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Lunar and Planetary Science XXXIII (2002)
1879.pdf
POLAR WANDER ON EUROPA: RESULTS FROM A SURVEY OF STRIKE-SLIP FAULTS. A. R. Sarid,
R. Greenberg, G. V. Hoppa, B. R. Tufts, P. Geissler, Lunar and Planetary Laboratory, University of Arizona,
Tucson Az 85721, ([email protected]), ([email protected])
Introduction
Strike-slip displacement is common on Europa [1,2,3] but relatively rare on most other planets.
A likely mechanism for driving such shear by a process of tidal "walking" has been identified [3,4]. That
mechanism requires that cracks penetrate to a low viscosity decoupling layer, such as a liquid water ocean.
In tidal walking, the changes in tidal stress over the
course of each day cause a net shear displacement of
the adjoining crustal plates.
The tidal walking process can be used to predict the sense of shear (left lateral vs. right lateral) for
any crack in Europa's thin ice shell [4], given any latitude, longitude and direction of "strike". In general,
right lateral shear is predicted to predominate in the
southern hemisphere, and left lateral in the north, with
a mix within 30o of the equator. The 800 km-long
strike-slip fault Astypalaea Linea, originally identified
by Tufts [5], fits this rule: It is located in the far south
and has a 40 km right lateral offset. Indeed, Astypalaea was the inspiration for development of the tidal
walking model [4,5]. Studies of several other locales
showed general agreement with the predictions of the
tidal walking theory [3,4].
Here we report on identification and mapping
of strike-slip features over a much larger portion of the
surface, specifically that imaged at 200-m resolution in
the Galileo "Regional Mapping" data set. These include RegMap01 from the far north to the far south in
the trailing hemisphere, and RegMap02 from the far
north to the far south in the leading hemisphere. We
have mapped the locations of all strike-slip faults
identified in this area, recording location, azimuth of
the fault (the "strike" direction), and the sense of the
displacement (right or left lateral).
Analysis of these data, through a comparison
with predictions of the tidal-walking theory, provides
evidence for polar wander, in which the ice crust appears to have shifted as a whole relative to the direction of the spin axis.
Results
Strike-slip displacement in the far north and
far south (more than about 45 o from the equator) is
found to be predominantly left and right lateral, respectively, as predicted by the tidal walking theory [4].
However, in the trailing hemisphere, right-lateral displacement is also found to dominate all the way up to
the equator, independent of fault azimuth. In contrast,
the theory predicts that within 35o of the equator, both
left- and right-lateral displacement is possible, de-
pending on azimuth. In this hemisphere, the zone of
left- and right-lateral displacement is shifted so that it
lies entirely in the northern hemisphere.
We find that the sense of displacement would
generally match the theoretical predictions if the terrain in the trailing hemisphere were shifted southward
by ~30o. Moreover, similar considerations in the
leading hemisphere indicate that the observed strikeslip displacements would fit the theory better if the
terrain in that hemisphere were shifted northward by a
comparable amount. Together, these results suggest
that the crust of Europa has slid as single unit relative
to the spin axis. If a marker had been placed at the
north rotational pole at the time that most of this strikeslip occurred, it would now lie ~ 30 o toward the south
in the leading hemisphere.
Discussion
The distribution of strike-slip in both the
leading and trailing hemispheres provides evidence for
polar wander. The direction of the polar migration is
found to be consistent in independent evidence from
each hemisphere, as described above.
We can estimate how recently this shift took
place, and hence how rapidly the poles can wander, as
follows. The age of the surface is probably less than
about 50 million years [6], but the recognizable strikeslip features are among the more recent tectonic features created during that time. Most of the tectonic
terrain is covered by older cracks, ridges, and bands,
much deeper in the crosscutting sequence of formation.
Strike-slip is harder to recognize in those older features, which tend to remain only as short segments and
which therefore are less likely to have identifiable
piercing lineaments to define displacement. The
strike-slip features described here are probably well
within the most recent 10% of the surface age, and
hence less than a few million years old. The polar
wander inferred from those features must therefore
have occurred within the past few million years.
What could drive such polar wander? On a
nearly spherical, spinning body, a very small local excess mass on the surface may cause such polar wander,
as the site of the anomaly spins outward toward the
equator [e.g., 7]. It is unlikely that the entire mass distribution of Europa is sufficiently symmetric. However, the ice crust is very thin and has fairly low topography. If it is entirely uncoupled from the interior, by
an underlying global ocean for example, an anomalously thick site might be pulled by centrifugal force
out toward the equator, dragging the entire ice shell
with it.
Lunar and Planetary Science XXXIII (2002)
1879.pdf
Polar wander on Europa: Results from a survey of strike-slip faults. Sarid et. al.
One mechanism (suggested by [8]) for driving
such a process is thermal thinning of the equatorial ice
crust relative to the poles, where tidal heating within
the ice is relatively weaker. In that model the thicker
ice near the poles then tends to spin out towards the
equator. Such events could readily occur within a few
million years, but might not give a full 90o flip, because the thick ice would thin as it is warmed approaching the equator. One difficulty with that model
is that it is possible that tidal heat from the interior
dominates the determination of ice thickness, so the
poles might not become significantly thicker than
elsewhere.
However, given the fairly smooth, uniform
appearance of Europa's ice shell, it is probably susceptible to polar wander if any local mass anomaly occurs,
whatever the cause. Causes might include local or
regional variations in ice thickness due to thermal
anomalies in the ocean or due to geological processes.
An important implication of our result that the crust
does seem to have wandered is that the ice layer must
be effectively decoupled from the interior, so that it
can slip as a single unit. Substantial grounding of the
ice layer on subsurface rocky continents seems to be
ruled out if the poles have in fact wandered.
Polar wander has also been suggested by several previous studies. A study of whether there was
any record in the orientations of tectonic lineaments on
Voyager images [9] was inconclusive, but hinted at a
shift similar to what we report here. Other evidence
comes from the distribution of chaotic terrain [10] and
of pits and uplifts [11,12] in the same parts of the
leading and trailing hemisphere investigated here. It
was found that each of those types of feature had size
and spatial distributions that were similar in the northern leading and southern trailing regions, with another
distinctly different distribution found in both the
southern leading and northern trailing regions.
Moreover, the qualitative character of the tectonic fabric in these regions displays a similar oblique antipodal
symmetry. The symmetry axes for all these types of
features seem to be similarly inclined by tens of degrees from the pole. Assuming that the character of
the distribution of such features was related to distance
from the equator (for example, if it depended on ice
thickness), this tilted symmetry of the distributions
could be explained by polar migration, consistent with
the polar wander inferred here from the distribution of
strike-slip faulting.
References
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1999. [3] Hoppa, G.V., et al. J. Geophys. Res.-Planets
105, E9, 22617-22627. 2000. [4] Hoppa, G.V., et al.
Icarus 141, 287-298, 1999. [5] Tufts, R., PhD Dissertation, Univ. Arizona, Tucson, 1998. [6] Zahnle, K.L.,
et al. 1998. Cratering rates on the Galilean satellites.
Icarus 136, 202-222. [7] Goldreich, P., and A. Toomre,
J. Geophys. Res 74, 2555-2567, 1969. [8] Ojakangas,
G.W., and D.J. Stevenson, Icarus 81, 242-270, 1989.
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Pits and Uplifts on Europa: Distributions and characteristics, Bull. Amer. Astron. Soc. 33, 1099, 2001. [12]
Leake et al., this conference, 2002.