Other
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p51e1165r&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P51E-1165
Other
[5450] Planetary Sciences: Solid Surface Planets / Orbital And Rotational Dynamics, [5475] Planetary Sciences: Solid Surface Planets / Tectonics, [6221] Planetary Sciences: Solar System Objects / Europa
Scientific paper
Europa’s obliquity was expected to be negligibly small due to tidal damping. However, gravitational interaction with Jupiter’s other large satellites may force the obliquity to ~0.1° or larger depending on the internal structure. A forced obliquity would alter the pattern of tidal stress on Europa and could thus influence the formation of tidally-driven fractures. We present evidence of a significant obliquity from our investigations of strike-slip fault patterns, global cycloid patterns, and modeling of individual cycloids. The mismatch between observations of strike-slip faults on Europa and the predictions from a tidal formation model has previously been interpreted as evidence of polar wander. However, obliquity was not included in the stress field calculations used to generate the predictions. Our preliminary results indicate that a tidal model that includes obliquity of ~1° can correctly predict the sense of slip of all the observed faults, eliminating the need to invoke polar wander. Cycloids have been proposed to be tidally-driven, and the general trends and overall shapes of cycloids have been reproduced using a tidal model that included the effects of Europa’s forced eccentricity. However, global cycloid maps created using the eccentricity-only model fail to reproduce two important characteristics of observed cycloids. First, the model predicts that boxy cycloids form along the equator. Although boxy cycloids are observed, they are actually offset from the equator. The discrepancy is inconsistent with the polar wander event previously invoked to explain strike-slip fault patterns, casting further doubt on the polar wander interpretation. We find that incorporating obliquity into the tidal model of cycloid formation offsets the latitudes at which these boxy cycloids form in a manner consistent with the observations. The second characteristic that cannot be explained by an eccentricity-only model is a set of arcuate cycloids that cross the equator. Again, we find that adding obliquity solves the problem; these cycloids are generated in our global cycloid maps. In addition to using global maps, we have also performed detailed fits to three southern hemisphere cycloids and three equatorial cycloids with various tidal models. To identify the best fits to observed cycloids, we developed a systematic algorithm to search parameter space, which quantitatively evaluates goodness of fit. Incorporating obliquity, we find better fits to southern hemisphere cycloids, in some cases with far fewer free parameters, than in previous work that neglected obliquity. We also successfully reproduced equatorial cycloids. These detailed fits imply a variable obliquity of order 1°, consistent with our results from strike-slip faults and with expectations for a mechanically decoupled Europa. An obliquity of ~1° would alter the pattern of tidal stress and thus affect the formation of tidally-driven fractures; we have now found evidence of a large and varying obliquity within both strike-slip fault and cycloid patterns. A useful goal of the future Europa mission would be measuring Europa’s current obliquity and spin pole direction.
Huff Eric
Hurford Terry A.
Manga Michael
Militzer Burkhard
Rhoden A.
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