46th Lunar and Planetary Science Conference (2015)
AND CASSINI INMS DATA. D. M. Hurley1, M. E. Perry1, J. H. Waite2, and R. Perryman2; 1Applied Physics Laboratory, Johns Hopkins University, Laurel, MD ([email protected]), 2Southwest Research Institute, San Antonio, TX.
Introduction: Since their discovery [1], the vapor
plumes emanating from the south pole of Enceladus
have been imaged and sampled with every opportunity.
Cassini has passed Enceladus 19 times, with most of
the trajectories going through the plume. The Cassini
Ion and Neutral Mass Spectrometer (INMS) has confirmed that the vapor plumes of Enceladus release water and other volatiles into the space over the south
pole of Enceladus[2]. The constituents of the plume
provide insight into the subsurface composition of Enceladus, as their sources are ultimately tied to some
region below the surface. Further, understanding the
dynamics of their release from the surface provides
information that can be related to the subsurface acceleration mechanisms.
Variability in the water vapor emanating from the
south polar region of Enceladus stems from factors that
affect the source rate or the release parameters of the
vapor. Understanding of the variability and the factors
that influence it provides insight into the mechanism
producing the plume. Variability was first based on
magnetometer data gathered during the first three Enceladus encounters[3] and later found in the molecular
emissions as measured by Cassini INMS (Ion and Neutral Mass Spectrometer) [4]. Recently, VIMS observations of the dust to link plume variability to the mean
anomaly of Enceladus indicates that tidal forces may
modulate the emissions [5].
The Cassini INMS profiles of volatiles in the
plumes of Enceladus show variations from flyby to
flyby [6]. Owing to the different geometries of the flybys, temporal and spatial variations are necessarily
convolved in the data. We present a model that can be
used to decouple spatial variability from temporal variability in the distribution of material in the vicinity of
Model: A model that was developed for the
Earth’s Moon [7] and applied to the vapor plumes resulting from spacecraft impacts on the Moon [8] was
adapted for application to the distribution of vapor at
altitude over Enceladus [9]. The model is a Monte
Carlo model that follows particles using the equation
of motion under Enceladus’ gravity. It neglects collisions, rotational effects, and Saturn’s gravity. This is a
simplification used to demonstrate some basic principles of the distribution of exospheric particles emitted
from Enceladus’ jets. The expected influences of these
simplifications on the model results are discussed.
A model run consists of initializing a set of usually 100,000 particles with position and velocity vectors
representative of the jets. These particles are propagated through time without collisions under the influence of Enceladus’ gravity using a 4th order RungeKutta algorithm. The particles continue until they
reach the Hill Sphere at a distance of 949 km from
Enceladus. The very few particles that return to the
surface of the moon before escaping are considered
trapped on the surface and are no longer followed.
Photodissociation, charge exchange and electron impact ionization loss processes are neglected.
A large number of discrete sources of varying
magnitude contribute to the plume[10]. [11] triangulated the sources from multiple views and reported 8 primary sources. In some model runs, the initial locations
are evenly distributed between those 8 sources. However, we have also programmed in the option of having
the source locations to be spread out along the lengths
of the tiger stripes [12]. The run that uses the tiger
stripes as the initial location is identified below.
The initial velocity for each particle is selected
from a drifting Maxwellian distribution, which is comprised of 2 components that are added in vector form.
The first component is the bulk velocity of the jet. For
every particle coming out of a single jet, the bulk velocity is set to the same magnitude and direction. The
default setting is to use the directions presented in [11].
However, this is allowed to change to better reproduce
observations. The second velocity component is the
thermal contribution. The thermal component varies
from particle to particle. The magnitude of the thermal
component is selected from a Maxwell Boltzmann
distribution at the assigned temperature of the jet gases. The direction is selected from an isotropic distribution in 4 pi steradians. The bulk velocity vector and
the thermal velocity vector are added for each particle
to comprise the initial velocity.
The primary constituent of the vapor plumes is
water [2]. However, the interaction between water and
the walls of the INMS complicates producing a spatial
profile of the water density using in situ sampling from
the INMS [13]. The spatial profiles of minor species
with molecular mass 44 and 28 are measured by
INMS. The u=44 species presumably is CO2. The
u=28 species may be N2 or CO, or some combination
of the two. The model does not include losses from
photolysis, charge exchange, or other collisional processes. Therefore the only important parameter to signify the species is its molecular mass because it influences the thermal velocity. The INMS measurements
of mass 44 and 28 species can be used as a proxy for
46th Lunar and Planetary Science Conference (2015)
Figure 1. Comparison of low Mach number model and high Mach number model to INMS data for 3 Casssini
passes. These passes are nearly parallel to each other and to the tiger stripes. There are density enhancements observed near closest approach that can’t be explained with either high Mach number flow coming from the tiger
stripes or low Mach number flows coming from the tiger stripes. Many of these line up with previously identified
water. The model is used to simulate each of these
three species separately. The relative source rates of
the different species can be folded in after the simulations by including a spatially constant scaling factor.
Results: Model runs have addressed some of the
enigmatic observations from the INMS, including differences in the temporal profiles of the ratio of mass 44
to mass 28. These are found to be the effect of proximity to source regions and the higher columnation of
higher mass material than lower mass material. Most
of the enhancements in the INMS observations correspond to crossing the tiger stripes or closest approach
to strong jets. However, some jets are clearly not observed in the INMS data, indicating that they are either
not directed in the previously inferred direction or are
not active during the Cassini flyby. The background
ramp up is used to determine the plumes emanating
along the length of the tiger stripes, which is found to
be substantial. The small scale structure is used to
identify specific jets and their relative strength. Differences in the mass 28 and mass 44 channels are used
to constrain the effective temperature of the vapor.
References: [1] Dougherty et al., Science 311,
2006. [2] Waite et al., Science 311, 2006. [3] Saur et
al.. GRL 35, 2008. [4] Smith et al., JGR 115, 2010. [5]
Hedman et al., Nature 456, 2013. [6] Perry et al., submitted to Icarus. [7] Crider and Vondrak, JGR 105,
2000. [8] Hurley, JGR 116, 2011 [9] Hurley et al.,
submitted to Icarus [10] Hansen et al., Nature 456,
2008. [11] Spitale and Porco, Nature 449, 2007. [12]
Spencer and Nimmo, Ann. Rev. Earth Plan. Sci. 41,
2013. [13] Teolis et al. JGR 115, 2010.