  Craters are probes of planetary surface and interior properties. Here we
measure depths, widths, and spacing of circumferential ring-graben surrounding
the two largest multiring impact structures on Europa, Tyre and Callanish. We
estimate formation conditions including the ice shell structure. The radial
extension necessary to form these graben is thought to be caused by
asthenospheric drag of warmer, more ductile ice and/or water flowing toward the
excavated center of the crater, under a brittle-elastic lithospheric lid.
Measurements of graben depths from stereo-photoclinometric digital elevation
models result in estimates of displacement, strain, and stress experienced by
the ice shell. Graben widths are used to estimate the intersection depth of the
bounding normal faults, a quantity related to the brittle-ductile transition
depth that approximates elastic shell thickness during crater collapse. Heat
flows at the time of crater formation as well as ice lithosphere and total
shell thickness are thus also constrained. Average widths and depths tend to
decrease with increasing distance from the structure center, while inter-graben
spacing generally increases. Varied assumptions yield plausible total
conductive ice shell thickness estimates between 4-8 and 2.5-5 km for Tyre and
Callanish, respectively, and heat flows of ~70-115 (+/-30) mW m^-2 for
realistic thermal conductivities, consistent with other geophysical estimates
for Europa. Higher heat flows are consistent with thin (<10 km), conductive ice
shells and impact breaching, or penetration of the stagnant lid for a
convecting ice shell. Callanish, geologically younger, formed in a time or
region of greater heat flow than Tyre.
