JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, A09220, doi:10.1029/2004JA010635, 2005
A self-consistent model of plasma and neutrals at Saturn:
Neutral cloud morphology
S. Jurac and J. D. Richardson
Center for Space Research, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA
Received 16 June 2004; revised 3 October 2004; accepted 2 June 2005; published 17 September 2005.
[1] We present a model of the plasma and neutral environment that treats plasma and
neutrals self-consistently, using Voyager plasma and ultraviolet H observations and
Hubble Space Telescope OH measurements as constraints. The neutral distributions are
determined with a Monte Carlo model, and the plasma distributions are determined with a
diffusive transport model including sources and losses. We find a larger percentage of
molecular ions than in previous work, that H2O is the dominant neutral near the satellite
sources but OH is dominant elsewhere, and that the total H2O source required is
1028 H2O/s, a factor of 3 larger than previously thought. The large amount of water
appears to emanate from Enceladus’s orbital distance, and the production mechanism
remains unclear. We produce density contours for each neutral species and discuss the
implications of these results in anticipation of new data from Cassini.
Citation: Jurac, S., and J. D. Richardson (2005), A self-consistent model of plasma and neutrals at Saturn: Neutral cloud morphology,
J. Geophys. Res., 110, A09220, doi:10.1029/2004JA010635.
1. Introduction
[2] The magnetosphere of Saturn contains satellites and
rings composed primarily of water ice. When the Voyager
spacecraft passed through the magnetosphere in 1980 and
1981, they detected a plasma consistent with a water source,
although the mass/charge of the heavy ion species was not
uniquely determined [Bridge et al., 1981, 1982; Lazarus
and McNutt, 1983; Richardson, 1986]. The source of the
water ions was thought to be sputtering of water from the
inner satellites and rings by energetic particles and corotating plasma and subsequent ionization [Lanzerotti et al.,
1983; Bar-Nun et al., 1985]. The sputtering yields and
energy distribution were used to calculate neutral densities
from the satellites [Johnson et al., 1989] and rings
[Pospieszalska and Johnson, 1991]. The plasma observations of density and temperature from the Voyager spacecraft were combined to create a model of the plasma density
throughout the magnetosphere [Richardson and Sittler,
1990]. A cloud of neutral hydrogen was also observed
and interpreted first as a torus surrounding Titan and
extending from 8 to 25 RS [Broadfoot et al., 1981] and
subsequently as a cloud extending in to Saturn with a local
time asymmetry [Shemansky and Hall, 1992].
[3] Combination of the neutral H density and the plasma
densities and temperatures gave an estimate of the ion
production rate and of the plasma transport rate. Although
these models did well at fitting the heavy ion density, the H
density derived from ultraviolet spectrometer (UVS) observations gave a proton source that seemed too large to match
the plasma observations. Shemansky and Hall [1992] predicted large neutral OH densities would be present; Hubble
Copyright 2005 by the American Geophysical Union.
0148-0227/05/2004JA010635$09.00
Space Telescope (HST) observations verified this prediction [Shemansky et al., 1993]. Subsequent observations
[Hall et al., 1996; Richardson et al., 1998] were combined
with a Monte Carlo model to construct a model OH cloud
morphology that is consistent with the HST observations
[Jurac et al., 2002]. The peak OH density is about
1000 cm3 at the equatorial plane at 4 RS. Ip [2000]
modeled the plasma composition assuming the peak density of the water group neutrals was 10,000 cm3 at
Enceladus and found that large neutral densities would
result in comparable molecular (OH+ and H2O+) and
atomic (O+) ion densities.
[4] In this paper, we combine the neutral model of Jurac
et al. [2002] and the plasma transport and chemistry model
of Richardson et al. [1998] to find self-consistent neutral
and plasma densities. The plasma source depends on the
neutral density, composition, and distribution. Similarly, the
neutral density, composition, and distribution are affected
by interactions with the plasma. We stress the heavy species
in this paper since almost all of the heavy species come
from sputtering of the rings and satellites, whereas H has
numerous sources, such as Titan and Saturn’s atmosphere
and ionosphere, which are not well constrained [Ip, 1996;
Smyth and Marconi, 1993; Shemansky and Hall, 1992;
Eviatar et al., 1990; Richardson and Eviatar, 1987]. These
models, combined with constraints from the HST and
Voyager observations, give self-consistent distributions of
O, OH and H2O.
