Tidally Driven Frictional Heating and the Seismogenic Zone on Enceladus'
Frictional strength of polycrystalline ice and ice-ammonia mixtures have
been measured in the laboratory in order to constrain fault behavior on
the south polar region of Enceladus. These faults, known as the tiger
stripes, are associated with anomalous temperature gradients and active
jets, which have been linked to tidally induced stresses on the faults.
The temperature dependence of fault stability is used to map a
seismogenic zone within the brittle layer of Enceladus’ icy shell which
is estimated as the upper 4 km. The results from our experimental study
and models of frictional heating are used to infer fault strength and
heat generation within the brittle layer. The friction experiments were
conducted using polycrystalline ice and ice-ammonia mixtures in a custom
servo-hydraulic biaxial deformation apparatus with a liquid
nitrogen-cooled cryostat at icy satellite conditions. The frictional
response was measured in steady-state as well as at dynamic conditions,
as a function of frequency, amplitude, normal stress, and temperature.
For the modelling, a simple 1-D numerical frictional heating model was
constructed showing the change in temperature on a fault during sliding.
The estimations were based on the solution for heat diffusion through a
fault of finite thickness, where temperatures were calculated for
different time/space combinations as functions of time during/after slip
and inside/outside the slipping zone. Using estimated fault depth and
slip distance from previous studies, as well as our experimentally
determined friction coefficient, frictional heating with depth was
determined with varying values for fault width and slip velocity.
Frictional heating models of pure polycrystalline ice will also be
compared to that of ice-ammonia partial melt in order to constrain the
depth of partial melt within the icy shell, characterizing a possible
oxidant transport mechanism into the subsurface ocean. Although we
consider sliding on Enceladus, results in this study are additionally
applicable to Europa, where instead sulfuric acid may provide the deep
eutectic second phase. The identification of the source of seismicity in
a tidally loaded fault system could benefit interpretation of seismic
observations in future missions to icy satellites.