Martian life is awfully cryptic. Thats a scientific term: It means life that is out of sight, below the surface, burrowed into ecological niches not easily scrutinized by robotic sentinels from the planet Earth.
Or perhaps its 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 materialize.
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 civilization 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, with Mars an obvious place to look. But its proving to be elusive. The latest buzz kill came Thursday when scientists announced that NASAs Curiosity rover had not detected methane in the atmosphere. Atmospheric methane is often a byproduct of living organisms.
The new finding wasnt a total showstopper, but scientists would have been thrilled by a different result.
Naturally, I was disappointed, said Michael Mumma, a planetary scientist at NASAs Goddard Space Flight Center. It would have been nice if they had detected an abundant signal for methane.
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 Marss atmosphere. A European orbiter around Mars also spotted methane. But the methane has proved ephemeral – now you see it, now you dont.
Mumma said he and his colleagues are reviewing their work to see whether there is some error in the mix. Perhaps the methane simply disappears quickly on Mars, through some unknown chemical process.
Its possible that we dont understand something thats going on in the Martian atmosphere, said Michael Meyer, lead scientist for NASAs Mars Exploration Program.
Jim Green, director of the planetary sciences division at NASA headquarters, said the goal of Curiosity is to understand the full history of Mars, not simply to take a momentary snapshot.
Today is a small microcosm of time, and Curiosity is going for the geological record of the planet, which includes maybe millions or hundreds of millions of years when life existed, Green said.
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.
Curiositys suite of instruments is not designed to detect life itself. NASA learned a lesson in the 1970s when it plunked two Viking spacecraft on Mars and performed much-ballyhooed tests that might have detected life. The results were ambiguous at best; most scientists interpreted the findings as negative. NASA learned that it is hard to get funding for future robotic missions when the first wave of probes saw only a cold, dry, dead-looking place.
In the past two decades, NASA has chosen to study Mars in a more incremental fashion, looking at the geology and chemistry and trying to understand the broader narrative of the planet.
One 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 in impacts from asteroids or comets, have wound up on Earth, and the same process has presumably happened in reverse. Scientists believe organisms can potentially survive inside ejected rocks even during long transits in space.