  Europa has been spotted to have water outgassing activities by the space and
ground-based telescopes as well as reanalysis of the Galileo data (Roth et al.
2014; Sparks et al. 2016, 2017; Paganini et al. 2020; Jia et al. 2018; Arnold
et al. 2019). However, these observations only provided limited information
about plume dynamics, which is critical in understanding the eruption mechanism
and preparation of future exploration. We adopt a 3D DSMC model to investigate
the plume characteristics of Europa assuming supersonic expansion originated
from the undersurface vent. The main goal is to understand the physical
processes and structures of Europa water vapor plumes, which can play a key
role on probing its undersurface vent condition and outgassing mechanism. With
a parametric study of the total gas production rate and initial gas bulk
velocity, the gas number density, temperature and velocity information of the
outgassing plumes from the various case studies are derived. Our results show
that the plume gases experience acceleration through mutual collisions and
adiabatic cooling when exiting and expanding from the surface. The central part
of the plume with the relatively large gas production rates (of 1029 and 1030
H2O s-1) is found to sustain thermal equilibrium and nearly continuum
condition. Column density maps integrated along two different viewing angles
are presented to demonstrate the importance of the projection effect on remote
sensing diagnostics. Finally, the density profiles at different altitudes are
provided to prepare for observations of Europa plumes including the upcoming
spacecraft missions such as JUICE and Europa Clipper.
