Since Schiaparelli’s 19th century description of “canals” on the Martian surface, Mars has been the most popular planet for speculation about life. Generations of astronomers and science fiction writers alike speculated about an advanced civilization struggling to save itself by constructing these canals as its planet’s water supply dwindled.
Today, we know that Schiaparelli’s “canals” were not the work of an advanced intelligence–in fact they were probably an optical illusion. But we also know that other parts of this tale were uncannily accurate. Mars did once have surface water, and that water did gradually disappear as Mars’ geology cooled and its atmosphere leaked away into space. Today, Mars still has enough water to meet the needs of a flourishing civilization–but what is left is frozen, in the planet’s polar ice caps and underground.
Now, the question–did Mars have liquid water long enough to develop life early in its history? And if it did, has any of that life survived its planet’s dessication? (Photo of sunset on Mars courtesy of NASA. Yes, Mars sunsets really are blue due to Raleigh scattering.)
Mars, like Earth, was once a ball of molten minerals. And like Earth, it went through a hot, geologically active period. Mars lost its geologic heat much faster than Earth due to its much smaller size. But before it did, the red planet passed through a phase during which would have been warm enough to support liquid water, and produced enough geologic gases to have a fairly substantial atmosphere. Life arose on Earth within 1 billion years of the planet’s formation–how long did Mars’ “habitable period” last?
Right now, we don’t know. Mars’ geology shows that the planet did harbor stable surface water for at least 10,000 years–the amount of time necessary to create some of the water-requiring mineral formations found on its surface. Many people think the period was far longer–10,000 years is an eyeblink in geologic time–but we don’t yet know enough about Mars’ geology to say for sure. In time, with a lot of study, we may date the planet’s wet period far more precisely.
We have studied Mars, our near neighbor, in much more depth than distant Europa. So is there any hard evidence of life on Mars?
In the 1990s, a shocking science discovery made global headlines–what looked like fossilized bacteria had been found in a meteorite from Mars. This rock was a chunk of the Martian surface blasted into space by an impact, then picked up by Earthly explorers after it landed in Antarctica. And under the electron microscope, this meteor contained rod-like structures that looked like tiny bacteria.
In the decades since this discovery, debate has raged within the scientific community over the meteorite ALH 84001. Some say it’s actually Earthly bacteria in the meteorite–that it was contaminated in Antarctica or in the lab. Others say it’s not bacteria at all, but rather an inorganic crystal structure that could have formed during the impact or in space. Still others say that it’s conclusive proof that life once existed on Mars. Many theories have been offered, but nobody has been able to prove their claims. (Wonderful photo of the meteorite by Kunalm.)
So in the face of this uncertainty, ALH 84001 has largely dropped out of public consciousness. If the scientific community can’t prove what it is, they may as well not use it. But there is recent evidence just as tantalizing, and just as ambiguous.
Researchers have detected methane in the Martian atmosphere. This is a little odd, because methane should break down quickly in that environment. So it stands to reason that something is replenishing it. Methane can be produced either by active geology–of which Mars has little or none–or by certain types of bacteria. That is, life.
The methane story gets stranger still. It is coming from the ground–that’s to be expected, whether the gas is geological or biological in origin. But it isn’t produced at a constant rate. Rather, the ground produces more methane during Mars’ warm season.
This is, in my opinion, the best evidence that life still exists on Mars. Warm seasons on Earth speed up metabolism and cause life forms to grow and proliferate. And, of course, to produce more waste. The increase in methane production with Mars’ warm seasons does not make a lot of sense if the methane is geologically produced. It is, however, exactly what you would expect to find if the methane were produced by life.
Scientists are hesitant to weigh in on the issue. Alien geologies have surprised us before (Venus, anyone?), so there could very well be some unknown geologic process happening on Mars. But some scientists have come forward to say that the methane gas mixture is not terribly consistent with geological processes–it lacks other trace elements that might be expected in that case. It’s more consistent with the molecular signature of life.
The prospect of finding Martian bacteria is tremendously exciting for biologists. Thus far, it’s been difficult for us to make any statements about the fundamental nature of life because we have a sample size of one–one origin of life from which all known Earth life forms are descended. If Mars had a separate origin of life, studying it could yield knowledge that we could never hope to get from studying Earth life.
It’s also possible that, if there is life on Mars, we may find it to be more than simple bacteria. No one knows how far life may have evolved during Mars’ wet period–we haven’t studied nearly enough of the Martian surface to hunt for fossils. And no one knows what may have survived the gradual dessication.
At this point we can mostly rule out the existence of an intelligent civilization. But whatever life forms existed on Mars’ surface during its wet period would have had millions of years to adapt to a vanishing atmosphere–they could have moved underground for shelter against the increasing cold, dehydration, and radiation. We know that liquid water still sometimes bursts from the Martian underground and leaves rivulets in the sand. Who knows what might exist alongside it?