Chemical clues can detect aliens unlike any life on Earth

A new method for recognizing the chemical properties of living things could help us discover alien life even if it works differently from life on Earth.

When searching for alien life, scientists usually rely on biosignatures – substances or patterns that can reliably indicate the presence of living organisms. Astronomers can analyze the atmospheres of distant planets to look for molecular biosignatures. But many molecules produced by living things can also arise through geological or chemical processes in the absence of life forms.

The new test, developed by Christopher Carr at the Georgia Institute of Technology and his colleagues, is based on amino acids. Amino acids are the building blocks of proteins, complex molecules on which all life on earth depends. However, amino acids are relatively simple molecules, and they can occur in the absence of life: for example, they are found in lunar soil and on comets and meteorites.

So, instead of just detecting amino acids, Carr and his colleagues reasoned that measuring the reactivity of the molecules in a sample would be a more reliable indicator of living things.

In a non-living system, molecules are formed and destroyed when they react with things in the environment, such as cosmic rays or other molecules, but the more reactive molecules are more likely to disappear. “If you don’t have a system in place to maintain what’s there, the things that tend to get broken are the ones that are more reactive,” says Carr. However, living systems will preferentially retain more reactive molecules because they require them for the chemical processes that support life, leading to a unique signature.

The reactivity of a compound is determined by the arrangement of electrons in the molecule. More reactive molecules have a smaller difference in energy between the outermost electron and the next available space that would be filled by an extra electron during a reaction.

Carr and his team calculated this difference in energy for 64 amino acids, including many not used by life on Earth. They then looked up amino acid abundances in known samples, which came from either abiotic sources, such as meteorites or lunar soil, or from living samples, such as fungi or bacteria, and used their molecular energy calculations to map the statistical distribution of amino acid reactivities. Based on this, they could then assign a probability that the sample was alive or non-living.

Using this method on more than 200 living and non-living samples, they found that it could correctly identify life 95 percent of the time. “The beauty of this approach is that it’s incredibly simple,” says Carr. “It’s very explainable and it’s directly related to physics.”

Life, if it exists elsewhere in the universe, is likely to be based on carbon chemistry and amino acids, operating according to the same chemical reactivity rules as life on Earth, Carr argues, so this method should work for extraterrestrial life, he says. “Life must inherently control when, how and where molecules interact and reactions take place, so that will involve having structures that can regulate the flow of electrons and how things interact electrically,” says Carr.

Using the reactivity of molecules to detect life is not a new idea, but measuring the reactivity in a statistical distribution is, says Henderson Cleaves at Howard University in Washington DC. The method could be part of a suite of life-detecting tools on a future space mission to Mars or one of Saturn’s moons, such as Enceladus, but it would require equipment that can accurately measure molecules and their abundance, which is not easy, says Cleaves.

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