February 11, 2005
The Mystery of the Martian Methane
In this excerpt from the new Forward to the paperback edition of "Lonely Planets", planetary scientist David Grinspoon ponders what the recent discovery of methane on Mars could mean for the possibility for life on the Red Planet.
Astrobiology Magazine -- "Lonely Planets: The Natural Philosophy of Alien Life," by David Grinspoon, is a humorous and balanced look at the history and science of astrobiology. Awarded the 2004 PEN Literary Award for Nonfiction, this book was described by Frank Drake as "Superb... Anyone, even a professional scientist, who wishes to become familiar with contemporary astrobiology should read this book. It's a prime place to become more than casually acquainted with one of the hottest, most interesting fields of science."
The European Space Agency's Mars Express, which arrived in Mars orbit on Christmas day, 2003, had caught a whiff of something in the air. Something that didn't belong there. Something that might indeed be a sign of life there today. The announcement received much less press attention than the Mars rovers which, after all, were taking cool pictures.
On board the orbiter, an "infrared spectrometer" - which precisely dissects the radiation leaking from the planet into a million distinct colors - had detected a most unexpected trace gas in the Martian air. Feeble signs of methane had been found on Mars. Methane is CH4, a carbon bonded to four hydrogens.
Chemically, it is out of place in an atmosphere like that of Mars, which is composed almost completely of carbon dioxide (CO2). Finding methane on Mars is like finding a gazelle strolling unnoticed through a pack of hungry lions.
To me this announcement was shocking, and seemed even more unreal than any of the strange postcards sent home by the rovers. Methane on Mars is the example I've always given of something that could change my mind about Mars today being a perfectly dead world. Why? Because as life evolves on a planet, the atmosphere evolves along with it. Chemically, the two become intimately coupled together.
On Earth, the oxygen we breathe, the protective ozone layer and, yes, the trace of methane in the air are all chemical byproducts of 4 billion years of biology. Life in turn has molded its chemistry to cleverly utilize the atmosphere pervading our world. It may be this way on all planets with life and air. If so, then a close study of a planet's air will always betray life or the lack of it.
More specifically, life is a process that consumes energy and produces gasses that are "out of equilibrium" with the rest of the atmosphere. These gasses don't last long because they react quickly with their surroundings. Methane does not belong in the atmosphere of Mars, just as oxygen doesn't belong in the atmosphere of Earth. Oxygen here is a product of green plants. What (or who) is producing the methane on Mars?
When I first heard about the methane observation, I was highly skeptical but at the same time I felt my pulse quicken. Could this really be the faint breath of underground colonies of Martians?
We've been wrong about Mars many times. Mars is so like Earth in some respects, and so close at hand. Like an only friend in a large, empty universe, we sometimes project too much onto poor old Mars. We are hungry for signs of life, and this hunger is dangerous. In science the desire to find a certain answer can lead us astray. I describe a few of these wrong turns in chapter three of my book, such as the late 1950's "discovery" of chlorophyll - the green stuff in green plants - on the red planet. This sensational announcement (in Science magazine) was greeted unskeptically at the time because of the prevailing view that the seasonal color changes observed through telescopes were caused by vegetation. (We now know that they are caused by windblown dust.)
The chlorophyll was later discredited, its "fingerprint" shown to be caused by compounds of deuterium (heavy hydrogen) in our own atmosphere. Half a century earlier the scientific community had been briefly enthralled, and then greatly embarrassed, by Percival Lowell's claims of finding intelligently designed canals criss-crossing the map of Mars. The canals either conveniently disappeared just as our telescopes and cameras improved enough to truly see them, or (more likely) they were never there at all.
Given this history of false starts and retreats, we've learned not to lightly declare that the Martians have at last been found. Or have we? In the spring of 2004 the teams reporting on the methane detections all openly speculated on underground Martian life as a likely source. In part this is facilitated by an attitudinal pendulum within science which has again swung toward acceptance of the possibility of life on our neighboring planet.