2. Models
2.1. Observational Constraints
[5] The Saturnian environment was sampled in situ by
Pioneer 11 and Voyager 1 and 2 and remotely by the UVS
on Voyager and by the Hubble Space telescope. These
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Table 1. Summary of HST Line-of-Sight OH Observations:
Distance From the Spin Axis R, Distance From the Equatorial
Plane Z, and Measured Brightness in Rayleighsa
Date
R(Rs)
Z(Rs)
Brightness
Source
24 – 26 Aug. 1992
17 Dec. 1994
18 Dec. 1994
20 Dec. 1994
9 Aug. 1995
9 Aug. 1995
9 Aug. 1995
10 Aug. 1995
10 Aug. 1995
4 Oct. 1996
4 Oct. 1996
4 Oct. 1996
12 Oct. 1996
12 Oct. 1996
12 Oct. 1996
12 Oct. 1996
12 Oct. 1996
4.5
4.5
6.0
10.0
1.9
1.9
1.9
2.1
2.3
3.0
3.0
3.0
4.5
4.5
4.5
4.5
4.5
0
0
0
0
0.28
0.40
0.60
0.28
0.28
0.125
0.000
0.125
0.125
0.000
0.125
0.254
0.375
37 ± 12
131 ± 12
22 ± 10
1±6
77 ± 17
61 ± 12
20 ± 13
85 ± 23
111 ± 12
248 ± 50
324 ± 33
282 ± 33
109 ± 7
122 ± 7
102 ± 7
98 ± 9
77 ± 7
Shemansky et al. [1993]
Richardson et al. [1998]
Richardson et al. [1998]
Richardson et al. [1998]
Hall et al. [1996]
Hall et al. [1996]
Hall et al. [1996]
Hall et al. [1996]
Hall et al. [1996]
Jurac et al. [2002]
Jurac et al. [2002]
Jurac et al. [2002]
Jurac et al. [2002]
Jurac et al. [2002]
Jurac et al. [2002]
Jurac et al. [2002]
Jurac et al. [2002]
a
Data are normalized to a Saturn-Earth distance of 8.9 AU. A detailed
description of previous measurements and the data reduction are given by
Richardson et al. [1998].
measurements provide constraints that must be accommodated by models of the near-Saturn environment.
[6] Voyagers 1 and 2 found a plasma composed of
protons, water group ions and possibly nitrogen ions. The
plasma instruments on the Voyager spacecraft could not
differentiate between the N+, O+, OH+, H2O+, and H3O+
ions thought to be present in Saturn’s magnetosphere. Thus
the observations give the proton density and temperature,
total heavy ion density and average temperature, and the
bulk speed of the plasma.
[7] The Voyager UVS experiment detected a large cloud
of neutral H surrounding Saturn. These were line-of sight
measurements, so the spatial resolution is not well determined, but a density of 100 H cm3 was estimated for near
the orbit of Dione [Shemansky and Hall, 1992]. Four sets of
OH measurements were taken by HST. These sets of
measurements generally consisted of several lines of sight
through the OH cloud at different radial distances from
Saturn and at different heights above Saturn’s equator. The
HST measurements are summarized in Table 1. The combination of these measurements provides fairly stringent
constraints on the density and morphology of the OH cloud
near Saturn [Jurac et al., 2002]. The peak OH density peak
is over 1000 cm3 near Enceladus and the OH density is
about 600 cm3 near Mimas.
2.2. Neutral Model
[8] The neutral model was described by Jurac et al.
