杏吧原创

Update, 24 January 2014: The results reported below have now been published in the journal 鈥� the day before the 10th anniversary of Opportunity鈥檚 landing on Mars

Original article, published 10 June 2013

An old Mars rover that landed nine years ago has found the latest evidence that drinkable water once flowed on the Red Planet. Combined with previous, similar discoveries, this suggests that neutral water was once present all over the planet, so life could have arisen in multiple places.

Winter is coming to Mars鈥檚 Southern hemisphere, where NASA鈥檚 Opportunity rover is exploring. But before heading for sunnier hills for the rest of the season, the solar-powered rover squeezed in an examination of an ancient rocky outcrop on Endeavour crater and hit pay dirt. The outcrop, called Esperance, contains aluminium-rich clay minerals, something that must have formed in the presence of neutral, life-friendly water billions of years ago.

鈥淲e鈥檝e been discovering evidence for water on Mars since Opportunity landed in 2004,鈥� , the rover鈥檚 principal investigator, said at a press briefing on 7 June. But the vast majority of that liquid would have been more like sulphuric acid, and only hospitable to extreme microbes.

This is the first time Opportunity has founds signs the planet once hosted water that would have been favourable to more familiar life forms. 鈥淭his is water you could drink,鈥� says Squyres, of Cornell University in Ithaca, New York. The result shows that habitable conditions probably existed all over Mars.

Similar stories

A 2010 analysis of data from Opportunity鈥檚 now defunct twin rover Spirit, which had been exploring Gusev crater on the opposite side of the planet, revealed a , a clay mineral that also needs water to form but that dissolves in acidic conditions. Hints of carbonates had previously been spied from orbit in a more northerly Martian region called Nili Fossae.

And earlier this year NASA鈥檚 roving science lab Curiosity used its more advanced instruments to drill into and examine rocks in the Gale crater. The results offered the first direct evidence of neutral, slightly salty water on Mars, which also appears to have contained compounds that early microbes could have used as energy sources.

鈥淚t鈥檚 really striking to me how similar the stories are that are being told by the rocks at Endeavour crater and the rocks being investigated at Gale crater,鈥� says Squyres. 鈥淚t is broadly consistent with patterns we see over the whole planet.鈥�

Esperance is part of a collection of fine-grained outcrops along the rim of Endeavour crater. Getting a good look at it was no easy task. 鈥淭his took us weeks,鈥� Squyres said. The task was interrupted by a dust storm, and a period of radio silence as Mars went behind the sun from Earth鈥檚 perspective. What鈥檚 more, the rock itself was a challenge to examine using the older rover鈥檚 tools.

Lumpy, bumpy, dirty

鈥淚t was lumpy, bumpy, dirty 鈥� covered with all sorts of Martian crud,鈥� says Sqyures. 鈥淚t took us seven tries.鈥�

Orbital data had hinted there might be clays known as smectites in the region. On Earth these form when basalt is exposed to relatively small amounts of water. Instead, the clays inside Esperance are closer to montmorillonite, a mineral that is rich in aluminium and lower in iron. This suggests that the clays formed as large amounts of water flowed through fractures in igneous rock, said Opportunity scientist Ray Arvidson of Washington University in St Louis, Missouri.

Esperance is one of the oldest rock outcrops that Opportunity has examined, probably dating from the first billion years of Martian history, says Sqyures. Finding hints of more acidic conditions in younger rocks would fit with the picture of Mars drying out and getting colder over time, so that its water became more concentrated. But without samples of rock it鈥檚 hard to pinpoint ages and link the different environments with periods of Mars鈥檚 past, says Squyres.

Opportunity is now on its way to Solander Point, a hill about 2.2 kilometres away that contains 50 metres of exposed layered deposits. 鈥淲e are hoping the stratigraphy and some interesting chemistry and mineralogy will be exposed, and maybe we will see the geological context in the older rocks,鈥� says Arvidson.

Race against time

The situation parallels the plan for NASA鈥檚 Curiosity rover, which landed in 2012, and is wrapping up its investigation of rocks at a site in Gale crater called Yellowknife Bay. In a few weeks it will begin its trek to Mount Sharp, a 5-kilometre mountain also boasting layered deposits.

The difference is that Opportunity needs to hurry. The dark cold of Martian winter will set in after August. Being on the north-facing slopes of Solander Point will give Opportunity the best chance of gathering enough sunlight to keep working through the season, says Arvidson. Nuclear-powered Curiosity will be able to take its time, meandering to the base of Mount Sharp over the next 10 months or so.

There鈥檚 also the constant worry that Opportunity 鈥� now almost 10 years into what was designed as a 90-day mission 鈥� could suffer a fatal failure. Unlike Curiosity, Opportunity was built without redundant systems, so if a component breaks, the mission is over, says John Callas, the project manager for Opportunity at NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California.

鈥淲ith each of the craters we鈥檝e visited it鈥檚 been like a whole new mission,鈥� says Callas. 鈥淏ut Opportunity could have a massive stroke at any time, so we treat each day preciously.鈥