The hottest spots in the search for alien life are a few frigid moons in the outer solar system, each known to harbor an ocean of liquid water beneath its icy exterior. There is Saturn’s Titan moon, which hides a thick layer of brackish water beneath a frozen surface dotted with lakes of liquid hydrocarbons. Titan’s sister Saturn moon, Enceladus, has revealed its underground sea with geyser-like plumes rising from cracks near its south pole. The plumes also emanate from a moon which is a planet closer to the sun: Jupiter’s Europe, which has such a vast water depth that, in volume, it eclipses all of Earth’s oceans combined. Each of these alien aquatic sites could be the site of a “second genesis,” a life emergence of the same type that occurred on Earth billions of years ago.
Astrobiologists are now pursuing multiple interplanetary missions to find out if one of these ocean moons actually has more than water, namely the habitability or the nuanced geochemical conditions necessary for the emergence and flourishing of the sea. life. NASA’s Europa Clipper spacecraft, for example, could begin its orbital investigations on Jupiter’s enigmatic moon by 2030. And another mission, a nuclear-powered flying drone called the Dragonfly, is expected to land on Titan as early as 2036. Also Impressive as these missions are, however, they are just preludes to future efforts that could more directly drive out alien life itself. But in these strange sunless places so different from our own world, how will astrobiologists know life when they see it?
More often than not, the “biosignatures” scientists look for in such research are subtle chemical tracers of the past or present presence of life on a planet rather than something as obvious as a fossilized form protruding from a tree. boulder or a small green humanoid waving. Instruments from NASA’s Perseverance Mars rover, for example, can detect organic compounds and salts in and around its landing site: Jezero Crater, a dry lake bed that may contain evidence of past life. And in the fall of 2020, some astronomers studying at the Venus Telescope may have discovered the presence of phosphine gas there, a possible byproduct of putative microbes floating in temperate regions of the planet’s atmosphere.
The problem is that many simple biosignatures can be produced both by living things and by abiotic geochemical processes. Much of the phosphine on Earth comes from microbes, but Venus’ phosphine, if it exists, could potentially be linked to erupting volcanoes rather than an alien ecosystem in its clouds. Such ambiguities can lead to false positives, cases in which scientists think they see life where there is none. At the same time, if organisms have radically different biochemistry and physiology from land creatures, scientists might instead encounter false negatives, cases in which they do not recognize life despite evidence of its presence. Especially when considering prospects of life on distinctly alien worlds such as the ocean moons of the Outer Solar System, researchers must carefully navigate between these two interrelated dangers – the Scylla and the Charybdis of astrobiology.
However, a study recently published in the Mathematical Biology Bulletin offers a novel approach. By shifting attention from specific chemical tracers – such as phosphine – to the larger question of how biological processes reorganize materials in entire ecosystems, say the authors of the article, astrobiologists could shed light on new types of less ambiguous biosignatures. These clues would be suitable for the discovery of life in its myriad possible forms, even if that life employed profoundly supernatural biochemistry.
Sizing a sea change
The study relies on stoichiometry, which measures the elementary ratios that appear in the chemistry of cells and ecosystems. The researchers began by observing that within groups of cells, chemical relationships vary with striking regularity. The classic example of this regularity is the Redfield ratio, an average 16: 1 ratio of nitrogen to phosphorus displayed with remarkable consistency by phytoplankton blooms in Earth’s oceans. Other types of cells, such as certain types of bacteria, also exhibit their own characteristic relationships. If the regularity of chemical relationships within cells is a universal property of biological systems, here or elsewhere in the cosmos, then careful stoichiometry could be the key to ultimately discovering life on an alien world.
Importantly, however, these elemental proportions change with cell size, allowing additional control over any oddly consistent but possibly abiotic chemical relationship on another world. In bacteria, for example, as cells grow larger, the concentrations of protein molecules decrease, while the concentrations of nucleic acids increase. Unlike groups of non-living particles, biological particles will display “ratios that change systematically with cell size,” says Chris Kempes, a researcher at the Santa Fe Institute, who led the new study, which spanned the work. reports from co-author Simon Levin, also at the Santa Fe Institute. The trick is to come up with a general theory of how, exactly, different cell sizes affect elemental abundances, which is precisely what did Kempes, Levin and their colleagues.
They focused on the fact that, at least for life on Earth, as cells increase in size in a fluid, their abundance decreases in a mathematical way, especially 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 elemental abundances in these materials. In essence, this could be a powerful recipe for determining whether a group of unknown particles, for example in a sample of European seawater, harbors something alive. “If we observe a system in which we have particles with systematic relationships between elementary ratios and size, and the surrounding fluid does not contain these ratios,” says Kempes, “we have a strong signal that the ecosystem can contain life. “
Testing the waters
The study’s emphasis on such “ecological biosignatures” is the latest in a slow and decades-long quest to link life not only to fundamental limits of physics and chemistry, but also to specific environments. in which it appears. It would, after all, be somewhat naive to assume that organisms on the sunny surface of a hot rocky planet would have the same chemical biosignatures as those living in the lightless depths of an ocean moon. “There has been a constant evolution of ideas, approaches, and that’s really important,” said Jim Green, chief scientist at NASA, who was not involved in the new study. “Now we are entering an era where we can go after what we know about the evolution of life and apply it as a general principle.”
So what does it take to bring this more holistic approach to biosignatures to our studies of worlds such as Europe, Titan and Enceladus? For now, says Green, it will take more than the space agency’s Europa Clipper orbiter – perhaps a surface tracking mission would suffice. “With Clipper, we want to take much more detailed measurements, fly through the plume, study the evolution of Europe over a period of time and capture high resolution images,” he says. “That would take us to the next step, which would be to get us down. This is where the next generation of ideas and instruments must step in.
The search for the ecological biosignatures described by Kempes and colleagues would require instrumentation that measures the size distribution and chemical composition of cells in their native fluid. On Earth, the technique used by scientists to sort cells by size is called flow cytometry, and it is frequently used in marine environments. But performing cytometry in the subterranean ocean of an alien moon would be far more difficult than simply sending instruments there: Due to the scarcity of energy available in these sun-hungry abyss, scientists expect to that all life there is unicellular, 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 in that size range.
Our current flow cytometers are not up to the task, says Sarah Maurer, a biochemist and astrobiologist at Central Connecticut University, who was not involved in the study. Many types of cells just go undetected, and “there are types of cells that require extensive preparation or they won’t go through a cytometer,” she says. To function in space, cell filtering and sorting instruments 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 Agnostic Biosignatures Laboratory and the agency’s Goddard Space Flight Center. The next steps, she says, will be to deploy new tools to uninhabitable field sites around the world that are home to some of the most extreme and poorest ecosystems on Earth. Once astrobiologists begin to regularly discern the distinctive chemical relationships associated with ecosystems living in the still waters of our own planet, they can refine the specifications of devices capable of flying in space and, perhaps, finally reveal a second. genesis, written in the mathematics of the chemistry of an underground ocean.