Mars is a tease.
It seems that discoveries hinting at life on the Red Planet are as recurrent as Kansas hay fever. Open up the science section of any periodical, and you'll invariably trip across new research encouraging us to believe that somewhere, skulking in the vast, dry landscapes of that desolate world, are small, wiggling creatures -- fellow inhabitants of the solar system.
Such enticing tidbits are nothing new. Their modern incarnation dates back to the early 1900s, when astronomer Percival Lowell promoted the existence of Martians who had trussed their planet with irrigation canals. This idea was well received by the public, but the astronomical community was at first skeptical, and eventually dismissive. By the First World War, these sluice-happy Martians were vaporware.
As the century ground on, additional see-saw arguments for martian life made regular appearances. In the 1970s, the Viking Landers, with the best science instrumentation NASA could launch, went looking for life in the martian dirt. The verdict was that they didn't find any. But one member of the Viking biology team doesn't agree. Was it a hit or a whiff? We still can't say for sure.
Then in 1996, claims of fossilized microbes in a meteorite known to come from Mars became the biggest science news story of the year. But were the seductive squiggles seen under the microscope really dead Red Planet microbes, or were they just inanimate features that mimicked croaked critters? Again, the jury has not returned to the court room.
This litany of teases continues today with the saga of martian methane.
Methane is best known on Earth as natural gas, and there's a good chance it's powering the device you're using to read this. It's the simplest of the organic molecules. "Organic," by the way, doesn't mean that it's necessarily the product of biology, or that it was grown on a farm that shuns pesticides. It just means that the molecule incorporates carbon as one of its constituent elements. Since carbon has four covalent bonds, the simplest molecule you can make with this stuff is by attaching a hydrogen atom to each of these "chemical arms." CH4 is the result, known to savvy 11th graders as methane.
But in the context of extraterrestrial life, methane is important as a possible biomarker. It's the exhaust gas of many forms of life on Earth -- bacteria, most notably, but also slightly bulkier organisms such as cattle and pigs. If you detect methane in a planet's atmosphere, you may have found pigs in space. Or more likely, microbes in space.
In 2004, the Europeans launched the Mars Express orbiter, and did just that. They claimed that their spacecraft had spectroscopically sniffed clouds of methane wafting above the Red Planet. American astronomers, using ground-based telescopes, also thought they had sensed this gas. The claim was important, if true, because CH4 could be caused by underground, martian bacteria. If so, this would be the first detection of life beyond Earth.
And even more, it would be living life. Not the dead microbes supposedly entombed in a meteorite, but metabolizing Martians that were still kicking. That's because ultraviolet light from the Sun, untroubled by an ozone layer that Mars doesn't have, would take apart any methane molecules in the atmosphere within 300 years or so. So if there's methane around, it's today's methane (note to reader: for astronomers, 300 years ago is the same as "today").
Given this back story, you can imagine the considerable interest when NASA's Curiosity rover bounced onto the sands of our little ruddy buddy in 2012, equipped with instruments that could also check for methane. The result, announced in September 2013, was that it couldn't find any at a level under a part per billion, or roughly ten times lower than expected on the basis of the earlier measurements. You might guess that maybe Curiosity had the bad luck to land in a spot far from the madding, methane cloud. Sure, but scientists figure that -- thanks to the circulation of its thin atmosphere -- any gas spewed out in one spot would get spread around the entire planet within months. You should be able to detect it anywhere, if there's enough of it.
The 2013 negative result from Curiosity was, indeed, both curious and a downer. But this week, researchers attending a conference of the American Geophysical Union in San Francisco heard of the detection of a sudden spike in methane. In a truly remarkable measurement by Curiosity, we find that the gas is back.
That's exciting news, but history cautions us not to party hearty just yet. Methane can be produced by geophysics as well as biology, when rocks and water interact chemically. Just because it smells like a duck, doesn't mean it's a duck.
So what gives? No one's sure yet; the obvious variability in the presence of methane suggests local sources, but the big question is whether the source is geophysical or biological.
Nathalie Cabrol, a SETI Institute astrobiologist who is especially interested in the habitability of Mars, said, "The good news is that we now know sources of methane exist. This is something that we've measured."
Cabrol is cautious about concluding that these latest discoveries are even semi-solid evidence for biology, but there's little doubt that such a scenario is possible.
"There may not be an easy way to untangle whether the source of the gas is geophysical or biological," Cabrol notes. "But if life evolved on Mars and survived eons of sudden and drastic climate changes, it might have evolved strategies analogous to dormant species on Earth. Bacteria can survive millions of years in terrestrial permafrost, awaiting the return of favorable conditions to start up their metabolism and multiply."
It might be life, or it might not be. But the good news is that we now have evidence of some sort of activity under the surface of Mars -- phenomena subject to solid, repeatable measurement.
Long everyone's favorite place to search for extraterrestrial life, the Red Planet continues to taunt us a century after Percival Lowell assured us that it was both inhabited and cultivated. At least the first is still possible.