Monte-Carlo Model of Europa's Water Vapor Plumes

Abstract
It has long been postulated that Europa might have a sub-surface ocean covered by an icy crust. First clues for the existence of such a sub-surface ocean were obtained by the Galileo magnetometer in the late '90s [1]. If such an ocean indeed exists, it might sporadically erupt in plumes. Indeed, in 2014, [2] reported on increased Lyman-α and oxygen OI130.4 nm emissions, which the authors interpreted as transient water vapor measurements resembling water plumes. In this presentation we analyze different plume models to determine which model would result in an observation as the one presented by [2], and analyze what implications this might have for the upcoming measurements of the Neutral and Ion Mass spectrometer (NIM) onboard JUICE.

1. Plume observations
[2] characterized the source of the increased Lyman-α and oxygen OI130.4 nm emissions through modeling. According to their results, the emissions are best described by two individual water vapor plumes that are located at 90°W/55°S and 90°W/75°S, respectively, both of which exhibit a radial expansion of ~200 km and a latitudinal expansion of ~270 km, and the surface densities of which amount to 1.3x10¹⁵ m^-3 and 2.2x10¹⁵ m^-3, respectively.

2. Plume model
The source of the observed plume might either be a liquid or a solid (icy) reservoir that evaporates or sublimates as it comes into contact with space. Figure 1 shows different scenarios which could lead to the localized release of H₂O particles resulting in a plume-like structure. In the first scenario, a surface, or near-surface, liquid reservoir is exposed to near-vacuum conditions upon which water directly evaporates into space. In the second scenario, a crack in the ice shell all the way to the bottom leads to the exposure of oceanic water to space, resulting in the formation of an oceanic plume. If the flow is chocked (by the conduit's geometry), the plume may become a supersonic jet. In the third scenario a rising diapir results in the warming of local surface ice (indicated by the shaded region in Figure 1), which sublimates into space. The water temperature was set to 280 K in scenarios one and two, whereas the ice temperature was set to 250 K in scenario three, and the reservoir areas were set to ~1'000 m² and ~20'000 m², respectively. 

Figure 1: The three analyzed scenarios: Scenario one shows evaporation of a surface, or near-surface, liquid, scenario two shows evaporation of oceanic water (in form of a jet), and scenario three shows the sublimation of surface ice heated by diapirs.

3. Monte-Carlo Model
To simulate Europa's plumes, we use a 3D Monte Carlo model originally developed to model Mercury's exosphere [3]. In this model particles are created ab initio, travel on collision-less trajectories, and are removed as they are either ionized, fragmented, or lost either to space or by freezing out on the surface. The grids are ~25x25x25 km³ in size, thus almost a factor 10 higher in resolution than the [2] measurements. For comparison with the [2] observations we also merged our model results into 200x200x200 km³ bins.

4. Model Results
Figure 2 shows our model results for the three different scenarios presented above. For each scenario, we present the [2] measurement on the left, the reduced resolution result in the middle, and the high resolution result on the right. All measurements are normalized to one and span six orders of magnitude. 

Figure 2: Model results for the three analyzed scenarios. The top row shows the surface liquid scenario, the middle row shows the oceanic water jet scenario, and the bottom row shows the diapir scenario. The [2] measurement is shown on the left, the reduced resolution model results are shown in the middle, and the high resolution model results are shown on the right.

5. Discussion
Whereas the observed ~200 km scale height is met by all three modeled scenarios, only scenario number two, the oceanic water jet scenario, results in a narrow enough plume structure. Both scenarios number one and three are too broad to be in good agreement with the [2] observations. It thus seems that for the high radial scale height but the low latitudinal expansion a narrowing factor needs to be present, as for example a conduit-like geometry provides. The presence of a nozzle does not necessarily require that the liquid stems from the ocean, though. If a throat exists close to Europa's surface, it is also possible that a water inclusion close to the surface results in a jet-like geometry.

6. NIM Plume Observations
NIM is a highly sensitive neutral gas and ion mass spectrometer designed to measure the exospheres of the Europa, Ganymede, and Callisto. The detection limit is at 10^-16 mbar for a 5 second integration time, which translates to a particle density of ~1 cm^-3. NIM's mass resolution is M/ΔM > 1100 in the mass range 1-1000 amu. In addition to the modeled 3D plume density profiles, we will also present modeled mass spectra for the individual scenarios, and discuss their implications for positive plume identification possibilities.

7. Conclusion
The origin of the observed Europa water vapor plumes is of high scientific interest because the plume's chemical composition directly represents the reservoir's chemical composition. If the plume thus indeed originates in the ocean expected to lie underneath Europa's surface ice layer, analysis of its chemical omposition with a mass spectrometer would offer us direct information about the chemical composition of the water ocean itself. Such information would in turn teach us more about Europa's habitability, and about the possibility of Europa harboring life.

[1] Khurana, K. K., et al.: Induced magnetic fields as evidence for subsurface oceans in Europa and Callisto, Nature, Vol. 395, pp. 777-780, 1998.

[2] Roth, L., et al.: Transient water vapor at Europa's south pole, Science, Vol. 343, pp. 171-174, 2014.

[3] Wurz, P., and Lammer, H.: Monte-Carlo simulation of Mercury's exosphere, Icarus, Vol. 164, pp. 1-13, 2003.