Eureka? Scientists’ first hints of life on other planets may not be so obvious

When Corrine Rojas saw the speckled rock on the surface of Mars for the first time, she didn’t say “Eureka!,” she said, “Now Percy’s cooking with gas!” Rojas, who is a mission science operations specialist at NASA Goddard and former instrument operations engineer for the Perseverance rover (fondly known as “Percy”), knew it was something special right away. “These types of features are often associated with microbial life here on Earth. It’s the kind of rock you can pick up around a river or stream.” And the rover’s science mission is just getting started, she emphasized.

Two major things make this rock, which has been nicknamed “Cheyava Falls” by the NASA Perseverance rover team, so special. The first is the small “leopard spot” specks visible above, between the thick bands of white. To the geologist’s eye, they resemble a phenomenon on Earth which occurs when subsurface microbes become fossilized, according to NASA’s press release. But we won’t know if they are fossils or not for sure until a return mission brings the rover’s sample back to Earth.

The second discovery is more subtle, but equally exciting: the presence of a nearby mineral called olivine. Olivine is a favorite of proponents of the theory that life began in the oceans around hydrothermal vents. When water from deep under the ocean floor meets the olivine inside deep-sea vents, the reaction creates a lot of hydrogen. It becomes much more alkaline, or basic, than the seawater outside the vent. Mixing two is like taking an antacid for heartburn: it produces a quick, warm chemical reaction as the acidic seawater is neutralized. This familiar process could have been the very first source of energy for rudimentary cells.

Cheyava Falls’ fascinating two-for-one geology is what scientists call a potential biosignature. Biosignatures are things that are produced by living creatures. A fossil is a biosignature. The carbon dioxide we exhale is a biosignature. The screen you’re reading this article on is a biosignature. 

Some biosignatures are more compelling than others. Carbon dioxide may be a waste gas here on Earth, but there’s also quite a lot of it on notoriously hostile Venus. On the other hand, if Perseverance rolled over a Nokia phone one day, that would be considered fairly compelling evidence for aliens on the cusp of discovering the Find My Phone feature.

But when you can’t dropship a rover to the planet you want to study, the potential for compelling biosignatures begins to dry up. Suddenly, a detection of carbon dioxide becomes extremely appealing. But carbon dioxide doesn’t mean life. And methane doesn’t mean life. Even things like water and oxygen don’t mean life. 

Here’s why: all these gasses have what we call abiotic, or non-living sources. Carbon dioxide is all over the place. So is water, found on every planet in our Solar System (though usually not in liquid form). Methane is a waste gas of Earth creatures, but it’s also put out by volcanoes. Oxygen is produced via photosynthesis, but also by a process called photolysis in which solar radiation breaks apart molecules, including splitting water into hydrogen and oxygen. If a planet once had an ocean, and that ocean evaporated, the world left behind might resemble Dune’s Arrakis: breathable, but without a drop to drink. In fact, there are quite a few scenarios, illustrated below, in which oxygen can appear without a biological origin.

This may seem like a dead end, but it’s not. An astrobiologist must consider the whole picture, not just one gas at a time. From this big picture, a solution emerges: biosignature pairs. Some gasses simply do not coexist in nature unless something is actively producing both. On our planet, we have a bit of methane, which comes from both natural processes and human endeavors like agriculture. But oxygen and carbon dioxide don’t like to hang out with methane. When they come in contact, they combine into other gasses, like water, carbon dioxide, and organic molecules. So if you see methane and oxygen together, or methane and carbon dioxide together, that is a far stronger indicator of potential life than either gas alone.

In the absence of our Martian cell phone, or a radio broadcast from ET, or a visit from the Vulcans, humanity’s first encounter with alien life may very well come in the form of a biosignature pair detection. And what a Eureka! moment that would be.

Or would it? Such a result would undoubtedly take years of careful data collection. Astrobiologists often rely on a method called transit spectroscopy to study planets beyond the Solar System, called exoplanets. This process analyzes starlight that passes through an exoplanet’s atmosphere on its way to Earth, and it is challenging work. Even for JWST, some molecules, like oxygen, possess signatures outside of its color range, just like if you tried to see infrared light with your eyes. For the pair without oxygen (methane and carbon dioxide), a strong detection à la the famous 2012 “five sigma” Higgs Boson detection would take — at the absolute barest, barest minimum — about 60 hours of JWST time spread over several years. 

What would that five sigma detection look like for scientists? “We would get super excited, and then very skeptical,” said Dr. Jane Greaves, an astrobiologist and professor at Cardiff University. “People would start looking immediately for abiotic sources of the signal.” Dr. Greaves hopes the follow-up would include not just more astronomical data but lab work, too. “In a lab, you can actually recreate the environment to see if it’s realistic or not,” she said. She’s also looking forward to NASA’s next great space telescope, the Habitable Worlds Observatory (HWO). HWO, which is slated to launch in the early 2040’s, has been designed with searching for life on Earth-like planets in mind, and will provide our clearest look at potentially living worlds yet.

Ultimately, the work of finding life on another planet may be far more complex than imagined, involving scientists of all stripes to get the big picture right. Whether we are roving the dunes of Mars or pointing a telescope at a distant star, the discovery of a lifetime may not be very cinematic at all. But hopefully, one day soon, missions like Perseverance and the Habitable Worlds Observatory will bring us a flurry of debate that settles on the side of life — history’s greatest Eureka!