Thinking Outside the Box in the Search for Extraterrestrial Life, Courtesy of Heavy-Metal Gases

When researchers start scanning the atmospheres of distant worlds for signs of life, the top target will naturally be the mix of gases blanketing our own life-brimming planet. Here on Earth, nitrogen and oxygen dominate, and along with traces of carbon dioxide and methane, offer up compelling evidence for life’s presence. Yet as promising as this brew of so-called biosignatures is, each gas can also be readily produced by abiotic processes—meaning that any detections of them in the air of exoplanets will require rigorous efforts to rule out non-living sources.

A pioneering line of research seeks to remove such ambiguity by identifying certain gases that, if observed, would essentially be a slam dunk for life. This research, led by Edward Schwieterman and Ziming Yang, focuses on organometallic gases. These rare substances paradoxically pair life-critical carbon with toxic heavy metals, including mercury, arsenic, and antimony.

The gases are astrobiologically striking for several reasons. For starters, although few microorganisms on Earth currently emit organometallic gases, there are no known abiotic sources. What’s more, there are examples of microbes across all three domains of life on Earth—Eukarya, Bacteria, and Archaea—that can make the gases, suggesting the biochemistry to do so is fairly basic and thus reasonably likely to emerge in life elsewhere. Furthermore, at points in our planet’s past when the producers of organometallic gases were possibly common, these noxious-to-most gases could have comprised a relatively larger portion of the atmosphere than the trace amounts found today.

“We anticipate that there's a huge variety of environments that are possible on exoplanets that may be conducive for producing these organometallic gasses,” says Schwieterman, an Assistant Professor of Astrobiology at the University of California, Riverside. “We tend to focus on oxygen as a biosignature, which makes a lot of sense, but it’s worth keeping in mind that for half the history of Earth, there was no free oxygen in the atmosphere.”

“Organometallic gases are a very new direction for biosignatures,” added Yang, who is an Associate Professor of Environmental Chemistry at Oakland University. “There are challenges ahead of us, but we think these gases are definitely worth investigating.”

The continuing research is being supported by The Kavli Foundation through Signatures of Life in the Universe, a Scialog initiative led by the Research Corporation for Science Advancement. Schwieterman and Yang were originally selected as Scialog Fellows following the second Scialog: Signatures of Life in the Universe meeting, held in June 2022.

In the time since, Yang and colleagues have conducted a series of lab experiments to help establish plausible atmospheric abundances of some organometallic gases. Those results have then been fed to Schwieterman and his students to build models for how future observatories could hope to detect the unorthodox biosignatures.

On the laboratory front, Yang’s team has collected tundra soil samples from Alaska to perform incubation experiments with different gas compositions. The Arctic samples are ideal for this kind of work because “we know many microbes in Arctic permafrost can do methylation very effectively,” said Yang, referring to the process whereby microbes biochemically add a methyl group (composed of one carbon and three hydrogen atoms) to the inorganic form of heavy metals such as arsenic and mercury. That methylation forms an organometallic gas that can then whisk the heavy metal away into the atmosphere. (While researchers still aren’t sure exactly why some microbes perform this organometallic gas generation, a leading theory is that the activity detoxifies their local environments.) A second benefit of Arctic soil is that it’s “more natural and less affected by human activities,” said Yang, yielding cleaner results.

By varying the gas compositions the samples are exposed to, Yang and colleagues are gauging organometallic gas production in different plausible exoplanetary atmospheres. The work is still in the early stages but has already started teasing out some surprising results. For instance, while less oxygen results in more organometallic gas production, as expected, higher methane concentrations tend to tamp down production of methylmercury, a precursor to dimethylmercury (a potential biosignature compound). That finding suggests to researchers that worlds with methane-rich atmospheres may not be good candidates for the detection of this particular high-percentage biosignature.

“Experimental data like those are useful for Eddie and his students to carry out and test new modeling on organometallic gases,” said Yang. “On our end, we're going to conduct more sets of incubations, changing the concentration of gases to see what could be the threshold for triggering or inhibiting the methylation process further.”

The models employed by Schwieterman’s team are used to identify minimum levels of gases in an atmosphere necessary for astronomical observatories to register a solidly positive detection and the corresponding production rates of those gases needed to maintain these concentrations. Known as photochemical models, the models simulate what happens to mixtures of gases over time as sunlight energizes them and causes chemical reactions. A key feature of any biosignature is that it must be enduring in an atmosphere in the sense that biological activity maintains a certain abundance of the gas, even as natural photodegradation breaks down the gas down.

Schwieterman and colleagues are considering planetary atmospheres representative of both modern-day Earth and a past-era Earth when organometallic gas producers might have had their heyday. Importantly for the modeling, “sunlight” does not only pertain to the specific spectrum emitted by our Sun which, in the grand scale of things, is a relatively uncommon star, comprising only about 12% of in the Milky Way Galaxy. Instead, around 70% of stars are smaller, dimmer, and redder than our Sun, therefore referred to as red dwarfs. Schwieterman’s modeling is accordingly taking these predominant stars into account. Another salient reason is that dim red dwarfs will be the first stars whose planets astronomers will be able to effectively scour for biosignatures.

Some of that groundbreaking work could be the province of JWST, the most powerful space telescope ever launched and which has been advancing the state of cosmic science over the last couple years. However, given the sensitivity likely needed to suss out organometallic gases, the effort may fall to next-generation instruments, such as the thirty-meter-class ground telescopes of the late 2020s and 2030s, along with an exoplanetary-dedicated Habitable Worlds Observatory proposed for the 2040s.

“We’re collecting fundamental data right now,” said Schwieterman. “This preparatory and precursor science is critical for unlocking the full potential of these future missions.”

Overall, the innovative research has made progress on the dimethylmercury front in particular, but with so many overlooked and unexamined organometallic gases still remaining, Schwieterman and Yang said that a community-wide effort will be needed to tackle the full suite.

“There's a wide variety of these organometallic gasses and we have barely scratched the surface of them,” said Schwieterman. “Many of these other ones are relatively unexplored, and that's sort of a big point that we're trying to make.”

To help the broader community collaborate, the researchers are preparing a hypothesis-style paper that will gather the in-many-instances scant information available on organometallic gases in biological (and astrobiological) contexts.

“It’s a really exciting time to be part of this initiative for moving the state of knowledge forward,” said Yang. “Eddie and I are thankful to The Kavli Foundation for helping us pursue this novel area of biosignatures, and we eagerly anticipate how our findings would aid in the search for life research in the years to come.”