Martian mystery: Why is an isotope of carbon depleted on the surface of Mars?

The NASA rover Curiosity has been tooling around Mars for nearly a decade now, over 3,300 Martian days. It's been examining the environment of Gale Crater, an enormous impact feature over 150 kilometers wide. Billions of years ago Gale was likely a lake of water, and it was chosen as a landing site specifically for Curiosity to examine conditions there and see if they could have once supported life.

It's not looking for little green Martian critters directly, but it's come up with something of a mystery. One possible — possible — solution is life, but don't jump the gun. It's still possible that abiotic (non-biological) processes are responsible for the anomaly.

Curiosity has a drill on board that allows it to burrow into rocks on the surface, and the samples can then be analyzed using a mass spectrometer, a device that can determine the abundances of various chemicals in the sample. For example it can look at how much carbon dioxide, methane, carbon monoxide, and other molecules are present relative to each other.

It can also look at isotopes of some elements as well. A single element, like carbon, is defined by having six protons in the nucleus of its atoms. If it has 6 neutrons as well then it's called carbon-12 (abbreviated ¹²C). If it has 7 neutrons it's carbon-13 (¹³C). These different isotopes of carbon can tell you different things about the origin and evolution over time of a sample, because they have slightly different properties.

Most carbon atoms have 6 protons and 6 neutrons in the nucleus (red and white balls in this illustration), called carbon-12. Some have an extra neutron, which is called carbon-13. Photo: Dorling Kindersley/Getty Images

For example, the Martian atmosphere has a ratio of ¹²C to ¹³C of about 78 to 1. That's actually a little lower than the natural ratio of those isotopes elsewhere. Why? ¹²C is lighter than ¹³C (by the weight of one neutron). Some processes in the Martian atmosphere give these atoms energy, and for the same amount of energy a ¹²C atom will get a higher velocity than ¹³C because it's lighter. At the top of the atmosphere that means ¹²C is more easily lost to space, increasing the ratio of ¹³C to ¹²C.

Here's where the weirdness comes in (link to paper). Curiosity has taken 30 different samples at five different spots on the Martian surface inside Gale Crater, and the ratio of ¹³C to ¹²C varies wildly from place to place. But in nearly all cases that ratio is lower than it is in the atmosphere.

In other words, some process on Mars is either lowering the amount of ¹³C or enhancing the amount of ¹²C.

A hole drilled into the surface of Mars by the Curiosity rover. The location was the Vera Rubin Hills in Gale Crater, which was once a huge lake. Photo: NASA / JPL-Caltech / MSSS

The scientists who made this study offer three plausible mechanisms to do this. One is that ultraviolet light from the Sun can split apart carbon dioxide molecules in the Martian upper atmosphere into carbon monoxide and an oxygen atom, which then recombine with other molecules to form more complex carbon-based molecules. These processes can very slightly favor the lighter-weight ¹²C atoms, so when these molecules fall to the surface they become incorporated into the rock, enhancing the amount of ¹²C relative to ¹³C.

Another possibility — and I like this one, for obvious reasons — is that every now and again, roughly every 100 million years or so on average, the Sun plows through a cloud of interstellar gas and dust, called a giant molecular cloud. Interstellar dust falls on Earth all the time, but during one of these cloud traversals that number could increase up to 100 times! It's still a tiny amount in total, but as it happens these clouds have more ¹²C in them than ¹³C compared to what we see on Mars. So if this dust rained down to the surface, it again would enhance ¹²C at the expense of the ¹³C ratio.

The third possibility is the most exciting, but to be very clear it's not at all proven. Like the others, it's just an idea: Biology. Ancient Martian bacteria could have produced methane biologically. If that life was like Earthly bacteria, that methane would preferentially have the ¹²C atom in it. The methane would then rise up in the air, get zapped by solar ultraviolet, and rain down as in the example above, becoming incorporated into rocks and raising the amount of ¹²C versus ¹³C in them. I'll note that weird plumes of methane have been detected on Mars, but an explanation for them — geologic or biologic — has yet to be found.

So which is it? Weird chemistry, weird collisions with giant dust clouds, or weird belching Martian bacteria?

Yeah, so that's the issue. It's not possible to tell which one is the best answer, just that they're all plausible. The scientists in the study are very careful to make that point, perhaps knowing that a lot of folks in the media will run away with the eructating bacteria idea.

A Curiosity rover measurement indicated methane in the Mars atmosphere at its location in Gale Crater that was supported by the orbiting Mars Express, but observations by ExoMars Trace Gas Orbiter show nothing. Credit: NASA/JPL-Caltech/MSSS / clipartmag.com / Phil Plait

It's worth noting that none of these processes works well on Earth, so whichever one it is, it's weird. Well, compared to what we're used to. But part of studying Mars is trying to figure out how it's different, and how those processes play out.

The real bummer is that as of right now the evidence is very iffy. It's possible that more samples analyzed by Curiosity can help. Also, the Perseverance rover is a few thousand kilometers from Curiosity in the crater Jezero, and it's not only looking for evidence to see if life was supported there eons ago, but it's also looking for life itself, fossilized or otherwise. Even if it doesn't find any, the samples it's analyzing could help break the stalemate.

Until then, the Mystery of the Martian Carbon Isotope Ratios will continue.

Mars, it turns out, is an interesting place. But it doesn't divulge its secrets easily. Hopefully, with Curiosity and Perseverance, we may just get our answers.