Icarus 157, 490–506 (2002)
doi:10.1006/icar.2002.6825
Nonsynchronous Rotation Evidence and Fracture History
in the Bright Plains Region, Europa
Simon A. Kattenhorn
Department of Geological Sciences, University of Idaho, P.O. Box 443022, Moscow, Idaho 83844-3022
E-mail: [email protected]
Received August 9, 2001; revised December 21, 2001
A geologic map for the Bright Plains in the Conamara Chaos
region of Europa is presented and is used to unravel a detailed
fracture sequence using cross-cutting relationships and fracture mechanics principles. Fracture orientations in the Bright Plains region
rotated with time, consistently in a clockwise sense. This conclusion
agrees with the observations of other researchers’ northern Europan
hemisphere investigations and points strongly toward the fracture
sequence being controlled by the effect of nonsynchronous rotation,
whereby the outer ice crust of Europa rotates slightly faster than the
satellite’s interior. This is convincing evidence that Europa’s crust
has been decoupled from the interior, possibly due to the presence
of a liquid ocean beneath the crust.
Tidal stresses induced in the ice crust by the combined effects
of nonsynchronous rotation and diurnal tidal flexing can be calculated using the assumption that the crust behaves elastically over
relatively short time scales (i.e., no viscous relaxation of stresses).
The fracture orientations in the Bright Plains area were compared
to a global scale tidal stress field to determine the longitudes at
which each fracture set developed. The fracture sequence points
strongly to the Bright Plains region of the crust having rotated
at least 720◦ (and perhaps up to 900◦ ) with respect to the satellite’s interior during the visible fracture history. This amount exceeds previously published estimates of nonsynchronous rotation.
The orientations of the most recent surface fractures are incompatible with the current state of stress in the Bright Plains region,
implying a period of a few thousand years since the most recent
fracturing events based on existing nonsynchronous rotation rate
estimates. c 2002 Elsevier Science (USA)
Key Words: Europa; surfaces, satellite; geological processes;
tectonics; tides.
I. INTRODUCTION
1998, Carr et al. 1998, Pappalardo et al. 1998a). Some researchers also advocate the existence of a warm ductile ice layer
below the brittle ice crust (Pappalardo et al. 1999). The thickness of the brittle ice crust is a topic of great contention and is
variably hypothesized to be between 0.2 and 30 km (Carr et al.
1998, Greenberg et al. 2000, McKinnon 1999, Moore et al. 1998,
Ojakangas and Stevenson 1989b, Pappalardo et al. 1998a, 1999,
Pappalardo and Head 2001, Tufts 1998, Williams and Greeley
1998).
Perhaps one of the most convincing lines of evidence for the
existence of a subsurface ocean on Europa is the evidence for
faster than synchronous rotation of Europa’s crust with respect
to its interior. This phenomenon is supported by the interpreted
stress history of the satellite and is the major focus of this paper.
The objective of this investigation is to provide a mechanical
rationale for the development of multiple fracture sets on the
Europan surface and thus to characterize and unravel the deformation and stress history of the fractured ice shell. Many
regions of Europa are ubiquitously fractured, and precise geologic mapping techniques can be used to unravel the fracture
sequence (e.g., Figueredo and Greeley 2000). I will show that
an almost complete history of surface stresses can be unraveled
from complexly fractured regions. Furthermore, these stresses
are clearly reconcilable with existing models of tidal stresses
induced by the combined diurnal and nonsynchronous rotation
effects. Detailed fracture mapping was undertaken in the pervasively fractured Bright Plains area in the Conamara Chaos region
of Europa’s trailing hemisphere (∼273◦ W/15◦ N) (Fig. 1). The
geologic mapping provides evidence for 720◦ –900◦ of rotation
of Europa’s ice shell with respect to its interior, indicating an
effective decoupling of the brittle ice shell from a low-viscosity
interior, ostensibly a liquid ocean, during rotation of the shell.
Galileo solid-state imaging (SSI) of Europa, Jupiter’s fourth
largest satellite, has revealed details of complex fracture sets
on the surface that attest to tensile stresses having developed in
the ice crust. Many of the characteristics of the fractures and
other structural features, in conjunction with gravity and thermal models, suggest that the ice crust of Europa is underlain
by a water layer approximately 100 km thick (Anderson et al.
II. FRACTURES AND GLOBAL STRESSES
A. Background
Voyager spacecraft images of Europa obtained in the late
1970s revealed Europa’s surface to be highly fractured
(Lucchitta and Soderblom 1982, Smith et al. 1979). The
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c 2002 Elsevier Science (USA)
All rights reserved.
BRIGHT PLAINS FRACTURE HISTORY, EUROPA
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FIG. 1. Regional Galileo image (E6DRKLIN) of the Conamara Chaos region at 180 m/pixel. The Bright Plains are located north of the intersection of Asterius
Linea and Agave Linea.
resolution of those images (>2 km/pixel) was insufficient to
accurately describe many of the surface fracture lineaments;
however, analysis of large-scale (hundreds of kilometers) lineaments in the early 1980s attempted to relate the fracture orientations to a quantifiable state of global stress characteristic of the
Europan crust. Fracture orientations provide direct evidence
of the principal stress orientations. The underlying assumption
is that the lineaments are essentially opening mode fractures,
meaning that the fractures opened in response, and at right angles, to the orientation of the local maximum tensile principal stress. These assumptions formed the foundation for initial
global stress models that were based on calculations of tidal
stresses induced on Europa, described more fully in the next
section.
The Galileo SSI mission that began in 1995 increased the
resolution of surface images of Europa to as little as 6 m/pixel.
Analysis and interpretation of these images during the past several years strongly suggest that the Europan fracture lineaments
form predominantly as opening mode fractures and several classification schemes have arisen as the various fracture types have
been examined (Figueredo and Greeley 2000, Geissler et al.
1998b, Greeley et al. 2000, Hoppa et al. 1999b, Lucchitta and
Soderblom 1982, Prockter et al. 1999a). Numerous examples of
strike–slip faults have also been described (Hoppa et al. 1999a,
2000, Prockter et al. 2000, Tufts et al. 1999); however, they are
interpreted as having initially formed as opening mode fractures
that were subsequently reactivated as strike–slip faults. Thus,
fault orientations may also provide evidence of the stress state
at the time of their development as opening mode fractures. Furthermore, the sense of slip along the faults is dictated by the shear
stress sense along the fault surfaces, providing additional constraints on stress field evolution through time. Tensile fracture
models have been challenged by a recent model that suggests
fracture formation predominantly through shear failure (Spaun
et al. 2001), perhaps facilitated by frictional melting (Gaidos
and Nimmo 2000). This model provides a rationale for fracture
lineament development in an all-round compressive stress field.
Galileo coverage of Europa has provided a wealth of information to be analyzed. However, there have been very few highly
detailed fracture mapping efforts (e.g., Figueredo and Greeley
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SIMON A. KATTENHORN
2000, Prockter et al. 1999a, Spaun et al. 1998). Most fracture
interpretations have thus been in the context of the global stress
models and have been based on the analysis of the major fracture lineaments. The work presented here represents a finer
scale of fracture mapping that reveals a greater degree of insight into the evolution of stresses at both a regional and local
scale.
B. Global Stress Models
The prevalence of large-scale fracture lineaments on Europa
provides direct evidence that tensile stresses, of sufficient magnitude to break the ice, have been produced in the Europan crust.
