* Astronomy

New gullies that did not exist in mid-2002 have appeared on a Martian sand dune.

That's just one of the surprising discoveries that have resulted from the extended life of NASA's Mars Global Surveyor, which this month began its ninth year in orbit around Mars. 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" - Tom Thorpe, project manager for Mars Global Surveyor at NASA's Jet Propulsion Laboratory, Pasadena.

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.
"Mars is an active planet, and over a range of timescales changes occur, even in the surface" - Dr. Michael Malin of Malin Space Science Systems, San Diego, principal investigator for the Mars Orbiter Camera on Mars Global Surveyor.

"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" - Michael Meyer, Mars Exploration Program chief scientist, NASA Headquarters, Washington.

Two gullies appear 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, and checked back at 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, vaporised 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,".

Some changes are slower than expected. 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.


Three views, one from Viking 2 orbiter on 28 September 1976, one from MGS MOC on 5 October 1999, and one from MGS MOC on 11 January 2005; the 1999 and 2005 views show the small, fresh crater that may have formed in the 1980s.

The camera has recorded seasonal patterns of clouds and dust within the atmosphere over the entire planet. In addition, other instruments on Mars Global Surveyor 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. However, the extent and duration of dust storms varied from year to year.


Scientifically, perhaps the most important result from use of the Mars Orbiter Camera on NASA's Mars Global Surveyor during that spacecraft's extended mission has been the discovery and documentation of a fossil delta. The feature is located in a crater northeast of Holden Crater, near 24.0 degrees south latitude, 33.7 degrees west longitude. Since the announcement of the discovery of the delta in November 2003, the International Astronomical Union has provided a provisional name (pending final approval) for the crater in which the landforms occur. The crater has been named Eberswalde, for a town in Germany.


Mars Orbiter Camera image mosaic of fossil delta in Eberswalde Crater, formerly 'Holden NE Crater' on Mars.

The image offers a higher-resolution view of a portion of the fossil delta than any seen earlier. North is up. At the bottom of the frame, the image includes the north end of a looping, inverted, meandering channel. The image covers an area of about 3 by 3 kilometres. It was produced using a technique called "compensated pitch and roll targeted observation," in which the rotation rate of the spacecraft is adjusted to match the ground speed under the camera. At full resolution, this map-projected image is at 50 centimetres per pixel.

Mars Global Surveyor was launched Nov. 7, 1996; entered orbit around Mars 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. JPL, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington. JPL's industrial partner is Lockheed Martin Space Systems, Denver, which built and operates the spacecraft.

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