Exploring Enceladus and other Ocean Worlds using Mass Spectrometry

Abstract:

Saturn’s icy moon Enceladus emits into space a plume of gas and ice grains formed from a subsurface salt-water ocean that interacts with a porous rocky core via hydrothermal vents. A similar plume phenomenon is suspected to occur on Jupiter’s moon Europa. Such plume material enables the internal exploration of satellites by spacecraft flybys, as undertaken at Enceladus by the Cassini spacecraft’s mass spectrometers, the Cosmic Dust Analyzer (CDA) and the Ion and Neutral Mass Spectrometer (INMS).

CDA and INMS detected H₂, CH₄ [1] and nano-silica [2], confirming active alkaline hydrothermal activity at Enceladus’ ocean-core interface, believed to be similar to the Lost City hydrothermal vent systems on Earth. The discovery of insoluble high-mass organic macromolecules [3] as well as soluble N-, O-, and aromatic low-mass organic compounds [4], some of which could act as amino acid precursors, revealed complex and reactive organic chemistry within the moon.

Interpreting ice grain mass spectra and inferring the biogeochemistry of Enceladus requires terrestrial calibration. Here, we report our results from Laser-Induced Liquid Beam Ion Desorption (LILBID) time-of-flight mass spectrometry in the laboratory, a proven analogue for CDA in situ ice grain mass spectrometry [5]. Geochemically-relevant salts and biologically-relevant organics, such as amino acids, fatty acids and peptides, have been tested and characteristic spectral signatures of these compounds could be identified [6]. Abiotic and biotic mass spectral fingerprints could be discriminated and detection limits of the organics were found to be at the µM or nM level in Enceladus-like solutions [7], while those for salts (including sulfates and phosphates) are currently being determined. Further biological samples, such as DNA and lipids extracted from bacterial cell cultures, are also under evaluation.

The results reported here aid planning for future space missions to Enceladus and other ocean worlds, such as the Europa Clipper mission to Europa.

References

[1] Waite et al., 2017 (Science)

[2] Hsu et al., 2015 (Nature)

[3] Postberg et al. 2018 (Nature)

[4] Khawaja et al. 2019 (Mon Not R Astron Soc)

[5] Klenner et al. 2019 (Rapid Commun Mass Spectrom)

[6] Klenner et al. 2020a (Astrobiology)

[7] Klenner et al. 2020b (Astrobiology)