Furthermore, the distribution and orientations of the fractures
suggest that tidal flexing of the crust is a likely explanation for
the origin of these stresses (Geissler et al. 1998b, Greenberg
et al. 1998, Helfenstein and Parmentier 1983, 1985, Leith and
McKinnon 1996, McEwen 1986).
Quantifying tidally induced stresses requires that the orbital
characteristics of a planetary body be accurately known.
Europa’s orbit is not circular but has an orbital eccentricity of
0.01 induced by a three-body gravitational resonance with Io
and Ganymede (Peale et al. 1979). As a result, the distance between Europa and Jupiter varies as a function of the position
within the orbit, attaining a minimum at perijove and a maximum at apojove. Consequently, the amplitude of the tidal bulge
induced in Europa’s ice shell by Jupiter varies throughout the orbital period, manifested as an oscillating flexing of the ice shell
above either a viscous or a liquid interior. The tidal bulge locations on Europa oscillate longitudinally about an axis radial to
Jupiter due to Europa’s eccentric orbit and the inability of the ice
shell to respond instantaneously to tidal forces. The associated
tidal stresses in Europa’s ice shell thus vary as a function of orbital position. This cyclic stress field is termed the diurnal stress
field and completes an entire cycle once per orbit (equivalent to
3.55 Earth days) (Morrison et al. 1977), attaining a maximum
tensile stress magnitude of approximately 0.04 MPa (Hoppa
et al. 1999b). At any point on the Europan surface, the principal
stress orientations rotate through 180◦ during each Europan day
(Hoppa et al. 1999a).
Although some large-scale fracture orientations on Europa
can be seemingly attributed to the current diurnal stress field pattern (Helfenstein and Parmentier 1980), most fractures appear to
be located in the incorrect longitudinal position to be attributable
to the current stress field (Geissler et al. 1998a, Greenberg et al.
1998, Helfenstein and Parmentier 1985, Leith and McKinnon
1996, McEwen 1986). A plausible explanation for these observations is that Europa’s ice shell has rotated with respect to
the satellite’s interior in response to the equilibrium spin rate of
Europa being slightly faster than synchronous (Greenberg
and Weidenschilling 1984, Helfenstein and Parmentier 1985,
McEwen 1986). In this hypothesis, Europa’s crust is decoupled from an underlying liquid ocean or ductile layer, enabling
the outer ice shell to rotate faster than the satellite’s interior.
This effect is driven by the induced torque on, and associated
acceleration of, Europa’s rotation in response to the tidal bulge
lagging behind the point on Europa’s surface that faces directly
toward Jupiter at perijove.
Consequently, a point on Europa’s surface directly facing
Jupiter migrates longitudinally with time. The Jupiter-facing
tidal bulge appears to migrate relatively westward as the surface migrates eastward with respect to a fixed reference frame.
In response, surface stress patterns caused by the tidal bulges,
and resultant fracture orientations at any point on Europa’s surface, have varied over time, although at much longer time scales
(>12,000 years) than diurnal variations (Greenberg et al. 2001,
Hoppa et al. 1997).
An important consequence of nonsynchronous rotation is the
production of a second component to the global stress state,
superimposed on the diurnal stress field. This nonsynchronous
rotation stress field, resulting from a reorientation of the crust
with respect to the tidal bulge, produces stresses of up to 30 times
greater magnitude (∼0.1 MPa) than the diurnal tidal stresses,
and with a similar global stress field pattern except that it is
shifted longitudinally with respect to the diurnal tidal stress field
(Greenberg et al. 1998). The actual magnitudes and patterns of
nonsynchronous rotation stresses are dependent on the angle of
rotation over which the stresses are able to accumulate, which
is limited by both the brittle strength and the viscous relaxation
time of the ice crust. Previous studies have generally assumed
sufficient stress build-up to fracture the ice over 1◦ of nonsynchronous rotation (e.g., Greenberg et al. 1998). The total global
stress state is composed of the superimposed nonsynchronous
rotation and diurnal tidal stress fields (Fig. 2). As a result of
FIG. 2. Global distribution of stresses in Europa’s crust at 1/8 orbit after apojove due to diurnal tidal stresses superimposed on stresses accumulated
over 1◦ of nonsynchronous rotation (Greenberg et al. 1998). Tics represent the
orientations and magnitudes of the maximum and minimum principal horizontal stresses. Tensile stresses are black and compressive stresses are gray. The
location of Bright Plains is shown by the gray box.
BRIGHT PLAINS FRACTURE HISTORY, EUROPA
493
the 180◦ rotation of the diurnal stress field during each Europan
orbit, the total global stress field changes as a function of
Europa’s orbital position, with maximum tensile stresses occurring at 1/8 orbit past apojove (Greenberg et al. 1998). Over long
time scales, nonsynchronous rotation causes the stress pattern in
Fig. 2 to migrate westward relative to the Europan surface, resulting in a progressive rotation of principal stresses and hence
fracture lineament orientations through time at any particular
location (clockwise in the northern hemisphere and counterclockwise in the southern hemisphere).
Broad-scale regional mapping (Geissler et al. 1999,
Helfenstein and Parmentier 1985, McEwen 1986) and detailed
mapping (Figueredo and Greeley 2000) of fracture sequences
at various longitudinal and latitudinal locations on Europa have
indicated nonsynchronous rotation of the Europan crust by up
to 360◦ with respect to the satellite’s interior. However, these
previous studies have not resulted in a consistent estimate of
the amount of nonsynchronous rotation of Europa’s crust. A detailed fracture sequence reconstruction of the Bright Plains area,
north of Conamara Chaos (Fig. 1), suggests 900◦ of nonsynchronous rotation has occurred, exceeding previously published
estimates.
III. GEOLOGICAL FEATURES IN THE BRIGHT
PLAINS REGION
The study area in the Bright Plains is centered at 14.80◦ N,
273.16◦ W and covers a roughly rectangular region approximately 33 km wide and 60 km long (Fig. 3). The region was
imaged during Galileo’s E6 orbit in February 1997 (image mosaic E6ESBRTPLN02), with an image resolution (∼20 m/pixel)
suitable for detailed fracture mapping. The spacecraft view direction in the orthographically reprojected photo mosaic is toward the north with an emission angle (the angle between the
normal to the surface and the spacecraft) of ∼45◦ . The sun angle
is from the east at ∼78◦ from vertical, resulting in long shadows
to the west of north–south trending ridges. Numerous geomorphic units were identified in this image area by Kraft and Greeley
(1997) and Sullivan et al. (1999); Head et al. (1999) describe
the characteristics of ridges in the area.
Fracture lineaments and other geological features were
characterized using a classification scheme modified after those
of Prockter et al. (1999a), Figueredo and Greeley (2000), and
Greeley et al. (2000). Numerous conceptual models have been
developed to explain the evolution of and relationships between
the various fracture lineaments (Geissler et al. 1998b, Greenberg
et al. 1998, Head et al. 1999, Kadel et al. 1998, Pappalardo et al.
1998b, Tufts et al. 2000). The types of features present in the
Bright Plains region are described below and illustrated in Fig. 4.
Crater material occurs in and around impact craters and is
assumed to have been ejected then deposited during the impact
event.
