"There's still quite a bit of discussion and debate about the formation of these gullies and the nature of the fluid that formed them," Mustard says. "What we see here today definitively shows that at least one type of gully can form under current conditions. That's very exciting."Fading crater
Impact cratering has been called the solar system's most common geological process, but planetary scientists almost never have a chance to see fresh craters. Mars Global Surveyor has found perhaps half a dozen small, obviously new, impact craters. The best example, is a 65-foot-wide (20m) bowl centered in a splash of dark, rayed ejecta. It lies near the rim of Ulysses Patera, the caldera of an ancient volcano.
"The rays are dark because of the ejecta disturbing the dust-covered surface," says Malin, which is also why the camera can spot the tracks of the Mars Exploration Rovers Spirit and Opportunity.
The crater is obvious in a 1999 Mars Orbiter Camera image, but in 2005, the ejecta has nearly faded into the surrounding terrain. Moreover, there's no hint of the crater in an earlier image, returned in 1976 by one of the Viking orbiters. "This was a crater that didn't exist 20 years ago," Malin says. It was probably created by an object just a few feet across.
Although MOC has observed only about 4 percent of the martian surface, Malin notes the number of fresh craters is lower than planetary scientists anticipated. Because scientists use the number of craters to infer the age of a planetary surface, a lower rate of crater formation would make surfaces on Mars older.
"I suspect that the reason why the cratering rate looks different is that impact cratering comes in clusters. There are pulses in that bombardment. There are times when the impact rate will be lower," Malin explains. "We're in a lull at Mars."
Other images included a recent rock fall, in which several boulders left tracks as they rolled and bounced down a crater wall. "It's the first we have found that had actually occurred while we were observing," Malin says. The rocks tumbled sometime between November 2003 and December 2004. Strong winds, a nearby impact, or an earthquake — or rather, a marsquake — could have dislodged them. "The rock fall is evidence that there was some kind of disturbance," Malin says.
Mars Global Surveyor arrived at Mars September 12, 1997. After gradually adjusting the shape of its orbit, it began systematically mapping the planet in March 1999 and completed its primary mission in 2001. Now halfway through its third mission extension, which ends October 2006, the durable spacecraft has been productive longer than any other sent to the Red Planet. To date, the craft has returned some 200,000 images — more than all previous Mars missions combined. The spacecraft is healthy, says Malin, and carries enough maneuvering fuel to last through the rest of the decade.
Its Mars Orbiter Camera acquires the highest-resolution images obtained by a Mars-orbiting spacecraft, revealing objects as small as 13 feet (4m) across. Scientists at Malin Space Science Systems
have developed techniques to double this resolution by rotating the spacecraft so it tracks a target as it orbits.
NASA's Mars Reconnaissance Orbiter (MRO) is expected to arrive at the Red Planet in March. It will join NASA's Global Surveyor and Mars Odyssey, as well as the European Space Agency's Mars Express, in their continuing missions.