NASA MARS Rover Opportunity: On verge of new discovery

A closeup of the Cape York rim segment of Endeavour Crater with the Opportunity rover's path shown. 

Tisdale, the rock Opportunity sampled earlier, is on the southern tip of Cape York.

Mars rover Opportunity was poised on the rim of the 22,000 meter-wide Endeavour Crater, preparing to sample a novel rock type.

Much older than the sedimentary samples the rover's "tasted" so far, this new sample is flush with the promise of revealing clues to the planet's environment when running rivers coursed the surface.

What was supposed to have been a 90- to 180-day exploration of two distinct regions of the red planet has turned into a saga that has become one of science's most compelling and long-lasting adventures (now into its eighth year), enthralling the public and the science communities alike.

Launched the summer of 2003 and landing in January 2004, the solar-powered Mars Exploration Rovers (MER) Spirit and Opportunity completed their intended basic missions in April 2004.

Raymond E. Arvidson, PhD, the James S. McDonnell Distinguished University Professor in Arts and Sciences at WUSTL, is the MER deputy principal investigator. Each continued roving until March 2010, when Spirit, mired in unexpected but scientifically interesting martian sand and pointed in an unfavorable direction to survive the winter dark, gave up the ghost.

Opportunity, on the other hand, remains active, having reached the rim of Endeavour Crater Aug. 9, 2011, knocking at the door of geology different from any it has explored during its first seven-plus years on Mars.

"Opportunity now is in a brand new mission," Arvidson says. "In late August, we looked at a rock named Tisdale, with a composition unlike any we've seen before. It has an enormous amount of zinc, bromine, phosphorus, chlorine and sulfur, all elements that are mobile in the presence of water.

The ancient rim of Endeavour represents a period when there was probably a lot more water on the surface," Arvidson says. "So, we're trying to get the chemical, mineralogical and geological setting to 'back out' those ancient conditions to reconstruct environmental conditions during this earlier time period."

The conditions that formed the sandstones Opportunity has sampled over the past seven years represent a kind of drying-out period of Mars.

Occasionally wet but usually dry and wind-blown, the sulphur-rich mineral grains formed vast dune fields that were cemented into sandstone over millions of years by occasional seeping groundwater.

But the terrain Opportunity now is sampling - largely buried by lake bed sediments - pops up in places like the Endeavour rim and is much older, going back to the earliest days of the planet.

That's some 3.5 to 4 billion years ago in the last stages of heavy bombardment, when Mars was sweeping up the last planetessimals - cosmic dust grains that collided and stuck to each other to form larger bodies. Endeavour is an impact crater produced during that heavy bombardment period.