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Holding Back the Ocean
Martian features that suggested the presence of an ancient ocean are most likely tectonic.
by Vanessa Thomas
An elevation map of Mars constructed from MOLA data
MOLA Science Team
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The northern plains of Mars are flatter than any known surface in the solar system. Planetary scientists have suggested that an ocean may have once covered the smooth martian lowlands, and in December 1999, a science team located candidate shorelines in data from the Mars Global Surveyor spacecraft. However, a second look at these supposed shorelines suggest that the features are actually tectonic ridges.
The Mars Orbiter Laser Altimeter (MOLA) aboard Mars Global Surveyor creates a topographic map of the martian surface by reflecting laser pulses off the planet and measuring the light's round-trip travel time. MOLA data from the edges of the northern plains revealed slopes with terrace- or step-like characteristics. Some planetary scientists interpreted these features as imprints from a receding ocean.
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This MOLA image reveals ridges near the Utopia impact basin. (Click the image to also see a Voyager image of the area.)
MOLA Science Team
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Last summer, Paul Withers of the University of Arizona and Gregory Neumann of the Massachusetts Institute of Technology and NASA's Goddard Space Flight Center decided to examine some of these shorelines. They focused on two candidate shorelines on opposite sides of the proposed ocean: one near the Utopia impact basin and another near the Alba Patera volcano. Upon close inspection of the MOLA data, the pair found that the ridges don't look like typical coastline formations, but do resemble tectonic features.
Withers and Neumann point out that most of the ridges are located near regions that have been placed under stress - the Utopia impact basin, the volcanic Tharsis Rise, and the Alba Patera volcano, for example. The ridges look just like "wrinkle ridges" created by compressive tectonic stresses and lie near areas known to have wrinkle ridges. The formations also tend to be perpendicular to the expected directions of the greatest compressive stresses, another observation the two scientists say indicate a tectonic origin.
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Click the image above to see plots of two slopes on Mars that have terrace features.
MOLA Science Team
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One more detail Withers and Neumann noticed is that the terrace features on the two sides of the proposed ocean face opposite directions. In the Utopia region, the vertical part of each "step" faces downslope. However, in the region near Alba Patera, the "backs" of the features face upslope.
"We believe that this morphology is hard to explain in terms of a shoreline-formation process, as is the reversal of shoreline morphology from one side of the ocean to the other," the team writes in the April 5 issue of Nature. "We favor the idea that these candidate shorelines were created by tectonic activity...."
Withers and Neumann say their findings do not dismiss the possibility that an ocean or small seas did exist in the northern martian hemisphere. The smooth, youthful appearance of the northern lowlands is still a good argument for the presence of an ancient ocean. However, the features formerly identified as possible shorelines are most likely tectonic features which may help us uncover even more about the Red Planet's past and present.
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04/12/2001
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