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In our hunt for life on other worlds, amino acids could be key. The subsurface oceans of icy moons such as Jupiter鈥檚 Europa and Saturn鈥檚 Enceladus might contain detectable amino acids, potential biosignatures of life. But amino acids can also be produced by geological processes, so we need to know if spotting them would really mean we have found aliens.

The first step is to rule out amino acids created when the solar system first formed, which have previously been found on comets. 鈥淎mino acids are the building blocks of protein, and the chemistry of life as we know it,鈥� says Ngoc Truong at Cornell University in New York. 鈥淚 wanted to see if amino acids can be found coming from the ocean of Europa, could they be the relics of primordial synthesis processes?鈥�

In laboratory experiments, Truong and his colleagues have found that certain amino acids, particularly aspartic acid and threonine, could only be present in the warm, hydrothermally active oceans of icy moons if they were produced in the last one million years.

This means that any that were detected would likely have been produced recently, cosmically speaking, ruling them out as relics of the primordial solar system, which formed around 4.6 billion years ago.

鈥淭his paper is exciting because it helps us understand which biosignatures we may wish to target for future missions to Enceladus or Europa to search for life,鈥� says Morgan Cable from NASA鈥檚 Jet Propulsion Laboratory in California.

Such missions include NASA鈥檚 Europa Clipper, on which Cable is a team member, which is planned to launch in 2023. It will have the ability to detect amino acids on these icy worlds using two mass spectrometer instruments by flying through plumes of ejected water from the oceans, or studying the ejecta from afar.

鈥淲e鈥檝e done experiments in the laboratory to see what kind of fingerprints you expect from all kinds of amino acids,鈥� says Frank Postberg from the Free University of Berlin, who is working on one of Clipper鈥檚 mass spectrometers. 鈥淚t鈥檚 a pretty clear fingerprint, even at low concentrations.鈥�

Detection of amino acids alone would not prove that life is in the oceans, as they would still need be distinguished from ones produced by geological processes, such as through the interaction of water and minerals. But it would be another important piece of the puzzle towards making such a suggestion. 鈥淚f we find them there that will be a really compelling reason to go back and dig deeper,鈥� says Cable.