For over a century, science fiction authors have imagined that humanities’ first contact with extraterrestrial life would come via intelligent humanoid aliens arriving on Earth in technologically advanced spaceships, likely in pursuit of interstellar conquest. If the latest Red Planet news from NASA turns out to be that first contact, the reality is far more mundane than the fiction. There may be Martians, but if so, we’ve only seen their gas, not their ships. Even calling it “their” gas may be getting ahead of ourselves.
Over the weekend, NASA’s Science Mission Directorate announced that the Mars Curiosity Rover has detected an increase in the methane gas concentration of the Red Planet’s atmosphere at its position. This marks the third such surge of methane observed by Curiosity, confirming its earlier observations and adding intrigue to the notion that the methane could be biological in origin.
But the default setting of scientists is skepticism. Measurements have to be defended. If there’s any reasonable doubt, then the results are not fully accepted. Indeed, the general philosophy in science is that even well-established, observed truths are only true to the extent that they have been tested. Not only were the first detections of methane in Mars’ atmosphere — 15 years ago, by an orbiter and Earth-based telescopes — not considered to be a detection of extraterrestrial life, they weren’t even accepted as a detection of methane!
Now that the search for life is heating up, what should we expect?
But then six years ago, in 2013, only one year into its mission, the Curiosity Rover began routinely measuring methane concentrations from the Martian surface. For the first eight months of its mission, it found a very low methane background level of less than one part per billion, or 1ppb.
Curiosity’s first detection of a methane spike occurred at Gale crater in June 2013 when it measured 6ppb. The next day, on June 16, 2013 the orbiting Mars Express Explorer measured 15ppb above Curiosity’s location in Gale crater. Six months later, Curiosity again detected increased methane levels up to 9ppb that lasted for two months before subsiding. Long term measurements showed that the background methane level was stable above zero and varied seasonally, increasing in concentration by more than a factor of three from ~0.2ppb to ~0.65ppb. Then, no more methane outbursts were observed for the next five years — until last week. The New York Times reported that Curiosity has measured a new methane spike at 21ppb, or three times the 2013 level.
This is exciting news — with some caveats. Some scientists have wondered whether the Curiosity’s methane readings originate from gas on Mars, or from the Rover’s own onboard methane. (Yes, Curiosity did bring with it a small amount of methane, even though it’s unmanned.) But an even bigger reason for the uncertainty is that the European-Russian ExoMars explorer, which carries the most sensitive instrument yet for detecting methane in Mars’ atmosphere, announced just two months ago that they’d measured only the tiniest traces of methane during its four months of observations. The background level measured from space by ExoMars Trace Gas Orbiter instrument (0.012ppb) is less than one-thirtieth of that observed by Curiosity!
So what’s really going on? Is methane being released on Mars? And if there is, is it detectable only at low altitudes? On balance, the evidence points toward a positive detection of methane on the Red Planet, though we still need a coherent story that accounts for the disparate observations. Are we receiving a false negative from ExoMars, or do the observations reflect the variability of nature?
And is the detection of methane a sign of life?
Methane is a potential biosignature — a molecule that may indicate the presence of life. It’s particularly interesting because chemical reactions and sunlight destroy it. It’s estimated that methane can only survive for about 400 years in the Martian atmosphere. This suggests that any methane detected has been added recently. So how did it get there?
Here on Earth, most methane is produced by life. You’re life. Methane is likely being produced in your gut right now by microbes called methanogens from the domain Archaea. They’re single-celled organisms like bacteria and also lacking a cellular nucleus; but Archaea were originally thought to be even more ancient.
Methane (CH4) consists of one carbon atom and four hydrogen atoms bonded together. Your gut provides a perfect environment (water, organic matter, and low oxygen) for Archaea to produce the simple methane molecule along one of several paths. In the simplest, one carbon dioxide molecule and four hydrogen molecules are broken apart and reassembled to form one methane molecule and two water molecules: CO2 + 4H2 → CH4 + 2H2O. Another process involves breaking down a single large organic molecule to form two simpler molecules: CH3COOH → CH4 + CO2. (And that’s as hard as I’ll geek out on the chemistry, I promise.)
The absence of oxygen is critical as it’s the greedy, badass bully atom that go where it want and do what it want like a young CEO when it comes to chemical reactions — just like in “Suge” by DaBaby. I know, I’m random. (Listen to the song; you’ll get it.)
But nature also has non-biological ways of producing methane. This type of methane is known as abiotic methane. We currently have not determined if Mars’ methane is biological or abiotic.
Abiotic methane differs from biological methane in that it’s generated from geological processes rather than by life. The two most common sources are geochemical reactions between rocks and water, and gasses from volcanic magma. Mars does not appear to have magma or any active volcanic activity at all, so the most likely geological source is geochemical reactions, e.g. a common methane-producing reaction that occurs between the rock-forming mineral olvine and water. Olvine is not the only mineral that supports such reactions. And we know that there’s water on Mars, though not necessarily in the locations where Curiosity is detecting methane. This all means we can’t yet rule out a geochemical explanation for Curiosity’s methane detections.
Another potential source of Mars’ methane could be reservoirs of fossil methane produced biologically millions or billions of years ago that are still trapped underground but occasionally burst to the surface. While certainly not as exciting as colonies of biomass currently living in the Martian crust, the conclusive detection of fossil life byproducts would surely usher in major scientific and cultural shifts worldwide.
Space agencies around the world are planning missions to solar system bodies known to harbor abundant liquids: Titan, Europa, Enceladus, Mars. We also have the Transiting Exoplanet Survey Satellite (TESS) searching out planets around the brightest stars in the sky. These are the stars that have enough light for us to use large ground-based telescopes — and when it becomes available, the James Webb Space Telescope — to search the planets’ atmospheres for biosignature molecules. Now that the search for life is heating up, what should we expect? Do we expect to find life? And do we expect to find intelligent life?
It appears that under the right conditions, the universe doesn’t mess around.
As a physicist, my job is to observe the universe and report what I see. I observe that life exists, and that intelligent life exists. So yes. I consider it 100% proven that both exist in the universe.
What makes Earth different — and possibly singular — is that its life is exposed to the outside universe. The universe is a very harsh environment for life. All but the hardiest living things are easily damaged by light and fast-moving matter particles, and can only exist in narrow ranges of temperature and pressure. This means that life can only exist in environments that are protected from space — in the nooks, crannies, and fluids of the universe. Earth’s thin atmosphere and strong magnetic field allows us to live under what is essentially a planetary layer of transparent protective glass. It may be that most of the universe’s life is locked away under thick atmospheres or in liquids trapped under miles of ice. But our atmosphere is a virtually transparent window that we can see through — and know there’s a larger universe out there to explore and understand.
Evidence suggests that life has been present on Earth almost from its formation. It appears that under the right conditions, the universe doesn’t mess around. The chemical reaction we know as life springs forth quickly and doggedly holds on — surviving bombardments, major extinction events, ice ages, and changes in the atmosphere’s chemical composition.
Perhaps life took hold on Mars around the same time as it did on Earth and is now holding on underground. I wouldn’t be surprised. But we only accept as true that which has been observed to be true. The results coming from Mars are intriguing. But for now we observe, analyze, plan, and keep exploring.