Fresh Mars: craft views new gullies, craters, and landslides.

Mars may have been cold and dry for billions of years, but it's still an active place. A comparison of images taken just a few years apart by a Mars-orbiting spacecraft reveals freshly carved gullies and recent landslides. It also shows that a recently found, 20-meter-wide crater is only about 25 years old.

The images, taken by a camera aboard the Mars Global Surveyor spacecraft, provide evidence supporting earlier observations that the Red Planet's south polar cap is shrinking at a rate of about 3 meters every 2 Earth years. That's an indication that the planet, though frigid, is significantly warmer than it was just a few centuries ago, when frozen carbon dioxide was deposited to create its pitted terrain.

Michael C. Malin of Malin Space Science Systems in San Diego announced the findings this week during a telephone briefing. Malin's team built the camera on Surveyor, which began orbiting Mars in 1997.

The most intriguing of the new features could turn out to be the gullies, which are located in sand dunes, comments planetary scientist Jack Mustard of Brown University in Providence, R.I.

Malin and his collaborators had previously found gullies on the slopes of craters near both poles. The team attributes those gullies to the seepage of groundwater (SN: 7/1/00, p. 5). Malin argues that the new gullies arose within the past few years when frozen carbon dioxide trapped in the sand vaporized during a Martian spring. A sudden release of the gas could have caused sand to flow like a liquid.

But Mustard notes that these gullies might instead have been carved by deposits of frozen water that briefly liquefied in the sand. If so, the dunes could supply "jugs of water" for astronauts that may someday land on the Red Planet, he says.

The gully debate may soon be settled. A spectrometer aboard the Mars Reconnaissance Orbiter, scheduled to arrive at the planet next March, is expected to discern whether the gullies contain frozen water or carbon dioxide.

A separate set of Surveyor images reveal that between November 2003 and December 2004, more than a dozen boulders slid from the wall of a crater to its bottom, leaving behind tracks on the dusty slope. Either strong winds or a quake could have caused this landslide, as well as others documented by Surveyor, Malin says.

Yet another cache of images from the craft is revealing surprising information on how infrequently the planet has been bombarded by meteorites over the past century, Malin says. Surveyor spotted a crater that wasn't there when the Viking orbiter imaged the same region in 1976. Examining Surveyor images of the crater recorded in 1090 and 2005, Malin's team measured the amount by which dark streaks of material ejected by the impact had faded. The rate of fading indicates that the crater formed in the early 1980s. In a similar analysis, the team identified several other craters that appear to have formed within the past 100 years.

This record, although limited to the 4 percent of the planet that Surveyor's camera has examined at high resolution, suggests that during the past century, the planet has been smacked by large chunks of space debris at only one-fifth the rate that theorists had estimated, Malin says.

However, Surveyor might not be privy to the whole picture, notes Mustard. Most of the craters seen by the craft come from bright areas where dark streaks of crater material are easiest to spot. Many craters may remain hidden because they lie on a dark background, Mustard says.

Surveyor is funded to continue its exploration of Mars through 2006.