Ridge-related flood deposits are smooth, featureless plains
that occur within local lows or trenches alongside ridges, and
which embay adjacent high-standing features. They are inter-
FIG. 3. Image mosaic of the Bright Plains region (E6ESBRTPLN02) at
approximately 20 m/pixel. The prominent ridge across the center of the area is
Androgeos Linea.
preted as fluidized slushy ice deposits that were extruded from
adjacent ridges or recent fractures.
Surface fractures represent the most recent structural features
and thus cross-cut all other lineaments. They are generally linear
but several exhibit broadly curving trends. Fractures within the
boundaries of the mapped area do not exceed lengths of ∼12 km
and some are segmented, forming en echelon geometries. Many
of the fractures appear to form wide crevasses at the surface,
referred to as troughs by Greeley et al. (2000). Surface fractures
formed in response to near-surface tension and are interpreted
to not extend deep enough into the Europan crust to tap an underlying liquid ocean, assuming one exists.
Double ridges (DR) are the most common class of fracture
in the mapped area. They consist of a dark lineament (a central
trough) flanked to either side by bright bands (ridges). They
criss-cross the mapped region in a range of orientations and with
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SIMON A. KATTENHORN
FIG. 4. Examples of geologic features in the Bright Plains region.
(a) Ridged plains. (b) Smooth band cut by normal faults. (c) Double ridge.
(d) Complex ridge. (e) Medial-trough ridge. (f ) Proto-ridge. (g) Surface fractures. (h) Normal faults crossed by surface fractures.
a range of ages and are thus particularly unambiguous and useful
indicators of the overall fracturing sequence. Double ridges vary
markedly in both height and width. The largest double ridge in
the mapped region, Androgeos Linea (Tufts 1998), is approximately 2.3 km wide and up to 100 m high (Greenberg et al.
1998). The weight of the largest double ridges has caused downward warping of the adjacent crust, forming troughs to either side
of the ridges. Ridge-parallel fracture sets were induced in the
flexing crust adjacent to the troughs in response to the bendingrelated stresses. Smaller double ridges can be observed within
relatively older plains material and are generally on the order of
500 m wide. These smaller double ridges are more numerous and
accordingly more closely spaced. Double ridges are interpreted
to be extensional fractures that acted as vertical pathways for the
extrusion of material along the fracture flanks, resulting in the
construction of the ridges to either side of the central fracture.
Proto-ridges (PR) are fracture lineaments that exhibit evidence of juvenile ridge development along the flanks of a central fracture. They are irregular in both height and width and the
edges of the proto-ridges are sinuous, suggesting an early stage
in the development of the flanking ridges. They are laterally discontinuous and may transform along strike into simple surface
fractures. Proto-ridges are interpreted as being transitional between surface fractures and double ridges, in agreement with the
evolutionary models of fracture evolution proposed by Geissler
et al. (1998b) and Head et al. (1999) in which double ridges represent a more evolved form of nascent fractures. Proto-ridges
may be analogous to the “raised-flank trough” of Pappalardo
et al. (1998b).
Medial-trough ridges (MTR) have a low relief and reduced
relative albedo that distinguishes them from double ridges. They
are described by Pappalardo et al. (1998b) and Figueredo and
Greeley (2000) as low relief bands of relatively featureless
smooth material flanking a central trough with a bilateral symmetry. They have been interpreted as evidence of symmetric
crustal spreading away from a central fracture. However, the associated significant component of dilation described by
Pappalardo et al. (1998b) is not present in the Bright Plains
MTRs, which appear to be primarily extrusive from a narrow
central crack.
Smooth bands (SB) are generally linear bands of smooth, featureless surface material. In the Bright Plains, their widths are
variable (2–13 km) and their margins may be highly irregular.
Smooth bands are interpreted as representing crustal spreading
features, analogous to mid-oceanic spreading centers on Earth
(Greeley et al. 2000, Prockter et al. 1999b).
Complex ridges (CR) have similar dimensions to smooth
bands as described above; however, they appear to be composed of numerous subparallel double ridges (Head et al. 1999,
Tufts et al. 2000). In the mapped region, the complex ridges are
relatively old features and the internal geometry defined by the
constituent ridges has been disturbed by subsequent deformation. In addition, the complex ridges display abrupt lateral terminations along lineaments, interpreted in this study as being
old strike–slip faults analogous to transform faults in terrestrial
spreading centers.
Ridged plains (RP) form the background upon which younger
fracture lineaments are superimposed. They are composed of a
dense network of small-scale double ridges that probably developed under mechanical conditions similar to the relatively
younger and more prominent double ridges. In some areas, the
background plains exhibit a low relief and lack a complex arrangement of double ridges. These are the oldest regions of RP
and are referred to as subdued plains.
Finally, the mapped region contains numerous examples of
both normal and strike–slip faults. The normal faults occur
almost exclusively within smooth bands. High-standing fault
blocks can be identified by the manner in which recent surface
BRIGHT PLAINS FRACTURE HISTORY, EUROPA
495
A. Fracture Sequence
FIG. 5. Tilted fault blocks in the SB2 unit. (a) Surface fractures are deflected
across the tilt block topography, highlighting the high-standing regions. Three
relative albedos are produced in response to the incident sun direction with
respect to the faults: high (bright footwall surfaces facing the sun direction),
medium (valley fill deposits), and low (shadows cast by fault scarps). (b) Cross
section through fault blocks showing infilled valleys.
fractures are deflected from a linear trace across the fault blocks
(Fig. 5a). Where the fault scarp faces the incident sunlight, the
scarp is bright and is mantled by a single dark band of shadow
on the uplifted footwall block. If the scarp faces away from incident sunlight, the scarp casts a shadow on the subsided hanging
wall block whereas the upper surface of the footwall appears
bright. As a result, the surface reflectance in normal faulted regions is composed of three relative albedo levels: high (bright),
medium (gray), and low (dark). The gray material separating the
dark shadows and bright surfaces consistently occurs within the
subsided hanging wall valleys suggesting that the fault-induced
valleys contain basin fill material (Fig. 5b). Strike–slip faults are
evidenced by lateral offsets of linear features to either side of
the fault trace. The strike–slip faults may have developed from
preexisting fractures that were subject to later shear stresses. The
relative timing of slip along strike–slip faults can thus be determined with respect to the stratigraphic ages of features offset by
the faults.
IV. GEOLOGIC HISTORY
Detailed fracture mapping was used to produce a geologic
map of the study area (Fig. 6) and to unravel the sequence of
fracturing using cross-cutting relationships.
Where more than one age of a particular type of structure
developed (e.g., double ridges), the different stages of development are numbered sequentially from oldest to youngest. In
some cases, it is not possible to directly correlate features in
the northern half of the mapped region (north of Androgeos
Linea: the prominent double ridge bisecting the field of view
in Fig. 3) with similar aged features in the south. Therefore,
a separate stratigraphic nomenclature is used for the northern
and southern regions, with possible age correlations indicated
in the stratigraphic chart (Fig. 6). This is only necessary for the
oldest stratigraphic features that lack broad-scale cross-cutting
relationships; younger features can be correlated across the entire mapped region. The stratigraphic history is described in
sequence below.
The oldest units are the low-lying, subdued plains, which are
overprinted by a dense network of variably oriented ridges of
the ridged plains unit. For the remainder of this paper, these
units will be collectively referred to as the background plains.
