The document summarizes findings from Cassini spacecraft observations of plumes emerging from Saturn's moon Enceladus. Analysis of particle impacts on the spacecraft detected three main compositional types of particles, including one rich in sodium and potassium salts. Close to the moon, the proportion of salt-rich particles increased while salt-poor particles decreased, indicating salt-rich particles dominate near the surface. Models accounting for particle size and ejection velocity match observations and suggest a salt-water reservoir beneath the surface of Enceladus provides nearly all matter in the plume.

1. LETTER doi:10.1038/nature10175
A salt-water reservoir as the source of a
compositionally stratified plume on Enceladus
F. Postberg1,2, J. Schmidt3, J. Hillier4, S. Kempf2,5,6 & R. Srama2,7
The discovery of a plume of water vapour and ice particles emerging terrain. Salt-poor type I grains can form by homogenous nucleation
from warm fractures (‘tiger stripes’) in Saturn’s small, icy moon from the gas phase7,8. In contrast, the salt-rich type III particles have to
Enceladus1–6 raised the question of whether the plume emerges from be formed from salt-ice condensation cores, presumably frozen spray
a subsurface liquid source6–8 or from the decomposition of ice9–12. of salt water8. The latter mechanism naturally forms larger grains,
Previous compositional analyses of particles injected by the plume which then receive lower average ejection speeds for a given density
into Saturn’s diffuse E ring have already indicated the presence of and speed of the carrier gas7. Indeed, measurements of CDA’s High
liquid water8, but the mechanisms driving the plume emission are Rate Detector18 and photometry of the plume in the near-infrared19
still debated13. Here we report an analysis of the composition of both indicate that the grain ejection velocity decreases with size. This
freshly ejected particles close to the sources. Salt-rich ice particles would lead to the compositional stratification of the plume, with an
are found to dominate the total mass flux of ejected solids (more (observed) increase of the proportion of salt-rich grains close to the
than 99 per cent) but they are depleted in the population escaping sources. This idea is supported by the generally larger yields of ions
into Saturn’s E ring. Ice grains containing organic compounds are from impacts of salt-rich particles recorded in the E ring (Sup-
found to be more abundant in dense parts of the plume. Whereas plementary Fig. 5). Proportionally fewer of the slower (larger) salt-rich
previous Cassini observations were compatible with a variety of grains escape the moon’s gravity and end up in the E ring, explaining
plume formation mechanisms, these data eliminate or severely the dominance of the smallest of the three types (type I) in the E ring8.
constrain non-liquid models and strongly imply that a salt-water We use a simple model (Supplementary Information) to test the
reservoir with a large evaporating surface7,8 provides nearly all of the hypothesis of whether a general size difference between salt-poor and
matter in the plume. salt-rich grains can cause the observed variations. We use the two-
During three traversals (E4, E5 and E7) of the Enceladian plume body gravitational interaction20 of plume particles with Enceladus to
(Supplementary Fig. 1), Cassini’s Cosmic Dust Analyser (CDA) pro- construct the particle number density expected along the spacecraft
duced time-of-flight mass spectra generated by high-velocity impacts trajectory. For this fly-by, we consider only grains larger than the
of individual grains onto a metal target14. A specific configuration of detector threshold of ,0.2 mm. In reality, the size distributions of
CDA at E5 (Supplementary Information) provided a sampling rate salt-rich and salt-poor grains overlap, but to model different average
high enough to infer a spatial compositional profile in the plume. sizes we assume that all grains larger than a certain radius (found to be
Most plume spectra can be assigned to one of three compositional 0.6 mm) are salt-rich, and those smaller are salt-poor. Before ejection,
types (Supplementary Fig. 4) previously detected in the E ring8,15,16. both species obey a single continuous power-law size distribution. We
One of these types (type III, ,6% of E-ring detections) is particularly assume a flux of particles emerging from many sources uniformly
rich in sodium and potassium salts (0.5–2% by mass). The composi- distributed across all four tiger stripes. Starting speeds are size-depend-
tion of these salt-rich grains8 closely resembles the predicted composi- ent because of the size-dependent frictional force governing accelera-
tion of an Enceladian ocean that has been in prolonged contact with tion by a gas7. This leads to the observed tendency of larger particles to
the rocky core of the moon17. Because of this compelling compositional preferentially populate the lower regions of the plume18,19. A fit of this
match, the grains are thought to form from frozen droplets present as simple model to the E5 data (Fig. 2) qualitatively reproduces the rise in
spray over a liquid reservoir close to the surface8. the fraction of salt-rich grains around closest approach.
