A fracture history on Enceladus provides evidence for a global ocean

Abstract

The region surrounding the south pole of Saturn's moon Enceladus shows a young, pervasively fractured surface that emanates enough heat to be detected by the Cassini spacecraft. To explain the elevated heat and eruptive icy plumes originating from large cracks (informally called ``tiger stripes'') in the surface, many models implicitly assume a global liquid ocean beneath the surface. Here we show that the fracture patterns in the south-polar terrain (SPT) of Enceladus are inconsistent with contemporary stress fields, but instead formed in a temporally varying global stress field related to nonsynchronous rotation of a floating ice shell above a global liquid ocean. This finding increase to at least three the number of outer planet satellites likely to possess a subsurface liquid water layer.

Full text

A fracture history on Enceladus provides evidence
for a global ocean
D. Alex Patthoff
1
and Simon A. Kattenhorn
1
Received 3 June 2011; revised 13 August 2011; accepted 18 August 2011; published 22 September 2011.
[1]TheregionsurroundingthesouthpoleofSaturn’s
moon Enceladus shows a yo ung, pervasively f ractured
surface that emanates enough heat to be detected by the
Cassini spacecraft. To explain the elevated heat and
eruptive icy plumes originating from large cracks
(informally called “tiger stripes” )inthesurface,many
models impli citly assume a global liqui d ocean beneath
the surface. Here we show that the fracture patterns in the
south‐polar terrain (SPT) of Enc eladus are inconsistent
with contemporary stress fields, but instead formed in a
temporally varying globalstressfieldrelatedto
nonsynchronous rotation of a floating ice shell above a
global liquid ocean. This finding i ncrease to at least three
the number of outer planet satellites likely to possess a
subsurface liq uid water layer.
Citation: Patthoff, D. A., and
S. A. Kattenhorn (2011), A fracture history on Enceladus provides
evidence for a global ocean, Geophys. Res. Lett., 38,L18201,
doi:10.1029/2011GL048387.
1. Introduction
[2] The geologically youngest region of Saturn’s small
moon Enceladus (∼250 km radius) is the heavily fractured
area surrounding the south pole [Porco et al., 2006], called
the south‐polar terrain (SPT). The numerous cracks in the
region are the source of eruptive plumes [Porco et al., 2006;
Hansen et al., 2006] and as much as 12.7–18.9 GW of thermal
energy emanating from the SPT [Spencer et al., 2006; Howett
et al., 2011]. These plumes and their associated energy have
been variably interpreted to be associated with tidal heating
[Porco et al., 2006; Spencer et al., 2006; Meyer and Wisdom,
2007; Roberts and Nimmo, 2008a], forced libration [Hurford
et al., 2009], clathrate decomposition [Kieffer et al., 2006],
frictional shear heating [Nimmo et al., 2007], or some com-
bination of these processes. These models, except the clath-
rate model, implicitly assume a body of liquid water beneath
the surface. The liquid layer has been suggested to be con-
fined to a localized sea beneath the SPT, based on the global
shape of Enceladus [Collins and Goodman, 2007] or through
numerical modeling [Tobie et al., 2008]. Other numerical
models have suggested the liquid is global in extent [Ross
and Schubert,1989;Schubert et al.,2007,Nimmo et al.,
2007] but do not provide geologic evidence for a global
liquid ocean. The estimated thickness of a hypothetical ice/
water layer is ∼80–100 km [Schubert et al., 2007] with the
liquid layer, if present, in the range ∼40–72 km thick [Olgin
et al., 2011].
[
3] A liquid layer is theoretically necessary to amplify tidal
distortion [Squyres et al., 1983; Ross and Schubert, 1989]
sufficiently to account for the large energy flux from the SPT
[Roberts and Nimmo, 2008a; Hurford et al., 2009]; however,
no direct evidence for such a liquid layer has been detected.
Indirect evidence for the phase state of the interior has pre-
viously been based on the composition of the plumes [Porco
et al., 2006; Waite et al., 2006; Matson et al., 2007;
Schneider et al.,2009;Kieffer et al.,2009,Waite et al.,
2009; Postberg et al., 2011], the salts they contribute to
the E‐ring [Postberg et al., 2009], and the existence of a
topographic depression centered on the south pole [Collins
and Goodman, 2007]; however, the extent of any potential
subsurface body of water is unconstrained by these methods.
