Article,
Putative ice flows on Europa: Geometric patterns and relation to topography collectively constrain material properties and effusion rates
Icarus, 177 (2): 413--424 (October 2005)
Abstract
Europa's surface exhibits numerous small dome-like and lobate features,
some of which have been attributed to fluid emplacement of ice or
slush on the surface. We perform numerical simulations of non-Newtonian
flows to assess the physical conditions required for these features
to result from viscous flows. Our simulations indicate that the morphology
of an ice flow on Europa will be, at least partially, influenced
by pre-existing topography unless the thickness of the flow exceeds
that of the underlying topography by at least an order of magnitude.
Three classes of features can be identified on Europa. First, some
(possibly most) putative flow-like features exhibit no influence
from the pre-existing topography such as ridges, although their thicknesses
are generally on the same order as those of ridges. Therefore, flow
processes probably cannot explain the formation of these features.
Second, some observed features show modest influence from the underlying
topography. These might be explained by ice flows with wide ranges
of parameters (ice temperatures >230 K, effusion rates >107 m3 year-1,
and a wide range of grain sizes), although surface uplift (e.g.,
by diapirism) and in situ disaggregation provide an equally compelling
explanation. Third, several observed features are completely confined
by pre-existing topographic structures on at least one side; these
are the best known candidates for flow features on Europa. If these
features resulted from solid-ice flows, then temperatures >260 K
and grain sizes <2 mum are required. Such small grain sizes seem
unlikely; low-viscosity flows such as ice slurries or brines provide
a better explanation for these features. Our results provide theoretical
support for the view that many of Europa's lobate features have not
resulted from solid-ice flows.
