Rows of barren sand dunes, sculpted by wind into dreamy ribbon-like ripples, punctuate the rust-coloured volcanic rocks and rubble formed from solidified lava. The Red Planet's desiccated and freezing landscape seems the unlikeliest of places to find life, even of the most rudimentary kind.
Yet might there be — or once have been — life on the fourth rock from the sun? It's the most tantalising of questions that scientists now believe they are a step closer to answering.
An artist's impression of the Curiosity rover at work on Mars.
Using its suite of high-tech instruments, NASA's Curiosity rover has succeeded in analysing the Red Planet's soil for the first time. And guess what? It contains a complex chemistry — including a sprinkling of what seem to be organic molecules, carbon-containing compounds that might be ingredients for life.
Among other things, water, sulphur and chlorine-containing substances appeared in samples scooped up by the rover's robotic arm and analysed by its sophisticated on-board laboratory. The simple organic compounds came from a drift of sandy soil found in a crusted dune in an area nicknamed Rocknest in Gale Crater, where Curiosity landed.
"We have no definitive detection of Martian organics at this point, but we will keep looking in the diverse environments of Gale Crater," says Paul Mahaffy of NASA's Goddard Space Flight Centre in Maryland.
NASA releases stunning high resolution images of Mars
Mars rover Curiosity sends back pictures of itself and the Mars landscape in amazing detail.
"Scientists need to test multiple samples to ensure they've not detected trace contaminants accidently brought from Earth," explains Guy Murphy, a director of the Mars Society Australia.
Strictly speaking, he reminds, the term 'organic' refers to compounds containing carbon and does not necessarily imply they are of biological origin. "But on Earth, life is based on organic chemistry and, if there is life on Mars, it may be carbon-based as well," Mr Murphy explains.
There's another possibility, too: the traces of carbon might also have come from meteorites, he adds.
Mars' Mount Sharp is pictured in this photo taken by the Curiosity rover.
CSIRO astrophysicist Kurt Liffman agrees. "It's certainly possible that such material might have been brought to the surface by meteorites. For example, meteorites known as carbonaceous chondrites tend to be rich in organic materials," he explains.
A subclass of chondrites, known prosaically as the CM class, is famous for its organic content. An example is the Murchison meteorite, named after the northern Victorian town near where the meteorite fell in 1969.
The Murchison meteorite brims with organic material, including a suite of amino acids, the building blocks of protein — the stuff of life. "The Curiosity team would be familiar with carbonaceous chondrites and would be undertaking further tests to ensure that they are not observing meteorite contamination," Dr Liffman says.
He expects it would take years before researchers are confident that the organic material is of Martian origin.
Testing en route
Curiosity's mission is to reach nearby Mount Sharp, a five-and-a-half-kilometre-high mountain with interesting geologies billions of years old, including a welter of clays and sulphates that might once have been associated with water.
On the way there, the intrepid rover is performing other checks, such as the calibration and testing of its bank of instruments.
"NASA scientists chose Rocknest as one of the first scooping sites because of its windblown fine dust and sand," says Catarina Ubach of Swinburne University. "This dust can be used to calibrate Curiosity's instruments and "dry wash" them from unwanted particles brought from Earth."
The dust consists mainly of common volcanic minerals and glasses. An organic molecule, and an oxygen and chlorine compound called perchlorate, also turned up in the soil sample, Ms Ubach says.
"Perchlorate is thought to be an "organic killer", suggesting that the carbon molecule was brought by Curiosity and thus probably of Earthly origin," she adds. "But at this early stage no one is ruling out other alternatives and we should wait for more results from the team before drawing conclusions. "
Gale Crater, where Curiosity landed, is a hotbed of scientific interest. "The crater hosts different geological and mineralogical features which provide evidence that liquid water once flowed there," says Francesco Pignatale, also of Swinburne University.
The crater has exposed sedimentation layers revealing its geological history, making it the ideal location for searching for organic compounds, Mr Pignatale points out. "Since Curiosity has already found the soil has a higher-than-anticipated water content bound to grains and dust, the chances of finding organic compounds are greater here than in previously explored areas."
NASA is now searching for ways to eliminate the possibility that the samples were somehow contaminated by equipment brought from Earth, says Mars Society geologist Jon Clarke.
The US space agency is optimistic about finding more organic compounds as its rover trundles towards Mount Sharp, searching for somewhere suitable to dig deeper down.
"Curiosity has a good chance of finding organic matter in the soils and sediments of Mount Sharp," Dr Clarke says. "In fact, it has a much better chance there than in the dune sands it is on at the moment. You'd not expect to find much organic matter in the dune sands any more than you would on Earth."
Although Mars is an unwelcoming place to Earthlings — its tenuous atmosphere is perishingly cold and contains no oxygen — it might once have harboured life of some sort.
"It might still contain life in some protected and hospitable environments, such as those deep underground," says Mark Sims, a professor of astrobiology and space instrumentation at Britain's University of Leicester and former mission manager of the European Space Agency's Beagle 2 mission.
Curiosity is tackling this question obliquely by trying to establish whether Mars has or once had potentially habitable environments.
The chances are good that scientists will find some carbonates and 'organics', says Australian National University astrophysicist and biologist Charley Lineweaver. "The tricky part will be trying to figure out what they mean," he explains. "For example, suppose they detect some methane. The hard part will be to determine if it came from life or from some other source."
Science is necessarily slow and methodical, Dr Lineweaver reminds. "The complexity of the organics will be important. We know what the organics from carbonaceous chondrites look like."
Finding these on the surface of Mars will come as no surprise, he says. "But if we find complex organics that we've never found in a meteorite, then that will be something special — so long as it is not the result of contamination," Dr Lineweaver notes.
Curiosity in profile
Curiosity has a six-wheel drive and a rocker-bogie suspension system, without axles or springs, enabling it to roll over obstacles up to 65 centimetres high — more than its forebear rovers could manage. The improved transport system lets Curiosity roam up to 200 metres a day over fairly arduous terrain, though in practice its cautious controllers are less ambitious.
High-resolution cameras, mounted on a central mast, assist mission controllers to select exploration targets and driving routes.
But unlike its predecessor rovers, Opportunity and Spirit, both of which relied on solar panels, Curiosity is powered by the radioactive decay of plutonium-238, enabling it to rove for at least one Martian year, or about 687 Earth days, with a minimum range of 20 kilometres.
Curiosity has a unique suite of scientific instruments, weighing more than 68 kilograms. These enable the rover to collect samples of rocks and soil, then process and distribute them to on-board test chambers inside analytical instruments. This helps pinpoint chemical elements, and their isotopes, in rock and soil samples.
That's not all. Curiosity can drill up to 10 centimetres into rocks to sample unweathered material. This will allow it to detect traces of organic chemicals lying deep within rocks — something its predecessor rovers were incapable of checking.
"If life ever existed on Mars, and has left traces of organic chemicals in near-surface rocks, Curiosity should be able to find them," an ebullient Mr Murphy predicts.
• Find out the latest on Curiosity's explorations at www.nasa.gov/mission_pages/msl/news/msl20121203.html
• View some sample soils at www.nasa.gov/mission_pages/msl/multimedia/pia16571.html
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