[
4] Through a detailed analysis of the fractures in the
SPT, we show that most of the fractures in the region can be
grouped into one of just four sets that exhibit different
orientations and relative ages. The pattern of fracturing
indicates that the moon has experienced nonsynchronous
rotation (NSR) made possible by a global liquid layer, most
likely water, beneath the ice shell. Subsurface oceans likely
exist on two other icy satellites: Jupiter’s moons Europa and
Callisto [Schubert et al., 2004] and possibly exist on Gan-
ymede [Schubert et al., 2004] and Titan [Sohl et al., 2010].
Our work suggests that Enceladus should be counted among
the tally of likely subsurface liquid layers in the outer solar
system with Enceladus being the smallest.
2. Categorizing SPT Fractures
[5] The most prominent fractures in the SPT are the four
so‐called tiger stripes [Porco et al., 2006] (Figure 1 and
auxiliary material).
1
These large fissures (∼130 km long,
2 km wide, and 500 m deep) cut across all other features,
have a higher temperature than the surrounding terrain, and
are the likely source of the wa ter‐ice plumes [Porco et al.,
2006; Spencer et al., 2006; Spitale and Porco, 2007]
related to tectonic activity. A cursory examination suggests
that within the heavily fractured SPT, the tiger stripes are the
only ordered set of fractures; however, we advocate that the
numerous other fractures record a long and ordered geologic
history of the SPT. Our detailed mapping of the SPT has
revealed systematic and pervasive fracture sets of differing
ages, some of which have tiger stripe‐like characteristics but
with different orientations related to fracture age. The
majority of SPT fractures are disaggregated remnants of
formerly prominent cracks that can be grouped into one of
four sets, each with a distinct orientation (Figure 1) and each
having a unique relative age determined by crosscutting
relationships (Figure 2).
1
Department of Geological Sciences, University of Idaho, Moscow,
Idaho, USA.
Copyright 2011 by the American Geophysical Union.
0094‐8276/11/2011GL048387
1
Auxiliary materials are available in the HTML. doi:10.1029/
2011GL048387.
GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L18201, doi:10.1029/2011GL048387, 2011
L18201 1 of 6

[6] To categorize the SPT fractures, a master orientation
for each set was identified based on the average orientation
of the longest and most prominent fractures. Shorter frac-
tures (old cracks that were extensively dissected by later
activity) were then assigned to a specific fracture set based
on their orientation relative to the master sets such that all
fractures of a single set are within ±5° of each other and the
master orientation. Most of the fractures are approximately
linear but range in length from tens of meters to tens of
kilometers, the shortest fractures only being visible on the
highest resolution (∼10 m/pixel) images from the Cassini
spacecraft. Mapping was limited to fractures with lengths
greater than three times their width to ensure an accurate
orientation could be established. Image resolution of the
SPT varies from hundreds to less than ten meters per pixel,
so different sections of the SPT must necessarily be mapped
with varying levels of detail. Fractures that could not be
grouped into one of the four sets are overall much shorter,
less pronounced, and often do not have clear relative age
indicators. Although some of these cracks share a common
orientation that falls between the orientations of sets 3 and 4,
these fractures are always older than the four designated sets
where relative ages are apparent. Too few of the fractures
that do not correspond to one of the four main sets have
consistent orientations and apparent relative ages to group
them into additional sets, and are likely remnants of old
fracture sets disaggregated by tectonic overprinting by later
fracturing episodes. Areas that contain very few fractures,
particularly the region between Baghdad and Damascus
sulci (Figure 1), may have experienced recent resurfacing
[Barr, 2008] or near‐surface disruption by folding [Barr and
Preuss, 2010] or other deformation, which appears to have
eliminated evidence of any shorter fractures.
[
7] Along with orientation, each fracture was assigned
arelativeagebasedoncrosscuttingrelationshipsand
mechanical interactions between individual fractures. Similar
techniques have been used to classify the sequence of joint set
development in sedimentary rocks [e.g., Cruikshank and
Aydin, 1995]. Age indicators include the merging of two
fracture sets, where a younger set follows partly along the
path of an older set, or where a younger crack cuts through an
older one, occasionally creating lateral offset through subse-
quent strike‐slip motion (Figure 2). Relative ages cannot be
deduced for all fractures due to inadequate image resolution,
and not all fractures can be seen to clearly interact with other
fractures. However, many individual fractures of each set do
interact with those of other sets, revealing the relative ages.