The background plains represent the greatest areal coverage of
all individual geologic units mapped in the Bright Plains area,
although the fracturing information they provide is the most
complex. There are few preferred orientations of ridges in the
ridged plains unit; local patterns exist where a NW–SE set of
ridges appears to postdate an ENE–WSW set. The ridges cannot be correlated across a wide area, precluding the determination of an unambiguous stress history during background plains
development.
Smooth Band 1 (SB1), occurring along the southern edge of
the mapped area, clearly postdates the background plains. Subtle internal lineaments may be ancient fault scarps. SB1 is overprinted by a network of relatively younger ridges, the Double
Ridge 1 unit (DR1), restricted to the area south of Androgeos
Linea. This is the oldest set of double ridges (i.e., independently
distinguishable from the background plains), some of which
cross-cut each other. Ridge orientations progress from 330 to
025◦ with time, implying the regional stress field rotated in a
clockwise sense through a 55◦ arc during DR1 time. The Complex Ridge 1 unit (CR1), occurring in the NW corner of the map,
is of age similar to that of DR1 in the south; however, its exact
position in the stratigraphic sequence is difficult to constrain due
to a lack of cross-cutting relationships with similar aged units.
CR1 terminates abruptly at its southern end, where it is truncated
by a relatively younger smooth band unit.
Smooth Band 2 (SB2) is a 10.5-km-wide band of crustal
spreading material near the southern edge of the study area.
SB2 clearly cross-cuts the older DR1 ridges but its age relative
to CR1 is unknown. SB2 appears to be internally dissected by
numerous normal faults striking at 075◦ and seemingly dipping
to the NNW (Fig. 6). Beyond the map boundaries, SB2 continues westward for ∼60 km; however, to the east the unit is
obliterated by lenticula development associated with diapirism
(cf., Pappalardo et al. 1999).
In the northern half of the map, Smooth Band 3 (SB3) crosses
the entire map region from east to west but bifurcates into two
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SIMON A. KATTENHORN
FIG. 6. Geologic map of the Bright Plains area (see photo mosaic in Fig. 3). Geologic units are described in the main text. The relative order and orientations of
fractures are as shown in the stratigraphic column. Some older geologic units cannot be directly correlated between the north and south areas, which are separated by
the double ridge Androgeos Linea. Separate north and south stratigraphies are therefore defined for the oldest geologic units. Younger units can be unambiguously
correlated.
∼2-km-wide strands about 5 km apart in the western portion.
The northern strand of SB3 is flanked to either side by a series of
left-stepping echelon fractures that suggest right-lateral shearing
and hence a clockwise rotation of the regional stress field after
the formation of SB3. The echelon fractures are overprinted
by several relatively younger ridge units, allowing them to be
differentiated from recent surface fractures.
Complex Ridge 2 (CR2), in the SE corner of the map, developed at a similar time to SB3 and represents the northern end
of a more extensive feature that can be traced to the south in
BRIGHT PLAINS FRACTURE HISTORY, EUROPA
regional images for a distance of at least 40 km to the intersection of Agave Linea and Asterius Linea. CR2 attains a maximum
width of ∼7 km and appears to have a bilateral symmetry about
a central ridge. The ridges within CR2 terminate abruptly at
their northern end along a relatively younger smooth band unit,
analogous to the southern termination of CR1.
A second period of double ridge development, Double Ridge 2
(DR2), overprinted CR2 and SB2 in the south and SB3 in the
north. DR2 ridges north of Androgeos Linea (DR2N) cannot be
unequivocally correlated with DR2 ridges south of it (DR2S).
Nonetheless, their relative ages are similar within the regional
stratigraphic sequence. As with DR1, DR2 ridges have a range
of orientations and thus represent an extended period of ridge
development during which time the regional stress field varied.
Relatively younger DR2 ridges are rotated successively more
clockwise than those preceding them (wherever relative ages are
apparent). At least one DR2N ridge developed along a preexisting plane of weakness along the western boundary of CR1 and
thus cannot be used as a stress indicator. Similarly, some DR2S
ridges utilize a preexisting lineament trend within SB2 and are
thus also unreliable stress indicators. The range of orientations
of DR2S ridges overlaps with the DR2N ridges, which supports
the notion that DR2N and DR2S are broadly contemporaneous.
DR2 ridge development was followed by another period of
crustal spreading, Smooth Band 4 (SB4). As with SB3, the width
of SB4 is variable (5.7–12.3 km) and the margins are irregular.
In regional images, SB4 can be traced beyond the map area
for ∼120 km to the west, where it is obliterated by a large region of chaos terrain, and to the east for at least 140 km (the
limit of photographic coverage of the Conamara Chaos region).
SB4 is thus an extensive spreading lineament, with a regionally consistent orientation (∼065◦ ) along a 260-km length. As
with the other smooth bands, SB4 is internally faulted by numerous normal faults oriented parallel to its trend. A relatively
younger fault population (035◦ –055◦ orientation) between the
older faults formed in a subsequently reoriented stress field.
SB4 crustal spreading was followed by another period of
broad-scale double ridge development, forming the Double
Ridge 3 set (DR3), which are not as numerous as the older ridge
sets. At least one, and possibly two DR3 ridges have offset older
ridge and smooth band material in a right-lateral sense by up to
700 m, suggesting strike-slip motion occurred along DR3 ridges
at some time after their formation. DR3 ridges were succeeded
by the development of comparatively lower relief medial-trough
ridges in bands up to 2.5 km wide, each with a narrow central
trough. In the northern area, a 9-km left step along a MTR has
resulted in the development of an eye-shaped feature bounded
by linking ridge segments.
A final episode of crustal spreading is evidenced by the
Smooth Band 5 unit (SB5). Only two examples of inchoate SB5
spreading exist, both south of Androgeos Linea. The wider of the
two is slightly more than 400 m wide and indicates pure opening offset of older ridges across the band. At its eastern end, the
largest SB5 band transforms into a double ridge, indicating the
genetic evolutionary relationship between these fracture types
497
(c.f., Geissler et al. 1998b). The second, more northerly SB5
band is <200 m wide and shows a small (∼100 m) amount of
left-lateral offset, which decreases to zero toward the eastern end
of the band. It cannot be determined if the lateral motion pre- or
postdates the crustal spreading across the band.
The next period of ridge development, Double Ridge 4 (DR4),
is represented by two ridges noticeably wider than older ridges.
DR4 ridges cross-cut DR3, SB4, SB5, and MTR and exhibit
a consistent orientation of 150◦ . A small portion of a ridge in
the WSW corner of the map (Fig. 6) represents the easternmost
ridge along the ∼10-km-wide Agave Linea (Fig. 1), which can
consequently be temporally placed into the overall fracture sequence. A DR4 ridge in the east-central portion of the mapped
area appears to offset an older DR2S ridge in a right-lateral sense
by up to 575 m, implying a clockwise rotation of the stress field
after the development of DR4.
The most geologically recent ridge-forming episode, Double Ridge 5 (DR5), produced two ridges in the map area, including Androgeos Linea. It is the single largest feature in the
Bright Plains area, traceable across the entire Conamara Chaos
region ∼35 km northwest of Asterius Linea, with which it may
be coeval. Vertical loading of the crust by this ridge resulted
in isostatic subsidence and downwarping along the ridge flanks,
producing a ridge-parallel set of bending-induced fractures. Subsequent infill of the marginal troughs by either material extruded
from Androgeos Linea or ice debris flows off the ridge slopes
produced flood deposits that embay along the surrounding highstanding ridges. The DR5 ridge in the south exhibits right-lateral
strike–slip offset of both a DR3 ridge and an MTR band by up
to 280 m, implying a clockwise rotation of the stress field after
the development of this DR5 ridge.
