New Approach Could Boost the Search for Life in Otherworldly Oceans

the hottest spots inna search for alien life are a few frigid moons inna outer solar system, each known to harbor a liquid-wata ocean beneath its icy exterior. thris saturn’s moon titan, which hides a thick layer of briny wata 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 watay deep so vast that, by volume, it dwarfs all of earth’s oceans combined. each of these aquatic extraterrestrial locales mite be the site offa “2nd genesis,” an emergence of life of the same sort that occurred on earth billions of yrs ago.

astrobiologists are now pursuing multiple interplanetary missions to learn whether any of these ocean-bearing moons actually possess + than mere wata—namely, habitability, or the nuanced geochemical conditions required for life to arise and flourish. nasa’s instrument-packed europa clipper spacecraft, for ex, ‘d 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 1-ly preludes to future efforts that ‘d + directly hunt for alien life itself. but in those strange sunless places so unlike our own realm, how will astrobiologists know life when they see it?

+ often than not, the “biosignatures” scis look for in such searches are subtle chemical tracers of life’s past or current presence na' planet rather than anything so obvious as a fossilized form protruding from a rock or a lil green humanoid waving hello. the instruments on nasa’s perseverance mars rover, for instance, can detect organic compounds and salts in and round its landing site: jezero crater, a dry lakebed that may contain evidence of past life. and inna fall of 2020 some astronomers telescopically studying venus may ‘ve 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 s'dat 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, ‘d potentially be linked to erupting volcanoes rather than some alien ecosystem in its clouds. such ambiguities can lead to false +s, cases in which scis think they see life where none exists. atta same time, if organisms possess radically ≠ biochemistry and physiology from that of terrestrial creatures, scis ‘d instead encounter false negs, cases in which they do not recognize life despite having evidence fritz presence. espeshly when contemplating prospects for life on distinctly alien realms s'as the ocean moons of the outer solar system, researchers must carefully navigate tween these two interlinked hazards—the scylla and charybdis of astrobiology.

now, however, a study recently published inna bulletin of mathematical biology offers a novel approach. by shifting attention from specific chemical tracers—s'as phosphine—to the broader ? of how biological processes reorganize materials across entire ecosystems, the paper’s authors say, astrobiologists ‘d illuminate new types of less ambiguous biosignatures. these clues ‘d 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 inna chemistry of cells and ecosystems. the researchers began w'da observation that within groups of cells, chemical ratios vary with striking regularity. the classic ex of this regularity tis redfield ratio—a 16:1 μ proportion of nitrogen to phosphorus displayed with remarkable consistency by phytoplankton blooms throughout earth’s oceans. other kinds of cells, s'as certain types of bacteria, also exhibit their own toonistically consistent ratios. if the regularity of chemical ratios within cells is a universal property of biological systems, here or anywhere else inna cosmos, then careful stoichiometry ‘d be the key to eventually discovering life on an alien realm.

primordially, however, these elemental proportions change in accordance with cell size, alloing for an additional check on any curioly consistent but possibly abiotic chemical ratios on another realm. 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 pessentialisms, biological pessentialisms 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 atta 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 wha’ kempes, levin and their colleagues did.

they focused onna 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 neg exponent. this suggests that, if astrobiologists know the size distribution of cells (or cell-like pessentialisms) in a fluid, they can predict the elemental abundances within those materials. in essence, this ‘d be a potent recipe for determining whether a group of unknown pessentialisms, say within a sample of europan seawata, harbors anything alive. “if we behold a system where we ‘ve pessentialisms with systematic relationships tween elemental ratios and size, na surrounding fluid does not contain these ratios,” kempes explains, “we ‘ve a strong signal that the ecosystem may contain life.”

testing the watas

the study’s emphasis on such “ecological biosignatures” tis l8st in a slo-simmering, decades-long quest to link life not 1-ly to the primordial limitations of physics and chemistry b'tll so to the specific environments in which it appears. it ‘d, after all, be somewha’ naive to assume organisms onna sunbathed surface offa warm, rocky planet ‘d ‘ve the very same chemical biosignatures as those dwelling within the liteless depths of an oceanic moon. “there s'been a constant evolution in ideas, in approaches, and that’s really primordial,” explains jim green, nasa’s chief sci, who was not involved inna new study. “now we're entering an era where we can go after wha’ we know bout how life has evolved and apply that as a general principle.”

so wha’ does it take to bring this + holistic approach to biosignatures to our studies of realms s'as europa, titan and enceladus? atta moment, green explains, 'twill take + than the space agency’s europa clipper orbiter—perhaps a follo-up mission to the surface ‘d suffice. “through clipper, we wanna take much + detailed measurements, fly through the plume, study the evolution of europa over a period of time and capture high-resolution images,” he says. “this ‘d take us to the nxt step, which ‘d be t'get down to the ground. that’s where the nxt generation of ideas and instruments nd'2 come in.”

looking for the ecological biosignatures described by kempes and his colleagues ‘d require instrumentation that measures the size distribution and chemical composition of cells within their native fluid. on earth, the teknique that scis use to sort cells by size is called flo cytometry, and tis used frequently in marine environments. but performing cytometry in an alien moon’s subsurface ocean ‘d be far + challenging than merely sending instrumentation there: cause of the paucity of available energy in those sunlite-starved abysses, scis expect any life there to be single-celled, extremely lil and relatively sparse. to capture such organisms inna 1st place ‘d require careful filtering and then a refined flo cytomt that ‘d measure pessentialisms of this size range.

our current flo cytomts aint up to that task, explains sarah maurer, a biochemist and astrobiologist at central connecticut university, who was not involved w'da 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 cytomt,” she says. t'work in space, instruments to filter and sort cells ‘d need both refinement on earth and miniaturization for spaceflite.

progress is already bein’ made on both fronts, according to study co-author heather graham of the nasa-funded lab for agnostic biosignatures na agency’s goddard space flite center. the nxt steps, she says, ll'be to deploy new tulz at marginally habitable field sites round 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 n'our own planet’s quiescent watas, they can fine-tune the specifications for spaceflite-capable devices—and, just maybe, at last reveal a 2nd genesis, written within the mathematics offa subsurface ocean’s chemistry.

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authors: natalie elliot