We observe that interacting fractures of a common set
consistently share the same relative age. The youngest fea-
tures (set 1, pink in Figure 1), which include the tiger stripes
(red in Figure 1), cut across all other features. Set 2 (yellow,
Figure 1) is older than the tiger stripes but many of its features
cut through the relatively older set 3 (green, Figure 1) and set
4 (blue, Figure 1) fractures. The oldest (set 4) are pervasively
Figure 1. Fracture history of the south‐polar terrain. The map shows four fracture sets with set 1 (youngest) in pink
(including the named tiger stripes, or sulci, in red), set 2 in yellow, set 3 in green, and set 4 (oldest) in blue. Figure 2
location is shown by the white box. Image courtesy CICLOPS; credit: NASA/JPL‐Caltech/SSI. Mosaic created by Roatsch
et al. [2009], centered on the south pole and obtained from NASA’s PDS node. The outer edge of the figure represents 65°
S latitude. See auxiliary material for map with no interpretation.
PATTHOFF AND KATTENHORN: EVIDENCE FOR A GLOBAL OCEAN ON ENCELADUS L18201L18201
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dissected and seemingly displaced by younger sets, making it
difficult to correlate individual fractures over long distances
(10s of km). Progressively older fractures are typically
shorter and less numerous, potentially due to resurfacing
[Ross and Schubert, 1989; Barr, 2008] and overprinting by
younger features. A greater amount of morphological deg-
radation of these fractures gives additional qualitative support
for their older relative age.
[
8] Approximately 450 set 1 fractures, 250 set 2 fractures,
290 set 3 fractures, and 250 set 4 fractures have been mapped
between the south pole and 55°S latitude. Only 115 fractures
were identified that do not correspond to one of the four main
fracture sets.
3. Ancient Tiger Stripes
[9] Most fractures are short (<30 km) and narrow (10–
100 s m) with a muted morphology. However, some fractures
of sets 2, 3, and 4 stand out from the other fractures of their
set and are morphologically similar to the present day tiger
stripes (Figure 3 and auxiliary material), particularly set 2.
These fractures are longer and wider than adjacent fractures
of similar orientation and age, and can be linked across
younger bisecting fractures. The segmented fractures have a
cumulative length and spacing that is similar to the four
active tiger stripes (Table 1) and ostensibly behaved analo-
gously before becoming inactive and undergoing modifica-
tion by later deformation.
[
10] These ancient tiger stripes suggest a long history of
tiger‐stripe‐like activity on Enceladus where the older tiger
stripes were similar in form and function to the current tiger
stripes. The older tiger stripes may have had plumes of
water‐ice erupting from the surface and contributed to earlier
versions of Saturn’sE‐ring in a manner similar to the way the
present day tiger stripes contribute to the modern E‐ring
[Porco et al., 2006]. Portions of the ancient tiger stripes of
set 2 (and perhaps even set 3) are warmer than other fractures
[Howe tt et al., 201 1] and a re still active today [Spitale and
Porco, 2007], partly controlling the locations of the water‐
ice plumes erupting from the SPT during the Cassini mission
observational period and suggesting that ancient tiger stripes
have the capability to be reactivated even when younger
fracture sets have already developed. Three of the current
tiger stripes (Damascus, Baghdad, and Cairo) also appear to
have utilized portions of the set 2 ancient tiger stripes as they
evolved, accounting for abrupt bends in the tiger stripes
where they coalesce with the ancient tiger stripes (Figure 3)
and the seemingly forked ends to some tiger stripes.
4. Discussion
[11] The tiger stripes have been hypothesized to have
formed, and later been modified, by dilation or shearing due
to a diurnal tidal stress field [Hurford et al., 2009] generated
by the eccentricity of the moon’s orbit around Saturn [Porco
et al., 2006]. During each orbit, the diurnal stress field south
of the equator in a tidally responding ice shell rotates 180°
clockwise [Greenberg et al., 1998; Hurford et al., 2009].