Nascent ridge development after the DR5 stage is manifested
by a set of proto-ridges. East–west trending PRs exhibit evidence
of left-lateral strike–slip motion (up to 430 m), offsetting DR1
and DR4 ridges as well as an SB5 band. Left-lateral motion induced tensile stresses north of the eastern tip of the PR in the
center of the mapped area (Fig. 6), resulting in a set of tailcrack
fractures that propagated linearly into the surrounding crust for
up to 5.4 km, cross-cutting an adjacent PR in the process, implying that opening of PRs predates strike-slip motion along
them.
The final stage of fracturing was the development of several
ages of surface fractures (Fig. 7), none of which appear to be
associated with intrusion or extrusion of material from below,
suggesting that they are all near-surface fractures. The relative
ages of these fractures with respect to each other are unclear.
Some overlap may have occurred between the time of surface
fracture growth and the development of PRs. Furthermore, some
fractures appear to be cross-cut by PRs, suggesting that some
surface fracturing preceded the formation of proto-ridges.
B. Fracture Orientations
Fracture orientations were measured on the Bright Plains mosaic (Fig. 3), which was orthographically reprojected about the
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SIMON A. KATTENHORN
tudes or strain rates associated with the development of the two
fracture categories.
C. Areal Coverage of Geologic Features
The areal coverage of the various geologic features (Fig. 9)
does not provide a definitive means of characterizing trends in
structural development, but nonetheless reveals several interesting details. When individual geologic units are considered, the
majority of the surface area of the Bright Plains (23%) is made
up of the background plains material (Fig. 9a) and indicates
that over three-quarters of the oldest ridge features have been
superposed or eradicated by subsequent ridge and band development. Considered individually, different fracturing episodes
FIG. 7. Geologically recent surface fractures in the Bright Plains showing a
range of orientations. Each fracture set is represented with a different linewidth.
At least five distinct orientations are identifiable but the relative ages cannot be
unambiguously determined.
center of the image area to minimize distortion effects. Angular
relationships are accordingly accurate to within a few degrees.
Rose diagrams showing the range of fracture orientations in the
Bright Plains region (Fig. 8) display preferred orientations of
similar fracture types and disparate orientations for different
fracture types (ridges versus bands).
For example, the category of fractures incorporating the different types of ridges (double ridges, complex ridges, medialtrough ridges, and proto-ridges) display orientations that are
typically clustered around either NW–SE or NE–SW. There is
very little variation in orientations within the individual ridge
units (other than the oldest ridge unit, DR1, which probably represents several consecutive ridge-forming episodes). In contrast,
the category of fractures incorporating bands (SB1—SB5) are
typically approximately E–W trending. This observation may
point to a strong correlation between the type of fracture developing in Europa’s crust and the characteristic global stress
field present at a particular location at a particular point in time.
This phenomenon may be indicative of significant differences in
structural style, or simply the effects of disparate stress magni-
FIG. 8. Rose diagrams in 10◦ increments showing the orientations of fractures in the Bright Plains. Ridges typically have NW–SE or NE–SW orientations
whereas smooth bands are typically in a more E–W orientation. The number of
orientation measurements (n) is shown for each unit. Arrows around the circle
edges represent average orientations for each unit.
BRIGHT PLAINS FRACTURE HISTORY, EUROPA
499
FIG. 9. Areal coverage of different geologic units in the Bright Plains. (a) Amount of area covered by individual units. Background plains are the most
extensive unit. (b) Areal coverage of units when combined into categories.
superimposed different amounts of surface area (Fig. 9a).
Double-ridge-forming episodes typically consumed <7% of the
existing surface area. Although the size of individual ridges has
noticeably increased through time (Fig. 6), there is no corresponding increase in amount of total surface area consumed
(assuming DR2N and DR2S can be combined), because the geologically recent larger ridges are fewer in number than smaller,
older ridges. In the absence of available elevation data, changes
in the volume of material extruded through time cannot be
estimated.
Unlike ridges, smooth bands appear to show a progressive
increase in the amount of surface area consumed through time
(Fig. 9a), peaking at 16% for SB4. The most recent smooth band
event (SB5) has consumed <1% of the surface area, perhaps indicating that SB5 bands are merely nascent smooth bands. In
Fig. 9b, all types of ridges (excluding complex ridges, which
are more synonymous with spreading features than individual
extrusive fractures) are combined into a single category and
compared with the areal coverage of complex ridges, smooth
bands, and background plains. Smooth bands have the greatest
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SIMON A. KATTENHORN
areal coverage (37%) when all episodes are combined. If double
and complex ridges are considered together, their areal coverage (39%) is similar. Considering that smooth bands appear to
represent new ice crust at Europan spreading centers, it remains
a challenge to explain the addition of 37% of new surface area
without direct evidence of an increased total surface area on
Europa, or the eradication of an equivalent amount of surface
area during the spreading process such as through folding of thin
lithosphere (e.g., Prockter and Pappalardo 2000).
D. Crustal Spreading and Fracture Reactivation
Several key features of fractures in the Bright Plains area
(Fig. 6) provide insights into the formation mechanisms and the
importance of reactivation of older structures during more recent
deformation episodes. Although double ridges are common features, it is readily apparent that ice crust separation and crustal
spreading is equally important in resurfacing Europa (Fig. 9b).
Complex ridges and smooth bands represent two contrasting
forms of crustal spreading in the Bright Plains. In previous
works, two prominent models for band evolution have been proposed. The first maintains that smooth bands form by repeated
intrusion of, and ridge development alongside, a central fracture
that progressively dilates, resulting in new ice crust to either side
of the central fracture and a bilateral symmetry of ridges to either side of the central fracture (Sullivan et al. 1998, Tufts et al.
2000). The second model describes smooth bands as representing new ice crust produced by buoyant upwelling of warm ice
beneath a zone of rifting, chilling, and fracturing near the surface, forming a system of graben or half-graben across the width
of the band (Prockter et al. 1999b). The smooth bands identified
in this study are clearly internally dissected by tilt-block faulting (half-graben), supporting the Prockter et al. (1999b) model.
Complex ridges are more adequately described by the Sullivan
et al. (1998) and Tufts et al. (2000) models, implying that the two
existing models describe different types of structural features,
both of which fall into the general category of crustal spreading.
In the Bright Plains, noticeable differences between smooth
bands and complex ridges are reflected in their disparate surface
albedos and internal geometries. Smooth bands display regularly
repeating bands of high, medium, and low albedo associated with
horsts, valleys, and fault scarps. Complex ridges have variable
albedo and resemble a multitude of deformed ridges alongside
each other. Regional images suggest bilateral symmetry about a
central trough in the case of CR2. The spacing between ridges
in the complex ridges is consistently less than the fault spacing
in the smooth band units.
