The hottest spots in the search for alien life are a few frigid moons in the outer solar system, each known to harbor a liquid-water ocean beneath its icy exterior. There is Saturn’s moon Titan, which hides a thick layer of briny water beneath a frozen surface dotted with lakes of liquid hydrocarbon. Titan’s sister Saturnian moon Enceladus has revealed its subsurface sea with geyserlike plumes venting from cracks near its south pole. Plumes also emanate from a moon that is one planet closer to the sun: Jupiter’s Europa, which boasts a watery deep so vast that, by volume, it dwarfs all of Earth’s oceans combined. Each of these aquatic extraterrestrial locales might be the site of a “second genesis,” an emergence of life of the same sort that occurred on Earth billions of years ago.
Astrobiologists are now pursuing multiple interplanetary missions to learn whether any of these ocean-bearing moons actually possess more than mere water—namely, habitability, or the nuanced geochemical conditions required for life to arise and flourish. NASA’s instrument-packed Europa Clipper spacecraft, for example, could begin its orbital investigations of Jupiter’s enigmatic moon by 2030. And another mission, a nuclear-fueled flying drone called Dragonfly, is scheduled to touch down on Titan as early as 2036. As impressive as these missions are, however, they are only preludes to future efforts that could more directly hunt for alien life itself. But in those strange sunless places so unlike our own world, how will astrobiologists know life when they see it?
More often than not, the “biosignatures” scientists look for in such searches are subtle chemical tracers of life’s past or current presence on a planet rather than anything so obvious as a fossilized form protruding from a rock or a little green humanoid waving hello. The instruments on NASA’s Perseverance Mars rover, for instance, can detect organic compounds and salts in and around its landing site: Jezero Crater, a dry lakebed that may contain evidence of past life. And in the fall of 2020 some astronomers telescopically studying Venus may have teased out the presence of phosphine gas there, a possible by-product of putative microbes floating in temperate regions of the planet’s atmosphere.
The trouble is that many simple biosignatures can be produced both by living things and through abiotic geochemical processes. Much of the phosphine on Earth comes from microbes, but Venus’s phosphine, if it exists at all, could potentially be linked to erupting volcanoes rather than some alien ecosystem in its clouds. Such ambiguities can lead to false positives, cases in which scientists think they see life where none exists. At the same time, if organisms possess radically different biochemistry and physiology from that of terrestrial creatures, scientists could instead encounter false negatives, cases in which they do not recognize life despite having evidence for its presence. Especially when contemplating prospects for life on distinctly alien worlds such as the ocean moons of the outer solar system, researchers must carefully navigate between these two interlinked hazards—the Scylla and Charybdis of astrobiology.
Now, however, a study recently published in the Bulletin of Mathematical Biology offers a novel approach. By shifting attention from specific chemical tracers—such as phosphine—to the broader question of how biological processes reorganize materials across entire ecosystems, the paper’s authors say, astrobiologists could illuminate new types of less ambiguous biosignatures. These clues would be suitable for discovering life in its myriad possible forms—even if that life employed profoundly unearthly biochemistry.
Sizing Up a Sea Change
The study relies on stoichiometry, which measures the elemental ratios that appear in the chemistry of cells and ecosystems. The researchers began with the observation that within groups of cells, chemical ratios vary with striking regularity. The classic example of this regularity is the Redfield ratio—a 16:1 average proportion of nitrogen to phosphorus displayed with remarkable consistency by phytoplankton blooms throughout Earth’s oceans. Other kinds of cells, such as certain types of bacteria, also exhibit their own characteristically consistent ratios. If the regularity of chemical ratios within cells is a universal property of biological systems, here or anywhere else in the cosmos, then careful stoichiometry could be the key to eventually discovering life on an alien world.
Importantly, however, these elemental proportions change in accordance with cell size, allowing for an additional check on any curiously consistent but possibly abiotic chemical ratios on another world. In bacteria, for instance, as cells get larger, concentrations of protein molecules decrease, whereas concentrations of nucleic acids increase. In contrast to groups of nonliving particles, biological particles will display “ratios that systematically change with cell size,” explains Santa Fe Institute researcher Chris Kempes, who led the new study, which expanded on prior work by co-author Simon Levin, also at the Santa Fe Institute. The trick is to devise a general theory of how, exactly, the various sizes of cells affect elemental abundances—which is precisely what Kempes, Levin and their colleagues did.
They focused on the fact that, at least for Earth life, as cell sizes increase in a fluid, their abundance decreases in a mathematically patterned way—specifically, as a power law, the rate of which can be expressed by a negative exponent. This suggests that, if astrobiologists know the size distribution of cells (or cell-like particles) in a fluid, they can predict the elemental abundances within those materials. In essence, this could be a potent recipe for determining whether a group of unknown particles, say within a sample of Europan seawater, harbors anything alive. “If we observe a system where we have particles with systematic relationships between elemental ratios and size, and the surrounding fluid does not contain these ratios,” Kempes explains, “we have a strong signal that the ecosystem may contain life.”
Testing the Waters
The study’s emphasis on such “ecological biosignatures” is the latest in a slow-simmering, decades-long quest to link life not only to the fundamental limitations of physics and chemistry but also to the specific environments in which it appears. It would, after all, be somewhat naive to assume organisms on the sunbathed surface of a warm, rocky planet would have the very same chemical biosignatures as those dwelling within the lightless depths of an oceanic moon. “There has been a constant evolution in ideas, in approaches, and that’s really important,” explains Jim Green, NASA’s chief scientist, who was not involved in the new study. “Now we are entering an era where we can go after what we know about how life has evolved and apply that as a general principle.”
So what does it take to bring this more holistic approach to biosignatures to our studies of worlds such as Europa, Titan and Enceladus? At the moment, Green explains, it will take more than the space agency’s Europa Clipper orbiter—perhaps a follow-up mission to the surface would suffice. “Through Clipper, we want to take much more detailed measurements, fly through the plume, study the evolution of Europa over a period of time and capture high-resolution images,” he says. “This would take us to the next step, which would be to get down to the ground. That’s where the next generation of ideas and instruments need to come in.”
Looking for the ecological biosignatures described by Kempes and his colleagues would require instrumentation that measures the size distribution and chemical composition of cells within their native fluid. On Earth, the technique that scientists use to sort cells by size is called flow cytometry, and it is used frequently in marine environments. But performing cytometry in an alien moon’s subsurface ocean would be far more challenging than merely sending instrumentation there: Because of the paucity of available energy in those sunlight-starved abysses, scientists expect any life there to be single-celled, extremely small and relatively sparse. To capture such organisms in the first place would require careful filtering and then a refined flow cytometer that would measure particles of this size range.
Our current flow cytometers are not up to that task, explains Sarah Maurer, a biochemist and astrobiologist at Central Connecticut University, who was not involved with the study. Many kinds of cells simply do not get picked up, and “there are cell types that require extensive preparation or they won’t go through a cytometer,” she says. To work in space, instruments to filter and sort cells would need both refinement on Earth and miniaturization for spaceflight.
Progress is already being made on both fronts, according to study co-author Heather Graham of the NASA-funded Laboratory for Agnostic Biosignatures and the agency’s Goddard Space Flight Center. The next steps, she says, will be to deploy new tools at marginally habitable field sites around the globe that play host to some of Earth’s most extreme and impoverished ecosystems. Once astrobiologists begin routinely discerning the distinctive chemical ratios associated with living ecosystems in our own planet’s quiescent waters, they can fine-tune the specifications for spaceflight-capable devices—and, just maybe, at last reveal a second genesis, written within the mathematics of a subsurface ocean’s chemistry.