If a Planet Has a Lot of Methane in its Atmosphere, Life is the Most Likely Cause

The ultra-powerful James Webb Space Telescope will launch soon. Once it’s deployed, and in position at the Earth-Sun Lagrange Point 2, it’ll begin work. One of its jobs is to examine the atmospheres of exoplanets and look for biosignatures. It should be simple, right? Just scan the atmosphere until you find oxygen, then close your laptop and head to the pub: Fanfare, confetti, Nobel prize.

Of course, Universe Today readers know it’s more complicated than that. Much more complicated.

In fact, the presence of oxygen is not necessarily reliable. It’s methane that can send a stronger signal indicating the presence of life.

Oxygen might seem like the obvious thing to look for in a planet’s atmosphere when searching for signs of life, but that’s not the case. Its presence or lack thereof is not a reliable indicator. Earth’s history makes that clear.

Modern Earth’s atmosphere contains about 21% oxygen, and we know that most of it comes from organisms in the planet’s oceans. But there’s a hitch: once cyanobacteria on ancient Earth started producing oxygen as a byproduct of photosynthesis, it still took an awfully long time before the atmosphere became oxygenated, possibly a billion years.

What if we examined an exoplanet, found no oxygen, then moved on, not realizing that there was life down there, at the beginning of oxygenating that world? What if we were a billion years too early, and life hasn’t oxygenated the exoplanet’s atmosphere yet? Rocky planets have many oxygen sinks, and biologically-produced oxygen wouldn’t be found free in the atmosphere until those sinks were becoming saturated.

That’s what happened on Earth, and that’s what we expect might happen on other rocky worlds. On Earth, geological activity churns magma up from the mantle onto the crust. Much of the mantle material, like iron, for example, bonds with atmospheric oxygen, pulling it out of the atmosphere.

This is one reason that planetary scientists focus on other things, like methane (CH4). In a new paper, researchers examined the potential for methane to signal biological activity. They say that abundant methane in a planet’s atmosphere is unlikely to come from volcanoes and most likely has a biological origin.

The paper’s title is “Abundant Atmospheric Methane from Volcanism on Terrestrial Planets Is Unlikely and Strengthens the Case for Methane as a Biosignature.” The lead author is Nicholas Wogan from the Dept. of Earth and Space Sciences, University of Washington, and from the Virtual Planetary Laboratory at the U of W. The paper is published in The Planetary Science Journal.

Detecting potential biosignatures like methane in the atmospheres of distant exoplanets is tricky. But once something like methane is detected, harder work awaits. Its presence must be investigated in the context of the planet itself.

Biosignature researchers haven’t been waiting idly for the James Webb Space Telescope to launch. They’ve put a lot of thought into detecting biosignatures with the telescope. Scientists have proposed that planetary atmospheres with abundant methane and carbon dioxide in disequilibrium could be a strong biosignature. In their paper, the authors point out that “…few studies have explored the possibility of nonbiological CH4 and CO2 and related contextual clues.” In this case, nonbiological means volcanoes.

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