In this picture, dated 8 May 2010, Opportunity looked back on its tracks during its journey to the Endeavour crater. Credit: NASA/JPL-Caltech

During its ten years on Mars — a milestone reached this week — NASA’s Opportunity rover has rolled well over 38 kilometres across the planet's surface, spotting everything from meteorites to wind-blown ripples to dust devils. But its streak of discoveries is not over yet. Opportunity has now explored rocks from the earliest era of the planet's history, showing that non-acidic, life-friendly water existed on Mars some 4 billion years ago.

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NASA's newest rover, the much larger Curiosity, which landed in August 2012, has already found much the same conditions on the red planet. But that was on a dried-up lake-bed 8,000 kilometres away and hundreds of millions of years later in Martian history. Together, Opportunity and Curiosity are showing not only that habitable environments existed on Mars, but also that they did so at several locations and in different geological settings.

“What’s interesting is that we’re seeing similar environmental conditions at both landing sites, even though they’re on completely opposite sides of the planet and separated in time,” says Bethany Ehlmann, a planetary scientist at the California Institute of Technology in Pasadena. “The further back we look, the more Mars starts to look like Earth — a complex place with multiple types of environments.”

Decade of travel

Findings from both Opportunity and Curiosity are published in the 24 January issue of Science1,2. The following day, Opportunity will mark its tenth anniversary on the red planet. Opportunity and its twin rover, Spirit, both landed in January 2004 and were originally designed to last just 90 Martian days, or about seven Earth weeks. Spirit became stuck in April 2009, and mission control at NASA's Jet Propulsion Laboratory in Pasadena lost contact with it on 22 March 2010.

Opportunity spent much of its decade travelling across the sulphate-rich plains of a region called Meridiani Planum. In August 2011, it arrived at Endeavour crater, a 22-kilometre-wide hole punched out by a meteorite impact some 4 billion years ago. After exploring part of Endeavour’s rim and surviving another martian winter, Opportunity started heading south.

Around the same time, Raymond Arvidson, the rover’s deputy principal investigator, had been looking at satellite observations of the region. A spectrometer aboard the Mars Reconnaissance Orbiter was repositioned so that it photographed the area at a higher resolution than usual. Arvidson, a planetary geologist at Washington University in St. Louis, Missouri, then spotted something intriguing. Just west of where the rover was rolling sat a pile of clay-rich rocks. Clays are formed in watery environments.

“I called [the mission head] and told him: turn right,” says Arvidson.

Opportunity climbed the small outcrop, called Matijevic Hill. Chemical analyses of the hill's rocks revealed a clay mineral, smectite, that is rich in aluminium. That and other minerals suggested that the rocks had formed in a watery environment with nearly neutral pH.

Spherules on this rock found at Mars's Matijevic Hill reveal that it formed after the impact that created the Endeavour crater (scale across the scene is about 5 centimetres). Credit: Science/AAAS

The Matijevic rocks lay beneath those kicked up by the meteorite impact that created Endeavour crater, suggesting that they must already have been there when the impact occurred. “We’re looking at the oldest rocks we’ve ever seen with Opportunity,” says Arvidson.

In other words, the oldest environment known on Mars was a watery, pH-neutral one that could have been habitable for life as we know it. Later, after the impact, the water became more acid- and sulphate-laden, and less suitable for life.

Both Opportunity and Curiosity confirm the idea that Mars formed lots of watery clay minerals early in its history, says Ehlmann3. The next task is to pin down how many different types of habitable environments it might have had, in different locations and at different times.

Roving on

For now, Curiosity is still trekking to its ultimate goal, a 5-kilometre-high mountain whose layers may contain geological clues to more of Mars’s past. Opportunity will shortly hunker down for another winter before making tracks farther south, to a possible mother lode of more clay outcrops, says Arvidson.

Meanwhile, on 8 January, a rock mysteriously appeared in front of the rover where no rock had been before. It is probably a piece of underlying bedrock that the rover’s wheels kicked up and that then tumbled downslope, says Arvidson. “Most of us call it the jelly doughnut,” he says, “because it has kind of a red centre.”

The researchers have found that the rock has an unusual composition, and are still sorting through what that might mean — but they are happy to have a new puzzle to work on for the rover’s anniversary.

These replicas show the relative sizes of three generations of NASA rovers: Sojourner (foreground, landed in 1997); Spirit/Opportunity (left, 2004); and Curiosity (right, 2012). Credit: NASA/JPL-Caltech