Both complex ridge units display abrupt terminations along
one end, perpendicular to the ridge orientations. In the case
of CR1, such an abrupt termination of a spreading center that
formed a 14-km-wide wedge of new crust can only be reconciled
with spreading along a transform fault, the evidence of which
was subsequently obliterated by SB3 along the southern margin
of CR1. Analogous spreading alongside transform faults occurs in the Wedges region of Europa (Lucchitta and Soderblom
1982, Prockter et al. 1999a, Tufts et al. 1997). The ridges within
CR2 all terminate abruptly at their northern end along a younger
smooth band unit (SB5), which probably developed by dilating
an old transform fault.
This dilation of an old transform fault provides one line of evidence of ongoing reactivation of older structures in the Bright
Plains, whether it be dilation along preexisting faults or shear
failure of preexisting ridges, as evidenced by strike-slip offsets
along some ridges. For example, SB3 truncates the southern edge
of the relatively older CR1 unit and is interpreted to have formed
along an older transform fault. This may explain the sudden dogleg turn along the southern strand of SB3, exactly at the location
where the tip of the preexisting transform fault would have been,
based on the geometry of CR1. An important implication of this
phenomenon is that band orientations should not necessarily be
used to deduce stress histories directly. Stress reconstructions
should incorporate data on piercing point vectors across a band,
which provide a more accurate stress indicator by revealing the
maximum tensile stress direction, assuming minimal reorientation of the stress field during the opening of the band.
Ridges also appear to utilize preexisting planes of weakness
in the Europan crust. For example, Androgeos Linea (DR5)
is one of the geologically youngest features in the Bright Plains,
yet many older ridges appear to terminate against its margins.
This implies that Androgeos Linea reactivated a preexisting lineament across the Bright Plains that may have existed as far back
as DR1 time.
V. DISCUSSION
A. Implications of Fracture Sequence for Stress History
The direct relationship between opening mode fracture orientations and the orientation of the maximum principal tensile stress at the Europan surface at the time of fracturing allows a complete stress history to be determined for the Bright
Plains region. Clockwise rotation of the stress field is indicated
fairly unambiguously during the major fracturing sequence. The
sequence of recent surface fracturing is unclear, and counterclockwise rotations between fracture sets cannot be ruled out,
although it would be inconsistent with the pattern of earlier fracturing, assuming a similar genetic origin. Clockwise rotation of
stresses is consistent with global stress models that consider the
combined effects of diurnal tidal stresses and nonsynchronous
rotation.
The global pattern of principal stresses (Fig. 2) computed by
Greenberg et al. (1998) for 1◦ of nonsynchronous rotation at 1/8
orbit past apocenter (where tensile stresses are maximized and
dilational fractures are thus most likely to develop) can be utilized to place the sequence of fracturing in the Bright Plains area
into a global stress evolutionary context (Fig. 10). The Bright
Plains region migrated over the global stress field pattern as the
crust rotated eastward faster than Europa’s interior. During the
fracturing history, the global stress field would thus have appeared to be moving relatively westward over a fixed Bright
BRIGHT PLAINS FRACTURE HISTORY, EUROPA
501
FIG. 10. Orientations of geologic units in the Bright Plains superimposed on the global tidal stress field at 15◦ N, as in Fig. 2. Unit colors are as in Fig. 6.
Recent surface fractures are shown in dark blue. Strike-slip events on existing fractures are labeled as LL (left-lateral) and RL (right-lateral). Black lines are tensile
stresses and red lines are compressive stresses. Longitudinal locations of units assume development by tensile fracturing perpendicular to the maximum tensile
stress. Successively older units are plotted progressively further west (to the left) on the diagram, moving backward in time. This back rotation accounts for the
progressive eastward motion of Europa’s crust due to nonsynchronous rotation. The current location and stress field in the Bright Plains area are shown by the red
box. However, to account for the orientations of the most recent fractures, an additional 180◦ shift to the east is required to match the current state of stress, placing
the Bright Plains in the location of the green box. Up to 900◦ of nonsynchronous rotation is implied moving backward in time from the green box.
Plains reference frame. Successively older fracture sets can thus
be placed into a time sequence of older states of stress occurring
when the Bright Plains region was positioned at successively
more westward longitudes (relative to the current tidal bulge locations) moving further and further back in time (cf. Figueredo
and Greeley 2000).
Consideration of the change in fracture orientations through
time (Figs. 5 and 10) indicates that the principal stress axes in
the mapped region have undergone at least two complete clockwise rotations (720◦ ) during the sequence of major fracturing
(excluding relatively recent surface fracturing and proto-ridge
development). This estimate assumes all lineaments to have originated as tensile fractures and is based on the greatest amounts
of possible temporal overlap between different units. If the temporal overlaps are less (i.e., greater time gaps between successive units), the actual stress rotations are potentially greater than
720◦ . Using these changing fracture orientations to deduce the
longitudes of fracture development through time, it can be determined that the outer shell of Europa must have completed a
minimum of two complete rotations about its fixed core prior to
DR5 development (Fig. 10). The background plains, which were
not included in the determination of this estimate, include several
older ridge sets that imply an even greater, but unknown, amount
of rotation of the principal stresses during the fracturing history.
The current state of stress on Europa at the latitude of the
Bright Plains region is shown along the top panel of Fig. 10,
with the longitude of Bright Plains (∼273◦ ) shown by the red
box. The current state of stress closely approximates (within
10◦ ) the state of stress that would have existed at the time of
development of the most recent ridge-forming event (DR5) and
hence possibly the nearby Asterius Linea. However, recent surface fracturing and proto-ridge development would require a
state of stress that is located as much as 110◦ east of the red box
location in Fig. 10. If the recent surface fracturing and protoridge development is attributable to nonsynchronous rotation
effects, additional rotation of Europa’s ice shell is required to
account for the development of these features.
Because global stress patterns are symmetric in opposing
hemispheres separated along any line of longitude, an eastward
longitudinal shift of 180◦ is necessary to place the Bright Plains
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SIMON A. KATTENHORN
where it is again in the correct current stress field (green box in
Fig. 10), implying a minimum total of 900◦ of nonsynchronous
rotation. This longitudinal shift would account for the development of recent surface fractures and proto-ridges, the last episode
of which would thus have occurred at a time when the Bright
Plains was located at least 70◦ west of the longitudinal position
of the green box in Fig. 10.
Previous regional mapping (Geissler et al. 1998a, 1998b,
1999, Greenberg et al. 1998, Helfenstein and Parmentier 1985,
McEwen 1986, Prockter et al. 1999a, Spaun et al. 1998) and
detailed mapping (Figueredo and Greeley 2000) of fracture sequences at various longitudinal and latitudinal locations on
Europa have indicated nonsynchronous rotation of the Europan
crust with respect to its core. However, estimates of the amount
of nonsynchronous rotation of Europa’s outer shell based on
the above work are variable: ∼25◦ –50◦ (McEwen 1986); ∼60◦
(Greenberg et al. 1998); 60◦ –90◦ (Geissler et al. 1998a); ∼95◦
(Geissler et al. 1998b); ∼360◦ (Geissler et al. 1999); 360◦ –720◦
(Figueredo and Greeley 2000). These inconsistencies are probably related to the variable scales over which fracture interpretation was carried out; however, diurnal stress field variations
could conceivably produce a range of fracture orientations over
short periods of time (Figueredo and Greeley 2000), complicating nonsynchronous rotation estimates. The best estimates
for nonsynchronous rotation will result from detailed fracture
mapping using high resolution SSI images of regions containing numerous ages of superimposed fractures, representative of
the regional stress history.
