The quest for signs of ancient life on Mars received a major boost today after NASA revealed that its Curiosity rover has found signs of organic molecules that date back at least three billion years. Though the space agency acknowledges that their origin may more likely be geological than biological, the presence of such molecules suggests that if life did once exist on the Red Planet, it will be possible to find traces of it.

The search for life on Mars over the past half century has been an exercise in lowering the bar combined with ever more sophisticated ways of seeking clues. In the 1950s, Mars was considered a dead planet, but in the sense that it might only run to mosses and lichens. After the first Mariner missions of the 1960s, even these were ruled out and the hope was that when the Viking landers touched down in 1976, they'd be able to find signs of biological processes. However, when soil samples were subjected to nutrients in the landers' automated laboratories, the results were, at best, highly inconclusive.

Today, the general consensus is that Mars is too hostile for life to exist on the surface and the only hope is that isolated pockets of bacteria might survive in exceptionally favorable conditions – but the chances of this are remote. Instead, scientists are looking more for signs that life might have once existed on Mars billions of years ago when the planet was young and the environment was warmer and wetter.

Key to this search into the distant past has been to test for the presence of ancient organic molecules in areas that were once home to liquid water, like mud flats and stream beds. Molecules made of carbon, hydrogen, oxygen, and nitrogen are literally vital to life as we know it and if these molecules could exist for two or three billion years, it might give us a clue as to whether Mars was once habitable.

According to NASA, samples collected by the unmanned Curiosity rover from drilling operations into the top 5 cm (2 in) of sedimentary rocks from four areas in Gale Crater were studied by the machine's onboard Sample Analysis at Mars (SAM) instrument suite. There, they were subjected to temperatures of over 900° F (500° C) – hot enough to break down more complex molecules into simpler fragments, including thiophenes, benzene, toluene, propane, and butene, that can be identified and measured.

The Gale Crater samples showed organic molecules in concentrations of over 10 parts per million – very similar to those found in ancient meteorites that originated from Mars billions of years ago and 100 fold greater than previous samples taken on the Martian surface. NASA says that these molecules may have survived because they bonded with sulfur atoms, which made the long, complex molecules much tougher in the same way as introducing sulfur into the rubber used in tires by vulcanization makes the tires durable.

Even if these ancient organics aren't of biological origin, the Curiosity team holds out hope in the search for life. The findings will help in planning future rover missions, including NASA's Mars 2020 and ESA's ExoMars.

"Curiosity has not determined the source of the organic molecules," says Jen Eigenbrode of NASA's Goddard Space Flight Center. "Whether it holds a record of ancient life, was food for life, or has existed in the absence of life, organic matter in Martian materials holds chemical clues to planetary conditions and processes."

The research appeared in Science and the video below provides an overview of the discovery.

Source: NASA

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