Volume: 04, Issue: 02 09/21/2005 
A new gully formed on a dune in an unnamed crater; this pair of images shows the dune as it appeared on July 17, 2002 (left) and April 27, 2005 (right). Image courtesy NASA/JPL/MSSS/ASU.
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Three wide angle views taken by the Mars Orbiter Camera on NASA's Mars Global Surveyor at intervals approximately one Mars year apart show similar spiral dust clouds over a volcano named Arsia Mons. Image courtesy NASA/JPL/MSSS.
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Images from Nov. 14, 2003 (left) and Dec. 4, 2004 (right) show more than a dozen new boulder tracks formed on the crater wall during the intervening time. Image courtesy NASA/JPL/MSSS.
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Scientists have discovered a crater that appears to have formed on Mars in the past 20 Earth years and have used it and other similar craters to estimate the present cratering rate on Mars. Image courtesy NASA/JPL/MSSS.
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Other Articles in This Issue:
NASA Unveils Plans for Return to Moon
Cassini Spots Ghostly Ring Spokes and Possible Titan Shoreline
Crash Claims Life of Dryden Chief Engineer Marta Bohn-Meyer
More Student Lessons on Hurricanes

Durable Orbiter Tracks Mars Changes

In its ninth year of orbiting the Red Planet, NASA’s Mars Global Surveyor is still going strong. The craft recently discovered gullies on a Martian sand dune that did not exist in mid-2002.

And that's just one of the surprising discoveries that have resulted from Mars Global Surveyor’s extended life. Boulders tumbling down a Martian slope left tracks that weren't there two years ago. New impact craters formed since the 1970s suggest changes to age-estimating models. And for three Mars summers in a row, deposits of frozen carbon dioxide near Mars' south pole have shrunk from the previous year's size, suggesting a climate change in progress.

"Our prime mission ended in early 2001, but many of the most important findings have come since then, and even bigger ones might lie ahead," said Tom Thorpe, project manager for Mars Global Surveyor at NASA's Jet Propulsion Laboratory (JPL). Thorpe said the orbiter is healthy and may be able to continue studying Mars for five to 10 more years.

Mars years are nearly twice as long as Earth years. The orbiter's longevity has enabled monitoring of year-to-year patterns on Mars, such as seasonal dust storms and changes in the polar caps.

"To see new gullies and other changes in Mars surface features on a time span of a few years presents us with a more active, dynamic planet than many suspected before Mars Global Surveyor got there," said Michael Meyer, Mars Exploration Program chief scientist.

Two gullies appeared in an April 2005 image of a sand-dune slope where they did not exist in July 2002. The Mars Orbiter Camera team has found many sites on Mars with fresh-looking gullies. They rechecked more than 100 gullied sites for possible changes between imaging dates, but this is the first such find. Some gullies on slopes of large sand dunes might have formed when frozen carbon dioxide, trapped by windblown sand during winter, vaporized rapidly in spring, releasing gas that made the sand flow as a gully-carving fluid.

At another site, more than a dozen boulders left tracks when they rolled down a hill sometime between the taking of images in November 2003 and December 2004. It is possible that they were set in motion by strong wind or by a "marsquake," according to Dr. Michael Malin of Malin Space Science Systems, principal investigator for the Mars Orbiter Camera on Mars Global Surveyor.

Some changes are slower than expected. According to Malin, studies suggest new impact craters might appear at only about one-fifth the pace assumed previously. That pace is important because crater counts are used to estimate the ages of Mars surfaces.

The camera has recorded seasonal patterns of clouds and dust within the atmosphere over the entire planet. In addition, other Mars Global Surveyor instruments have provided information about atmospheric changes and year-to-year patterns on Mars as the mission has persisted. Daily mapping of dust abundance in Mars' atmosphere by the Thermal Emission Spectrometer has shown dust over large areas during three Mars southern hemisphere summers in a row. The extent and duration of dust storms, however, varied from year to year.

Mars Global Surveyor was launched Nov. 7, 1996. It entered the Martian orbit Sept. 12, 1997 and returned the first Mars data from its science instruments Sept. 15, 1997. Beyond its own investigations, the orbiter provides support for other Mars missions, such as landing-site evaluations, atmospheric monitoring, communication relay, and imaging of hardware on the surface.

For newly released images on the Internet, visit the following websites:

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