A novel technique for discerning the chemical characteristics of living entities offers the potential to pinpoint extraterrestrial life, even if its operations diverge from terrestrial biology. The findings of this new study have been documented on arXiv.
In the quest for alien organisms, researchers typically rely upon biosignatures—substances or characteristic patterns that serve as dependable indicators of biological presence. Astronomers currently examine the atmospheres of distant celestial bodies looking for molecular biosignatures. However, a number of molecules produced by living systems can also arise through purely geological or chemical phenomena where no life exists.
The new assay, devised by Christopher Carr of the Georgia Institute of Technology and his collaborators, focuses on amino acids. These compounds are the fundamental components of proteins, the intricate molecules upon which all known life on Earth depends. Nevertheless, amino acids are relatively straightforward molecules, and they can be present without life; for instance, they have been detected in lunar regolith, comets, and meteorites.
Consequently, instead of merely detecting the presence of amino acids, Carr and his associates posited that gauging the reactivity of the molecules within a sample would constitute a more dependable marker for the existence of living organisms.
In an abiotic setting, molecules are both created and degraded through interactions with environmental factors, such as cosmic rays or other compounds, but more reactive molecules are more prone to degradation. “If you don’t have a system maintaining what’s there, the things that are more reactive are going to disappear,” states Carr. Biological systems, conversely, will favor the preservation of more reactive molecules, as these are crucial for the chemical operations that sustain life, resulting in a distinctive signature.
A compound’s propensity to react is determined by the arrangement of electrons within its structure. Species that are more reactive exhibit a smaller energy gap between their outermost electron and the next available orbital space that an additional electron would occupy during a reaction.
Carr and his team computed this energy gap for 64 amino acids, encompassing many not utilized by Earth-based life. They subsequently characterized the amino acid composition of known samples derived either from lifeless origins, such as meteorites or lunar material, or from living entities, like fungi or bacteria. Employing their molecular energy calculations, they mapped the statistical distribution of amino acid reactivity. Based on this, they could ascertain the probability of a given sample being biogenic or abiogenic.
Applying this methodology to over 200 samples, both alive and inert, they found it accurately classified life 95% of the time. “The beauty of this approach is its sheer simplicity,” notes Carr. “It is easily rationalized and directly tied to physics.”
Life, regardless of where it might exist throughout the cosmos, is likely grounded in carbon and amino acid chemistry and operates under the same governing principles of chemical responsiveness as life on our planet, Carr suggests, implying this technique should be effective for detecting extraterrestrial biology. “Life inherently needs to control when, how, and where molecules combine and reactions occur, so this necessitates architectures capable of governing electron flow and how substances interact electrically,” Carr elaborates.
Utilizing molecular reactivity to identify life is not an entirely novel concept, but quantifying reactivity across its statistical distribution is a fresh contribution, comments Henderson Cleaves of Howard University in Washington. This methodology could be incorporated into the suite of life-detection instruments on future deep-space explorations targeting Mars or one of Saturn’s moons, such as Enceladus. However, this prospect hinges upon the development of apparatus capable of precisely measuring molecules and their concentrations, which presents a significant technical hurdle, Cleaves observes.
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