More than a year after their official mission ended the two Mars Rovers are still exploring the surface of Mars, and they continue to send back more data than even the most optimistic space enthusiast could have imagined.

The most recent pictures from Mars show two small impact craters, formed by impacts so recent that they are still "fresh", not filled in by sand or dust. On Earth, people tend to think of craters as giant holes in the ground like Meteor Crater in Arizona, ancient features too big to miss that mark the site of a catastrophic collision with an asteroid or comet. But craters can also be small objects, like the two discovered recently by the Opportunity rover on the plains of Meridiani on Mars. Both are less than half an inch deep and clearly visible in images taken by the rover's navigation cameras.

On Earth, small craters are not commonly recognized because they're quickly filled with dirt, pine needles, or other debris carried by water and wind. On Mars, craters are primarily filled by wind-blown sediment, though in the past they may also have been filled by lava, melting ice, or flowing water.

"Given that these two craters haven't been covered by sand even though they are surrounded by sand ripples on a flat plain lends support to the idea that they're fairly recent," said Golombek. "Of course, recent might mean any time from yesterday to 100 million years ago."

NASA's Durable Mars Rovers Get Approval for 18 Additional Months of Exploration

"The rovers have proven their value with major discoveries about ancient watery environments on Mars that might have harbored life," said Dr. Ghassem Asrar, deputy associate administrator for NASA's Science Mission Directorate. "We are extending their mission through September 2006 to take advantage of having such capable resources still healthy and in excellent position to continue their adventures."

The rovers have already completed 11 months of extensions on top of their successful three-month prime missions. "We now have to make long-term plans for the vehicles because they may be around for quite a while," said Jim Erickson, rover project manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

Erickson cautioned though, "Either mission could end tomorrow with a random part failure. With the rovers already performing well beyond their original design lifetimes, having a part wear out and disable a rover is a distinct possibility at any time. But right now, both rovers are in amazingly good shape. We're going to work them hard to get as much benefit from them as we can, for as long as they are capable of producing worthwhile science results."