'Tantalizing' organics discovered on Mars but still no proof of life

NASA's Curiosity rover has detected a form of carbon on Mars that is found in life on Earth, but the intriguing results do not prove there is life in the Martian soil.

Curiosity had been exploring Mars since 2012, driving around Gale Crater, which is believed to have been a lake billions of years ago when Mars was a warmer, wetter world. A recent report says soil samples taken by the rover contain high levels of carbon-12, a form or isotope of carbon that is taken up preferentially by life on Earth.

Recently retired Curiosity scientist Paul Mahaffy was quoted in a NASA release calling it "tantalizingly interesting."  NASA scientists suggest the carbon might have formed through biological processes of life in the Martian soil. But like other past discoveries, there could be less exciting ways to explain the findings that don't invoke a biological source.

In this case, the high levels of carbon-12 could have come from interactions between ultraviolet radiation from the sun and Mars' carbon dioxide atmosphere, or even an galactic molecular cloud that passed over our entire solar system billions of years ago that deposited the carbon on the surface.

This debate about the biological versus non-biological source of potentially fascinating extraterrestrial discoveries has been ongoing for more than 40 years as the search for life on Mars continues.

Evidence with differing opinions

Back in 1976, Viking, the first Mars lander designed to specifically look for life, carried a small laboratory where soil samples were fed nutrients, then heated. The gases released by the soil suggested there was microbial activity within it.

But other instruments on Viking were unable to detect any organic chemicals, those based on carbon. So the results of the experiment were deemed to be due to chemistry rather than biology, an assertion that remains controversial to this day. 

Some scientists maintain the best explanation for Viking's findings is biological in nature and that other researchers should consider it as a possibility. 

Ongoing quest for proof of Martian life

Now that those organics have been found in Martian soil by the more sensitive instruments on Curiosity, that still doesn't prove life exists there. What is needed is more direct proof, such as fossils.

That opportunity seemed to have arrived on a meteorite found in Antarctica in 1984 that had been blasted off Mars billions of years ago and landed on Earth. The rock was found to contain what look like worm-shaped "fossils" made of magnetite crystals similar to those made by some aquatic life on Earth.

There was such great excitement over the discovery that NASA made a public announcement in 1996 about the first evidence that "strongly suggests" life on another planet.

Shortly after, however, other scientists chimed in saying the same structures could be made without involving biology. The debate over whether the meteorite contains proof of life on Mars or not remains a hot topic.

'Extraordinary claims require extraordinary evidence'

This is how good science works. Any new idea is immediately held up to scrutiny through the process of peer review. In the words of Arthur Conan Doyle in The Case-Book of Sherlock Holmes: "When you have eliminated all which is impossible, then whatever remains, no matter how improbable, must be the truth."

Finding life on another world is a holy grail in science, a fundamental question of whether we are alone in the universe. Whomever makes that discovery will go down in the history books. But before anyone makes that extraordinary claim, they must be absolutely certain of their evidence.

So far, the case for life on Mars is incomplete. Until we find a fossilized skeleton, footprints in the sand or iron-clad indirect evidence, the question remains unanswered for now and so the search continues. 

Who knows? Perhaps Martian life will announce itself one day when an alien creature walks up to one of our rovers and looks into the camera. After all, our robots would be UFOs to them.

Now that would be interesting.