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A. R. Sarid, R. Greenberg, P. Geissler (LPL, Univ. of Arizona), G. V. Hoppa (Raytheon Missle Systems)
From Galileo's first (“G1”) orbit, multi-spectral imaging (~ 1.6 km/pixel) of a broad region in Europa's northern hemisphere centered near the intersection of the linea Cadmus and Minos revealed cross-cutting relationships among sets of lineaments (Geissler et al., Icarus 1998), which displayed a progressive change in azimuthal orientation in a clockwise sense. Geissler et al. (Nature 1998) noted that such a progression is consistent with expected stress variations that would accompany plausible non-synchronous rotation (NSR) (Greenberg & Weidenschilling, Icarus 1984). We have re-examined the tectonic sequence in the same region using regional mapping images from orbit E15, which provide better illumination and resolution (~ 200 m/pixel) for morphological studies. Determination of the cross-cutting order at each intersection among significant lineaments provides a more complete record than the G1 data did. We find that to fit the sequence into a continual clockwise change of orientation would require at least 1080o (6 cycles) of azimuthal rotation. If due to NSR of Europa, this result implies that we are seeing back further into the tectonic record than the G1 results had suggested (with only about 90o of azimuth change). The three sets of orientations found by Geissler et al. now appear to have been spaced over several cycles, not within one cycle. While our more complete sequence of lineament formation may be consistent with NSR, it cannot be construed as evidence for NSR, because an azimuthal sequence in the opposite direction from theoretical predictions would fit equally well. Similar results were found in the southern leading hemisphere (Sarid et al. LPSC 2003). However, several other lines of evidence do support NSR (Greenberg et al., Cel. Mech. & Dyn. Astron. 2002). The results reported here are consistent with Hoppa et al.'s (Icarus 2001) interpretation from cycloid evidence that, if there is NSR, only a couple of cracks form in a given broad region in any given NSR period.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.