During the next couple weeks, NASA's Mars rover Opportunity is likely to reach the rim of a crater wider and deeper than any it has visited; known as "Victoria," it is approximately half a mile wide and 230 feet deep. Examination of the rocks exposed in the walls of the crater are expected to greatly increase the understanding of past conditions on Mars and the role of water.

NASA's Mars rover Opportunity is closing in on what may be the grandest overlook and richest science trove of its long mission.

During the next week or so, the robotic geologist is likely to reach the rim of a hole in the Martian surface wider and deeper than any it has visited. The crater, known as "Victoria," is approximately half a mile wide and 230 feet deep.

Images from NASA's Mars Global Surveyor orbiter show the crater walls expose a stack of rock layers approximately 100 feet to 130 feet thick. Opportunity will send back its initial view into the crater as soon as it gets to the rim. Scientists and engineers will use Opportunity's observations from points around the rim to plot the best route for entering the crater.

"Victoria has been our destination for more than half the mission," explains Ray Arvidson of Washington University, St. Louis. Arvidson is deputy principal investigator for Opportunity and its twin rover, Spirit. "Examination of the rocks exposed in the walls of the crater will greatly increase our understanding of past conditions on Mars and the role of water. In particular, we are very interested in whether the rocks continue to show evidence for having been formed in shallow lakes."

The NASA rovers have been exploring landscapes on opposite sides of Mars since January 2004. Their prime missions lasted three months. Both rovers still are investigating Mars' rocks, soils and atmosphere after more than 30 months. Opportunity works in a region where rock layers hundreds of yards in thickness cover older, heavily cratered terrain.

"We have a fully functional vehicle with all the instruments working. We're ready to hit Victoria with everything we've got," says Byron Jones, a rover mission manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

Though it's still winter in Mars' southern hemisphere, days have begun getting longer again, and Opportunity's power supply from its solar panels is increasing daily.

During its first two months on Mars, Opportunity examined a 1-foot stack of rock layers at its landing site inside "Eagle Crater" and found geological evidence that water had flowed across the surface long ago. The rover spent the next nine months driving to and exploring a larger crater, "Endurance." There it examined a stack of exposed layers 23 feet thick. Over the drive from Endurance to Victoria, the rocks tell a history of shallow lakes, drier periods of shifting dunes and ground water levels that rose and fell. Minerals indicate the ancient water was very acidic.

The much thicker stack of revealed rock layers at Victoria beckons. Arvidson reveals, "We want to examine the thick section of rocks exposed on the walls in Victoria crater to understand whether the environment that produced these materials was similar to the environment recorded in the rocks that we have seen so far. Is there a record of a different type of deposition? Was there a wet environment that was less acidic, perhaps even more habitable? Where do the layers from Endurance fit in this thicker sequence?"

NASA's Mars Reconnaissance Orbiter reached Mars in March 2006. It will begin its primary science phase in November, offering higher resolution images and mineral mapping than have been possible with previous orbiters. Victoria will be one target for the orbiter. "By combining the data from Opportunity and Mars Reconnaissance Orbiter, we'll be able to do some fantastic coordinated analysis," Arvidson says. Such analysis will enhance the science return of both missions and aid in interpreting orbiter data taken of potential landing sites for future missions elsewhere on Mars.