Methane on Mars: Life or Just Gas?
NASA’s Curiosity rover has measured methane—often a sign of life—on Mars. Could it be?
Methane is a familiar chemical, whether you know it by that name or not. It’s the major component of natural gas, which heats my house and possibly yours too. Methane is also a large part of human gas, which means I could start this article with a fart joke if I really wanted to. (However, it’s not the smelly part, which is provided by sulfur compounds.) Lakes on Titan are full of methane, and the chemical is a major component of the giant planets Jupiter, Neptune, and so forth.
Mars is a different case, and an interesting one: it doesn’t have a lot of methane in its atmosphere at any given moment. However, several probes—most recently the Curiosity rover—have measured methane in the Martian atmosphere. Methane on Mars could possibly reveal that the planet is more active geologically than it seems, or even that it harbors microscopic life.
While the most exciting option would be Martian life, what we have is the chemistry, which isn’t enough by itself to tell us where the methane came from. The authors of the new study, published this week in Science, are rightfully agnostic on that point, even though others aren’t being so cautious.
Curiosity researchers analyzed the chemistry in Mars’ atmosphere over 20 months, and found four measurements with more than 10 times the normal level of methane. The increased amount was present for a period of about two months with normal amounts before and after. That indicated a relatively rapid increase in methane, followed by an equally fast decrease. These results are consistent with results from probes such as the orbiting Mars Express spacecraft and the Mariner 7 mission in 1969, as well as Earth-based telescopes. Whatever caused the increase is unknown.
All of this depends on Martian methane being a real thing, and that’s where it could get sticky. Kevin Zahnle, Richard S. Freedman, and David C. Catling argued persuasively in 2010 that there’s no realistic way for Mars’ atmosphere to get rid of that much methane that quickly, even if it can be produced on a short timescale. Instead, they argue that the methane analysis from other probes is based on assumptions of how the data is processed, and could be entirely an artificial result.
That doesn’t necessarily mean the methane isn’t real, but it does mean researchers have more work to do to figure out exactly what could cause rapid chemical fluctuations in Mars’ atmosphere. (Zahnle has critical commentary on the new Curiosity results as well, but as of the time I’m writing this article, it wasn’t yet linked from the Science website. I’ll try to update this piece with a link when it becomes available.)
Methane (chemical formula CH4) is one of the simplest hydrocarbons, which literally means “containing hydrogen and carbon.” It reacts very readily with oxygen by burning smokelessly, with carbon dioxide and water as its byproducts. That reactivity paradoxically is what makes its sudden coming and going mysterious. On Earth, there’s plenty of oxygen, but Mars has relatively little in its atmosphere, and what it has seems stable over time. Without lots of organisms to inhale it (like animals on Earth) or exhale it (like plants), there’s nothing really to make oxygen levels fluctuate. However, sudden changes in methane content should be coupled with similar variation in oxygen—and that doesn’t seem to be the case for Mars.
The Curiosity researchers took care to demonstrate the methane they measured was real, and actually on Mars as opposed to a chemical reaction produced by carried aboard the rover. And if the Martian methane is a real thing, it has some interesting implications. Much of Earth’s natural gas was produced when organisms died and were buried or flooded by water. Their decay proceeded without a ready supply of oxygen, producing hydrocarbons like methane instead of oxygen-bearing molecules. Other methane is exhaled by microscopic organisms directly, as in the human gut. (Now you can insert your own fart jokes.) Methane could be produced by microbes on Mars, too, if they exist in enough numbers.
However, life isn’t the only possible source of methane. Saturn’s moon Titan, for example, has lakes, clouds, and weather patterns similar to Earth’s, only involving methane instead of water. Methane is also present in clouds in the atmospheres of Jupiter and Saturn, and in the cold oceans of Uranus and Neptune. None of these places are strong candidates for life, with the possible exception of Titan, so it isn’t as though methane implies living things everywhere.
If methane has a non-biological origin on Mars, though, it’s still very interesting. The relatively rapid fluctuations would mean significant geological activity, even though Mars seems to be pretty inactive for the most part. None of its volcanoes have erupted for billions of years, and it doesn’t have plate tectonics like Earth to provide vents for chemical activity beneath the surface. Other possibilities could be chemical havoc brought by bombarding comets or meteors, or the gradual release of gas trapped underground from earlier, more chaotic days.
Methane wasn’t the only organic compound measured on Mars using Curiosity data. (“Organic” just refers to a molecule with carbon in it, not necessarily referring to living organisms and definitely having nothing to do with food.) The others are difficult to identify, since they reacted with other oxygen-bearing molecules in the soil. However, their presence shows that Mars could have a more complex and evolving chemical story. Perhaps the mysterious Martian methane, and its strange fluctuations, are part of that story.
Many thanks and tips of the bowler hat to Sarah Hörst, who helped with the atmosphere stuff I don’t understand. All errors of course are mine.