The E5 fly-by crossed the plume almost from north to south, with a Inspired by the observation in the ultraviolet21 of both slow, diffuse
closest approach to Enceladus of 21 km; this closest approach was located sources and supersonic, collimated gas jets, we add eight jet-like par-
at the fringe of the plume (Fig. 3, Supplementary Fig. 1). The proportions ticle sources (Supplementary Fig. 2), at locations identified in Cassini
of the three main compositional types exhibit significant variations with images22 and combine them with the uniformly distributed sources in
position in the plume (Fig. 1). Shortly before closest approach, the pro- a bi-component model. For the observed high gas speeds in these jets21
portion of salt-rich grains increases steeply from E-ring background (.1,000 m s21), abundant vapour condensation is expected in the
levels, reaching .40% just after closest approach, followed by a shallower vents7, resulting in a large fraction of small salt-poor grains, which is
decrease towards the dense plume. Simultaneously, the proportion of reflected by using a steeper size distribution for the jets. The bi-
salt-poor type I grains starts to decrease shortly before closest approach. component model further improves the fit to the observations
Furthermore, there is a less pronounced increase in the proportion of (Fig. 2). Establishing a link between size, ejection speed and composi-
type II grains (containing organic compounds and/or silicates16) after tion, our results indicate not only compositional but dynamical sub-
closest approach when Cassini enters the denser parts of the plume. The structures in the ice plume, with larger salt-rich grains ejected mainly
absolute number of all three types steadily increases until ,20 s after from slower sources distributed across the tiger stripes.
closest approach (Fig. 2a). The models permit us to extrapolate the measured abundance of salt-
The most plausible explanation for the simultaneous increase of rich grains down to the densest, near-surface regions of the plume (Fig. 3),
type III and decrease of type I proportions is that salt-rich grains representing the conditions immediately after ejection. We find that slow
become more abundant close to the sources located in the south polar sources (combining contributions from all tiger stripes) dominate the
1
Institut fur Geowissenschaften, Universita Heidelberg, 69120 Heidelberg, Germany. 2Max-Planck-Institut fur Kernphysik, 69117 Heidelberg, Germany. 3Institut fur Physik und Astronomie, Universita
¨ ¨t ¨ ¨ ¨t
Potsdam, 14476 Potsdam-Golm, Germany. 4Planetary and Space Sciences Research Institute, The Open University, Milton Keynes MK7 6AA, UK. 5IGEP,Technische Universitat Braunschweig, 38106
¨
Braunschweig, Germany. 6LASP, University of Colorado, Boulder, Colorado 80303, USA. 7IRS, Universitat Stuttgart, 70569 Stuttgart, Germany.
¨
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2. RESEARCH LETTER
a a 100.00
No useful
Spectra of type I (%)
spectra
60
10.00
40
R > 0.2 μm
N(>R) (m–3)
20 (all grains)
1.00
b 0
Spectra of type II (%)
60
0.10 R > 0.6 μm
(salt-rich)
40
20 0.01
b
CDA data No useful
spectra
c 0
40 Jets and all
Spectra of type III (%)
40 tiger stripes
Fraction of salt-rich (%)
30 Tiger stripes
30
only
20
10 20
0
–60 –40 –20 0 20 40 60
10
Time from closest approach (s)
Figure 1 | Proportions of particles of different spectral type detected during 0
the central period of the E5 fly-by. The measured frequencies of particles of –60 –40 –20 0 20 40 60
different spectral types (a, type I; b, type II; c, type III) along the E5 trajectory
Time from closest approach (s)
reflect proportions and not absolute abundances. Type I spectra imply almost
pure water ice grains with an average Na/H2O mixing ratio of about 1027, Figure 2 | Modelling of the E5 measurements. a, The expected total particle
whereas type II spectra exhibit contributions from organic and/or siliceous number densities along the E5 fly-by trajectory for particles .0.6 mm
material8,16. Type III exhibit drastically increased alkali salt content. They on (representing salt-rich grains; red dashed line) and .0.2 mm (all particles,
average contain 0.5–2% sodium and potassium salts by mass, mainly NaCl, including salt-poor; solid black line). In b, the resulting compositional profiles
NaHCO3 and/or Na2CO3 (ref. 8). Each data point represents an interval of are compared to the type III particle profile, measured by CDA (open
64.5 s and includes ,40 spectra. Error bars, standard error of the mean derived diamonds). The dashed line shows the compositional profile obtained from a
from counting statistics. The E-ring background dominates the particle flux model employing a uniform particle flux emerging from all four tiger stripes. The
until ,15 s before closest approach and is already dense enough to trigger CDA solid line shows a model including eight faster and more collimated jet-like
spectrum recording at its maximum rate (in the optimized configuration this is particle sources14. Although both models fit the observations well, the
about nine spectra per second, data for five of which could be transmitted), so contribution from the jet particles helps fit the rapid decrease of salt-rich grains
the detection rate could not increase further during plume crossings. During about five seconds after closest approach, as well as the relatively low level of
the period of highest impact rate, between ,18 s and ,35 s after closest these grains after 40 s from closest approach, when the spacecraft was still within
approach, too few evaluable spectra were obtained (hatched region) owing to range of the jets (Fig. 3 and Supplementary Figs 1, 2). For the uniform tiger stripe
overload of the instrument electronics (Supplementary Information). Similarly, sources, we use a gas speed26 of 500 m s21 and a relatively broad distribution of
unspecified selection effects may have led to fluctuations in the type statistics ejection angles, with a mean ejection angle of 35u measured from the surface
starting from ,11 s after closest approach. Although the data obtained during normal. For the fast jets, we use a gas speed of 1,200 m s21 and a mean ejection
this time interval (grey) may therefore have been affected by instrument angle of 15u (Supplementary Information). Comparison of the model particle
performance issues, they exhibit a stable trend matching the model predictions number densities to measurements performed with the CDA’s High Rate
presented in this work (Fig. 3). Detector1 allows us to include a background flux of E-ring particles, dominating
the signal of particles .0.2 mm before ten seconds before closest approach. Error
flux of particles above the detection threshold of the CDA. In both model bars, standard error of the mean derived from counting statistics.
