Detecting cellular biosignatures from single salt-rich ice grains emitted from Enceladus or Europa

Icy moons with a liquid subsurface ocean, such as Europa⁷ and Enceladus¹⁶, are prime candidates in the search for extraterrestrial life. For future spaceflight missions, the identification of biosignatures with onboard mass spectrometers (MS) will be essential. NASA and ESA’s Cassini-Huygens spacecraft, which passed nearby to Enceladus and sampled ice grains from the icy moon’s cryovolcanic plume using low resolution impact ionization mass spectrometry, has already identified a variety of organic material originating from the moon’s subsurface ocean^6,11. In upcoming missions, such as NASA’s Europa Clipper, the use of higher resolution mass spectrometers will be crucial for clearer identification of organic compounds^5,13.

The Planetary Sciences group at Freie Universität Berlin is specialized in simulating mass spectra obtained from ice grains in space by a Laser Induced Liquid Beam Ion Desorption Mass Spectrometer (LILBID-MS)¹⁷. Such experiments have proven capable of accurately identifying and quantifying biomolecules in a water-ice matrix, such as amino acids, peptides, sugars, and fatty acids from within cells⁴, while distinguishing between molecular patterns with biotic or abiotic origins⁸. They have also demonstrated how to identify cell material from within a single pure-water ice grain emitted from Enceladus or Europa⁹. However, there are still gaps in our knowledge of to what extent salts affect the spectrometric fingerprints of biosignatures.

Such salt concentrations are highly relevant for the ocean and surface of icy moons, where potential biosignatures might be found in water or ice with salinity levels similar to or greater than that of Earth’s oceans. On Enceladus, the ocean contains salt concentrations ranging from 0.07 - 0.3 M NaCl, dependent on depth and ocean layer^2,12. On Europa, the abundance and composition of salts in the ocean is not well defined, but existing estimates suggest that the ocean is dominated by NaCl and MgSO4 ³. Previous research indicates that the total hydrosphere of Europa contains ∼0.6 M of total dissolved salts¹⁴, ranging from 0.01 M MgSO4 and NaCl in less salty ocean waters¹⁰ to 2.4 M in mantle pore fluids or localized hyper-saline regions¹⁵. Biosignatures entrained in ice grains originating from these regions of Europa or Enceladus would contain far higher salinity levels than have previously been studied, and these levels of salinity are known to interfere with the visibility of organics in mass spectral data^{1,8, 18, 19}. 

This research project studies how salinity levels and various types of salts comparable to those found on the icy moons may affect mass spectral biosignature readings. To achieve this, the psychrophilic extremophile Sphingopyxis alaskensis is submerged in water-based solutions with salt concentrations similar to that of Enceladus and Europa. The mass spectra of the cells are then measured using MS laboratory instruments to determine what biosignatures might be detectable in single salt-rich ice grains that contain cell material.  

Understanding how salts affect the way cell material is presented in mass spectral data is conducive to  the identification of organic biosignatures in future spaceflight missions, such as Europa Clipper. This research project will help to strengthen our understanding of biosignature detection, enhancing the capability of spaceborne instruments to detect the presence or absence of biosignatures with high confidence.

 

• (1) Annesley, T. M., 2003, Clinical Chemistry, vol. 49, pp. 1041–1044, https://doi.org/10.1373/49.7.1041. 
• (2) Bouffard, M. et al., 2023, (preprint) https://doi.org/10.21203/rs.3.rs-2398898/v1. 
• (3) Carlson, R. W. et al., 2009, Europa, 283.
• (4) Dannenmann, M. et al. 2023., Astrobiology, 23(1), 60-75.
• (5) Kempf, S., 2024, https://doi.org/10.5194/egusphere-egu24-17570. 
• (6) Khawaja, N. et al., 2019, Monthly Notices of the Royal Astronomical Society, vol. 489, no. 4, pp. 5231–5243, https://doi.org/10.1093/mnras/stz2280. 
• (7) Kivelson, M. G. et al., 2000, Science, vol. 289, no. 5483, pp. 1340–1343, https://doi.org/10.1126/science.289.5483.1340. 
• (8) Klenner, F. et al., 2020, Astrobiology, vol. 20, no. 10, pp. 1168–1184, https://doi.org/10.1089/ast.2019.2188. 
• (9) Klenner, F. Janine Bönigk, et al., 2024, Science Advances, vol. 10, no. 12, https://doi.org/10.1126/sciadv.adl0849.
• (10) Melwani Daswani, M. et al., 2021, Geophysical Research Letters, vol. 48, no. 18, https://doi.org/10.1029/2021gl094143. 
• (11) Postberg, F. et al., 2018, Nature, vol. 558, no. 7711, pp. 564–568, https://doi.org/10.1038/s41586-018-0246-4. 
• (12) Postberg, F. et al.,  2009, Nature, 459(7250), 1098-1101.
• (13) Reh, K. et al., 2016 IEEE Aerospace Conference, 2016, https://doi.org/10.1109/aero.2016.7500813. 
• (14) Spiers, E. M., Schmidt, B. E., 2023, vol. 128, no. 11, https://doi.org/10.1029/2023je008028. 
• (15) Stephens, D. W., 1990, (Utah, USA), 1847–1987. Hydrobiologia, 197(1), 139–146, https://doi.org/10.1007/BF00026946.
• (16) Thomas, P.C. et al., 2016, Icarus, vol. 264, pp. 37–47, https://doi.org/10.1016/j.icarus.2015.08.037.
• (17) Klenner, F. et al., 2019, Rapid Communications in Mass Spectrometry, vol. 33, no. 22,, pp. 1751–1760, https://doi.org/10.1002/rcm.8518.
• (18) Napoleoni, M. et al., 2023, ACS Earth and Space Chemistry, 7(9), 1675-1693.
• (19) Napoleoni, M. et al., 2023, ACS Earth and Space Chemistry, 7(4), 735-752.