Future and its syndication partners may earn a commission when you buy through links in our articles.
Machine learning could help in the search for extraterrestrial life. |Credit: Robert Lea (created with Canva)
This article was originally published at: conversation. This publication contributed the article to Space.com. Expert Voices: Editorials and Insights.
When NASA scientists opened the sample collection container, osiris rex During an asteroid sample mission in late 2023, they discovered something surprising.
Dust and rocks collected from the asteroid Bennu Contains many of the building blocks of lifeall five nucleobases used in DNA and RNA, 14 of the 20 amino acids found in proteins, and a rich collection of others. organic molecules. They are made primarily from carbon and hydrogen and often form the backbone of life's chemistry.
For decades, scientists had predicted that early asteroids might have delivered the ingredients for life to Earth, and these discoveries seemed like promising evidence.
Even more surprisingly, these amino acids benne It was split almost evenly between “left-handed” and “right-handed” forms. Amino acids have two configurations that are mirror images, just like our left and right hands. chiral form.
Almost every biology on Earth requires a left-handed version. If scientists had discovered a strong left-handed excess in Bennu, it would have suggested that life's molecular asymmetry may have been inherited directly from space. Rather, the nearly equal mixtures tell a different story. In other words, life's left-handed preference was likely not imprinted in material transported by the asteroid, but instead emerged later through processes on Earth.
A “chiral” molecule is one that cannot be superimposed with another molecule that is its mirror image, even if it is rotated. |Credit: NASA
If space rocks contain familiar ingredients but no chemical “signatures” left behind by life, identifying the true signature of biology becomes extremely complicated.
These discoveries raise deeper questions with even greater urgency as a new mission. target marsand the moons of Mars sea world of our solar system: How will researchers detect life when only chemistry begins to look “living-like”? If inanimate objects can produce rich, organized mixtures of organic molecules, the traditional labels we use to recognize biology may no longer be sufficient.
as computational scientist As someone who studies biological characteristics, I face this challenge firsthand. My astrobiology research asks how, when exploring other planets, we determine whether a collection of molecules was formed by complex geochemistry or by extraterrestrial biology.
In a new study published in the journal PNAS Nexusmy colleagues and I developed a framework called LifeTracer to answer this question. Rather than looking for a single molecule or structure that proves the existence of biology, the authors looked at the complete chemical patterns of mixtures of compounds preserved in rocks and meteorites to sort out how likely those mixtures contained traces of life.
Identification of potential biosignatures
The key idea behind our framework is that life produces molecules with a purpose, but inanimate chemistry is purposeless. Cells need to store energy, build membranes, and transmit information. non-biological chemistry Substances produced by abiotic chemical processes, even if abundant, follow different rules because they are not formed by metabolism or evolution.
Traditional biosignature approaches rely on specific compounds, such as specific amino acids or lipid structures, or Chiral preferences such as left-handedness.
These signals are powerful, but they are entirely based on molecular patterns. used by life forms on earth. If we Assume that extraterrestrial life forms also use the same chemicals.we risk overlooking biology that is similar but not identical to our own biology, or mistaking the chemistry of inanimate objects for signs of life.
Bennu's results highlight this issue. Although the asteroid samples contained molecules familiar to life, there did not appear to be anything living inside them.
To reduce the risk of assuming that these molecules represent life, we have constructed a unique dataset of organic materials that lie at the border between life and non-life. Eight different samples were used carbon-rich meteorite They preserve 10 samples of Earth's soils and sediments, including abiotic chemicals from the early solar system and the decomposed remains of biomolecules from past or present life forms. Each sample contained tens of thousands of organic molecules, many in low abundance and many whose structures could not be fully determined.
at NASA Goddard Space Flight CenterOur team of scientists crushed each sample, added a solvent, and heated it to extract the organic matter. This process is like brewing tea. They then took the “tea” containing the extracted organic matter and passed it through two filtration columns. separated complex mixtures of organic molecules. The organic matter was then forced into a chamber and bombarded with electrons until it broke into smaller pieces.
Traditionally, chemists used fragments of these chunks as puzzle pieces to reconstruct the structure of each molecule, but the challenge was that each sample contained tens of thousands of compounds.
life tracer
life tracer is a unique approach to data analysis. Rather than reconstructing each structure, it works by taking fragmented puzzle pieces and analyzing them to find specific patterns.
These puzzle pieces are characterized by mass and two other chemical properties and organized into a large matrix that describes the set of molecules present in each sample. A machine learning model is then trained to distinguish between meteorites and terrestrial material on Earth's surface based on the types of molecules present in each.
One of the most common forms of machine learning is called supervised learning. We will take many input-output pairs as examples and learn the rules for proceeding from input to output. Even though there were only 18 samples in these examples, LifeTracer performed very well. It consistently separated abiotic from biological sources.
What mattered most to LifeTracer was not the presence of specific molecules, but the overall distribution of chemical fingerprints found in each sample. Meteorite samples tend to contain more volatile compounds, which evaporate or break down more easily. This reflects the type of chemistry most common in the cold environment of the universe.
Several types of molecules called polycyclic aromatic hydrocarbons were present in both groups, but they had distinct structural differences that the model could resolve. The sulfur-containing compound 1,2,4-trithiolane emerged as a powerful marker for abiotic samples, whereas terrestrial materials contained products formed by biological processes.
These findings suggest that the contrast between life and non-life is not defined by a single chemical cue, but rather by how a whole set of organic molecules is organized. Approaches like LifeTracer open new possibilities for evaluating samples returned by organisms by focusing on patterns rather than assumptions about which molecules life “should” use. mission to mars, The moons Phobos and Deimosa satellite of Jupiter europa and Saturn's satellites Enceladus.
Future samples may contain a mixture of organic matter from multiple sources, both biological and non-biological. Rather than relying only on a few well-known molecules, we can now assess whether the entire chemical landscape is closer to biology or more like random geochemistry.
LifeTracer is not a universal life detector. Rather, it provides a basis for interpreting complex organic mixtures. Bennu's discovery is a reminder that life-friendly chemistry may be pervasive around the world. solar systemBut chemistry alone is not equivalent to biology.
To tell the difference, scientists need all the tools we can build. We need not only better spacecraft and equipment, but also smarter ways to read the stories written in the molecules scientists bring back.