[2002, 2001b]; a Monte Carlo code follows ejected neutrals
as they orbit in Saturn’s gravitational field from their
injection until they are lost either by ionization, by collisions with rings, moons, or Saturn, or by escape from the
Saturnian system. The model includes the effects of plasma
chemistry and both neutral-neutral and plasma-neutral collisions and thus tracks the dynamic evolution of the water
group neutrals in Saturn’s magnetosphere. The dominant
neutral dissociation channels H2O ! OH + H, H2O ! O +
H2, and OH ! O + H are considered and energy balance is
maintained. For example, when H2O is photodissociated to
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form OH, the newly formed OH picks up 1/17 of the excess
energy of 3.41 eV, or 0.2 eV. The same energy is given to
electron-dissociated OH in the absence of a more reliable
estimate. Here only the dominant dissociation channels are
modeled; Ip [1997] lists other reactions that occur. The
lifetimes of the neutrals against loss to charge exchange,
dissociative recombination and electron impact dissociation
depend on the local plasma energies and densities.
[9] The source rates of water molecules ejected from each
moon are free parameters in the model and these rates are
adjusted to give the best fit to the observations. Like the
moons, the ejected neutrals orbit with speeds close to the
local Keplerian speed, while the plasma ions are frozen in to
the magnetic field and rotate with Saturn’s spin rate. This
velocity difference between fast ions and slower neutrals
results in charge exchange and also in collisions in which
momentum is transferred to the neutrals, expanding the
cloud and causing the loss of neutrals. The mean free path
between collisions, as well as the distance of the closest
approach between molecules, depends critically on the ion
and neutral densities. A Monte Carlo transport simulation
with ZBL collisional potentials [Ziegler et al., 1985;
Eckstein, 1991] was employed to describe the ion-neutral
and neutral-neutral collisions. The distance of closest approach for each collision is chosen by a Monte Carlo
procedure that depends on the mutual velocity between
the colliding particles; thus the collisional frequency is
heavily influenced by the velocities and densities of the
colliding species. The H2O and OH collisional interaction
potentials are approximated with a collisional atomic potential for oxygen (O). The substantially larger mass and
size of O compared to H makes its contribution to the
collisional cross section dominant. Since the molecular
collisions are not elastic, an inelastic energy loss of 30%
was assumed for ion collisions with H2O and OH on the
basis of a separate molecular dynamic calculation (M. Liu,
University of Virginia, private communication, 2001). BarNun et al. [1985] show that the sputtering process releases a
fraction of the water constituents in atomic form. In our
previous model [Jurac et al., 2002], a 100% H2O source
was assumed. Here we assume that 5% of the total source
rate of 1028 mol/s consists of H released from the surface in
atomic form, i.e., 284 kg/s H2O and 0.8 kg/s H.
[10] The momentum transfer collisions, especially those
between fast O+ ions and heavy neutrals, produce a substantial inflation of the cloud around the source region (as
shown for other neutral clouds by Smyth and Marconi
[1993] and Decker and Cheng [1994]). The plasma ions
that corotate with the magnetic field collide with slower
neutrals that orbit with Keplerian velocities. Momentum
transfer is particularly important for heavy species that
dominate the neutral population (H2O and OH) and have
larger cross sections than H. Outside Enceladus, the ion and
neutral densities decrease but the difference between the
corotational and Keplerian velocities increases; the peak
collisional dispersion of the neutral cloud occurs between
Enceladus and Tethys. The ion-neutral collisions substantially inflate the cloud in latitude and cause the loss of
neutrals. When a close collision with a heavy ion (O+, OH+,
H2O+) occurs, a neutral can be ejected from Saturn’s
magnetosphere. More frequently, the collisions are glancing
blows where only part of the collisional energy is trans-
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the OH cloud critically depends on the cloud’s vertical
profile, since OH lifetimes are much shorter near the
equatorial plane. Figure 1 compares observed OH brightnesses off the equatorial plane (Figure 1a) and in the
equatorial plane (Figure 1b) with model values (lines),
assuming that the neutral cloud is a steady state feature.
The model values show good agreement with data over four
years of observations, which suggests OH densities are
relatively constant over that time period.
