Modeling insights into the locations of density enhancements from the Enceladus water vapor jets

Abstract

Monte Carlo modeling of the vapor erupting from Enceladus' south polar region is presented to demonstrate the influence of physical characteristics of the emitted vapor on the distribution of particles at altitude. The modeled sources include localized jets and eruptions distributed along the surface features labeled “tiger stripes.” The modeling reveals that density enhancements are displaced from the source location caused not only by the angle of emission but also by superposition of material from nearby sources. The altitude of the superposition is mass dependent and contributes to differences observed in the mass 28 and mass 44 channels of the Cassini Ion and Neutral Mass Spectrometer (INMS) during Enceladus encounters. INMS data are well modeled using only the tiger stripe sources; however, certain regions require additional sources to reproduce some features of the data. In particular, an excess source is required for 44 unified atomic mass unit (u) on the Saturn‐facing hemisphere of Baghdad Sulcus. It is apparent in three parallel Cassini flybys. A relative decrease in the source rate is observed for mass 28 u for E14. The more diffuse nature of the 28 u INMS observations compared to the more collimated structure of the 44 u INMS observations is consistent with increased thermal spreading for low‐mass constituents of the plume.

Enceladus Plume Structure and Time Variability: Comparison of Cassini Observations

“…, 66 km E17, 18 flybys) due to the angular spread of the emission and, therefore, INMS is to a large degree measuring a summation over multiple tiger stripes and gas jets. We also show in Figure 3 the modeled H 2 O densities along the E14, 17, 18 trajectories, estimated on the basis of the model fits to CO 2 , after correcting for the relative mixing ratios of CO 2 (0.4% ± 0.1%, depending on the flyby) and H 2 O (∼90%) and the molecular mass dependence (m H2O /m CO2 ) 1/2 of the thermal Mach number (Hurley et al , 2015 ; Perry et al , 2015 ). The significant disagreement of the INMS H 2 O measurement with the model is due to H 2 O adsorption in the instrument, which distorts the signal by delaying the transmission of water vapor through the INMS (Teolis et al , 2010 ).…”

“…Left: INMS-measured H 2 O densities (blue points), and the CO 2 -based model prediction for the H 2 O density (red line). The prediction takes into account the relative mixing ratios of CO 2 (∼0.24%, 0.37%, and 0.24% at E14, 17, and 18) and H 2 O (∼90%) in the plume, and corrects for the anticipated (m CO2 /m H2O ) 1/2 thermal Mach number mass dependence (Hurley et al , 2015 ; Perry et al , 2015 ). Measured and modeled H 2 O disagree due to H 2 O adsorption in the INMS, which distorts the signal by (1) delaying H 2 O transmission through the gas inlet to the ion source and (2) causing residual H 2 O to persist in the instrument even after Cassini has exited the plume.…”

“…The best fit values are 18% (at Mach 0), 18% (at Mach 2), 52% (at Mach 4), and 12% (at Mach 16), with ∼±30% uncertainty on each contribution. The Mach numbers 0, 2, 4, and 16 apply to CO 2 ; for H 2 O, we assume the bulk gas speed and temperature to be equilibrated with CO 2 and, accordingly, scale the thermal Mach numbers down in inverse proportion to the (mass dependent) thermal velocity, that is, by (m H2O /m CO2 ) 1/2 = 0.64 (Hurley et al , 2015 ; Perry et al , 2015 ). The Mach number distribution is described by the source flux weights C M ( Eq.…”

“…The CO 2 E3 and E5 data show an approximate inverse square decay of the plume density with distance from the south polar terrain, which is consistent with collisionless vapor expansion from Enceladus well in excess of the 240 m/s escape speed. Following up on early UVIS-based modeling, Burger et al ( 2007 ), Tian et al ( 2007 ), Tenishev et al ( 2010 ), Dong et al ( 2011 ), Tenishev et al , (2014), Hurley et al ( 2015 ), and Yeoh et al ( 2017 ) applied analytical and Monte Carlo modeling to estimate plume source properties on the basis of these INMS data, that is, source rate and gas velocity, by fitting to these data the eight major grain jets identified from preliminary, low-resolution Cassini imaging (Spitale and Porco, 2007 ). Smith et al ( 2010 ) combined Monte Carlo models of the plume and Saturnian magnetosphere and concluded (similarly with Dong et al [ 2011 ], Tenishev et al [ 2014 ], and Yeoh et al [ 2017 ]) that the INMS data were consistent with an increase, by a factor ∼4, in the plume source rate from the E3 to the E5 flyby.…”

“…In Figure 2 , we show a three-dimensional (3D) projection of these data over the Enceladus south polar terrain to illustrate how the jet structure observed by INMS is spatially distributed relative to the tiger stripes. Using these E14, E17, and E18 INMS data, Hurley et al ( 2015 ) compared Monte Carlo plume models fed by (1) the eight (Spitale and Porco, 2007 ) sources and (2) a source continuously distributed along the tiger stripes, and concluded that data are best explained by a continuous source with location-dependent emission strength. In this study, we test the viability of a plume model fed by the 98 jets identified and precisely located in high-resolution imaging by Porco et al ( 2014 ), with spatially and temporally variable vapor jet sources consisting of both a broad (slow, isotropic emission) and a sharp (fast, directed emission) component.…”