There is one thing we are sure of: life leaves traces. Some may be gases in an atmosphere, others certain organic molecules, some are based on technology… But there is a much more subtle signal: a geometric characteristic that appears in much of the chemistry of living beings and that could survive even when life itself has already disappeared.
It’s called chirality. Some molecules can exist in two versions that are mirror images of each other, like a right hand and a left hand. They have the same atoms and the same chemical formula, but their spatial arrangement is different. Amino acids, the building blocks of proteins, are a good example. In terrestrial chemistry, living beings almost exclusively use the L form of amino acids. Non-biological chemical processes, on the other hand, tend to produce both versions, L and D, in similar proportions.. Therefore, finding a significant imbalance between both forms in an extraterrestrial sample could become a biosignature: it would not in itself prove that there was life, but it would point to a chemistry that would deserve special attention.
Small parenthesis. The letters L and D come from Latin laevus (left) and dexter (right), and originally described the sense in which a substance bent the plane of polarized light: the first deflected the polarized light to the left and the D to the right, logically. We close parentheses.
The problem is how to recognize these two molecules when they are mixed with many others. Traditional methods can do this, but they often rely on relatively complex separation and analysis techniques.. The Osaka University team, led by Masateru Taniguchi, has looked for an alternative that transforms the molecule itself into an electrical signal. The results are explained in a study published in Nature.
The idea seems almost microscopically simple. Two gold nanowires are placed a tiny distance apart, like a ring that doesn’t close completely. When a molecule passes through this space, a tunneling current can be established between the electrodes. The current is not exactly the same for the two versions of an amino acid: each produces a certain form of electrical signal. That’s where artificial intelligence comes in. The team of Taniguchi trained algorithms to recognize those differences in the signals produced by individual molecules. The result was a high accuracy rate for distinguishing the L and D forms.
“By combining our technique with artificial intelligence, we were able to distinguish the L and D forms of amino acids with an accuracy greater than 80% – says Takahito Oshiro, co-author of the study -. “This is the first discrimination of amino acid chirality on a single-molecule scale and a fundamental advance in chemical analysis.”
But there remained a much more difficult test. A sample collected on Mars, an asteroid, or an icy moon would not contain a single amino acid carefully prepared in a laboratory. It would be a mixture of molecules, possibly altered by billions of years of radiation and geological processes. For this reason, Taniguchi’s team analyzed natural samples from the Murchison meteorite, which fell in Australia in 1969 and is known to contain a wide variety of organic compounds, and soil samples from the Atacama Desert, one of the most extreme terrestrial environments and used as an analogue of Mars. The new technique managed to recover the main characteristics of the amino acid composition and offered results comparable to those obtained through conventional methods.
The importance of the work is precisely in what could come next. A detector capable of analyzing individual molecules using electrical signals could be more compact than some conventional analytical systems and, by not necessarily depending on certain reagents or optical systems, offer advantages for instruments intended for space travel. The authors themselves propose that the technology could contribute to the development of compact electrical devices for the search for extraterrestrial life.
That does not mean that we have a machine capable of introducing a Martian sample and showing “life found” on a screen. The technique still recognizes a signal consistent with biology, not biology itself. Besides, Some molecules can be formed through abiotic processes and also present characteristics that complicate the interpretation of a biosignature.
But there’s a powerful advantage: Instead of asking an alien sample what life looks like, the detector could ask something much smaller and more fundamental: Which way is its chemistry turned? If the answer were unequivocally different from what lifeless chemistry produces, perhaps an electric current from just a few molecules could become one of the first clues. that, somewhere in the solar system, life also learned to leave its signature.