On Mars, scientists' search for life goes on, despite no proof Hopeful: Seth Shostak is a senior astronomer at the SETI Institute. Photo: Andrew Taylor Martian life is awfully cryptic. That's a scientific term: It means life that is out of sight, below the surface, burrowed into ecological niches not easily scrutinised by robotic sentinels from the planet Earth. Or perhaps it's not anywhere. Mars may be dead as dead can be. Going back to the 19th century, a persistent feature of hypothetical Martian life has been the way it has bewitched and teased earthlings but then refused to materialise. Time and again, scientists have detected signatures of Martian life, only to discover that they were written in vanishing ink. Most notorious were the canals on Mars, promulgated in the 1890s by the great astronomer Percival Lowell, who saw them as evidence of an ancient civilisation struggling to survive on a desert world. They were purely an optical illusion. Extraterrestrial life is one of the greatest unknowns in all of science, and many scientists are sure it has to be out there, somewhere. But it's proving to be elusive. The latest buzz kill came on Thursday when scientists announced that NASA's Curiosity Rover had not detected methane in the atmosphere. Atmospheric methane is often a byproduct of living organisms. The new finding wasn't a total showstopper, but scientists would have been thrilled by a different result. "Naturally, I was disappointed," said Michael Mumma, a planetary scientist at NASA's Goddard Space Flight Centre. Mumma had high hopes for a positive result because he and his colleagues believe they have detected methane on Mars remotely, from telescopes on Earth that can discern the chemical nature of Mars' atmosphere. A European orbiter around Mars also spotted methane. Mumma said he and his colleagues are reviewing their work to see if there is some error. "It's possible that we don't understand something that's going on in the Martian atmosphere," said Michael Meyer, of NASA's Mars Exploration Program. Investigations by Curiosity, other landers and orbiters have led to a consensus that, billions of years ago, Mars was warmer and wetter, with the conditions necessary for life. Although the planet dried out and lost most of its atmosphere, life could have adapted and migrated to subsurface environments. On Earth, organisms thrive in exotic realms deep below the surface, even beneath ice caps. The astrobiological truism is that life finds a way. Curiosity's suite of instruments is not designed to detect life itself. NASA learnt a lesson in the 1970s when it plunked two Viking spacecraft on Mars and performed tests that might have detected life. The results were ambiguous at best; most scientists interpreted the findings as negative. In the past two decades, NASA has chosen to study Mars by looking at the geology and chemistry and trying to understand the broader narrative of the planet. An intriguing possibility is that life originated on Mars and spread to Earth through a meteorite - or vice versa. Chunks of Mars, blasted off the surface from asteroids or comets, have wound up on Earth, and the same process presumably happened in reverse. Scientists believe organisms can potentially survive inside ejected rocks. The very definition of life is tricky, since scientists have only Earth's example of life. The origin of life remains another enduring mystery. At one level, life is astonishing, for even the simplest organisms on Earth require a complicated genetic code. And yet life is also fashioned from common elements, the stuff that is lying around in the universe. Astrobiologists have a working definition: "Life is a self-sustained chemical system capable of undergoing Darwinian evolution." It's a subject fraught with conjecture, and perhaps, in some cases, wishful thinking. Seth Shostak, senior astronomer at the SETI Institute, related one of the many false-start Martian-life stories when Mars and the Earth came unusually close in 1928. Some got the notion that anyone with a radio transmitter should turn it off, believing that the silence would allow signals from Mars. In fact, some people did detect radio signals. Explained Shostak: "They weren't actually Martians. They were merely Canadians.