Status Report

Mars Odyssey THEMIS Image: Lava Flows in Eastern Tharsis

By SpaceRef Editor

May 31, 2002

Filed under Daily Themis Pictures, Mars Odyssey THEMIS, US

Image Context:

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

This image may at first appear somewhat bland — there is little
contrast in the surface materials due to dust cover, and there are few
impact craters — but there are some very interesting geologic features
here. The great Tharsis volcanoes have produced vast fields of lava flows,
such as those shown in this image, to the east of Tharsis Tholus. The
flows in this image have moved from west to east, down the regional
topographic slope. The lobate edges of the flows are distinctive, and
permit the discrimination of many overlapping individual flows that may
represent tens, hundreds, thousands, or even millions of years worth of
volcanic activity (overlapping relationships are especially evident at the
bottom of the image). Viewed at full resolution, the image reveals
interesting patterns and textures on the top surfaces of these flows. In
particular, at the top of the image, there are numerous parallel curved
ridges visible on the upper surfaces of the lava flows. These ridges make
the flow surface look somewhat ropy, and at smaller scales this flow might
be referred to as pahoehoe, indicative of a relatively fluid type of lava
flow. At the scales observed here, however, these features are probably
better referred to as pressure ridges. Pressure ridges form on the surface
of a lava flow when the upper part of the flow is exposed to air, freezing
it, but the insulated unfrozen interior of the flow continues to move down
slope (and more material is pushed forward from behind), causing the
surface to compress and pile up like a rug. Rough-looking flows with less
distinct (more random) patterns on their surfaces may be flows that are
more like terrestrial a’a flows, which are distinguished from pahoehoe
flows by their higher viscosities and effusion rates. Near the center of

the image there is an east-west trending, smooth-floored depression. The
somewhat continuous width of this depression suggests that it is not simply
formed by the edges of two higher-standing flows on either side; rather it
may be a leveed channel created by more fluid lava flows. Faint east-west
trending linear to arcuate features in the lower third of the image
separate rougher and smoother surfaces, and may be fractures that guided or
were barriers to later flows.

[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