Hypervelocity Impact Effect of Molecules from Enceladus’ Plume and Titan’s Upper Atmosphere on NASA’s Cassini Spectrometer from Reactive Dynamics Simulation

The NASA/ESA Cassini probe of Saturn analyzed the molecular composition of plumes emanating from one of its moons, Enceladus, and the upper atmosphere of another, Titan. However, interpretation of this data is complicated by the hypervelocity (HV) flybys of up to $\sim 18\mathrm{km}/\sec$ that cause substantial molecular fragmentation. To interpret this data we use quantum mechanical based reactive force fields to simulate the HV impact of various molecular species and ice clathrates on oxidized titanium surfaces mimicking those in Cassini’s neutral and ion mass spectrometer (INMS). The predicted velocity dependent fragmentation patterns and composition mixing ratios agree with INMS data providing the means for identifying the molecules in the plume. We used our simulations to predict the surface damage from the HV impacts on the INMS interior walls, which we suggest acts as a titanium sublimation pump that could alter the instrument’s readings. These results show how the theory can identify chemical events from hypervelocity impacts in space plumes and atmospheres, providing in turn clues to the internal structure of the corresponding sources (e.g., Enceladus). This may be valuable in steering modifications in future missions.

Figure 1

Predicted and INMS fragment data from ${\mathrm{H}}_2\mathrm{O}$ ice clusters of radius 10.5 to 13.0 Å in increments of 0.5 Å, ${\mathrm{CO}}_2$ molecules and ${\mathrm{CO}}_2[20{\mathrm{H}}_2\mathrm{O}]$ ice water clathrate (dotted lines) impacts as a function of encounter velocity. Accurate fractions and rates of change predicted for ${\mathrm{H}}_2\mathrm{O}$, ${\mathrm{H}}_2$, and CO production, compared to INMS data from Ref. 2. Correct trend for ${\mathrm{CO}}_2$ and ${\mathrm{O}}_2$ production, except for a decrease in ${\mathrm{CO}}_2$ between $E3\mathrm{\text{−}}E5$ and albeit overpredicted magnitudes.

Figure 2

Primary predicted fragments for methane, ethylene, and benzene molecular impacts. H, OH, ${\mathrm{CH}}_2$, and ${\mathrm{CH}}_3$ radicals not included. Each case corresponds to one randomly oriented molecule per ${\mathrm{TiO}}_2$ surface impact over 100 trajectories per impact velocity. Curves correspond to reflected species. Chemisorbed molecules not included.

Figure 3

Simulated mass spectra for hexane isomers shows topological effects on fragmentation patterns after impact and the possibility of disambiguating overlapping spectral peaks from INMS data (e.g., mass 28, CO, and ${\mathrm{C}}_2{\mathrm{H}}_4$).

Figure 4

RMD simulated ${\mathrm{TiO}}_2$ (110) crater radius versus ice cluster impactor radius is consistent with experimental data from Kearsley et al. [18] associated with NASA’s Stardust mission. Atomistic crater profiles shown as insets. The ratio of crater ($r_c$) to cluster radius ($r_p$) is linear over the range of impact velocities and its slope ($s$) provides an indication of increasing or decreasing damage (i.e., at $E5$ $s=3.6$; therefore, a larger diameter damage is expected than at $E3$ with $s=3.7$). Kearsley et al. report an $s=2.2–4.63$.