Non-synchronous Rotation of Europa: Indications from Galileo Observations

P.E. Geissler, R. Greenberg, G. Hoppa, P. Helfenstein*, A. McEwen, R.
Pappalardo|, R. Tufts, M. Ockert-Bell*, R. Sullivan§, R. Greeley§, M.J.S.
Belton¿, T. Denk¿, B. Clark*, J. Burns*, J. Veverka* and the Galileo
Imaging Team

Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ. 85712
USA *Laboratory for Planetary Science, Cornell University, Ithaca, NY
14853 USA |Department of Geological Sciences, Box 1846, Brown University,
Providence, RI 02912 USA ¿National Optical Astronomy Observatories, P.O.
Box 26732, Tucson, AZ 85726 USA §Department of Geology, Arizona State
University, Box 871404, Tempe, AZ 85287 USA ¿DLR-Berlin-Adlershof,
Institute for Planetary Exploration, Rudower Chaussee 5, 12489 Berlin,
Germany

   Non-synchronous rotation was predicted on theoretical grounds [1] by
considering the torque exerted on Europa's tidal bulges by Jupiter,
averaged over an orbital period of the satellite. If Europa's orbit were
circular, or the satellite were a frictionless fluid without tidal
dissipation, this torque would average to zero. However, Europa has a
small forced eccentricity e ~ 0.01 [2], generated by its 3-body resonance
with Io and Ganymede, which causes the equilibrium spin rate of the
satellite to be slightly faster than synchronous unless there is a
permanent interior mass distribution asymmetry large enough to offset the
tidal torque. Even then, the surface may rotate asynchronously if it is
decoupled from the interior, for example by a subsurface layer of liquid.
Non-synchronous rotation was invoked to explain Europa's global system of
lineaments and an equatorial region of rifting seen in Voyager images
[3,4].  Here, we report new Galileo observations which test the hypothesis
that Europa spins faster than the synchronous rate (or has done so at some
point in its past) and comment on the implications of the results for the
interior structure of the satellite. 

Several testable predictions can be based on the hypothesis of
non-synchronous rotation. First, systematic changes in the orientation of
lineaments with age may be expected as the surface reorients relative to
fixed global patterns of tidal stress. Second, crustal extension is
predicted in the regions west of the sub- and anti-Jupiter points as the
surface stretches to accommodate the changing tidal figure. Third, no
leading/trailing asymmetry in impact crater density should be found if
Europa's surface has rotated with respect to Jupiter. Finally, if the
rotation rate is large enough, the locations of surface features seen in
Galileo images might be displaced eastwards relative to their positions in
Voyager data. Galileo Solid State Imaging (SSI) data represent a
substantial improvement over Voyager in terms of spatial resolution,
spatial coverage and spectral range of the observations. After two close
passes of Europa and several non-targeted encounters [5], samples of the
surface at various locations have been imaged at resolutions down to 20
m/pixel, coverage has been expanded especially on the trailing hemisphere,
and observations of Europa's surface at near-infrared wavelengths have
revealed lineaments invisible in Voyager bandpasses. Our preliminary
assessment of these new data, pending further Galileo observations, is
that Europa must have rotated non-synchronously at some point in its past
(and may be doing so today). 

Galileo color data provide the first test of non-synchronous rotation. 
Multispectral imaging of Europa's northern high-latitude region was acquired 
at 37 degrees phase angle from a nontargeted flyby during Galileo's first 
orbit of Jupiter [5]. The imaged region extends across the trailing side of 
the antijovian hemisphere, centered at 45N, 221 W. 4-color coverage extends 
across the entire region, obtained with the violet, green, 756 and 968 nm 
filters, and data from two additional filters were partially returned. 
False-color composites made up from these images show at least three distinct 
classes of linear features on Europa's surface (Figure 1). The most prominent 
linear features are the dark triple-bands such as Cadmus and Minos Lineae. 
The triple-bands cross-cut older lineaments (some with morphologies similar 
to triple-bands) which are intermediate in color between the triple-bands and 
the icy plains, but are brighter than the triple bands and the surrounding icy 
plains at near-infrared wavelengths. These older lineaments include a bright 
wedge at 60 N, 200 W, which is somewhat similar to the gray bands identified 
in the southern polar regions in Voyager images [6]. The youngest features
appear to be incipient fractures, less than a pixel (1.6 km) wide, which 
cross-cut the triple bands. 


These three classes of features apparently represent different stages of
development of tectonic lineaments on Europa. Their distributions are
shown in Figure 2, derived from photogeologic and spectral mapping
(supervised classification) of the photometrically corrected 4-color data.
Bands with spectral reflectance similar to the bright wedge (A) make up
the stratigraphically oldest lineaments and generally have
southwest-northeast trends. The intermediate-aged triple-bands (B) trend
roughly east-west, with the younger of the two prominent criss-crossing
bands near the center of the image (Cadmus Linea, labeled "2") having the
more northwesterly trend. Rhadamanthys Linea ("R"), presumably a
triple-band in the early stages of formation [5], has spectral
characteristics similar to fully-developed triple bands but an orientation
closer to that of the youngest fractures (C), which run
northwest-southeast. The morphology of Rhadamanthys is transitional
between the young fractures and the triple-bands, suggesting that these
features are all formed by the same tectonic processes. If so, then a
clockwise rotation of stress direction has taken place in this region over
time. 

