appearance and erosions of the Ulysses Crater over time
These images show the appearance and erosions of the Ulysses Crater. The crater does not appear in a 1976 image by NASA's Viking spacecraft at far left, but is observed by the Global Surveyor in 1999 and 2005.
updated 9/22/2005 10:02:47 PM ET 2005-09-23T02:02:47

NASA’s Mars Global Surveyor spacecraft has spotted new gullies and a fresh crater — in astronomical terms — etched into the red planet’s surface, mission scientists said Tuesday.

Now in its eighth year in orbit around Mars, the Global Surveyor found the new gullies cutting through a sand dune, as well as numerous other signs that the planet is far from a static, unchanging world.

“[The gullies] are probably not the result of water action on the sand dune,” said Michael Malin, principal investigator for the Mars Orbiter Camera aboard Global Surveyor, during a teleconference with reporters. “What we think is going on here is that carbon dioxide snow has been incorporated into the sand dune.”

As the snow melts and evaporates into gas, it allows the sand around it to fluidize and run down the dune slope, Malin added.

Images from July 2002 and April 2005 show new Mars gully
Researchers with NASA's Mars Global Surveyor mission released these images showing a new gully that formed between July 2002 and April 2005.
The gullies are unique in that they’re carved in sand, and not in the rock faces of crater walls, researchers said. [An animation of the gullies spied by Global Surveyor is available here.]

Meanwhile, researchers poring through Global Surveyor images by hand found a fresh crater that apparently formed in the 1980s.

The crater, a 65-foot (20-meter) pit carved into the southern rim of the Martian volcano Ulysses Patera, did not appear in photographs taken by NASA’s Viking orbiter in 1976. But the Global Surveyor camera found the crater, ringed by a dark ejecta blanket and radial lines, in 1999. By 2005, the ejecta blanket was nearly faded, but the crater remained.

Based on the amount of ejecta fading, researchers estimated the crater formed between 1980 and 1985, and believe it is the remains of an impact by a small hunk of rock a few feet in diameter.

“Actually, we have five or six of these types of craters,” Malin said, adding that unlike the Ulysses crater, most of the others were not in areas also studied by Viking. “The number [of young craters] we’ve seen is much less than the number we’d predicted.”

Before the Global Surveyor arrived at Mars in 1997, researchers expected to find much more evidence of impact craters, given the planet’s location near the asteroid belt, researchers said. But the Global Surveyor has only photographed about 4 percent of the Martian surface, and there may be many more young craters waiting to be found, they added.

“I suspect that the reason why the cratering rate looks different is that the impact rate comes in clusters, and not a constant, uniform bombardment,” Malin said.

Rockfalls and ice caps
Mars scientists also revealed Global Surveyor images of a recent rockfall, in which many boulders broke free and rolled down a crater wall sometime between November 2003 and December 2004.

Researchers are unsure whether the rockfall was caused by strong winds, an nearby impact event or seismic activity.

“These images describe a dynamic surface of the planet Mars,” said Jack Mustard, an associate professor of geological sciences at Brown University, during the telecon. “They catch Martian geology in action.”

If the rockfall was caused by seismic activity, such as a marsquake, it could support views that Martian tectonics and volcanism are still active at the planet, Mustard added.

The spacecraft also observed a gradual evaporation of carbon dioxide ice in one of Mars’ polar caps, pointing to a slowly changing Mars climate.

“They way these polar pits are retreating is absolutely astounding,” Mustard said.

But like the rockfalls, researchers were unable to account for the gradual climate change.

“Why is Mars warmer today that it was in the past, we really have no way of knowing why,” Malin said.

An orbiter’s long life
NASA’s Global Surveyor mission is in its third extended mission phase, and the added years have been crucial to its ability to explore Mars, researchers said.

“Some of the things [observed] changed in few weeks or months, and others have changed over years,” Malin said. “Most of the discoveries that we’ve made came during the extended mission.”

The spacecraft has returned more than 250,000 images of Mars and a wealth of other data from its onboard instruments, NASA officials have said.

In addition to its Mars-watching role, the Global Surveyor has spied two of its fellow red planet orbiters — NASA’s Mars Odyssey and the European Space Agency’s Mars Express spacecraft — during its mission.

Michael Meyer, NASA’s Mars Exploration Program chief scientist, said that overall, the Global Surveyor remains in good health and still has years of red planet exploration ahead of it.

NASA has budgeted about $9.5 million for the Global Surveyor mission this year, and the spacecraft has enough propellant to last well into the next decade, Meyer said.

“I think we have much more science to be done by this spacecraft itself, and it will serve an important function as a communications asset [for future missions],” Meyer said.

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