a different surface “texture” than the
surrounding caldera floor. This could be due to compressional features that
formed during the lobe emplacement, or to contrasting surface properties that
cause the flows to be eroded differently than the caldera floor. In the lower
portion of the image, there is a cluster of small circular features in the
southernmost part of the central caldera. These features may be layered
material that has since been eroded into circular plateaus, or they may be
degraded volcanic cones, which would indicate a later stage of smaller-scale
volcanism within the caldera. Volcanic cones are common in many calderas on
Earth, and are formed after the initial stage of volcanic activity in that
caldera. Finally, in the southern wall of the caldera, there is classic
“spur-and-gully” morphology. This type of morphology is often formed on steepslopes, where variations in wall resistance cause the surface to be eroded
more easily in some areas.

[Source: ASU THEMIS Science Team]

Note: this THEMIS visual image has not been radiometrically nor geometrically calibrated for this preliminary release. An empirical correction has been performed to remove instrumental effects. A linear shift has been applied in the cross-track and down-track direction to approximate spacecraft and planetary motion. Fully calibrated and geometrically projected images will be released through the Planetary Data System in accordance with Project policies at a later time.

NASA’s Jet Propulsion Laboratory manages the 2001 Mars Odyssey mission for NASA’s Office of Space Science, Washington, D.C. The Thermal Emission Imaging System (THEMIS) was developed by Arizona State University, Tempe, in collaboration with Raytheon Santa Barbara Remote Sensing. The THEMIS investigation is led by Dr. Philip Christensen at Arizona State University. Lockheed Martin Astronautics, Denver, is the prime contractor for the Odyssey project, and developed and built the orbiter. Mission operations are conducted jointly from Lockheed Martin and from JPL, a division of the California Institute of Technology in Pasadena.

Image Credit: NASA/JPL/Arizona State University