Fractures growing in such a stress field should be cuspate,
like Europa’s cycloids [Marshall and Kattenhorn, 2005;
Hoppa et al., 1999]; however, the vast majority of fractures
in the SPT are linear, except where they interact with older
sets. Cairo sulcus has been suggested to have arcuate seg-
ments associated with tidal stresses [Hurford et al., 2007],
although the curving of such fractures may simply result
from mechanical interactions between fractures that propa-
gated toward each other [Helfenstein et al., 2011]. Addi-
tionally, the tensile strength of ice is likely ∼1–3 MPa
[Schulson and Duvall, 2009] whereas the maximum pre-
dicted diurnal tensile stresses are an order of magnitude less
(∼ 0.1 MPa) [Hurford et al.,2007;Smith‐ Konter and
Pappalardo, 2008]. The fractures are thus unlikely to have
initially formed in response to diurnal stresses, implying
some additional source of stress to create the fracture sets.
[
12] The orientations of the four fracture sets are not
random but instead show a consistent counterclockwise
change in orientation from the oldest (set 4) to the youngest
(set 1) fractures. Starting with the oldest, between set 4 and
set 3, the dominant fracture orientation rotated 78° counter-
clockwise when looking down on the south pole. Continuing
forward through time, between sets 3 and 2, the fracture
orientation rotated an additional 47° counterclockwise.
Between set 2 and set 1, the dominant orientation rotated an
additional 28° counterclockwise. These changing orienta-
tions point to unique stress states at different points in time in
the SPT that differed (relative to the Saturn direction) from
the contemporary stress state. The pattern of tidal stress
Figure 2. Crosscutting relationships (image location in
Figure 1). In the center of the image, the tiger stripe Cairo
Sulcus (red) cuts across and displaces the set 3 fractures
(green), creating ∼5 km of right‐lateral offset and indicating
the tiger stripe is younger than the set 3 fracture. Near the
top, a set 2 fracture (yellow) can also be seen to cut across a
set 3 fracture, indicating that the set 2 fracture is younger.
Use of such crosscutting relationships, along with observ-
able mechanical interaction effects on fracture propagation
paths, allows the relative ages to be established throughout the
SPT. Image courtesy CICLOPS; credit: NASA/JPL‐Caltech/
SSI. Mosaic created by Roatsch et al. [2009].
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during the course of the orbit is fixed relative to the Saturn‐
Enceladus system and so cannot explain the development of
systematic, linear fracture sets with different orientations
through time. However, if the ice shell experiences NSR,
whereby the location on Enceladus directly facing Saturn
changes longitudinally over time, the movement of the ice
shell about a fixed pole of rotation causes the tidal bulge to
migrate across the surface creating an additional component
of global stress (the NSR stress) that could exceed the
magnitude of the diurnal tidal stresses [Wahr et al., 2009].
[
13] Given the slow rate of NSR relative to the diurnal
time scale (perhaps tens of thousands to millions of years for
one NSR period as opposed to 1.37 days per orbit), the
relatively static and dominant NSR stress field would form
systematic fracture sets at any point in time (cf., the major
lineaments on Europa [Helfenstein and Parmentier, 1985;
Geissler et al., 1998; Kattenhorn, 2002]), consistent with
the fracture patterns in the SPT. Furthermore, although the
stress field pattern remains constant relative to the Saturn
direction, it migrates relative to the surface in response to
shell reorientation, resulting in fractures in any one location
forming with different orientations at different points in
time. Looking down on the south pole of Enceladus, the
outer shell would experience a clockwise sense of rotation
due to faster than synchronous rotation, creating a relative
counterclockwise rotation of the diurnal plus NSR stresses
through time [Greenberg et al., 1998]. In this event frame,
older fracture sets must be rotated more clockwise relative to
younger sets about the current pole of rotation, as we
describe to be the case.
Figure 3. Ancient tiger stripes. Some fractures of older sets have tiger stripe‐like characteristics with the most prominent
ancient tiger stripe‐like fractures belonging to set 2 (yellow). Individual fractures of set 2 appear to have been intersected by
the tiger stripes and can be matched across them to define features that have a similar length and spacing to the present day
tiger stripes (Table 1). The trends of the tiger stripes appear to be influenced by the older set 2 tiger stripes where inter-
sections occur. Four additional potential ancient tiger stripes, two belonging to set 3 (3A and 3B) and two to set 4 (4A and
4B), are shown in green and blue respectively. Image courtesy CICLOPS; credit: NASA/JPL‐Caltech/SSI. Mosaic created
by Roatsch et al. [2009] shows the south pole at the center of the image and 65°S at the outer edge.
