The search for life on other worlds needs a way to sift through the chemistry of their atmospheres.
If another species observed Earth to search for life, they’d look for “smoking gun” chemistry in the atmosphere.
That includes looking for oxygen, since it is created through photosynthesis by plants and some bacteria. So, the key is to look for life-dependent chemical “signals” at exoplanets.
The James Webb Space Telescope (JWST) has studied exoplanet atmospheres, opening up new avenues in the search for life elsewhere. With this and other powerful observatories, scientists are gaining the tools to fine-tune the search for life – and to determine where it doesn’t exist.
However, aside from oxygen gas in our atmosphere, they still haven’t defined any other chemical signals that definitively say “Here’s life!” on other worlds. Maybe searching for the “holy Grail” single signature isn’t the right approach.
In a new paper, researchers propose that the best way forward is to watch the interplay of chemicals in the atmosphere of a planet, developing a system that can even find “life as we don’t know it.”
That’s because there may be cases where other planets may have developed life forms that aren’t exactly like the ones we know from here on Earth.
Taking a Networking Approach
Aside from using oxygen as a prime signal for the existence of life, there are other chemicals that signal life that we know about based on Earth experience.
One is methane, which is generated by life forms here on Earth. However, it also exists in large quantities here as well as on the worlds of the outer solar system, and not all of it results from life processes.
That means remotely detected biosignature gases could be false positives in the search for possible life. For that reason, depending on just one gas as a search tool isn’t going to be very helpful.
In a recently published paper (see link below), authors Theresa Fisher, Estelle Janin, and Sara Imari Walker state that scientists need to develop methods to figure out first whether a gas comes from a biotic source or is produced abiotically (without the need for life).
They propose a chemical reaction network (CRN) approach to analyzing atmospheric gases.
A CRN is basically a set of chemical compounds that react together, and as they do, they are transformed. Scientists can analyze the structure of the network to understand its present and potential behaviors, and use what they learn to create models of the systems they’re studying.
Such analysis could give a great deal of insight into various questions about the search for life elsewhere. For example, a network approach could tell something about the possible life forms generating chemical compounds on a given world.
It could also tell observers if there is combustion of fuels (for example) on the surface or in the air of a planet.
It might give insight into the metabolisms of any life forms on a world. It can even be used to determine if there’s NO life on a world, or if that life isn’t anything like what we know here on Earth.
Examples
In their work, the authors wanted to distinguish biological, abiotic, and anomalous (unidentifiable) sources of biosignature gases in an atmospere using the networking approach. They simulated 30,000 terrestrial (Earth-type) atmospheres. They further divided them into two data sets.
The first set was labeled “Archaean-Earth-like worlds” and “modern Earth-like worlds.” The first type contains worlds with atmospheres very much like Earth’s during the Archaean Eon that occurred between 2 and 4 million years ago. There was little oxygen in the atmosphere, which was mostly methane and ammonia. There may have been a global ocean with a few landmasses, and life forms were limited to bacterial colonies. Archaean Earth conditions could exist on other exoplanets, and they could be habitable to simple forms of life.
Modern Earth-type worlds would have conditions very like our planet today, which has a nitrogen/oxygen atmosphere rich in hydrocarbons released by technological activity. The team simulated such worlds with and without chlorofluorocarbons (which are being phased out on Earth). They examined the network properties in the atmospheres of all these types of workds, to look at – for example – the production of methane as a biotic marker. It can be generated by life in a process called methanogenesis. It’s also the end result of a geological process where hydrothermal activity can transform minerals such as olivine and pyroxene into so-called “serpentine” minerals. Potentially habitable worlds that have methane in their atmospheres could get it from either process, so it’s not a definitive marker for life.
In their paper, the team stated that “Network analysis also distinguishes modern-Earth-like atmospheres with CFC-12 from those without it. Using Bayesian analysis, we demonstrate how atmospheric network statistics can provide stronger confidence for ruling out biological explanations compared to gas abundance statistics alone. Our results confirm how a network theoretic approach allows distinguishing hypotheses about biological, abiotic, and anomalous atmospheric drivers and, importantly, allows ruling out life as we know it as a possible explanation. We conclude with a discussion of how further developing statistical inference methods for spectral data that incorporate network properties could significantly strengthen future biosignature detection efforts.”
Using the Systems Approach to Finding Life
A unified systems approach depends on reliable and ongoing data from exoplanet studies to help scientists figure out the possibilities for life on other planets. Applying it to the study of a biosignature gas (methane) and a technosignature gas (CFC-12) shows that using a CRN analysis can pick up system-level changes that could indicate the presence of life. Those life forms could emit a single gas (such as the bacterial release of oxygen on early Earth), or they could be producing complex chemical mixtures more indicative of technological civilization.
In other words, analyzing the atmosphere by looking at the networking activity between atmospheric chemicals could provide insight into single gases as well as more complex chemical by-products. This analytic approach would also be quite helpful when our telescopes study a world where we don’t have enough information about the underlying biosphere, or if an existing technosphere isn’t well understood.
CRN analysis would help scientists suggest a number of possibilities, including whether or not the biology is more “Earth-like” or more “alien”. It would also help them rule out anomalous signals in the search for habitable worlds and the life that inhabits them.
Written by Carolyn Collins Petersen/Universe Today.
