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Composition of Organic Material in Freshly Ejected Ice Grains in Enceladus’ Plume

Presentation #303.02 in the session Enceladus (Oral Presentation)

Published onOct 23, 2023
Composition of Organic Material in Freshly Ejected Ice Grains in Enceladus’ Plume

Enceladus’s plume of gas and ice grains emerges into space through cracks in the icy crust at its south polar region. The ejected material, sampled both in the plume and in Saturn’s E ring by Cassini’s Cosmic Dust Analyzer (CDA) and the Ion and Neutral Mass Spectrometer (INMS) mass spectrometers, is considered a direct sample of the moon’s subsurface ocean. CDA’s detection of salty grains, nano-silica particles and organic enriched ice grains strongly indicated water-rock interactions and hydrothermal activity at the sea floor [1,2,3].

Previous analyses of organic material in E ring ice grains led to the discovery of (i) complex macromolecular compounds with masses > 200 u and (ii) volatile compounds with masses < 100 u, both contain various moieties e.g., N-, O- and aryl groups [4,5]. The presence of complex and volatile material in the ice grains indicates rich organic chemistry in Enceladus’s subsurface ocean e.g., Friedel-Crafts and serpentinisation reactions.

In this work, for the first time, we attempt a detailed compositional analysis of organic material in ice grains measured directly in the plume by CDA just after ejection. Previously these spectra have only been assessed as “organic bearing ice grains (Type 2)” without a detailed compositional investigation of the organic species [6]. Spectra of freshly emitted grains sampled at 17 km/s during the E5 21 km close flyby in 2008 were recorded at fluxes of up to 5 s-1, albeit with reduced mass resolutions and mostly ranges (2-110 u). At this speed, organic molecules fragment more when impacting the instrument’s target, providing a new perspective of their molecular structures in comparison to previously sampled and analysed CDA E ring Type 2 spectra. Importantly, spectra produced at 17 km/s do not exhibit water cluster peaks which often hamper spectral analysis at lower speeds (< 10 km/s).

The reanalysis of the composition of freshly ejected plume grains at a higher impact speed revealed spectral signatures corresponding to organic compounds e.g., alkene (CnH2n) and esters (RCOOR´) unidentified in previous studies [4,5]. The reanalysed spectra also confirm the presence of moieties (N- or O- and aryl groups), as found by Khawaja et al. (2019) and Postberg & Khawaja et al. (2018), in spectra of organic-enriched ice grains collected in the E ring.

[1] Postberg et al. (2009) Nature 459, 1098-1101

[2] Hsu et al. (2015) Nature 519, 207-210

[3] Waite et al. (2017) Science 356, 155-159

[4] Khawaja et al. (2019) MNRAS 489, 5231-5243

[5] Postberg & Khawaja et al. (2018) Nature 558, 564-568

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

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