Mars rover reaches rim of vast, ancient crater

THREE years of trundling across treacherous dunes has brought NASA's Opportunity rover to its most significant target yet - a huge crater called Endeavour that was once soaked with water and could hold clues as to whether there was ever life on Mars.

Orbital observations suggest the rocks on Endeavour's rim are more than 3.5 billion years old and so date from the earliest, wettest phase of Martian history, when water carved out vast drainage channels across the planet. Until now, neither Opportunity nor its now-defunct sister Spirit (see "The rovers at a glance") had examined rocks that clearly date from this period.

"This is potentially the most exciting scientific opportunity for the rover mission yet," says John Callas, mission manager at NASA's Jet Propulsion Laboratory in Pasadena, California. That's because mineralogical studies from orbit suggest these ancient rocks formed in a cosy environment for life.

The rovers have previously studied rocks that were once immersed in acidic, salty water (see "Blueberry bonanza"). The 20-kilometre Endeavour, by contrast, seems to have harboured water friendlier to life, since the crater contains clay minerals that require a relatively neutral pH to form. What's more, orbital measurements do not indicate that the ancient water was salty - though salty water may be flowing on Mars today (see "Dark streaks point to salty flows").

Opportunity's arrival at Endeavour marks a huge milestone for the mission. The goal seemed "almost unbelievably audacious" when it started heading there, says James Wray of the Georgia Institute of Technology in Atlanta.

The rover was only designed to last three months and in 2008, when it set out from a smaller crater called Victoria, it had already been on Mars for more than four years (see its route here). "I have gained a wife, lost a grandfather and moved twice [since then]," Wray says. "From that perspective, it does feel like a lot of time has passed."

The rover might reveal what form the water at Endeavour took. If it finds rocks bearing the imprint of ripples, that would suggest that water pooled on the surface, while if it spots rocks threaded with veins of clay minerals, that would point to water percolating underground, Wray says.

Opportunity entered Victoria crater but is likely to spend all its time at Endeavour on the rim. Endeavour's interior is less enticing because sediment from a later, drier period of Martian history has buried the old rocks there.

If it is still functioning a few years from now, the rover could set off for another, smaller crater called Iazu, with rocks that are just as old. "But holy smoly, that's like 15 kilometres away," nearly as far as the three-year trek to Endeavour, says Ray Arvidson of Washington University in St Louis, Missouri. He is content to see Opportunity live out the rest of its days scrutinising rocks and capturing eye-popping vistas on Endeavour's rim. "That's a spectacular way to end the mission," he says.

Blueberry bonanza

Almost immediately after it landed in 2004 in a region of Mars called Meridiani Planum, Opportunity made a watershed discovery: rocks at its landing site had formed in ancient lakes.
The evidence came in part from tiny "blueberries" (see image) made of haematite, which almost always forms in water. Curved lines of sediment pointed to the sweeping motion of a water current, while sulphate salts and the mineral jarosite, which forms in dilute sulphuric acid on Earth, suggested that the water was briny and acidic.

Dark streaks point to salty flows

Mars's image as a dust bowl may need a makeover. Dark streaks seen forming in summer and fading in winter might be signs of water flowing just beneath the surface (see image).
The appearance of streaks on sloping ground, including light streaks seen by NASA's Mars Global Surveyor spacecraft, has been attributed to present-day liquid water. But the link is not watertight - avalanches of dust could also be to blame.
Now, NASA's Mars Reconnaissance Orbiter (MRO) has revealed a previously unknown group of seasonal dark streaks in Mars's southern hemisphere that may be caused by flowing water. Alfred McEwen of the University of Arizona, Tucson, and colleagues found slopes where dark streaks appear every spring and disappear each winter (Science, DOI: 10.1126/science.1204816).
The seasonal streaks, which the team call recurring slope lineae, show no preference for dusty areas, where dust avalanches would be more likely. They are, however, found where radar observations show evidence for underground glaciers.
One possibility is that they result from meltwater that drains down slopes when ice thaws in the spring. But the researchers believe any flowing water lies below the surface - if it were above, MRO probably would have spotted its spectral signature, they say.
Some of the streaks form at -23 °C, well below the freezing point of pure water. Salty water, however, can remain liquid at such temperatures, and if it is flowing just beneath the surface, it might shift dust grains above, causing the dark streaks. "The best explanation we have for these observations so far is flow of briny water, although this study does not prove that," says McEwen.
The discovery of what might be liquid water on present-day Mars raises the possibility that life may have a toehold there. "It is our first chance to see an environment on Mars that might allow for the expression of an active biological process," says Lisa Pratt of Indiana University in Bloomington.