Abstract
1 The "Mitten'' (provisionally named Murias Chaos by the International
Astronomical Union) is a region of elevated chaos-like terrain in
the leading hemisphere of Europa. Its origin had been explained under
the currently debated theories of melting through a thin lithosphere
or convection within a thick one. Galileo observations reveal several
characteristics that suggest that the Mitten is distinct from typical
chaos terrain and point to a different formational process. Photoclinometric
elevation estimates suggest that the Mitten is slightly elevated
with respect to the surrounding terrain; geologic relations indicate
that it must have raised significantly from the plains in its past,
resembling disrupted domes on Europa's trailing hemisphere. Moreover,
the Mitten material appears to have extruded onto the plains and
flowed for tens of kilometers. The area subsequently subsided as
a result of isostatic adjustment, viscous relaxation, and/or plains
loading. Using plate flexure models, we estimated the elastic lithosphere
in the area to be several kilometers thick. We propose that the Mitten
originated by the ascent and extrusion of a large thermal diapir.
Thermal-mechanical modeling shows that a Mitten-sized plume would
remain sufficiently warm and buoyant to pierce through the crust
and flow unconfined on the surface. Such a diapir probably had an
initial radius between 5 and 8 km and an initial depth of 20-40 km,
consistent with a thick-lithosphere model. In this scenario the Mitten
appears to represent the surface expression of the rare ascent of
a large diapir, in contrast to lenticulae and chaos terrain, which
may form by isolated and clustered small diapirs, respectively.
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