2.3. Plasma Transport Model
[ 13 ] The plasma transport model is described by
Richardson et al. [1998]. Plasma transport is governed by
the diffusion equation
@Ni L2
@ DLL @Ni L2
þ Si Ri
¼ L2
@t
@L L2 @L
Figure 1. (a) Brightness profile of the model OH cloud
compared to HST brightness observations at different
vertical distances from Saturn’s equator. Symbols represent
observed brightnesses, and lines represent the modeled OH
emission. (b) Same as Figure 1a for radial distance from
Saturn. HST observational uncertainties are given as 1
standard deviation levels.
ferred to the neutral and the neutral ends up in a more
eccentric orbit, with a higher likelihood of intersecting and
being absorbed by the main rings. Both collisional and
dissociative momentum transfer are crucial for understanding the neutral cloud morphology.
[11] Neutral lifetimes are critically important for determining the total neutral densities, and the plasma source
depends on the neutral density, composition and distribution. Neutral lifetimes are shortest in the equatorial plane
where the plasma density peaks, ranging from few days for
O to a few months for OH. As described later, selfconsistent iteration between the plasma and neutral densities
is used to find the plasma composition, and once that is
determined, the neutral lifetimes are calculated. These lifetimes are then used in a Monte Carlo procedure to determine
the interaction likelihood for each species in two-dimensional (2-D) boxes (dimensions of latitude and radial
distance) at each time step of the simulation.
[12] The HST observations discussed above give the total
OH content along a line of sight through Saturn’s magnetosphere; the model OH densities are adjusted to provide a
good fit to these observations, as done by Jurac et al.
[2002]. Analogous to tomography, many line-of-sight
observations are used to build a 3-D picture of the OH
cloud morphology. These vertically resolved brightnesses
allow us to deduce a fairly complete picture of the latitudinal neutral distribution and associated scale heights of the
neutrals. The off-equatorial observations are especially
important because the source strength needed to produce
where Ni is the number of ions in a magnetic flux shell per
unit L, DLL is the diffusion rate and Si and Ri are source and
loss terms [Fälthammar, 1968; Siscoe, 1978]. The diffusion
rate has the form DLL = KLm, where K and m are constants;
we set m = 3, consistent with both theoretical predictions for
atmospherically driven diffusion [Brice and McDonough,
1972] and observations [Hood, 1989]. The boundary
conditions are that the plasma density go to zero at the
outer edge of the main rings (L = 1.5) and at the
magnetopause (L = 20). The source and losses are due to
the chemistry, ionization of neutrals, charge exchange,
recombination, and dissociation. We set the electron and ion
temperatures equal to those observed and include a
suprathermal electron component based on the observations
[Sittler et al., 1983; Richardson and Sittler, 1990]. We use
the neutral densities from the Monte Carlo model and the
initial plasma composition (the total plasma density is fixed
by the measurements) to calculate the ion source distributions and rates; we must use the ion source rates as the input
since starting from the neutral densities does not produce a
stable solution [Huang and Siscoe, 1987]. The diffusion
equation is then solved numerically, stepping forward in
time until a steady state is reached.
[14] We adjust the diffusion coefficient to obtain a good
match to the data; since the Voyagers measured the total
heavy ion density, we sum the ion species in the model for
this comparison. A value of DLL = 9 109 R2S s1 L3 gives
a good fit to the data and is consistent with the limits for
transport rates of 2 109 R2S s 1 L3 established by
Paonessa and Cheng [1986] and the value of 108 R2S s 1
L3 given by Richardson et al. [1998]. Roughly 30% of the
neutrals are lost via ionization and thus contribute to the
plasma source. About 20% of the plasma is lost via
transport and the rest via recombination and charge exchange. We then use the new plasma composition to
calculate a new ion source rate and iterate until the composition converges. This new plasma distribution is then used
to recalculate the neutral lifetimes used in the Monte Carlo
model (only the composition of the plasma changes significantly since the other parameters are constrained by the
Voyager observations). The Monte Carlo model then is run
again, with the source distribution adjusted to give a good
fit to the HST data, and the new neutral densities are then
used to calculate plasma source rates. We iterate until the
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Figure 2. Model cross sections of the neutral cloud
densities in Saturn’s magnetosphere for each species. Strong
peaks in neutral density are found near Enceladus and
Mimas, while other satellites produce minor density
enhancements.
