Laser Ablation Ionization Mass Spectrometry: A Space Prototype System for In Situ Sulphur Isotope Fractionation Analysis on Planetary Surfaces

Abstract

The signatures of element isotope fractionation can be used for the indirect identification of extant or extinct life on planetary surfaces or their moons. Element isotope fractionation signatures are very robust against the harsh environmental conditions, such as temperature or irradiation, which typically prevail on solar system bodies. Sulphur is a key element for life as we know it and bacteria exist, such as sulphur reducing bacteria, that can metabolize sulphur resulting in isotope fractionations of up to −70‰ δ34S. Geochemical processes are observed to fractionate up to values of −20‰ δ34S hence, fractionation exceeding that value might be highly indicative for the presence of life. However, the detection of sulphur element isotope fractionation in situ, under the assumption that life has existed or still does exist, is extremely challenging. To date, no instrument developed for space application showed the necessary detection sensitivity or measurement methodology for such an identification. In this contribution, we report a simple measurement protocol for the accurate detection of sulphur fractionation δ34S using our prototype laser ablation ionization mass spectrometer system designed for in situ space exploration missions. The protocol was elaborated based on measurements of five sulphur containing species that were sampled at different Mars analogue field sites, including two cave systems in Romania and the Río Tinto river environment in Spain. Optimising the laser pulse energy of our laser ablation ionization mass spectrometer (LIMS) allowed the identification of a peak-like trend of the 34S/32S ratio, where the maximum, compared to internal standards, allowed to derive isotope fractionation with an estimated δ34S accuracy of ∼2‰. This accuracy is sufficiently precise to differentiate between abiotic and biotic signatures, of which the latter, induced by, e.g., sulphate-reducing microorganism, may fractionate sulphur isotopes by more than −70‰ δ34S. Our miniature LIMS system, including the discussed measurement protocol, is simple and can be applied for life detection on extra-terrestrial surfaces, e.g., Mars or the icy moons like Europa.

Reduction of surface charging effects in laser ablation ionisation mass spectrometry through gold coating

“…The instrument design and measurement principle are described in detail in previous publications. 3,4,11–16…”

“…This explains why in the past numerous studies on non-conductive samples were conducted successfully with our LIMS instrument. [17][18][19][20][21][22][23] For certain samples, however, surface charging might have limited the maximally possible laser pulse energy and hence the detectability of elements and isotopes at trace level abundances.…”

Frontiers of astrobiology and the humanities

“…These subdisciplines have developed in a terrestrial context. But in order to attempt to give an answer to astrobiology's first chapter for the Jovian moon Europa with a specific space mission -JUICEthese two areas go beyond each other to form a new holistic approach to be tested in the near future on the chemical elements on the icy surface of this satellite (Riedo et al, 2021).…”

Instrumentation for Detecting Sulphur Isotopes as Biosignatures on Europa and Ganymede by Forthcoming Missions

“…There has been recent progress in miniaturised laser ablation ionization mass spectrometry (LIMS) for in situ measurements of sulphur isotope fractionation on planetary or icy surfaces of ocean worlds [48]. In situ measurements of sulphur isotopic fractionation excursions on Europa's icy surface have now been shown to be feasible, having identified a trend of the 34 S/ 32 S ratio, which allows them to derive isotope fractionation with an estimated δ 34 S accuracy of ~2‰.…”

“…In situ measurements of sulphur isotopic fractionation excursions on Europa's icy surface have now been shown to be feasible, having identified a trend of the 34 S/ 32 S ratio, which allows them to derive isotope fractionation with an estimated δ 34 S accuracy of ~2‰. However, since sulphate-reducing microorganisms are capable of fractionating sulphur isotopes by more than δ 34 S = −70‰, the accuracy achieved by Riedo and co-workers [48] is sufficiently precise to differentiate between abiotic and biotic signatures.…”