Analysis of Minor and Trace Organic Species in Enceladus’ Plume Gas

Abstract:

Ocean compositional information at Enceladus has been derived from both the Cassini Ion Neutral Mass Spectrometer (INMS) and Cosmic Dust Analyzer (CDA) experiments. Analysis of the major volatile content of the plume gas (Waite et al., 2017) and an inventory of potential minor and trace species (Magee and Waite, 2017) led to the discovery of High Mass Organic Compounds (HMOC) (Postberg et al., 2018). More recent insights into nitrogen, oxygen, and aromatic organic compounds have been obtained from the CDA data sets (Khawaja et al., 2019).

The analysis of minor species in Enceladus’ plume gas is complicated by the spectral complexity of the rich organic mixture, the unit mass resolution of the INMS mass analyzer, and by the varying degrees of molecular fragmentation that can take place at encounter speeds above 5 km/s (Waite et al., 2009). The E5 spectra indicated an unprecedented abundance of organic compounds above 50 u in the spectra, which have been correlated with molecular break up products of HMOC (Postberg et al., 2018).

A framework for identifying potential minor and trace species in the INMS observations of the low-speed flybys highlighted the degree of ambiguity in characterizing such species given the limitations of the INMS instrument and its dataset (Magee and Waite, 2017). With this as a backdrop we concentrate on the analysis of data from both the high-speed flyby E5 and a series of low-speed flybys (E14, E17, and E18) to extract information about minor and trace organic species in the plume gas which appear as signal from fragment ions above 24 m/z.

We present results of our investigation into how the nature of the minor and trace species can be constrained, identifying trends and limitations on interpreting the observed composition and its source, and possibilities for future improvements.