The 900◦ of nonsynchronous rotation interpreted from the
Bright Plains region exceeds the 360◦ –720◦ estimate of
Figueredo and Greeley (2000) deduced from detailed fracture
maps of the leading hemisphere of Europa. This may be a consequence of the fracturing density being higher in the Bright Plains
region than in other previously mapped areas of Europa. A more
complete global stress history may therefore be indicated by the
Bright Plains fracturing sequence described here and indicates
that the amount of nonsynchronous rotation of Europa’s outer
shell may be significantly higher than previously suggested.
Using the estimate of Hoppa et al. (1997) that a full revolution
of the outer shell would occur over a minimum of 12,000 years,
the most recent surface fractures and proto-ridges developed
a minimum of ∼2300 years ago, with older fracture sets only
developing intermittently prior to that with intervening time intervals of perhaps several thousands of years. This estimate has
important implications for evaluating active tectonics on Europa.
If the most recent fracturing occurred more than 2300 years
ago (or significantly longer if nonsynchronous rotation is much
slower than implied by the lower bound placed by Hoppa et al.
(1997)), this may imply that there is no active low latitude fracturing on Europa where the stress field is similar to that of Conamara Chaos (i.e., in the leading and trailing hemispheres around
∼090◦ and ∼270◦ longitude, respectively).
This is somewhat enigmatic considering that at each point
in the fracturing history of the Bright Plains when the stress
field was identical to its current configuration, episodes of ridge
development always occurred. An identical trend can be seen
in the data presented by Figueredo and Greeley (2000) for 90◦
longitude. The implications of this observation are unclear. One
possibility is that the red box in Fig. 10 describes the true current
Bright Plains location with respect to the fracturing history and
that recent proto-ridges and surface fractures cannot be explained using nonsynchronous rotation models. Alternatively,
if the green box in Fig. 10 is the correct current location of
the Bright Plains, this may be an indication of incipient (in the
geologic sense) fracture activity. A third possibility is that the
past fracturing activity cannot be used to predict current tectonic
trends on Europa, perhaps due to some long-term change in the
nature of the Europan crust, such as crustal thickening, that has
affected fracture activity.
A change in tectonic behavior may be supported by largescale observations of features in the Conamara Chaos region
(e.g., Spaun et al. 1998) that indicate the most recent geologic
activity to be disruption of the Europan crust by the formation of
domal structures, possibly related to diapirism or cryovolcanism
(Collins et al. 2000, Pappalardo et al. 1998a, Wilson et al. 1997),
resulting in chaotic terrain. This type of deformation appears unprecedented in Europa’s visible geologic history. Chaotic terrain
is not present in the high-resolution Bright Plains image (Fig. 3);
however, low-resolution images of the Bright Plains beyond the
bounds of the mapped area described in this study clearly reveal
that chaos formation postdates the development of DR5. For
example, Androgeos Linea is disrupted by chaos activity a few
kilometers beyond the eastern boundary of the mapped area.
The above-mentioned time interval estimates for fracture development would require a minimum time interval of ∼6300 years
for the 190◦ of rotation since the development of Androgeos
Linea (DR5). Based on a model by Pappalardo and Coon (1996)
for ridge development through diurnal cycling (i.e., ridge growth
by repeated opening, extrusion from, then closing of the ridge
fracture during each orbit), Greenberg et al. (1998) estimate
that a 1-km-wide, 100-m-high ridge would form over a period
of about 30,000 years. The 2.3-km-wide, ∼100-m-high Androgeos Linea ridge would presumably have taken a relatively
longer time to develop. This time estimate for DR5 development
is almost five times greater than the 6300-year estimate given
above, which may be an indication that: (1) the period of nonsynchronous rotation is significantly greater than the 12,000-year
lower bound suggested by Hoppa et al. (1997); (2) the ridge
growth rate of Greenberg et al. (1998) is too slow; or (3) more
than one rotation of Europa’s shell occurs between successive
fracturing episodes, in disagreement with the assumption of this
study that fracturing occurs during each nonsynchronous rotation cycle.
B. Global Stress Models Revisited
The Bright Plains observations presented here and inferences
about frequency of fracturing events do not necessarily hold true
at high latitudes (>50◦ ), where maximum principal stresses are
BRIGHT PLAINS FRACTURE HISTORY, EUROPA
tensile at all longitudes (Fig. 2). Nearer the equator, such as
in the Bright Plains area at ∼15◦ N, tensile stresses are very
small for large portions of the evolving stress history (e.g.,
between 110–200◦ and 290–020◦ longitude in the current pattern
of stress in Fig. 2). In fact, at latitudes of less than 40◦ , the stresses
remain compressive throughout these longitude intervals
(Greenberg et al. 1998). Additionally, the interpreted 70◦ of
nonsynchronous rotation since the most recent surface fracturing corresponds closely to that portion of the stress field in which
the principal stresses are all compressive, which may explain the
lack of fracture activity during this time interval.
Nonetheless, many of the older fracture lineaments in the
Bright Plains area appear to have formed within these compressive portions of the stress field, particularly the double ridges.
If the double ridges formed by tensile fracturing, as their geometries imply, the stresses must have been sufficient to cause
fracturing throughout the full rotation period of the stress field
(i.e., tensile driving stresses at all longitudes). A similar effect
can be seen in the low-latitude observations of Figueredo and
Greeley (2000) in the leading hemisphere of Europa.
These observations imply that the current models for the combined effect of nonsynchronous rotation and diurnal stresses for
surface stress magnitudes are not sufficient to explain the initiation and propagation of all tensile fractures on Europa. Spaun
et al. (2001) suggest that the double ridges may represent shear
failure features that form in a compressive stress field. Their
model arose out of the need to explain the high frequency of
NW- and NE-trending lineaments on Europa, which are only
expected in regions of all-round compression in tidal stress models (Fig. 2). However, their observations indicate inconsistent strike–slip sense along some NW- and NE-trending strike–
slip faults, in apparent conflict with their shear fracture model.
Nonetheless, Spaun et al. (2001) correctly emphasize the need
to account for the occurrence of NW- and NE-trending lineaments. Therefore, additional sources of tensile stresses may be
needed to explain their existence.
One possibility arises from the fact that the global stress
model in Fig. 2 only represents the diurnal tidal stress field superimposed on a presumed 1◦ nonsynchronous rotation stress
field at 1/8 orbit past apocenter. This point in Europa’s orbit
is where the greatest magnitude tensile stresses would occur;
however, there is no basis for presuming that fracture initiation
only begins at this point in Europa’s orbit. The diurnal stress
field is variable depending on Europa’s orbital position with respect to Jupiter (Greenberg et al. 1998); hence, the superposed
diurnal/nonsynchronous stress field is also variable during each
3.55-day orbit. Prior to apocenter, the stresses are least favorable
for fracture production at 1/4 orbit past pericenter (i.e., 1/4 orbit
prior to apocenter), where the diurnal stress field almost negates
the nonsynchronous rotation stress field (Greenberg et al. 1998).
