Detection of molecular biosignatures in polar ices with mass spectrometry: implications for Europa Clipper

The subsurface oceans of Enceladus and Europa are presumably the most habitable places in the solar system beyond Earth, and may host extant life [1]. Putative molecular biosignatures could be transported from the ocean to the surface, where they would be detectable by spaceborne instruments such as the SUrface Dust Analyzer (SUDA [2]) onboard Europa Clipper [3]. Indeed, recent laboratory experiments [4,5] showed that SUDA-type mass spectrometers could detect molecular biosignatures in ejected ice grains, with lipids providing some of the most notable and characteristic spectral fingerprints. Lipids (i.e., cell membrane-derived organic compounds based on carbon-carbon chains) are some of the most robust molecular biosignatures that could be detected on ocean worlds [6,7,8]. Ubiquitous in all life on Earth, lipids are considered as universal biomarkers of life [9] thanks to their effective membrane-forming properties even under geochemically hostile conditions [10] - a feature that is expected to be upheld in extraterrestrial environments. Lipids are therefore prime targets for life detection missions on ocean worlds.

 

Spaceborne impact ionization mass spectrometers such as SUDA could directly analyze fresh ocean material that may contain lipids among other organics [4,5,11,12]. However, the performance of such spaceborne instruments strongly relies on analogue data obtained from laboratory experiments. The calibration of spaceborne mass spectrometers can be achieved by using Laser Induced Liquid Beam Ion Desorption (LILBID), a well-established method which allows the simulation of ejected ice grains’ mass spectra. Many LILBID spectra have already been recorded to complement an expanding reference database [13] for Europa Clipper and other missions to ocean worlds. However, environmental samples from terrestrial locations analogous to icy moons have never been analyzed with LILBID so far, although they allow a more realistic assessment of the detection capabilities of spaceborne instruments as compared to experiments with prepared synthetic samples of well-defined compositions. Specifically, natural ice analogues from polar locations offer some of the most realistic representations of icy moons as these ices are subject to similar environmental conditions, leading to complex biochemical compositions that include extremophilic ecosystems - which could be the case for surface or plume ice samples from icy moons.

 

Here we present the first analysis of natural ice analogues with LILBID combined with a detailed analysis of lipid biomarkers. With support from the Instituto Antártico Uruguayo, ice samples were collected from key locations in the Collins (a.k.a. Bellinghausen) Glacier on King George Island, Antarctica, with several environmental conditions (including intense UV radiation, saline aerosols, low temperature) analogous to specific processes on ocean worlds. Our analytical study includes (1) Gas Chromatography coupled to Mass Spectrometry (GC-MS), which is a well-established, standardized and optimized technique already used on King George Island for the detailed examination of lipids [14] and (2) LILBID analyses providing SUDA-type mass spectra of the same samples. Combining these two analytical methods allows a novel assessment of the detectability of lipid biomarkers from icy moon analogues with spaceborne instrumentation. Preliminary results including the first data of each analytical technique will be presented, as well as future steps of sample processing and analysis. Overall, this work will highlight the possible limitations that spaceborne dust analyzers can potentially encounter when dealing with complex-matrix samples, and will allow to acquire a more profound knowledge of potential lipid biomarkers that could be encountered by SUDA onboard Europa Clipper.

 

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