plasma and neutral densities converge, giving a self-consistent distribution of plasma and neutrals. Convergence occurred relatively rapidly; four steps were used to converge
to the results shown in this paper. As shown below, the
results of the iterations were mainly an adjustment of the
ratios of the ion species, with more molecular and less
atomic ions present than previous work suggests.
3. Results
[15] Figure 2 shows the neutral density profiles derived
from the model. The top panel shows the OH density, which
is the species best constrained by the observations. The peak
OH density of over 1000 cm3 is near Enceladus and a
density peak of about 600 cm3 is associated with Mimas.
The other satellites produce smaller density peaks since they
are only minor sources (Table 2). The other major neutral
species are shown in the next three panels of Figure 2. The
peak O densities are just over 100 cm3 near the orbit of
Mimas. The H2O density is over 1000 cm3 from Mimas to
Enceladus with maxima at each satellite. The H density also
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has maxima at each satellite, with densities of over
200 cm3 near Enceladus and over 100 cm3 near Mimas.
[16] The neutral densities decrease rapidly inside Mimas
because of collisions of the neutrals with the particles in the
main rings. The densities also decrease rapidly outside
Enceladus because of the larger volume, larger distance
from the source, and shorter neutral lifetimes.
[17] The more processed the neutrals are, and the longer
they remain in the system, the more they spread in radius
and especially in Z. H2O is the neutral most tightly confined
about the equator. The OH daughter molecules formed by
the dissociation of H2O gain an average energy of 0.2 eV
from the dissociation process in addition to their orbital
energy. This excess energy is large enough to displace the
OH molecules from the equatorial plane, but not to otherwise substantially change their orbits. Outside 3 RS, collisions with corotating ions play an important role in
depleting equatorial densities and inflating the cloud.
[18] The O is formed by dissociation of OH and more
rarely H2O, in each case picking up energy that extends the
size of the O cloud. The O has a relatively short lifetime
against charge exchange with O+. These effects combine to
lower the O density in the magnetosphere and distribute the
O in a broad cloud about the equator.
[19] The H has two components. The H formed from
dissociation of H2O and OH gains energy from the dissociation process comparable to the Keplerian energy; some is
ejected from the system and some forms a large, tenuous
cloud surrounding Saturn. The H released in atomic form
makes a narrow H cloud near the equator.
[20] Table 2 shows a satellite source distribution that
provides good agreement with the data. The model indicates
that about 80% of the water comes from the Enceladus/E
ring region, about 10% from near Mimas, and only a few
percent from the other larger moons. Rhea’s contribution is
poorly determined since the only observation near its orbit
had a large uncertainty. A total H2O source rate of 1028
H2O/s is required to maintain the observed neutral densities,
compared to recent estimates of 3.75 1027 [Jurac et al.,
2002] and 1.4 1027 H2O/s [Richardson et al., 1998] based
on models where self-consistency between plasma and
neutrals was not required. Our model does not include
any contribution from the main rings, since neutrals formed
in the rings remain confined to that region [Pospieszalska
and Johnson, 1991]. Brightnesses of OH were small and
increase with distance from Saturn above the main rings
[Hall et al., 1996], suggesting that these rings are not a
significant source of OH for the region beyond 3 RS.
