Was there once life on the Red Planet? A definitive answer still eludes us, yet every sample NASA's Curiosity rover takes scientists a step closer to deciding whether Mars – today freezing cold, bone dry and bombarded by radiation – might once have been habitable.
Using its sophisticated on-board laboratory, the rover's latest discovery is of clay minerals containing the chemical elements sulphur, nitrogen, hydrogen, oxygen, phosphorus and carbon. They were found in powder drilled recently from a sedimentary rock near a former streambed.
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Mars shows signs of liveability
NASA scientists say tests on a rock sample taken from Mars indicate the planet could once have supported primitive life.
Elements like these represent a cross-section of key ingredients for life, suggesting Mars may once have sported an alien community of living microbes.
That said, the new results fall far short of evidence for life itself – past or present. All the same, the picture now emerging of conditions on the fourth rock from the sun is encouraging. And scientists are excited.
The latest findings show that habitable environments existed on Mars, says CSIRO astrophysicist Kurt Liffman. "It really is an important result: there are signs of water alteration, where the water was relatively neutral."
The environment would have been conducive to primitive life-forms, Dr Liffman explains, although there is no evidence for these life-forms so far.
The Earth boasts similar environments: dried lakebeds, for example, where ancient bacteria reside beneath the surface.
Swinburne University astrophysicist Chris Fluke agrees. The clay material Curiosity has uncovered, he says, contains many of the basic chemical building blocks that are present in Earth-based life: including oxygen, nitrogen and carbon.
More importantly, the clay minerals below the surface have not been oxidised. "This means that simple microbes could, in principle, 'feed' off it," Associate Professor Fluke explains. "It is not evidence that Martian life-forms did exist in the past, but it's a positive sign that conditions earlier in Mars' history may have been suitable for simple microbial life to prosper."
In itself, the presence of oxygen is not especially significant. Mars already has a small quantity of free oxygen present in its tenuous atmosphere – 0.13 per cent by volume. "So, finding more would not be terribly surprising," Professor Fluke points out.
Today, the surface of Mars is 'red and dead', with most of the surface rocks gaining their reddish colour from iron oxide. "This is not an environment conducive to life. But by drilling below the rusty surface, Curiosity has exposed 'grey clay'."
Guy Murphy, vice-president of the Mars Society Australia, takes a similar view. "As one of the components of water, oxygen is everywhere on the planet. But the discovery demonstrates conditions at this location could have sustained life on Mars in the distant past."
In particular, it suggests there once ran water that was neutral or only slightly alkaline. "I could have drunk a glass of that water and been fine," Mr Murphy says.
There was also a range of chemicals available which microbial life could potentially use for sustenance. These included substances with varying degrees of oxidation, which organisms on Earth are known to use as energy sources.
"We have long speculated these favourable conditions existing on early Mars," Mr Murphy explains. "This is the best evidence found to date by testing rocks containing a chemical record of that era."
Yet it's not the first time that Curiosity has analysed material on the Martian surface. This happened late last year when the rover's instruments found the soil its robotic arm had scooped up contained a complex chemistry – including some organic molecules, carbon-containing compounds such as sulphur and chlorine-based substances, that might be ingredients for life.
The simple organic compounds came from a drift of windblown sandy soil found in a crusted dune in an area nicknamed Rocknest in Gale Crater, where Curiosity landed. Such sandy, dusty material is distributed across the desiccated surface by wind and is relatively uniform in composition everywhere.
"The earlier testing of soil was of windblown dust and sand that is relatively consistent across the planet," Mr Murphy says. "This new result is of an ancient rock tied to a specific location – and so gives us historical information the windblown sample could not."
Curiosity's overall 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 – and hence life.
It's early days yet, but Curiosity's journey is sure to be full of eastern promise. Stay tuned.
Watch scientists discuss the landing of Curiosity and what it means for the hunt for life on Mars here.
Check out a Mars fact sheet here.
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