NASA's Mars rover Curiosity isn't the only vehicle driving on the Red Planet this week. A long-lived sibling rover is closing in on what may be its most scientifically interesting target since arriving on Mars more than eight years ago.
The golf-cart sized Mars Exploration Rover Opportunity is expected to soon reach a patch of bedrock on the rim of Endeavour Crater, located on the opposite side of the planet from Curiosity's landing site. The rock is believe to contain clay minerals, which form in the presence of water.
Clays also are believed to exist in the lower layers of Mount Sharp, the three-mile high mound of sediment that rises from the floor of Gale Crater, where Curiosity touched down on Aug. 6.
The timing is ironic, and possibly fortuitous. Opportunity, along with its twin rover Spirit, were only designed for 90-day missions to determine if their landing sites showed signs of past water.
The answers from both were a resounding "yes," paving the way for the much better equipped and more robust Curiosity geochemistry robot, which just began a two-year quest to assess the planet's potential for microbial life.
"One way I like to think about it is that Opportunity is the field geologist doing the exploration type of stuff, and Curiosity is the geologist who then takes samples back to his lab to do more detailed analysis on them," planetary scientist Diana Blaney, with NASA's Jet Propulsion Laboratory in Pasadena, Calif., told Discovery News.
The different missions grew out of different, but complementary science goals.
"When Spirit and Opportunity were built, we had places (on Mars) that we knew had had water, but we really didn't have any evidence for any place where water had been around doing chemistry on the surface for a long time.
"In that era the big questions were 'What is the history of water at these particular locations on Mars?'" Blaney said.
With its 10 science instruments and a two-year design life, Curiosity, which is about the size of a small car, is intended to address two difficult follow-on questions: whether Mars had ingredients besides water, such as organics, necessary for life and whether it had the means to preserve it.
"The missions inform each other to a limited extent," said Cornell University's Steve Squyres, the lead scientist on the Opportunity mission and a participating investigator on Curiosity's.
"Obviously, both (Opportunity and Curiosity) are on Mars and we're interested in similar scientific questions with both. But they're very different landing sites and Mars is so variable in its character from one place to another that you have to be very cautious about taking results from one site and applying them to another," Squyres told Discovery News.
Opportunity and Spirit, which succumbed to the harsh Martian environment two years ago, also weren't designed to travel nearly as far as Curiosity. Though Opportunity has racked up 22 miles on its odometer, scientists had to plan for a mission to be conducted within about 550 yards of its landing site.
"We didn't know what we were going to see on the ground," said Blaney, a deputy project scientist on the Opportunity and Spirit missions.
With Curiosity, she added, scientists wanted to go to the place they believe had the best chance for preserving a record of habitability in the rocks.
"When you start dealing with habitability and the potential for life, you raise the bar for what kind of measurements you have to make," Blaney said. "You need more capable instruments. You have to be a lot more careful with contamination. It's just a harder measurement to make."