You may have heard that the scientific community is buzzing with excitement over the discovery of Gliese 581g, an Earth-sized planet circling the red dwarf star Gliese 581, 20 light-years from Earth in the constellation Libra. Five other planets orbiting this star were already known, but what’s exciting is that the new one is smack in the middle of the star’s habitable zone, making it the best candidate ever discovered for an extrasolar planet with liquid water. And where water flows, is it possible that life follows?
This finding is a major blow to the creationists who insist that Earth must be the only life-supporting planet that exists. As the scientists who discovered Gliese 581g pointed out, finding a habitable planet this easily means that we’ve either been incredibly lucky, or such planets are common.
But I wanted to turn my attention a little closer to home for the moment. You might think, given the effort scientists are putting into finding Earth-like planets beyond the solar system, that we’ve exhausted all possibilities for discovering alien life any closer to home. Surprisingly, not only is that far from the truth, we have evidence which could imply the existence of life dwelling on our very nearest planetary neighbors.
Take Venus. Despite being Earthlike in size and composition, Venus has a surface of crushing pressure and 900-degree temperatures, making it almost certain that no life could survive – on the surface. But the surface isn’t the only environment on the planet where life could conceivably exist. The planetary scientist David Grinspoon, in a daring feat of imagination, hypothesized that free-floating microbial life could exist in Venus’ atmosphere. In the upper reaches of the Venusian atmosphere, the temperature is a far more hospitable 80 degrees, with the same pressure as Earth, and some evidence even suggests the presence of water. It’s possible that life began on Venus’ surface billions of years ago, but as the steadily increasing greenhouse effect turned the surface into an inferno, it escaped into the atmosphere, drifting high above the killing heat.
And this hypothesis isn’t pure speculation. There’s some tantalizing evidence which could indicate the presence of life in Venus’ clouds.
The astrobiologists Dirk Schulze-Makuch and Louis Irwin pointed out that Venus’ atmosphere contains very little carbon monoxide. This is curious, because lightning and ultraviolet radiation should be producing this gas in large amounts. Even more suggestively, Venus’ atmosphere contains significant amounts of three other molecules – hydrogen sulfide, sulfur dioxide, and carbonyl sulfide – which, at least on Earth, are only produced by life or by volcanic activity. Venus does have active volcanoes, but not as many as Earth (since it has no plate tectonics), and it’s not certain whether it’s enough to account for the measured abundances. It’s possible that some as-yet-unknown chemical pathway is breaking down carbon monoxide and producing these other compounds. But it’s also possible that what we see in our spectroscopes is the metabolic signature of Venusian life, drifting in the planet’s clouds and thriving to such an extent as to alter the balance of its atmosphere.
However, Viking’s gas chromatograph found no evidence of organic compounds in the soil. That seemed to be the death knell for possible life – until, in 2008, NASA’s Phoenix lander discovered a compound called perchlorate in Martian soil. Perchlorate becomes a strong oxidizing agent when heated, as the gas chromatograph does, and some scientists feel that this would have rapidly broken down any organic molecules and would explain why they didn’t show up in the analysis. The evidence from the Viking experiments is still much-debated and ambiguous, but it certainly doesn’t rule out the possibility of life.
Also, like Venus, Mars has anomalous chemical compounds in its atmosphere: in this case methane, which was detected by the European Space Agency’s Mars Express spacecraft. This molecule would rapidly decompose under Martian conditions, so for it to exist there means that something must be continually replenishing it. As with the anomalous compounds on Venus, it could be released by volcanic activity – except that Mars has no known volcanism, and is believed to be geologically dead. It’s possible that the methane is being produced by a geologic process called serpentinization. But it’s also possible that Mars is home to methanogenic bacteria, producing the gas as a product of their metabolism. Most likely, Martian methanogens would live far below the surface, deep underground where it’s warmer and there may be liquid water – similar to archaea on Earth that live in similar conditions deep within the crust.
The idea of life existing on either or both of these planets shouldn’t be too surprising. Although Venus is a suffocating inferno and Mars a freezing dry desert, both planets had clement pasts with surfaces where liquid water flowed. Both these planets, during the formation of the solar system, presumably received organic molecules from the same source as Earth’s. Depending on your assumptions about how likely abiogenesis is, life could well have started on all three planets at about the same time. We won’t know for sure, of course, until we’ve had a more detailed look – but it’s worth remembering that even the nearest shores of our vast and awesome cosmos may yet contain marvels we haven’t dreamed of.