FOR a meeting supposedly devoted to planets, moons, comets and asteroids, there was just one topic that seemed to be on everybody’s mind as some 1200 scientists gathered in Houston last week – Mars.
More spacecraft are circling around and roving over the Red Planet than ever before, sending back an unprecedented torrent of new information. This year’s Lunar and Planetary Science Conference, held on the outskirts of the Johnson Space Center (NASA-JSC) was the first opportunity for researchers from NASA’s Mars Rover missions and Europe’s Mars Express orbiter to present detailed data to their colleagues. It was also a chance for teams studying Martian meteorites and Mars-like environments on Earth to add their discoveries to the mix. What emerged was a picture of a planet once encircled by briny, acidic lakes and rivers – an environment ideally suited for preserving signs of life.
The meeting’s star attraction was the session devoted to the latest results from the Mars rovers, and the air was heavy with anticipation. Sure enough, the standing-room-only crowd was well rewarded for putting up with the discomfort of the packed, muggy room. Steven Squyres, lead scientist for the twin Mars rovers Spirit and Opportunity, reviewed the lines of evidence his team has been following. He built a solid case that the ground at Meridiani Planum where Opportunity landed was once sopping wet, and that at least some water had flowed at Gusev crater too (New Scientist, 13 March, p 13). And he ended with a flourish, revealing that the elusive repository of haematite, the mineral usually formed in the presence of water that drew them to the site in the first place, has now been found, in the tiny perfectly round spherules the team has dubbed “blueberries”.
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It is now clear that the spheres are concretions of haematite, slowly built up in layers as mineral-rich water flowed through layers of loose sediments. But that was no surprise to geologists who presented their findings from fieldwork in the arid Navajo sandstone formations of Utah. They showed samples of perfect spherules virtually identical in size, appearance and, it turns out, composition to those Opportunity found littering the floor of the 22-metre-wide crater where it has spent the past two months.
The terrestrial haematite spherules formed when very salty, mineral-rich and highly acidic waters flowed through the Utah soil, suggesting that a similar environment was present on Mars.
This idea is supported by several other observations. For example, Opportunity’s spectrometers found high concentrations of sulphur and bromine in the Meridiani bedrock. These are typical signatures of evaporites, and one of the key lines of evidence that water – very briny water – once saturated the site.
Meanwhile Mark Bullock of NASA’s Ames Research Center in California reported that when his team made “soil” that closely matched the mineral composition of Martian meteorites found on Earth, and soaked it in water for up to a year, it produced acid brines containing the same minerals now being found on Mars. Such acidic water would also solve a riddle that has long puzzled Mars researchers: why have no significant amounts of carbonates, which usually form in water, been found on Mars? Their absence has been cited as evidence that no large bodies of water could have existed there for long. But Bullock argues that a highly acidic environment would have stopped any carbonates forming.
Finally, Opportunity’s discovery of the mineral jarosite in the bedrock, perhaps the clincher in building the case for water on Mars, points to the same conclusion, as it only forms in acid brines.
This also supports the findings of studies of another Mars-like site, in Spain. Carol Stoker, also of NASA-Ames, reported on work that she and a team of US and Spanish scientists carried out last autumn at Rio Tinto, a very acidic river that flows from the world’s largest deposits of sulphide minerals. The area is rich in jarosite.
Such an acidic environment might seem harsh, but there is evidence that it is perfectly hospitable for life. In the Mars-like conditions at Rio Tinto, Stoker’s team found previously unknown – and still unidentified – bacteria thriving far underground without light or oxygen. “It’s a kind of biosphere that is suitable for Mars,” she says.
What’s more, if life did ever exist on Mars, the briny environment means we would have a very good chance of finding it. The evaporitic minerals Opportunity has found in the Mars bedrock are exceptionally well suited to preserving fine details. The rover team has been coy about discussing the possibility of fossils on Mars, but Squyres did go as far as saying that the site has great potential for “trapping whatever was going on in it, possibly biologically”.
In fact, the rovers have already seen structures in the rocks that some observers suggest might be signs of past life, including microscopic, thread-like filaments and a rotini-like shape (see “Alien lifeforms?”). But with the instruments available on the rovers, even if these are fossils there is no way to be sure. The features are “tantalising”, says Everett Gibson, a geologist at NASA-JSC but whatever tests the rovers might carry out, “there is nothing the community would accept as definitive”.
Even identifying fossils of early organisms in rocks from Earth is hard enough, and it has been the subject of disputes almost as intense as those that followed the reports of “fossils” found in the Martian meteorite ALH84001, eight years ago. The debate continued last week as a team reported to the conference on small filamentary fossils found in Australia and elsewhere, that bear a strong resemblance to the formations discovered in ALH84001.
So short of finding Martians alive and wriggling, proof of past life sufficient to convince most scientists will almost certainly require hands-on analysis by humans. But that won’t happen any time soon. There are no plans for any mission to bring samples back to Earth, and only vague plans for human exploration on Mars decades from now.
In the meantime, however, there are still some major opportunities awaiting the spacecraft now orbiting and roving on Mars. Both Opportunity and Spirit have found overwhelming evidence for liquid water in the ground, a discovery that Jeff Kargel of the US Geological Survey describes as “historic”. But, at the time of the meeting, neither had yet proved conclusively that the water flowed and accumulated above ground, forming rivers, lakes and seas.
The evidence is growing. Phil Christensen, an Arizona State University geologist on the rover science team, presented a very strong case that a lake larger than the UK may once have occupied the site where Opportunity now roams. He cited orbital evidence that shows the haematite-rich region has an extremely sharp boundary, at what seems from the rock topography to be a lake edge. Others at the meeting outlined strong evidence for a past ocean that may have filled much of Mars’s northern hemisphere: networks of river channels there all seem to vanish at the same elevation, at the boundary of a vast, smooth-floored basin.
Not everyone is yet convinced about Martian oceans, but that may not last for long. The rover team hinted that soon – possibly by the time you read this – it would announce evidence to convince even the sceptics, perhaps including distinctive patterns of sedimentation that can only form in flowing or standing water.
And the European orbiter Mars Express could complete the picture by showing once and for all whether liquid water still exists today just beneath the soil. When its radar system is turned on next month, it will penetrate beneath the planet’s surface to reveal any ice or water present now, even if it is hidden deep underground.
If it turns out liquid water has been a feature of Mars both past and present, the odds that life could have arisen there, and perhaps even persisted, would rise dramatically. No wonder everyone at the meeting was so excited.
