A New Look at Enceladus’s Organic Inventory

Enceladus ejects subsurface material into space in the form of ice grains and gas from fractures in the moon’s south polar region, the Tiger Stripes (Porco et al. 2006; Hansen et al. 2006). Most of the ice grains fall back to the surface and only a fraction escapes the moon’s gravity and populates Saturn’s E ring (Kempf et al. 2010). One of Cassini’s mass spectrometers — the Cosmic Dust Analyzer (CDA)— sampled emitted ice grains in the E ring, at velocities typically below 13 km/s and revealed both high-mass refractory and low-mass volatile organic compounds in these grains (Srama et al. 2004; Khawaja et al. 2019; Postberg & Khawaja et al. 2018). Cassini’s flybys of the Enceladus plume, however, provided a unique opportunity for CDA to collect freshly ejected subsurface oceanic material, most notably organic compounds prior to their distribution away from Enceladus in the E ring.

Previously, data from plume flybys was only used to classify ice grains based upon their spectral type, without performing in-depth compositional analysis (Postberg et al. 2011; Ershova et al. 2024). In this work, for the first time, we analyse time-of-flight mass spectral data from CDA of freshly ejected organic-bearing ice grains sampled at the higher velocity of nearly 18 km/s (the E5 flyby in 2008), significantly faster than Cassini’s E ring traversals. Our results confirm the presence of aromatic and oxygen-bearing species in freshly ejected ice grains, with their characteristic spectral features appearing even at such high impact velocities, the same compounds that were previously observed in the E ring at much lower impact velocities. In addition, CDA spectra of these freshly ejected organic-bearing grains also exhibit spectral features related to esters/alkenes, ethers/ethyl and N-O bearing compounds, which were not observed in the lower impact speed spectra of older E ring ice grains.

These new findings have implications regarding the subsurface geochemistry of Enceladus and hence impose further constraints on the moon’s habitability. This work will also provide a complementary framework for the detection of organics in ice and dust grains using impact ionisation mass spectrometers for ongoing and future space missions, such as the SUrface Dust Analyzer (SUDA; Kempf et al. 2025) and the Destiny Dust Analyzer (DDA; Simolka et al. 2024) onboard NASA’s Europa Clipper and JAXA’s Destiny+.

References

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Hansen et al. Science (2006), DOI: 10.1126/science.1121254

Kempf et al. ICARUS (2010), DOI: 10.1016/j.icarus.2009.09.016.

Srama et al. Space Sci Rev (2004), DOI: 10.1007/s11214-004-1435-z

Khawaja et al. MNRAS (2019), DOI: 10.1093/mnras/stz2280

Postberg & Khawaja et al. Nature (2018), DOI: 10.1038/s41586-018-0246-4

Postberg et al. Nature (2011), DOI: 10.1038/nature10175

Ershova et al. A&A (2024), DOI: 10.1051/0004-6361/202450429

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