However, as Europa moves toward apocenter, two N–S oriented
bands of tensile stresses are produced across the equatorial zone
(in the current longitudinal ranges 055◦ –145◦ and 235◦ –325◦ ),
which subsequently migrate eastward until centered at 055◦ and
503
235◦ longitude respectively by 1/8 orbit past apocenter (Fig. 2),
where tensile stress magnitudes are maximized. Conceivably,
fracturing may begin during this diurnal migration of the stress
field as soon as the tensile strength of the ice is surpassed,
which may occur for tensile stress magnitudes as low as 0.3 MPa
(Mellor 1986), or lower if preexisting fracture lineaments are reactivated, such as occurs in some instances in the Bright Plains.
This may explain the development of tensile fractures in the
compressive zones in Fig. 10.
This stress field variability on the diurnal time scale implies
that there may be some variability to the longitudinal positions
of fractures in the reconstructed fracture sequence (Fig. 10) depending on the timing of fracture development during the diurnal and nonsynchronous rotation cycles. If this is the case, the
fracture interpretation techniques described in this study could
potentially overestimate the amount of nonsynchronous rotation. However, the observed characteristic linearity of all fracture types would not be expected if fractures were propagating in
a temporally changing stress field associated with the diurnal cycle, unless fracture propagation was extremely rapid. For example, highly curved cycloidal ridges are interpreted to propagate
on the diurnal time scale (Hoppa et al. 1999b). The characteristic
fracture linearity thus strengthens the arguments for the nonsynchronous rotation estimates presented here because it implies
fracture growth at the same point during each diurnal cycle. In
addition, the locations of the most recent surface fractures and
proto-ridges in the reconstructed fracture sequence (Fig. 10) at
up to 110◦ east of the red box cannot be adequately explained by
diurnal variations in the stress field because the range of longitudinal migration of the N–S band of tensile stresses during the
diurnal cycle is less than 45◦ . This supports the inference of at
least 70◦ of nonsynchronous rotation since the most recent surface fracturing events, placing the Bright Plains at the location
of the green box in Fig. 10.
Nonetheless, the largest stress magnitudes induced by coupled
diurnal/nonsynchronous rotation effects do not exceed a few
tenths of a megapascal, which may be too small to induce new
fracturing. For example, the tensile strength of water ice close to
the freezing point may be as high as several megapascals (Cassen
et al. 1982) and increases with decreasing temperature. Thus,
coupled diurnal/nonsynchronous stresses alone are unlikely to
be the dominant control on fracture initiation and growth.
Assuming that the majority of Europan fracture lineaments
are in fact opening mode tensile fractures, which are intrusive in
nature, it is probable that the driving stress for fracture growth is
contributed by fluid pressures within the fractures themselves.
Internal fluid pressures of several megapascals, for example,
would greatly surpass the contribution of diurnal and nonsynchronous rotation stresses to the fracture driving stress. However, because fluid pressures are isotropic, fracture orientations
would nonetheless be governed by the anisotropic stress field induced by diurnal/nonsynchronous rotation effects, with opening
mode fractures forming perpendicular to the maximum tensile
principal stress orientations exactly as shown in Fig. 2. Thus,
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SIMON A. KATTENHORN
irrespective of stress magnitudes in Fig. 2, the principal stress
orientations are valid for predicting tensile fracture orientations
at various times in Europa’s past for all longitudes, including
those within the compressive regions.
Finally, fractures that were intruded by a pressurized fluid
must have either grown from the surface downward to penetrate
the entire crustal thickness to tap a subcrustal ocean or initiated
at the base of the crust and grew upward, assisted by the internal fluid pressure. The magnitude of tensile stresses calculated
for Europa’s surface (in the 0.1 MPa range) (Greenberg et al.
1998) is insufficient to allow downward growth of fractures from
the surface to depths greater than ∼65 m (Hoppa et al. 1999b)
as a result of the increasing lithostatic compression. How such
fractures penetrate the entire crustal thickness cannot be adequately explained by existing global stress models. Fractures
that initiate at the base of the crust require significantly higher
tensile stresses than those occuring at the surface to overcome
the lithostatic compression (e.g., ∼2.4 MPa at a depth of 2 km,
assuming an ice density of 0.9 g/cm3 ), emphasizing requisite
high fluid pressures to initiate fracturing. In summary, although
existing global stress models can be used to account for observed fracture orientation distributions on Europa, there are
still problems when one attempts to reconcile calculated stress
magnitudes with those required to fracture the entire Europan
crustal thickness.
VI. CONCLUSIONS
Detailed fracture mapping in the Bright Plains area of Europa
indicates a fairly unambiguous fracturing sequence. In response
to tensile stresses in the crust, the Bright Plains region experienced repetitive episodes of fracturing over a period of at least
several tens of thousands of years. The fracturing events included
multiple episodes of double ridge growth, sea-floor spreadinglike smooth band and complex ridge development, surface fissuring, and normal faulting. Shear reactivation of many tensile
fractures resulted in strike–slip fault motions. The stress history deduced from the interpreted fracturing sequence implies
a consistent clockwise rotation of principal stresses throughout the deformation history of the region. This observation is
in agreement with fracture sequence reconstructions elsewhere
in Europa’s northern hemisphere and strongly points toward
nonsynchronous rotation being the dominant control on crustal
fracture patterns.
Comparison of fracture orientations at various times in the
Bright Plains fracture history with global tidal stress models that
incorporate diurnal and nonsynchronous rotation stresses enabled the determination of the longitudes at which each fracturing episode occurred, moving backward in time from the present
stress configuration. These deductions indicate that the outer
shell of Europa has undergone at least two and possibly more
revolutions (∼900◦ ) about its interior during the visible fracture
history, providing strong evidence that the ice shell is decoupled from an underlying ocean. This nonsynchronous rotation
estimate is more than twice that of previous estimates made by
other researchers and indicates that a longer period of Europa’s
geologic history may be preserved at the surface than was previously determined. Furthermore, fracturing appears to have occurred in punctuated episodes mutually separated by perhaps
several thousands of years of inactivity. The most recent surface
fracture orientations are not consistent with the current state of
global stress, indicating that active fracturing may not have occurred in the Bright Plains region for several thousands of years.
Nonsynchronous rotation models are the most plausible explanation for the variation in fracture orientations in the Bright
Plains through time, despite unresolved issues with regard to
insufficient tensile stress magnitudes in such models to account
for crust-penetrating fractures. The addition of an isotropic stress
component to the driving stress for fracture development, such
as would be produced by an internal fluid pressure, may provide
sufficient tensile stress to promote fracturing through the crust
with orientations identical to those predicted by nonsynchronous
rotation stress models. Such models are therefore adequate for
unraveling the stress and nonsynchronous rotation history at any
location on Europa based on detailed fracture mapping and interpretation.
ACKNOWLEDGMENTS
This research was funded by grants from the NASA–Idaho Space Grant Consortium and NASA Grant Number NAG5-11495. Thanks are owed to Sandi
Billings for assisting in the compilation of a library of Galileo SSI images and
to Kenneth Sprenke for providing the impetus for this study. The Bright Plains
image mosaic was orthographically reprojected by Jim Klemaszewski and made
available by Jody Kerr at the NASA Space Photography Laboratory at Arizona
State University. Rose diagrams were produced using Georient software. Thanks
are also owed to Louise Prockter, Patricio Figueredo, and Rob Sullivan for stimulating discussions about Europa geology and map projections. I am extremely
grateful to Cynthia Phillips and Patricio Figueredo for their careful reviews of
the original manuscript and their insightful comments which led to a greatly
improved final product.
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