Ten years after it landed on Mars—a milestone in the annals of endurance—NASA's Mars Opportunity rover is still turning up evidence that the red planet was habitable 3.7 billion years ago. (See: "Exploration: Mars Rovers.") Its latest discoveries have been detailed in a news briefing and a Science magazine study released on Thursday.
Opportunity landed on January 25, 2004, inside an airbag that bounced across the Martian plain. Its robotic twin, Spirit, which landed on January 4 of that same year, ended its mission in 2011. But Opportunity is still trundling across the deserts and craters of Mars.
Both rovers far surpassed their projected three-month lifetimes, uncovering evidence of ancient water in rock layers and making unprecedented scientific observations of the planet. The six-wheeled Opportunity has gone on to make hundreds of pit stops on its dusty journey, breaking the record for distance traveled on another world—24 miles (38.7 kilometers).
Now investigating the Endeavor crater, in the Meridiani Planum region, Opportunity has discovered deep rock layers indicating that watery conditions favorable to early life once existed there.
In the just released study, led by Ray Arvidson of Washington University in Saint Louis, Missouri, the rover science team shows that these layers were deposited by less acidic, life-friendly waters, and then covered by rock layers deposited by more acidic, less hospitable waters—in the drama that built the Martian plain over several billion years.
In a related series of reports, NASA's Curiosity rover, now at Mars's Gale crater, offers evidence of similar habitable watery conditions there, in lake deposits that formed more recently than the ones at Meridiani Planum. Curiosity landed on Mars in 2012. (See: "Mars Curiosity Milestone: Top Five Discoveries.")
"These results demonstrate that early Mars was habitable, but this does not mean that Mars was inhabited," writes Caltech's John Grotzinger, head scientist on the Curiosity rover team, in a commentary accompanying the reports. And since the surface rocks now being explored by the rovers were first exposed by weathering some 80 million years ago, he suggests that future Mars missions looking for evidence of ancient microbial life face a daunting challenge.
Until that mission is undertaken, we can celebrate Opportunity's explorations. It has snapped tens of thousands of high-resolution images. This one, taken in November 2004, shows a stunning panoramic view of a massive outcropping along the wall of Endurance crater.
Forty-six individual images were stitched together to create this true-color composite (pictured above) showing the impressive formation called Burns Cliff.
In December 2004 Opportunity visited the heat shield it had jettisoned during its descent to Mars. This panoramic view shows shiny, twisted metal and insulation from the shield (visible on the left and at center), along with the crater (on the right) that was dug when the shield hit the ground.
Mission team members asked Opportunity to make the trek to the site to learn more about how their landing hardware had performed and to investigate the light-colored sand the impact had splashed around the crater.
Less than a week before this detailed image of Victoria Crater was captured by NASA's Mars Reconnaissance Orbiter, Opportunity drove up to the crater's rim.
The rover can be seen as a tiny, pixelated dot at about the ten o'clock position on the edge of the crater.
The crater stretches more than a half mile (800 meters) across. Heavy erosion along its rim has exposed sedimentary layers of the inner wall; undulating sand dunes cover its floor.
While exploring Victoria Crater in June 2007, Opportunity took detailed images of prominent outcrops along the western rim of the crater.
Known as Cape St. Mary, the peninsula-like, cross-layered rock formation rises some 45 feet (15 meters) above the crater floor. It's thought to have formed from sand dunes that once migrated across this region.
Meandering tracks left by Opportunity, seen in this April 2007 photo, showcase its autonomous hazard-avoidance system at work, letting the rover plot and then follow a safe, curving path around the pile of rocks visible on the right.
In the distant background the rocky rim of Victoria Crater rises above the flat, dusty Martian plain.
After making a precarious drive of 199 feet (60.8 meters) on July 19, 2009, Opportunity used its navigation cameras to look back at its own tracks.
Mission engineers asked the intrepid rover to detour around the deep sand dunes visible on the right, where it could have gotten stuck.
In May 2010 Opportunity stopped in its tracks and snapped this poignant shot of the distinctly reddish-brown dunes that lay ahead of it, just as the late afternoon sun set off to the right.
The iron-laced, wind-formed sand dunes, made up of grains of volcanic rock, blanket most of the red planet.
In honor of its seventh landing anniversary, back in January 2011, Opportunity captured a series of high-resolution images that were stitched together to produce this breathtaking panoramic vista of Santa Maria Crater.
The false-color view of the oval, football-field-size crater allowed scientists to pinpoint water-bearing mineral deposits exposed along the inner wall.
Heading into its fifth Martian winter in December 2011, Opportunity took this downward-looking self-portrait.
Rust-colored dust coats the rover's solar panels, testimony to its many years of exploration in the barren desert.
The dust buildup hinders the rover's ability to power its instruments. Yet wind storms have blown them clean in the past, allowing Opportunity to keep on roving.
In June 2013 Opportunity's Pancam captured this spectacular view of the rise in the foreground, called Nobbys Head. It's on the western rim of Endurance crater, which the rover had spent the previous two years exploring.
The robotic geologist's next destination is visible in the distance on the right—Solander Point, where scientists investigated clay minerals that indicated the environment had been wet billions of years ago.