On the Composition of Organic-bearing Plume Ice Grains Detected during Cassini’s E5 Enceladus Flyby

The ice grains emitted from the south polar region of Enceladus are direct sample of the moon’s subsurface ocean material [1]. A substantial fraction of Enceladus’ plume grains escapes the moon’s gravity and becomes part of Saturn’s E ring [2]. The Cassini’s Cosmic Dust Analyzer (CDA) [3] sampled these ice grains directly from the plume during spacecraft flybys of Enceladus as well as from Saturn’s E ring. The detection of salty grains, nano-silica particles and organic enriched ice grains strongly indicated water-rock interactions and hydrothermal activity at the sea floor [1, 4, 5, 6]. Understanding the composition of the ice grains is key to finding out whether or not signs of life exist in Enceladus ocean.

The CDA mass spectrometer worked on the principle of impact ionization where an ice particle vaporizes and ionizes after hitting on the instrument’s metal target at velocities ≥ 1 km/s and generating Time of Flight (ToF) mass spectrum of the cations. One major type of Enceladean ice grains (i.e. Type 2) detected by the CDA instrument is enriched with organic species [7]. Laser Induced Liquid Beam Ion Desorption (LILBID), a laboratory technique proven to simulate impact ionization mass spectra of the ice grains recorded in space [8], is used to identify the composition of organic species in the ice grains. This led to further classification of CDA Type 2 spectra into: (i) Type 2 HMOC (High Mass Organic Cations), indicating complex, probably refractory and hydrophobic, macromolecular organic compounds with masses > 200 u. The ice grains of this type formed when organic condensation cores are generated by the dispersion of a putative organic layer by bubble bursting on top of Enceladus’ ocean surface [5]; and (ii) Type 2 VOC (Volatile Organic Compounds), indicating volatile low mass N- and O-bearing as well as single ring aromatic (< 100 u) organic compounds that condense onto pre-existing water ice grains in the icy vents of Enceladus [6].

In this work, we analysed CDA Type 2 mass spectral ToF data of freshly ejected plume ice grains during Enceladus flyby E5 in 2008 [9]. The closest approach of the Cassini spacecraft was 21 km from Enceladus surface at a velocity 17 km/s. Following the flight path from north to south, after the closest approach at the fringe of the plume, the spacecraft then passed through the dense plume region directly above the plume source. In the previous compositional analysis of E5 CDA data, Postberg et al. (2011) only assessed whether an ice grain is organic enriched or not without a detailed compositional investigation of organic species. The higher degree of fragmentation of organics inside these ice grains and the resulting spectral appearance (e.g. absence of water clusters and hence no interference with organic signatures) at higher impact speeds (17 km/s) provides a different perspective on the composition of the organics as compared to previous analysis of CDA Type 2 spectra recorded at lower impact speeds before and after the E5 flyby. Here, we relate the composition of these grains with the possible organic species. The analysis of freshly ejected organic enriched ice grains confirms the findings of Khawaja et al (2019) and Postberg et al. (2018), where most of the organic enriched ice grains were collected in the E ring. In addition, spectra of these freshly ejected organic-bearing grains also exhibit certain spectral features, which were not observed in E ring ice grains spectra.

References

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[7] Postberg, F. et al. (2008) Icarus 193, 438-454.

[8] Klenner, F. et al. (2019) Rapid Commun. Mass Spectrom. 33, 1751-1760.

[9] Postberg, F. et al. (2011) Nature 474, 620-622.