Table 2. Source Region, Sum of the Heavy Neutral Density (H2O,
OH, and O) at the Orbital Distance of the Major Satellites in the
Equatorial Plane, Satellite Distance, Satellite Radius, and Source
Contribution From Each Region as a Fraction of the Total Source
Rate of 1028 H2O/s
Source Region
Mimas/G ring/F ring
Enceladus/E ring
Tethys
Dione
Rhea
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Heavy Neutral Density Distance Radius, Source,
Near Equator, cm3
Rs
km
%
3400
5900
900
300
50
3.08
3.95
4.89
6.26
8.74
197
250
524
559
764
11
82
3.5
2.5
1
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JURAC AND RICHARDSON: SATURN’S NEUTRAL CLOUDS
Figure 3. Fraction of each ion species in the equatorial
plane versus radial distance: OH+ (solid line), H2O+ (dashed
line), O+ (dash-dotted line), and H+ (dotted line). Around
the source region, from 2.5 to 7 RS, H2O+ ion dominates
OH+, while the opposite is the case away from the source.
[21] The previously inferred neutral source rates were
difficult to understand with the hypothesized water production mechanisms [Jurac et al., 2001a; Ip, 1997; Johnson et
al., 1993; Pospieszalska and Johnson, 1991]. The even
larger source now required magnifies the question of the
origin of the water injected into Saturn’s magnetosphere.
Not only does the total source rate remain a problem, but so
does the distribution of the source. The Mimas region
appears to produce about 4 times more H2O than Tethys
or Dione and the Enceladus region about 30 times more. If
either sputtering or micrometeorite bombardment is the
principal mechanism for producing water, much larger
surface areas are needed near Enceladus, where the E ring
resides, and near Mimas. Thus collisions between the ice
particles near Enceladus is a likely production mechanism
[Jurac et al., 2002].
[22] Figure 3 shows the fraction of each ion species in the
equatorial plane. Figure 4 shows the fraction of each neutral
species near the equator (0 – 0.5 RS, top panel) and off the
equator (0.5 – 1 RS, bottom panel). Both the neutral and ion
profiles are dominated by the molecular species. In the main
H2O source region, from 2.5 to 4.5 RS, water is the
dominant neutral near the equator, with local peaks near
the main satellites, especially near Enceladus where more
than 80% of the water is produced. Outside the source
region, OH is the main neutral, constituting about half of the
neutral population. Similarly, in the source region H2O+
densities are larger than those of OH+, while OH+ densities
are larger away from the source region. H2O+ dominates
from 2.5 to 7 RS and then falls slightly below OH+ outside
this distance. O+ makes up at most 15% of the ion
population. Off the equatorial plane, the H2O and O are
roughly equal in density out to 6 RS. The fraction of O
increases with distance, both in the equatorial plane and off
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the equator, because as the H2O produced mainly near
Enceladus collisionally drifts out, a larger fraction gets
dissociated.
[23] The predicted fraction of H has the largest uncertainty, both near the equator and off the equator. Near the
equator it depends on the amount of low-energy H entering
the system in atomic form. Higher-energy H is produced
mainly by dissociation, and how far it drifts depends on the
energy acquired in the various dissociation processes because its collisional cross section is small. Since H is far
lighter than the other water-like neutrals, a small uncertainty
in the energy it obtains via dissociation will significantly
influence its orbit.
[24] These results are quite different from those of
Richardson et al. [1998] and show the importance of
(1) using a Monte Carlo model to treat the neutral population and (2) requiring the plasma and neutral results be selfconsistent. The major differences in the current model are
that (1) the neutral O has lower densities and has a wide
latitudinal spread, lowering the O+ source, (2) larger H
densities deplete O+ via charge exchange, and (3) most of
the H formed from H2O is lost from the system reducing the
H+ densities.
4. Discussion
[25] The most perplexing issue, the source of the water,
becomes an even larger puzzle as a result of this study. The
Figure 4. Fraction of each neutral species (top) near the
equator (0 – 0.5 RS) and (bottom) off the equator (0.5 –1 RS).
In the main H2O source region, from 2.5 to 4.5 RS, water is
the dominant neutral near the equator, especially near
Enceladus, where more than 80% of the water is produced.
Outside the source region, OH is the main neutral,
constituting about half of the neutral population.