The suggested explanation for the reorientation of these lineaments is
non-synchronous planetary rotation. As Europa's surface revolves relative
to the tidal figure, the pattern of stresses experienced in the northern
hemisphere should rotate in a clockwise sense, consistent with the
observations. This is equally true whether the fractures are produced by
the long term stresses induced by non-synchronous rotation or by much
shorter time scale tidal distortions (both are biaxial distortions
centered on the equator). Polar wandering, in contrast, could not produce
the observed reorientation [7]. An estimate of the amount of eastwards
rotation implied by the observations can be obtained by matching the
orientations of the lineaments to theoretical biaxial stress trajectories,
shifted in the longitude direction until the lineaments are perpendicular
to the maximum extensional stress. About 60 degrees of eastwards
reorientation is required to explain the rotation inferred between the
oldest lineaments (Figure 2A) and the younger of the two prominent
intermediate aged triple bands, Cadmus Linea ("2" in Figure 2B). A further
rotation of more than 30 degrees is required if the youngest fractures
(Figure 2C) formed by the same process. 


The above analysis does not assume that Europa's lineaments necessarily
formed in response to the stresses associated with rotational
reorientation, but merely that they were carried along as the surface
subsequently shifted eastwards. In fact, the orientation of the youngest
fractures (incipient cracks, along with Rhadamanthys Linea) is roughly
radial to the antijove point (Figure 2C). This is not consistent with the
stress pattern predicted for non-synchronous rotation, which is displaced
45 degrees to the east [3], but instead fits the current tidal stress
regime. These youngest lineaments are apparently produced by diurnal tidal
stresses due to Europa's forced eccentricity, and shifted eastwards by
non-synchronous rotation over much longer time scales. Tidal flexing is
predicted to produce periodic bending and tensional stresses radial and
concentric to the sub- and antijove points, along with longitudinal
oscillations in the orientation of the tidal bulges [8].  Extension along
small circles concentric to the antijove point occurs during half of each
3.6 day orbit, as Europa recedes from Jupiter and the tidal bulges relax.
Fractures radial to the antijove point may form as the ice fails in
tension, perhaps incrementally over many thousands of orbits. Tidal
flexing produces stresses which alternate in sign each Europan "day",
providing a possible mechanism for ridge formation by repeatedly opening
and closing the cracks and pumping icy material from Europa's interior to
the flanks of the fractures. Ridge-building is the dominant process
shaping the moon's surface, and every landscape that has escaped erasure
by heating from below is imprinted with generation after generation of
intersecting ridges at all scales and orientations. The constant
reorientation of the tidal stress field due to non-synchronous rotation
should produce continual migration of the local stress directions over
time, explaining the observed sequential changes in ridge direction with
stratigraphic age. If so, then the multiple generations of lineae seen in
high resolution images of Europa would seem to suggest that the process
has gone on for many rotational cycles over the age of the surface. 


A second test of non-synchronous rotation was provided by Galileo imaging
of the trailing hemisphere of Europa. Concentrations of extensional
stresses west of the sub- and antijove points are expected from
non-synchronous rotation, as the surface stretches over the tidal bulges.
A large region near Europa's equator to the west of the antijove point is
characterized by relatively short, dark wedge-shaped bands which are
morphologically distinct from the global lineaments discussed above. From
Voyager observations these bands were believed to be due to crustal
extension on the basis of their polygonal boundaries [9] and the fact that
the brighter pre-existing crustal plates could be reconstructed by
rotation and translation with little loss or overlap [10].  The region was
only partially imaged by Voyager, however, leaving ambiguous the cause of
the extension; it had been mapped [6,9,10] at longitudes ranging from 170
W to 220 W. With the more complete coverage from Galileo, the zone is now
known to be centered near 210 W, significantly displaced from the apex of
tidal distortions, and extend westwards to at least 240 W. 

