In fact, the Global Surveyor has been so successful that it earned an extension following conclusion of its prime mapping mission early in 2001.

The second extension began in April 2002 and will continue the mission into late 2004.

"Things are going well," said Tom Thorpe, Mars Global Surveyor project manager at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "We've accomplished all of our primary mission objectives to date and the science instruments have returned a tremendous amount of data. Now we're looking forward to all the science to come in the second extension."

In addition to mapping operations, the spacecraft is targeting images of potential landing sites for the 2003 Mars Exploration Rovers. In its extended mission, the Surveyor has been given latitude to take pictures at different angles and target areas missed in the prime mission.

One of the tasks that scientists want to do is obtain stereo (three-dimensional) images of some areas already covered. To do this, the spacecraft has to be pointed off-nadir. Nadir is the point directly below the observer; hence if the spacecraft is "tipped," the target can be imaged up to 30 degrees away from the original ground track. When a place imaged from one angle is pictured again from another angle, the images can be overlaid to create a stereo picture.

This technique is useful for providing dramatic views of the planet's surface and to study the vertical profile of its atmosphere. For example, scientists want as complete a picture as possible of potential landing sites for the 2003 Mars Exploration Rovers. The stereo images taken by Global Surveyor will help substantially in the final site selection.

"There is still so much to learn about Mars," said Mars scientist Dr. Ken Edgett, Malin Space Science Systems, San Diego, Calif. "Our high-resolution camera is testing hypotheses formulated over the past two decades using Mariner 9 and Viking data and even new theories from Surveyor and Mars Odyssey images. We're seeking to understand previous observations, and to monitor changes that are taking place due to weather and changes in polar frost."

So How's the Weather on Mars?

Mars has seasons, just as Earth does. Using the Mars orbiter camera on Surveyor, scientists are now able to monitor the red planet's weather changes from one martian year (about twice as long as an Earth year) to the next. One of their discoveries has been that the southern polar ice cap, long thought to be permanent, isn't so permanent.

"What we're finding is just short of incredible," said Edgett. "For most of the Mars year, the weather patterns are very predictable. Last year, in late June, we had global dust storms that obscured the planet for three months - an event that did not fit the patterns we'd otherwise seen. We found that there were lots of storms going on at once, not that there was one gigantic global dust storm, as was thought during previous events."

Thorpe said, "The weather reports are very important, since weather will affect future spacecraft landings and operations on the surface of Mars, including the 2003 Mars Exploration Rovers."

"One of the reasons I'm so very excited about the second extension is because every week there is something new and surprising in our data," said Mars scientist Ken Edgett. "And what's really cool is that every four to six months we discover something totally amazing. Last year, we were flabbergasted to find that the southern polar "permanent" ice cap isn't so permanent. We're now tracking changes to the cap on shorter time scales."

Each winter, frost forms a seasonal polar cap covering everything from 60 degrees latitude to 90 degrees latitude; it retreats in spring. The permanent ice cap, which is mostly carbon dioxide, remains through the entire summer and was previously thought to be permanent.

"We now know that even in summer the ice is subliming (converting directly from solid to vapor) at a rate that suggests the entire cap could disappear in a few thousand to tens of thousands of years," said Edgett. "There's a lot of carbon dioxide in the permanent cap, but we're finding that it is going away on a larger time scale [than the seasonal frost], independent of season."

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