Table 1. Fracture Characteristics for Tige r Stripes and Ancie nt
Tiger Stripes
a
Fracture Name
Length
(km)
Maximum Width
(km)
Spacing
(km)
Damascus 128 2 35
Baghdad 158 2 35.5
Cairo 154 2 34.5
Alexandria 111 2 33
Set 2 Damascus 105 1.2 33
Set 2 Baghdad 126 1.2 37.5
Set 2 Cairo 90 1.3 42
Set 3 A 85 1.1 31
Set 3 B 78 1.4 31
Set 4 A 107 1.6 40
Set 4 B 21 1.3 40
a
The seven mapped ancient tiger stripes (Figure 3) and their character-
istics are compared to the present day tiger stripes. The lengths are the sum
of the components of each of the tiger stripes measured from tip to tip along
each fracture. Maximum width refers to the largest distance between the
edges of the fracture. The spacing is the average distance between the
named feature and the ancient tiger stripe fractures within its own set that
are closest to it on either side.
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[14] Polar wander has been proposed to explain the ori-
entation of the tiger stripes [Matsuyama and Nimmo, 2008];
however, this cannot account for the different orientations
and relative ages of the other three fracture sets. We
therefore advocate that the change in orientation of the
fracture sets over time can only be explained by a rotation of
the stress field relative to the surface of the SPT in response
to a faster than synchronous rotation about the current
rotational pole. Such a rotation is possible if Enceladus does
not possess a sufficient mass asymmetry to overcome the
tidal torque induced by its eccentric orbit [Greenberg and
Weidenschilling, 1984]. Between sets 4 and 1, the ice
shell has rotated 153° clockwise through the extant stress
field to create the visible fracture sets (i.e., almost half a
rotation of the outer ice shell relative to the solid interior
during the SPT geologic history, although this is a lower
limit). As on Europa, the NSR of the ice shell requires a
global liquid ocean between the icy outer layer and the
silicate interior to decouple the shell and allow for it to
freely rotate. SPT fracture patterns thus provide compelling
evidence for a global ocean during the fracturing sequence
at the SPT and provide a rationale for the high heat pro-
duction in the SPT being related to diurnal tides, which are
greatly enhanced if there is a decoupled shell [Nimmo et al.,
2007; Roberts and Nimmo, 2008a].
[
15] The creation of each fracture set helps to relax the
buildup of NSR stress as the ice shell continues to rotate. The
fractures likely continue to be active, even after the ice shell
has rotated the cracks relative to the stress orientations in
which they initially formed, by undergoing shear motions.
Accordingly, strike‐slip offsets are relatively common along
fracture sets of all ages. Ultimately, the resolved stress on an
older fracture that has rotated away from its original for-
mation orientation relative to the tidal bulges will be insuf-
ficient to cause the fracture to continue relieve the buildup of
NSR stress. At this point, it becomes more difficult to fric-
tionally shear the preexisting fractures than to create new
ones. Consequently, a new set of fractures will form in a new
orientation relative to the old fracture set (but in the same
orientation relative to Saturn at which the older fracture set
originally formed) to relieve the NSR stress, although the
most prominent cracks in older sets (ancient tiger stripes)
may remain partially active and continue to focus some plume
activity. As the activity on a new set becomes dominant, the
older set becomes progressively overprinted and dis-
aggregated. The amount of shell rotation at which new frac-
turing occurs progressively decreases between successive
fracture sets and may indicate ice shell weakening through
time [Roberts and Nimmo, 2008b], or thinning through time,
which we infer may be due to a change in heating driven by an
evolving orbital eccentricity [Ross and Schubert, 1989;
Zhang and Nimmo, 2009]. A more eccentric orbit (or a
thinner ice shell) could generate m ore heat [ Roberts and
Nimmo, 2008b] through larger tidal forces, causing thinning
and weakening of the ice, whereas a more circular orbit would
produce less heat and stronger ice.
[
16] Acknowledgments. This work was funded by the NASA Outer
Planets Research program grantNNX08AQ946andNASAEarthand
Space S cience Fellowship Program g rant number NNX10A079H. W e
thank Wes Pa tterson and an anonymous reviewer for their helpful com-
ments on the original manuscript.
[17] The editor thanks Wes Patterson and an anonymous reviewer.
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S. A. Kattenhorn and D. A. Patthoff, Department of Geological Sciences,
University of Idaho, PO Box 443022, Moscow, ID 83844‐3022, USA.
(patt0436@vandals.uidaho.edu)
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