 The extensional stresses expected for an incremental eastwards rotation
of the surface are centered 45 degrees west of the tidal axis, with
corresponding compression to the east [3]. Adding to this are the diurnal
stress components due to libration that tug the tidal bulges westwards
during the half-orbit centered about apojove (which of course subtract
during the other half-orbit).  The distribution of dark wedges (Figure 3)
matches the predicted extent of extensional stresses in longitude,
especially if allowance is made for eastwards rotation subsequent to
formation. In particular, the region closely corresponds to the zone in
which both principal horizontal stresses are tensional (blue in Figure 3).
However, the pull-apart zone appears offset from the equator, centered at
a latitude of about 15 S. This displacement is not predicted by
non-synchronous rotation, but may be due to polar wander (rotation about
an axis through the sub- and anti-jove points [11]) or to irregularities
in the interior of Europa. High resolution images of the dark wedge-shaped
bands suggest that the rifting occurred episodically in both the NE-SW and
NW-SE directions, with local rotation and translation of the rigid
pre-existing crustal blocks [12]. No tectonic features indicative of
compression have been recognized to the east of the antijove point,
perhaps because ice is stronger in compression than it is in tension. 


The two remaining predictions of non-synchronous rotation can be quickly
dealt with. No leading/trailing asymmetry in impact crater density should
be found if Europa's surface has rotated with respect to Jupiter, and
indeed imaging observations obtained during Galileo's seventh orbit show
no apparent excess of impact craters at the apex of the satellite's
leading surface. No discernible change took place in the positions of
surface features with respect to the terminator between the Voyager and
Galileo eras after allowance is made for synchronous rotation, allowing us
to place a lower limit of 104 yr on the rotation period of Europa with
respect to Jupiter [13]. 

 Confirmation of non-synchronous rotation of Europa's surface may have
important implications for the structure of the satellite's interior.
Anderson et al. [14] postulated significant non-hydrostatic components to
the gravity field of Europa to account for discordant results obtained
from doppler data during Galileo's two close flybys of the satellite.
Although it isn't possible to determine moments of inertia from the
gravity field alone, the departures from hydrostatic equilibrium are
inferred to be comparable to the gravitational effects of the tidal
bulges. For comparison, a difference in the equatorial moments of inertia
(B-A)/C only 10-2 to 10-3 of the hydrostatic value (due to the equilibrium
tidal bulges) would be sufficient to lock Europa into synchronous rotation
[1]. If Europa's interior possesses a sufficient mass distribution
asymmetry to tidally lock the moon towards Jupiter, as seems indicated by
the gravity results, and yet the surface shows evidence of asynchronous
rotation, then the crust is required to be mechanically decoupled from the
interior, for example by a subsurface ocean or a layer of warm and ductile
ice. 

The hypothesis of non-synchronous rotation will be further tested by
moderate resolution global color imaging during the extended mission of
Galileo.  Lineaments are predicted to rotate in an anticlockwise sense in
the southern hemisphere, but the limited southern hemisphere observations
to date have lacked the spectral and spatial resolution required to
discern detailed superposition relationships. Multispectral mapping of
Europa's high southern latitudes will begin with the spacecraft's 12th
orbit in December of 1997.  Another expectation is that a corresponding
equatorial rift zone similar to the region of dark wedge-shaped bands
should be found to the west of the sub-Jupiter point, an area not yet seen
by Galileo. Unfortunately the Jupiter-facing hemisphere will be the last
to be imaged by the spacecraft, not until November of 1999. Global mapping
of the lineaments and quantitative comparison of their orientations with
theoretical stress trajectories remain important tasks for the future. 

References: 


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13. Hoppa, G., R. Greenberg, P. Geissler, J. Plassmann, B. R. Tufts,
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Correspondence should be addressed to P.E.G. (geissler@pirl.lpl.arizona.edu).

Acknowledgments: We thank Bill McKinnon and an anonymous reviewer for
helpful comments, and Cynthia Phillips (University of Arizona) for
providing the combined Galileo/Voyager mosaic used for Figure 3. The
efforts of David Senske and Kenneth Klaasen contributed greatly to the
successful acquisition of the data used in this study. 

Figure Captions:

1. False color composite of northern high-latitude region of Europa,
produced from the 968 nm, 756 nm and Green filter images. Labelled on the
figure, in order of age, are young fractures (F), developing band
Rhadamanthys Linea (R), intermediate-aged triple bands Minos Linea (M) and
Cadmus Linea (C), and a wedge-shaped ancient band (AB). 

2. Distribution of ancient bands and bright wedges (A), intermediate aged
triple-bands and similarly colored materials (B), and young fractures (C)
which cross-cut the triple bands. In (B), "1" and "2" refer to the order
of placement of the two prominent triple bands near the center of the
scene, while "R" marks Rhadamanthys Linea, a feature evidently
transitional between the young fractures and the fully developed
triple-bands. 

3. Principal stress directions for an incremental eastwards shift in the
orientation of the surface with respect to the tidal figure of Europa are
overlain on a combined Galileo/Voyager mosaic of the antijovian
hemisphere.  The maximum extensional stresses (blue) are reached in the
region 45 to the west of the antijove point, where both of the horizontal
principal stresses are tensile. A rift zone centered at 15S, 210W may have
formed as the surface stretched over the tidal bulge. Note that the
principal stress directions rotate clockwise towards the east at high
northern latitudes, but rotate in an anticlockwise sense in the southern
hemisphere.