When NASA scientists opened the sample container from the OSIRIS-REx asteroid sample mission in late 2023, they found something astonishing.
Dust and rock collected from the asteroid Bennu contained many of the building blocks of life, including all five nucleobases used in DNA and RNA, 14 of the 20 amino acids found in proteins, and a rich collection of other organic molecules. These are mainly built of carbon and hydrogen, and they often form the backbone of life’s chemistry.
For decades, scientists have predicted that early asteroids may have delivered the ingredients for life to Earth, and these findings seemed like promising evidence.
Even more surprisingly, these amino acids from Bennu were split almost evenly between “left-handed” and “right-handed” forms. Amino acids come in two mirror image configurations, just like our left and right hands, called chiral forms.
On Earth, almost all biology requires the left-handed versions. If scientists had found a strong left-handed excess in Bennu, it would have suggested that life’s molecular asymmetry may have been inherited directly from space. Instead, the nearly identical mix points to a different story: Life’s left-handed preferences likely emerged later, through processes on Earth, rather than being imprinted into the material delivered by asteroids.
A ‘chiral’ molecule is one that cannot be superimposed on another that is its mirror image, even if you rotate it. NASA
If space rocks can carry known ingredients but not the chemical “signature” that life leaves behind, then identifying the true signatures of biology becomes extremely complicated.
These discoveries raise a deeper question – one that becomes more pressing as new missions target on MarsThe Mars moons and sea worlds of our solar system: How do scientists discover life when chemistry alone begins to look “natural”? If non-living materials can produce rich, organized mixtures of organic molecules, the traditional signs we use to recognize biology may no longer be enough.
Seam a computational scientist When I study biological signatures, I face this challenge head-on. In my astrobiology work, I ask how to determine whether a collection of molecules was formed by complex geochemistry or by extraterrestrial biology, when exploring other planets.
In a new study in the journal PNAS Nexusmy colleagues and I developed a framework called LifeTracer to answer this question. Rather than searching for a single molecule or structure that proves the presence of biology, we sought to classify how likely mixtures of compounds preserved in rocks and meteorites were to contain traces of life by examining the complete chemical patterns they contain.
Identifying potential biosignatures
The key idea behind our framework is that life produces molecules with purpose, whereas non-living chemistry does not. Cells must store energy, build membranes and transmit information. Abiotic Chemistry produced by non-living chemical processes, even when abundant, follows different rules because it is not shaped by metabolism or evolution.
Traditional biosignature approaches focus on searching for specific compounds, such as certain amino acids or lipid structures, or chiral preferences, such as left-handedness.
These signals can be powerful, but they are based solely on the molecular patterns used by life on earth. If we assume that alien life uses the same chemistrywe risk missing biology that is similar—but not identical—to our own, or misidentifying nonliving chemistry as a sign of life.
The Bennu results highlight this issue. The asteroid sample contained molecules known to support life, but nothing in it appears to have been alive.
To reduce the risk of assuming these molecules indicate life, we assembled a unique dataset of organic materials right on the dividing line between life and non-life. We used samples from eight carbon-rich meteorites which preserves abiotic chemistry from the early solar system, as well as 10 samples of soils and sedimentary materials from Earth, which contain the degraded remains of biological molecules from past or present life. Each sample contained tens of thousands of organic molecules, many present in low abundance and many whose structures could not be fully identified.
At NASA Goddard Space Flight Centerour team of scientists crushed each sample, added solvent and heated it to extract organic matter – this process is like brewing tea. We then took the “tea” containing the extracted organics and passed it through two filtration columns which separated the complex mixture of organic molecules. The organic substances were then pushed into a chamber where we bombarded them with electrons until they broke down into smaller fragments.
Traditionally, chemists use these mass fragments as puzzle pieces to reconstruct each molecular structure, but having tens of thousands of compounds in each sample presented a challenge.
LifeTracer
LifeTracer is a unique approach to data analysis: It works by taking in the fragmented puzzle pieces and analyzing them to find specific patterns, rather than reconstructing each structure.
It characterizes these puzzle pieces by their mass and two other chemical properties and then organizes them into a large matrix that describes the set of molecules present in each sample. It then trains a machine learning model to distinguish between the meteorites and the terrestrial materials from the Earth’s surface, based on the type of molecules found in each.
One of the most common forms of machine learning is called supervised learning. It works by taking many input and output pairs as examples and learning a rule for going from input to output. Even with only 18 samples like these examples, LifeTracer performed remarkably well. It consistently distinguished abiotic from biotic origins.
What mattered most to LifeTracer was not the presence of a specific molecule, but the general distribution of chemical fingerprints found in each sample. Meteorite samples tended to contain more volatile compounds—those that vaporize or break apart more easily—reflecting the kind of chemistry most common in the cold environment of space.
This figure shows compounds identified by LifeTracer, highlighting the most predictive molecular fragments that distinguish abiotic from biotic samples. The compounds in red are linked to abiotic chemistry, while the blue compounds are linked to biotic chemistry. Saeedi et al., 2025, CC BY-NC-ND
Some types of molecules, called polycyclic aromatic hydrocarbons, were present in both groups, but they had characteristic structural differences that the model could analyze. A sulfur-containing compound, 1,2,4-trithiolane, emerged as a strong marker for abiotic samples, while terrestrial materials contained products formed through biological processes.
These findings suggest that the contrast between life and non-life is not defined by a single chemical clue, but by how a whole series of organic molecules are organized. By focusing on patterns rather than assumptions about which molecules life “should” use, approaches such as LifeTracer open up new possibilities for evaluating samples returned from mission to Mars, its moons Phobos and DeimosJupiter’s moon Europa and Saturn’s moon Enceladus.
The Bennu asteroid sample return capsule used in the OSIRIS-REx mission. Keegan Barber/NASA via AP
Future samples are likely to contain mixtures of organics from multiple sources, some biological and some not. Instead of relying only on a few known molecules, we can now assess whether the entire chemical landscape looks more like biology or random geochemistry.
LifeTracer is not a universal life detector. Rather, it provides a basis for interpreting complex organic mixtures. The Bennu discoveries remind us that life-friendly chemistry may be widespread throughout the solar system, but that chemistry alone does not equal biology.
To tell the difference, scientists will need all the tools we can build — not just better spacecraft and instruments, but also smarter ways to read the stories written in the molecules they bring home.
By Amirali Aghazadeh, Assistant Professor of Electrical and Computer Engineering, Georgia Institute of Technology. This article is republished from The Conversation under a Creative Commons license. Read original article.
