Abstract
Europa's orbital eccentricity, driven by the resonance with Io and
Ganymede, results in "diurnal" tides (3.5-day period) and possibly
in nonsynchronous rotation. Both diurnal variation and nonsynchronous
rotation can create significant stress fields on Europa's surface,
and both effects may produce cracking. Patterns and time sequences
of apparent tectonic features on Europa include lineaments that correlate
with both sources of stress, if we take into account nonsynchronous
rotation, after initial crack formation, by amounts ranging up to
several tens of degrees. For example, the crosscutting time sequence
of features in the Cadmus and Mines Linea region is consistent with
a combined diurnal and nonsynchronous tensile-stress field, as it
evolves during tens of degrees of nonsynchronous rotation. Constraints
on the rotation rate from comparing Voyager and Galileo images show
that significant rotation requires >10(4) yr, but could be fast enough
to have allowed significant rotation since the last global resurfacing,
even if such resurfacing was as recent as a few million years ago.
Once cracking is initiated, diurnal tides work cracks so that they
open and close daily. Although the daily effect is small, over 10(5)
yr double ridges could plausibly be built along the cracks with sizes
and morphologies consistent with observed structures, according to
a model in which underlying liquid water fills the open cracks, partially
freezes, and is extruded during the daily closing of the cracks.
Thus, several lines of observational and theoretical evidence can
be integrated if we assume nonsynchronous rotation and the existence
of a liquid water layer. (C) 1998 Academic Press.
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