The methane claim was quickly bolstered by several independent observations. In addition to the detection by Mars Express in orbit, it has now been seen by two different groups of ground-based observers using some of the best telescopes on Earth, in Hawaii and Chile. So, the methane, it seems, is there. But, does it really mean life on Mars?
Here, when you actually look at the numbers, the evidence is not immediately convincing, because the quantity of methane is so tiny. Seen in the infrared, methane has a distinct and strong signature. In an atmosphere of carbon dioxide it stands out like a bloodstain in a fresh snow bank. So, why haven't we found it before, since we've been scrutinizing Mars in the infrared for decades now?
The signal is, in fact, very weak, implying that the methane is extremely scarce. The data suggest something like 10 parts per billion (ppb) of methane in the Martian air, so for every one billion molecules of carbon dioxide (CO2) there are, apparently, 10 molecules of methane (CH4). That's hardly a methane mother lode. Yet, there must be a source. And that is the part that sets our minds spinning.
On Earth the main sources of methane are biological ones. Methane doesn't last long in our air either, but bacteria living in rice paddies and in the guts of cows (for example) supply a constant trace. Could underground bacteria on Mars be the culprits here?
Given "Sagan's law" that "extraordinary claims require extraordinary evidence," we are obliged to search for other, more mundane, explanations before we trumpet the news (once again) of finding life. How do we know whether we should accept, or rule out, an alternative explanation for something as potentially important as methane on Mars? Many times a "back of the envelope" calculation is sufficient - an exercise in which we plug in rough but reasonable estimates for the important quantities and see if the answer we get is in, or at least near, the ballpark.
For example, a friend e-mailed me, suggesting that maybe the tiny residue of methane is simply leaking from the small collection of derelict spacecraft we've left on the planet. Yet this can quickly be ruled out with the nearest envelope: given that the entire Martian atmosphere weighs about 2.5 x 1016 kilograms, or about 25 thousand trillion kilos, this means that 10 parts per billion methane, as small as that sounds, still adds up to about 90 million kilos of methane.
So, if we had 90,000 spacecraft on Mars (as opposed to about a dozen), each weighing 1000 kilos and each composed entirely of methane gas (not a recommended construction material), then this could work as an explanation. Envelopes are good for reductio ad absurdum arguments, which tell us where not to waste our time.
A more promising possibility is volcanic venting. On Earth volcanoes burp great quantities of methane into the atmosphere. However, Earth is a volcanically active planet at present and Mars, overall, is not.
My favorite candidate for a nonbiological source is the steady rain of meteors. Organic material falls from space all the time on Mars, as on all other planets. We don't know the precise rate at which this space gunk is entering Mars' atmosphere, but we can make some reasonable inferences based on the observed rate at Earth and apply what we know about orbits and the gravitational reach of Mars. When I put this all together on the back of a nearby envelope, I find that the amount of organic carbon landing on Mars each year is likely close to the needed supply rate for the observed methane.
Decaying organic matter is a classic source of methane - think swamp gas. My oversimplified calculation tells me that the infall of meteors, and the subsequent release of organic gas as they break down in the atmosphere, could provide the right trickle of methane. No bugs required.
Is the methane falling from the sky? Maybe. Maybe not. With future space missions we'll eventually be able to measure the rate at which meteors supply Mars with organics. Much sooner than that we'll have better observations of the methane. Where, exactly, in the atmosphere does it reside? Is it concentrated near the surface (implying an underground source)? Is it coming from specific locations? The latter is the most important observation we can make in the near future.
If we find it venting from certain surface features, this destroys my "falling from the sky" idea, and gives us important clues to the conditions below ground which may be fostering the chemical activity - be it geological or biological - supplying the doomed molecules.
In my view, the methane is probably not a sign of life. (I'd sure love to be wrong about this.) For reasons I describe in my book, I don't expect that the signs of life on a planet with an atmosphere will be feeble or subtle. I believe that if life has survived on a planet for many billions of years (as it has on Earth, and must have on Mars for there to be life today) then it will have become deeply intertwined with the atmosphere of that world in a way that will make the atmospheres of living worlds distinct -flagrantly distinct - from those of nonliving worlds.
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