Just about all the important news from Mars over the past year and a half has come courtesy of the six-wheeled, instrument-laden, SUV-size Mars Science Laboratory, better known as the Curiosity rover. Almost from the moment it executed its improbable plunge to the surface in August of 2012, Curiosity has churned out one important result after another—including evidence of an ancient, rushing stream; a small but measurable amount of water bound up in Martian soil; strong evidence for a previously life-friendly environment in sediments from a dried-up lake—and of course, a rich gallery of amazing images.
But while Curiosity has been getting all the glory, one of its older cousins is still chugging along gamely, a full ten years after it began its own crawl across the Martian surface. And for a few days, at least, the venerable Opportunity rover has been stealing back the limelight and getting the kind of press it enjoyed in the glory days of the mid-2000s.
First came a photograph of a mysterious “ghost rock” that appeared, seemingly out of nowhere (it was probably kicked up by the rover itself). And now, just a couple of days later, scientists are reporting that Opportunity, too, has found evidence of a previously water-rich, hospitable environment, thousands of miles from where Curiosity is sniffing around. “When you combine it with the recent results from Curiosity,” says Raymond Arvidson, of Washington University, in St. Louis, lead author of a new paper in Science, “it tells you that relatively benign conditions once existed for extended periods in different parts of Mars.”
The new discovery was Opportunity’s, but it wouldn’t have happened without an assist from yet another vintage Mars probe. The Mars Reconnaissance Orbiter (MRO) has been circling the planet since 2006, imaging the surface in stunning detail and doing some impressive science of its own. Mission controllers have also used it to run reconnaissance for Opportunity, imaging the rim of Endeavor Crater, where the rover has been nosing around for more than two years, to pinpoint promising places for close-up inspection. “The rim is ancient,” says Arvidson, “and mostly covered by newer sandstone formations.”
This sandstone, Opportunity had determined, formed in highly acidic conditions, and in the presence of highly reactive oxygen-rich compounds. “Extremophiles might have lived there” Arvidson says, referring to organisms that love hostile environments, “but it wouldn’t really have been a good place for life.”
Not long ago,MRO observations persuaded Opportunity’s controllers to turn the rover to a particular spot in a region of the crater rim known as Cape York. Thanks to a new image-processing technique that has sharpened MRO’s resolution from 60 ft. (18 m) to more like 15 ft. (4.5 m), the orbiter revealed that the surface in that region is eroded away, revealing what might be especially intriguing minerals—specifically hints of smectite, a type of clay that forms in wet conditions. Opportunity happened to be driving along the eastern side of Cape York last year when MRO’s instruments picked up the smectite readings. Says Arvidson: “I told Squyres [Cornell’s Steve Squyres, principal investigator for the Opportunity mission] ‘holy smokes, man, let’s turn right and go uphill!’”
It was a good move: there was smectite aplenty, along with calcium sulfate deposits—which indicated not just that water was present in the panet’s past, but that traces of it are still hanging around. Best of all, the minerals had clearly formed in a neutral, rather than an acidic, environment, and without the reactive oxidants found in the sandstone. That would have made Mars a much nicer place for microbes to live about four billion years ago—assuming such organisms existed.
Similarly life-friendly conditions established by Curiosity last month at Gale Crater were probably hundreds of millions of years younger, showing that Mars was hospitable—at least in certain places—for an extended period. That doesn’t guarantee that life ever did exist on Mars, but it sure doesn’t hurt. Neither Opportunity nor Curiosity is equipped to search for life directly, or even for traces of ancient bacteria or other organisms.
That’s the job of NASA’s next rover, dubbed Mars 2020, for its anticipated launch date. It would not only carry out active searches for extant life, it would also gather a cache of Mars rocks for retrieval and transport back to Earth by a subsequent mission. Congress just awarded the project $65 million in research funding, and these remarkable discoveries should make it easier to shake more money loose over the next few years. No lawmaker, after all, wants to be the one who said no to meeting a Martian.