Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, Hawaii, USA

摘要

Nonsynchronous rotation of Europa should result in a periodic change in stress azimuths at any given location, which may result in a corresponding change in crack azimuths. An early study seemed to provide evidence for such variation within a crack sequence in the northern trailing hemisphere. However, more recent work with higher-resolution imagery resulted in a more complete sequence of cracks in the region and showed that the azimuth variation with time was not systematic. Similar results were found in the southern leading region. Here we investigate whether the northern leading region contains a crack record, on the basis of crosscutting relationships, that is adequate to indicate nonsynchronous rotation. As in the previous studies of other regions, we find no clear evidence of nonsynchronous rotation within crack azimuth patterns. However, there are other lines of evidence in support of nonsynchronous rotation. Therefore the lack of a continuous pattern of azimuth change with time is likely due to only a few cracks forming per cycle, thus hiding any discernable signal from nonsynchronous rotation in those regions. In that case, the cracks would have formed over numerous cycles of nonsynchronous rotation. The study also reveals a distinctive distribution of crack azimuths, with gaps at certain values. This distribution may reflect a combination of observation selection due to illumination and less-favorable cracking orientations.