New observatories and spacecraft missions are probing environments in our solar system that could potentially host life but have long remained hidden. Icy moons like Saturn’s Enceladus and Jupiter’s Europa likely contain oceans beneath frozen outer shells. But a layer of ice prohibits space probes from sampling them directly.
Exploring these icy moons is almost forensic: Their surfaces keep a partial record of inaccessible interiors. Scientists need tools that can help them figure out whether evidence of life lies beneath without observing it directly.
I’m a planetary scientist, and my colleagues and I have developed a tool that could help evaluate whether an environment has the right conditions for life, based on patterns in the types of molecules found in a sample.
Looking for life’s fingerprints
The search for life often begins with organic molecules: the carbon-based molecules from which life on Earth is built. Two especially important families of molecules are amino acids, which cells use to build proteins, and fatty acids, which help form cell membranes.
Yet these molecules are not unique to life – they can also form through nonbiological chemistry. Scientists have previously detected them in asteroids and meteorites.
Because detecting amino acids or fatty acids in a planetary environment alone will not tell researchers whether they are produced by life or by nonlife, they must seek additional evidence.
One clue is molecular handedness, or “chirality.” Certain amino acids occur in two mirror-image forms. Nonbiological processes often produce both forms in similar amounts, whereas life on Earth uses almost exclusively the left-handed forms. A strong excess of one form can point toward biology.
Another clue is found in the balance between the heavier and lighter forms of the same element within molecules. Usually, life prefers to use the lighter form.
Both of these clues are powerful indicators but difficult to measure in space. They require sensitive instruments, clean samples and often more material than a spacecraft can obtain.
That said, current and planned missions may provide a more limited – but still valuable – kind of measurement: a list of molecules and the proportions in which they are found. Our study demonstrates how researchers can use this simpler information to learn more about the molecules’ chemical origin.
Investigating diversity
Life does not merely produce certain molecules – it produces them in arrangements of unique patterns. Living systems invest energy into making molecules that serve specific functions, even when those molecules are complex and harder to form. Proteins, for example, require a broad set of amino acids, including relatively complex ones. Nonbiological chemistry can also make amino acids, but typically it makes simpler ones.
In our study, we investigated whether these molecules leave a statistical pattern that could serve as a biosignature: a measurable clue that may point toward life.
To quantify this idea, we used a method from ecology called diversity theory. Ecologists do not only ask how many species exist in a particular ecosystem, but also how those species are distributed: whether the community is dominated by a few very common species or by many species occurring in comparable numbers. The point of diversity theory is to both compile a list of species and capture the prevalence of each.
We applied the same logic to molecules. Within a family, such as amino acids, we treated each molecule like a species in an ecological community and measured its abundance. We wanted to know: Is a given mixture of molecules distributed evenly across different types or dominated by only a few of them? And could that pattern reflect the process that produced those molecules, whether biological or nonbiological?
Testing the framework
To test this idea, we compiled a deliberately broad dataset that included amino acids from a variety of sources: meteorites, samples from asteroid missions, laboratory simulations of nonbiological chemistry, modern organisms, sediments, ancient fossils and samples from various environments on Earth. We later did the same with fatty acids.
For amino acids, we found a clear distinction. The biological samples tended to contain many complex amino acids, in proportions similar to those of simpler ones. Nonbiological samples were usually sparser – that is, more strongly dominated by simple molecules.
This result makes sense. If biology can overcome the chemical bottlenecks necessary to create more complex molecules, you’d expect to see more of those molecules. On the other hand, nonbiological chemistry is more limited and dominated by molecules that form randomly. Complex molecules are far less likely to form under nonbiological conditions.
Fatty acids showed an opposite but equally informative pattern. Chains of fatty acids make up the outer membranes of living cells. We found that in biological samples, the fatty acid chains were all a similar length. In contrast, nonbiological samples had a wider distribution of chain lengths.
Innerstream/Wikimedia Commons
Even though, unlike the amino acid results, the nonbiological samples showed greater fatty acid diversity, this chain length finding supported the main idea behind our research: Life shapes molecular mixtures according to function.
Taken together, our results suggest that molecular diversity can serve as a new kind of biosignature. It cannot prove the presence of life on its own, and it should be interpreted alongside other measurements. But it offers a practical way to use the kind of data spacecraft are most likely to obtain: the proportions of molecules.
Searching for life in the solar system and beyond
Future spacecraft are unlikely to find pristine biological material, even if it exists. More likely, they will encounter the chemical traces of molecules, altered by the harsh conditions on planetary surfaces.
Next, we wanted to know how long the diversity signal could survive in the type of harsh environment where scientists may look, such as the surface of Europa. Its surface is continually being bombarded by energetic particles trapped in Jupiter’s magnetic field, which can break different organic molecules apart at different rates.
NASA/JPL-Caltech
We modeled how these molecules would degrade under such conditions and found that the diversity signal could remain recognizable for thousands of years when the molecules are buried under a few centimeters of ice. The signal is not indestructible, but it does not require an exceptionally fresh sample.
Our results suggest that in some cases the pattern left by life may still be recognizable even after the individual molecules have begun to break down.
The take-home message from our study is that life organizes chemistry in ways that could persist even after those ingredients are altered. Living systems arrange molecules according to biological needs, while nonbiological chemistry usually follows what is easiest to produce. If this organization can survive in planetary materials, future spacecraft may search not only for the building blocks of life but for the deeper statistical pattern that life leaves behind.
