Washington - NASA’s Mars rover Opportunity is approaching what might be the richest science finding of its long mission.

During the next two weeks, 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, called Victoria, is 750 meters wide and 70 meters deep, according to a September 6 press release from NASA’s Jet Propulsion Laboratory (JPL) in California.

The NASA rovers have been exploring landscapes on opposite sides of Mars since January 2004. Their prime missions lasted three months. Both rovers are still investigating Mars' rocks, soils and atmosphere after more than 30 months on the planet.

Images from NASA's Mars Global Surveyor orbiter show the crater walls expose a stack of rock layers 30 to 40 meters 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," said Ray Arvidson of Washington University in Missouri, 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,” he added. “In particular, we are very interested in whether the rocks continue to show evidence for having been formed in shallow lakes."

Opportunity works in a region where rock layers hundreds of meters thick cover older, heavily cratered terrain.

"We have a fully functional vehicle with all the instruments working,” said Byron Jones, a rover mission manager at JPL. “We're ready to hit Victoria with everything we've got."

Though it is 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 30-centimeter 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 7 meters thick.

Over the drive from Endurance to Victoria, the rocks tell a history of shallow lakes, drier periods of shifting dunes and groundwater levels that rose and fell. Minerals indicate the ancient water on Mars was very acidic. The much thicker stack of revealed rock layers awaits inspection at Victoria.

"We want to examine the thick section of rocks exposed on the walls in Victoria crater,” Arvidson said, “to understand whether the environment that produced these materials was similar to the environment recorded in the rocks that we have seen so far.”

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 a target for the orbiter. (See related article.)

"By combining the data from Opportunity and Mars Reconnaissance Orbiter, we'll be able to do some fantastic coordinated analysis," Arvidson said.

Such analysis will enhance the science return of both missions and help interpret orbiter data taken of potential landing sites for future missions elsewhere on Mars.

Meanwhile, on the other side of the Red Planet, Spirit remains in good working order and continues to make progress on its winter science campaign. The rover spent part of its time this week filling in sections for the McMurdo panorama - a 360-degree full-color panorama of its surroundings in the Columbia Hills in Gusev Crater on Mars.

Additional information, including rover images and the full text of the September 6 press release, is available on the JPL Web site.