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changes in lifetimes due to the change in plasma composition and the addition of the H source, requires a source rate
almost three times larger than in previous work [Jurac et al.,
2002]. Satellite sputtering can produce only a fraction of the
water needed [Jurac et al., 2001a; Johnson et al., 1989; Shi
et al., 1995]. Sputtering is a process in which energetic
plasma ions bombard the icy surfaces of the moons and ring
particles and knock out water molecules. The deficiency of
the sputtering source became even more apparent after the
recent reanalysis of energetic charged particle composition
observed by Voyager 1 and 2 [Paranicas et al., 2004]. They
find lower amounts of energetic O+ than originally thought
[Krimigis et al., 1983]. Since energetic O+ is the dominant
sputtering agent, this new energetic ion composition would
produce much less water vapor and thus worsen the H2O
source problem. Ion sputtering of submicron E ring grains,
which would have to be much more abundant than currently
estimated, was suggested as a possible H2O production
mechanism [Jurac et al., 2001b]. In sputtering calculations,
an order of magnitude uncertainty can arise depending on
surface properties, the temperature of the irradiated ice,
porosity, and the grain size distribution. Because of the
Paranicas et al. [2004] findings and the larger H2O source
rate required here, we estimate that a surface area more than
2 orders of magnitude larger than that currently estimated
for the E ring would be needed. Consequently, E ring
sputtering as a principal H2O production mechanism
appears unlikely, although it cannot be completely ruled
out. Since a typical grain size distribution decreases with
grain radius as r3.5, and the surface area of an individual
grain with r2, the smallest grains contain the largest
surface area. In the (unlikely) case that E ring is dominated
by <0.01 micron grains, enough surface area could be
contained in these tiniest grains to produce the required
H2O source.
[26] Impacts on the moons and rings by micrometeorites,
the small debris from asteroids and comets, was suggested
as an H2O production mechanism [Ip, 1997]. However,
micrometeorite bombardment would produce peak densities
near the main rings [Pospieszalska and Johnson, 1991] and
near the larger moons, which are not observed.
[ 27 ] Hamilton and Burns [1993] proposed a selfsustained mechanism for generating the E ring, a diffuse
tenuous ring, with peak density at Enceladus’ orbital distance. They suggested that the E ring regenerates itself,
producing the water vapor as part of this process. The E ring
grains that end up on eccentric orbits would collide with the
moons, producing both fresh grains and water vapor
[Hamilton and Burns, 1994].
[28] Paranicas and Cheng [1997] inferred from the
Voyager 2 energetic particle data that a large absorbing
surface area might exist between Enceladus and its trailing
Lagrange point. Roddier et al. [1998] observed a bright arc
at Enceladus’s orbital distance, which they interpreted as a
signature of a collision between two pieces of icy debris. On
the basis of these findings, Jurac et al. [2001b] suggested
that a large amount of debris residing near Enceladus’
trailing Lagrange point, remnants of a former moonlet that
broke apart, could be the reason for both the existence of the
E ring and the neutral cloud. As subsequent HST data
revealed a larger water source, Jurac et al. [2002] concluded
that direct water evaporation in collisions between these
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debris might play a significant role in H2O production in
addition to ion sputtering. In light of the Paranicas et al.
[2004] findings and the larger required source rate, collisional impact evaporation [Eichhorn and Grun, 1993]
between debris (grain-grain, grain-fragment and fragmentfragment) could be the dominant H2O production mechanism, if debris does exist near Enceladus’ Lagrange point.
[29] Many of the predictions made in the paper are being
directly tested by CASSINI. The CAPS instrument will
measure the ion composition. The UVS instrument should
produce better estimates of the neutral H and perhaps O.
These new data will help us to adjust our model to predict
the species, especially the neutrals, which are not directly
observed.
[30] Acknowledgments. This work was supported under NASA
contract 959203 from the Jet Propulsion Laboratory to the Massachusetts
Institute of Technology and NASA planetary atmospheres grant
NNG05GB27G.
[31] Lou-Chuang Lee thanks John Cooper and Wing Ip for their
assistance in evaluating this paper.
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