Investigation of Arctic Ice in preparation for the Future Exploration of Biosignatures on Enceladus and other icy moons

Introduction. Seep and vent environments on Enceladus would be ideal to sample by future astrobiological missions, although this may not be possible due to technological limitations [1, 2]. Searching for biosignatures in the more readily sampled Enceladus’ icy shell is preferable. In this regard, the Arctic Ocean is a unique terrestrial analogue of Enceladus. For this reason here we try to determine whether any geochemical biosignatures associated with methane cycling can be detected in Arctic ice using mass spectrometric techniques similar to those likely to be included in the payloads of planned missions to Enceladus.

Study Area and samples. The site investigated lies at the southernmost extent of the Vestnesa Ridge, on the Svalbard continental slope (Figure 1) and it was visited in May 2022 during the AKMA2-OceanSenses Research Expedition [3]. Gas hydrates are very common in the area as revealed by several seismic studies and direct sampling [4]. The Vestnesa Ridge system interacts with warm fluids whose circulation is driven by hydrothermal activity [5].

Ice and surface water from the drill holes were sampled from the ice pack (Figure 1). Samples of deep water were collected from another location nearby at a depth of 1392 m using a conductivity, temperature, and depth (CTD) probe (Figure 1). Shallow (<1 m from the sediment-water interface) gas hydrates were sampled in the same area using a gravity corer (Figure 1).

Figure 1. Study area and sampling sites: IC = ice samples; CTD = water samples collected using a CTD probe; GC = gas hydrate sample. From [6].

 

Methods. The hydrogen and oxygen stable isotopic compositions of the melted ice samples were determined at the HUN-REN Institute for Nuclear Research in Debrecen, Hungary, using off-axis integrated-cavity output spectroscopy (OA-ICOS).

The relative composition of gases emitted during the melting of the Arctic ice samples was measured via 70 eV electron impact quadrupole mass spectrometry (QMS) across a mass range of 1-50 amu at the HUN-REN Institute for Nuclear Research (Figure 2). Ice samples were analysed following an ad hoc protocol presented in Franchi et al. [6]. QMS analysis was performed by measuring integer masses sequentially using an ion counting secondary electron multiplier (SEM).

 

Figure 2. Apparatus used to measure the mass spectra of gases emitted by melted ice samples (top-right) and seawater and gas samples (bottom-right) using an ultrahigh-vacuum chamber fitted with a QMS (left).

 

Results and Interpretation. The measured isotopic compositions of the ice and seawater revealed that the surface water sample exhibited δ²H and δ¹⁸O values comparable to the standard. The ice sample collected at the water – ice interface exhibited significant enrichments of ²H and moderate-to-large enrichments of ¹⁸O, with δ²H and δ¹⁸O values of 17.43‰ and 1.959‰, respectively. These results are likely indicative of the isotopic fractionation effect that occurs during the freezing of water in which heavy isotopes are preferentially incorporated into the ice phase [7]. The deep ocean water exhibited noticeably negative δ²H and δ¹⁸O values. On Earth, this may be attributed to biological activity in the oceans, since living organisms preferably built in the lighter isotopes in their body, and after their death, the dead matter enriches the deep waters at the bottom with light isotopes.

The abundances of several gas species of interest (CH₄, C₂H₆, H₂, N₂, O₂, CO₂, Ar and Ne) trapped or dissolved within the Arctic water and ice samples was assessed by QMS. The composition of the gases emitted from one ice sample collected at the water – ice interface showed close similarity to that of air, with the notable exception of a relatively high molecular hydrogen abundance, and small quantities of methane and ethane.

Two samples of ice collected from another location, one acquired from the ice-seawater interface and one at the air-ice interface, showed higher concentrations of CO₂ consistent with their relatively low pH values. This observation is consistent with either: (i) these ice samples containing atmospheric CO₂ dissolved in the ocean water; or (ii) the oxidation of CH₄ released at the seafloor [6]. To ascertain the source of C, the δ¹³C values of carbon dioxide were studied using the signal intensities at the 44 (¹²C¹⁶O₂) and 45 (¹³C¹⁶O₂) amu mass channels. All Arctic samples exhibited δ¹³C values ranging between -2.4 and +2.3‰. Negative value was obtained for the deep sea water sample, which is in line with biological activity [8].

Our experiments have tentatively demonstrated that the concentration of molecular hydrogen in one ice sample is ca. 3.6%, which is about 175 times higher than its concentration in laboratory air control samples, and ca. 2.5-10 times higher than concentrations observed in the plume of Enceladus [9]. This high concentration of molecular hydrogen in Arctic ice could possibly be sourced from hydrothermal activity linked to serpentinization along the Arctic mid-oceanic ridge. Hence, hydrothermal activity within the global ocean of Enceladus might be inferred from excesses of molecular hydrogen within its ice shell.

Furthermore, our results demonstrate that QMS measurements similar to ours, which were and will be obtainable by past and future space missions at icy moons can detect possible biosignatures when gas components and isotopic ratios are carefully extracted from the data.

 

References: 1. McKay, CP., et al. 2012. Planetary and Space Science 71, 73; 2. Carrizo, D., et al. 2022. Astrobiology 22, 552; 3. Panieri, G., et al. 2022. CAGE22-2 Scientific Cruise Report: AKMA 2/Ocean Senses. CAGE – Centre for Arctic Gas Hydrate, Environment, and Climate Report Series 10; 4. Panieri, G., et al. 2017. Marine Geology 390, 282-300; 5. Bünz, S., et al. 2012. Marine Geology 332-334, 187-197; 6. Franchi, F., et al. 2025. Planetary and Space Science 257, 106051; 7. Toyota, T., et al. 2013. Journal of Glaciology 59, 697-710; 8. Kroopnick, P. M. 1985 Deep-Sea Research, 32, 57- 84,.9. Waite, J.H., et al. 2017. Science 356, 155.