A revolutionary method for detecting chemical properties of living organisms could unlock the secrets to identifying extraterrestrial life forms, even those with biochemical processes distinct from life on Earth.

In the quest for extraterrestrial life, scientists traditionally depend on biosignatures—substances or patterns that reliably signify the presence of life. By analyzing the atmospheres of distant planets, astronomers search for molecular biosignatures. However, many molecules associated with life can also arise from geological activities, suggesting a careful approach to interpretation.

A novel test developed by Christopher Carr and colleagues from Georgia Tech focuses on amino acids, which serve as fundamental components of proteins that sustain all known life forms. While amino acids can also be produced in lifeless environments, they have been uncovered in lunar soil, comets, and meteorites.

Given this, Carr and his team proposed that analyzing the reactivity of molecules within samples could provide more reliable biological indicators than merely detecting amino acids.

In non-living systems, molecules are continuously formed and destroyed as they react with environmental factors like cosmic rays. The more reactive a molecule, the more likely it is to decompose. “Without stable systems to maintain molecules, their reactivity increases,” explains Carr. However, living systems require reactive molecules, therefore they retain more reactive ones, creating distinct biochemical signatures.

The reactivity of compounds hinges on the arrangement of electrons in the molecules. More reactive molecules exhibit smaller energy differences between their outermost electron and the next available electron space during reactions.

Carr and his team calculated energy differences for 64 amino acids, including those not present in Earth’s biosphere. They analyzed the prevalence of these amino acids in samples sourced from both abiotic processes (like meteorites and lunar soil) and biotic sources (like fungi and bacteria), employing molecular energy calculations to establish a statistical framework for amino acid reactivity. This allowed them to estimate the probability of a sample being alive or inorganic.

After testing over 200 living and nonliving samples, they found their method could accurately identify life with 95 percent certainty. “This approach is remarkably straightforward,” Carr asserts. “It’s easily explainable and directly linked to the principles of physics.”

This reactivity-based method is applicable to the search for extraterrestrial life, as Carr posits that if life exists elsewhere, it likely relies on carbon-based chemistry and amino acids, governed by the same principles of chemical reactivity present on Earth. “Life inherently requires control over the timing, methods, and locations of molecular interactions. Therefore, structures that facilitate electron flow and molecular interactions are essential,” Carr notes.

While utilizing molecular reactivity to identify life isn’t new, measuring reactivity through statistical distributions is an innovative advancement. Henderson Cleaves from Howard University suggests that this method could enhance the toolkit of life-detection instruments on forthcoming space missions to Mars or the moons of Saturn, most notably Enceladus. However, Cleaves notes that the technology to accurately measure molecular abundance is a significant challenge.

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