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 scrutinized 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 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 it's proving to be elusive. The latest buzz kill came 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 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 Mars's atmosphere. A European orbiter around Mars also spotted methane. But the methane has proved ephemeral - now you see it, now you don't.

Mumma said he and his colleagues are reviewing their work to see if there is some error in the mix. Perhaps the methane simply disappears quickly on Mars, through some unknown chemical process.

"It's possible that we don't understand something that's going on in the Martian atmosphere," said Michael Meyer, lead scientist for NASA's 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.

Curiosity's 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.

Robert Zubrin, head of the Mars Society, which advocates for human missions to Mars, says that this is a mystery worth drilling into, literally.

"If we can bring up groundwater, find life, and analyze it, we will be able to determine if it contains more primitive examples of Earthlike (RNA/DNA based) biology," Zubrin said by email. "If so, that would show we came from Mars." Or perhaps, he said, the results would show that Mars life evolved from Earth life. If the two types of life were different, that would mean independent origins and would suggest that life is a cosmic imperative.

Although our solar system abounds with seemingly uninhabitable worlds, there are several places other than Mars that pique the interest of astrobiologists. Europa, a moon of Jupiter, apparently has a deep subsurface ocean and is a likely site of future robotic exploration. Saturn's small moon Enceladus has drawn attention since the 2005 discovery that it has ice geysers erupting from what is likely a liquid source beneath the icy crust.

"The interior of Enceladus would be dark and cold and not very pleasant for us humans," Chris McKay, a NASA planetary scientist, said in an email. "But we find microbes [on Earth] living under ice in remarkably similar conditions."

Titan, the largest moon of Saturn, has an atmosphere thick with hydrocarbons and could conceivably have life forms with biochemistry that is different from what we see on Earth, Green said.

"We call that 'weird life' because it doesn't conform to anything we know," Green said. "Weird life has no bounds. There might be weird life in a variety of places in our solar system. There may even be life under the ground in Venus."

The very definition of life is tricky, since scientists have only one example from which to draw conclusions, and that's Earth 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 everywhere in the universe. Life appeared quite early in Earth's history, as if it couldn't wait to get in the game.

Astrobiologists have come up with a working definition of life: "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: In 1928, when Mars and the Earth came unusually close to one another, some people in the United States got the notion that anyone with a radio transmitter should turn it off. The radio silence would presumably make it easier to detect any radio signals coming from a civilization on Mars.

In fact, some people did detect radio signals coming from somewhere out there. Explained Shostak: "They weren't actually Martians. They were merely Canadians."