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How We Might Spot Biosignatures in the Solar System

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Cassini spacecraft image of the surface of Saturn's moon Enceladus
This composite visible, infrared, ultraviolet, and radar image taken by the Cassini spacecraft shows the long fissures in the southern hemisphere of Saturn's moon Enceladus. The radar data are shown as the long light blue arc. [NASA/JPL-Caltech/ASI/Space Science Institute]

The icy moons of the outer solar system are a promising place to look for life beyond Earth, and a new research article shows that we already have the tools to start our search.

Biosignature Possibilities

Cassini image of plumes on Enceladus

An image of Enceladus’s water plumes taken by Cassini. The small moon has more than a hundred such geysers. [NASA/JPL/Space Science Institute]

While the search for life beyond Earth often focuses on our neighboring planet Mars or the promise of distant, Earth-like exoplanets, icy moons in our solar system have recently come to the forefront of this discussion. Researchers suspect that many of the moons in the outer solar system have oceans hidden beneath their icy or rocky crusts — and the plumes of water ice observed shooting from fissures in the surface of Saturn’s moon Enceladus may provide an excellent way to study one of these oceans directly.

If life is present in the oceans of Enceladus or other icy moons in the solar system, it’s possible that plumes could deposit life-signaling molecules — biosignatures — on the surfaces of these worlds. A recent publication tests our ability to detect a certain class of biosignature compounds: lipids.

chemical structure of cholecalciferol

The chemical structure of a lipid called cholecalciferol. Cholecalciferol is better known as vitamin D. [Wikipedia user Calvero; Public Domain]

Measuring Samples with Mass Spectrometry

Though the term lipids might not be familiar, the molecules themselves likely are: lipids include fats, waxes, and certain vitamins. Lipids are also important components of cell membranes, making them essential to all life on Earth. Their size and complexity mean they’re unlikely to form through simple chemistry, making them potential biosignatures. But how would we detect these molecules on another world?

Rather than attempt to detect these molecules from afar, we’ll likely need to sample them directly by sending an instrument down to the surface of the planet or moon we want to study. In a recent publication, a team led by Nikita Boeren (University of Bern, Switzerland) tested the abilities of a candidate lipid-detecting spectrometer called the ORganics Information Gathering INstrument (ORIGIN).

ORIGIN is a laser desorption ion mass spectrometer, a type of instrument that uses a pulse of laser light to remove (desorb) molecules from a surface and ionize them. These electrically charged molecules are then channeled toward a detector by electric and magnetic fields, and the time that each molecule takes to reach the detector is related to the ratio of the molecule’s mass to its electrical charge. Using the mass-to-charge ratio of the molecule and any fragments it might have split into, researchers can determine which molecules were present in the sample.

Looking for Lipids

Boeren and collaborators tested ORIGIN’s ability to detect pure samples of six varieties of lipids, as well as combinations of those same lipids with other organic molecules: amino acids (compounds often referred to as “the building blocks of life”) and polycyclic aromatic hydrocarbons (molecules containing rings of carbon atoms).

mass spectra for mixtures of different substances

Mass spectra for a mixture of six lipids (top), four lipids mixed with two amino acids (middle), and four lipids mixed with two polycyclic aromatic hydrocarbons (bottom). Click to enlarge. [Boeren et al. 2022]

The team’s results show that the instrument is capable of detecting and differentiating between several different lipid molecules, even when those substances were mixed with other compounds. While more work remains to be done, including testing the procedure on other lipids and exploring the effects of using different types of materials to hold the sample, ORIGIN already shows great promise as a way to detect biosignature compounds. In fact, the detection limits determined in this study meet the requirements of the Enceladus Orbilander, a high-priority mission proposed in the 2023–2032 Planetary Science Decadal Survey.

Citation

“Detecting Lipids on Planetary Surfaces with Laser Desorption Ionization Mass Spectrometry,” Nikita J. Boeren et al 2022 Planet. Sci. J. 3 241. doi:10.3847/PSJ/ac94bf