Opportunity has been travelling to Victoria crater for about half its mission ( Nasa/JPL/Malin Space Science Systems)
Nasa's robotic Mars rover Opportunity is closing in on what could be the richest scientific "treasure trove" of its mission so far.
Within the next two weeks, Opportunity should reach the rim of a crater wider and deeper than any it has visited in more than two-and-a-half years on Mars.
Rocks exposed in the walls of Victoria Crater could open a new window into the geological history of the Red Planet.
Opportunity has been exploring Mars' Meridiani Plains since January 2004.
Its "twin", the Spirit rover, continues to explore Gusev Crater on the other side of the Red Planet.
Images from Nasa's Mars Global Surveyor spacecraft show a stack of layered bedrock about 30-40m (100-130ft) thick in the walls of Victoria.
"We have a fully functional vehicle with all the instruments working. We're ready to hit Victoria with everything we've got," said Byron Jones, a rover mission manager at Nasa's Jet Propulsion Laboratory in California.
Surviving the winter
Ray Arvidson, deputy principal investigator for the rovers, said that exploring the rocks in Victoria Crater would greatly increase understanding of past conditions on Mars, including 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," said the scientist, who is based at Washington University in St Louis, US.
It is still winter in the Martian southern hemisphere where Opportunity is exploring. But the days have started to get longer again, so Opportunity's power supply from its solar panels is increasing day by day.
During its first two months on the Red Planet, Opportunity examined a pile of rock layers 30cm thick inside "Eagle Crater" and found geological evidence that water had flowed across the Martian surface many millions of years ago.
The rover spent the next nine months driving to and exploring a larger crater called "Endurance".
On the drive from Endurance to Victoria, the rocks told a history of shallow lakes, drier periods of shifting dunes and groundwater levels that rose and fell. Minerals indicated the ancient water was very acidic.
Professor Arvidson said mission scientists wanted to compare the rock layers at Victoria Crater with those seen so far to see if the conditions that created them were different.
He mused: "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, which reached the Red Planet in March this year, will assist the rover's task by mapping Victoria Crater at high resolution from orbit.
Spirit and Opportunity had primary missions lasting just three months. Though both are showing signs of wear, they are roving the Martian surface after more than 30 months.
Opportunity encountered a rock outcrop resembling a cobblestone road on its journey to "Victoria" (Nasa/JPL/Cornell)