versions, ,70% of ejected grains are salt-rich. Owing to their larger sizes,
they completely dominate the mass flux (.99%). The increased abund- N2, CO, CH4). However, sublimation of sodium-rich grains would lead
ance of type II ice grains in the core region of the plume, even at high to the release of sodium into both the plume’s gas phase and the E ring,
altitudes, cannot be reproduced with a size-dependent model, and which is not observed13. Models employing sublimation from warm ice
implies a general enrichment of ice grains containing organic material, currently cannot account for the predominant formation of salt-rich ice
probably associated with fast collimated jets. grains. As all sodium compounds should have been originally bound in
Our conclusion that a stratified plume, which primarily produces rock after the moon’s formation, the salt in plume grains must have
salt-rich grains, is feeding a salt-poor E ring is a direct consequence of been extracted from Enceladian rocky material by water. To preserve
the measurements and independent of model parameters. If the 6% of high salt concentrations in ice, rapid freezing would be necessary. Thus,
salt-rich grains detected in the E ring were also typical of the plume to reconcile our measurements with ice sublimation as a dominant
production region, then contributions from ‘dry’ sources (such as ice plume source, it is necessary to invoke a flash-frozen salty ice layer
sublimation or clathrate decomposition) would be viable. In contrast, a across the entire active region. We regard this scenario as physically
plume source dominated by salt-rich grains, as reported here, elimi- unlikely. Sublimation scenarios in which ice grains are predominantly
nates the possibility of significant contributions from dry, sodium-poor generated by the recondensation of a fraction of the sublimated water
sources and severely constrains or rules out non-liquid models in their vapour (see, for example, refs 11, 12) can be entirely eliminated, as such
present form. In a clathrate decomposition scenario9,10, a substantial a process cannot form salt-rich grains. A geyser-like scenario6 with
part of the water vapour in the plume gas forms from the sublimation of violently boiling salt-water is excluded by the absence of sodium in
ice grains entrained in a flow of initially released volatile gases9,23 (CO2, the vapour phase13.
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3. LETTER RESEARCH
a 200 –10 s b 100
Ma
ss
in s
9.7 alt-
CA rich
13.2 0
Proportion of salt-rich
So
17.9
uth
80
23.4
grains (%)
29.3 –200 +10 s
35.6
42.2 VI VIII
+20 s 60 Jet
Fraction (%)
49.2 III
Kilometres
–400 particles
56.3
70.7
Nu
+30 s Diffuse
mb
–600 source
er
40
of
+40 s
sa
CASSINI
lt-
–800
chri
+50 s 20
<R> = 0.40 μm
–1000
<R> = 0.45 μm
<R> = 0.60 μm +60 s
<R> = 0.70 μm
<R> = 1.00 μm –1,200 0
–200 0 200 400 600 1 10 100 1,000
Kilometres Altitude above south pole (km)
Figure 3 | Compositional and size profile of the ice plume. a, Background (III, VI, VIII in ref. 22; Supplementary Fig. 2) are clearly visible in both the
colours show the proportion of salt-rich grains obtained from the model. compositional profile and size contours. Note that the model only considers
Overlaid are contours of constant mean particle radius, ,R.. In contrast to the particle sizes above the instrument’s detection threshold (0.2 mm). b, The
compositional profile, the size contours show the pure plume emission without modelled contribution from jets versus slow, diffuse sources and the fraction of
the E-ring background. The projection used is in the plane of the E5 spacecraft salt-rich ice as a function of logarithmic altitude above the south pole. Near the
trajectory. We expect to see both the largest particles and the highest fraction of surface salt-rich particles account for about 70% of all grains .0.2 mm, but
salt-rich grains a few seconds after closest approach (CA) to Enceladus. owing to their larger average size they account for almost all the solid mass
Structures of the three most relevant localized supersonic jets for this projection created by Enceladus’ active region.
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origin of the plumes and heat flux on Enceladus. Nature 447, 289–291 (2007). Author Contributions F.P. led the analysis and write-up of the manuscript. J.S. led the
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238–241 (2009). measurement.
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1102–1104 (2009). Author Information Reprints and permissions information is available at
14. Srama, R. et al. The Cassini cosmic dust analyzer. Space Sci. Rev. 114, 465–518 www.nature.com/reprints. The authors declare no competing financial interests.
(2004). Readers are welcome to comment on the online version of this article at
15. Hillier, J. K. et al. The composition of Saturn’s E ring. Mon. Not. R. Astron. Soc. 388, www.nature.com/nature. Correspondence and requests for materials should be
1588–1596 (2007). addressed to F.P. (Frank.Postberg@mpi-hd.mpg.de).
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