Enceladus Plume Composition

Abstract

Abstract Apparent changes in the plume volatile composition measured by the Cassini Io n Neutral Mass Spectrometer as a function of the flyby speed have resulted in a more complete understanding of the plumeOs volatile and granular compounds. We present a baseline volatile makeup and suggest the existence of condensed high -molecular weight organic compounds in the plume mixture as well. 1. Introduction Waite et al. [1] reported a plume composition for Cassini Enceladus flyby E5 that was dominated by water with an admixture of CO 2 , CO, NH 3 , CH 4 , H 2 CO and a host of organic species , reminiscent of the volatile composition of a comet (Table 1) . The comet analogy was bolstered by a fortuitous measurement of D/H in water of 3 ! 10 -4 . However, Waite et al. noted a change in composition with flyby velocity that warranted further study. 2. Elemental Co mposition A subsequent flyby at low velocity (E7) reinforced the notion of apparent changes in the plume composition as a function of the spacecraft velocity (Fig. 1). This figure shows that as the speed of the flyby increases the oxygen content goes down, and the carbon and hydrogen content goes up. Nitrogen does not demonstrate a clear depend ence. We explain these changes as follows: 1) the oxygen decreases due to reaction with raw titanium vaporized from the antechamber walls by high -speed ice grains act ing on water vapor ; (This enabled the determination of the D/H in water.) 2) the increase in carbon comes from the surface induced dissociation of organic macromolecules (> 100 Da) or condensed organic materials (OgrainsO), which we detect as smaller organ ic fragments. The organic -bearing particles have been reported among E-ring particles originate d at Enceladus [2] and should be sampl ed by the INMS . Figure 1: Apparent changes in elemental abundances of plume volatiles with flyby speed.