  The discovery of a large putative impact crater buried beneath Hiawatha
Glacier along the margin of the northwestern Greenland Ice Sheet has
reinvigorated interest into the nature of large impacts into thick ice masses.
This circular structure is relatively shallow and exhibits a small central
uplift, whereas a peak-ring morphology is expected. This discrepancy may be due
to long-term and ongoing subglacial erosion but may also be explained by a
relatively recent impact through the Greenland Ice Sheet, which is expected to
alter the final crater morphology. Here we model crater formation using
hydrocode simulations, varying pre-impact ice thickness and impactor
composition over crystalline target rock. We find that an ice-sheet thickness
of 1.5 or 2 km results in a crater morphology that is consistent with the
present morphology of this structure. Further, an ice sheet that thick
substantially inhibits ejection of rocky material, which might explain the
absence of rocky ejecta in most existing Greenland deep ice cores if the impact
occurred during the late Pleistocene. From the present morphology of the
putative Hiawatha impact crater alone, we cannot distinguish between an older
crater formed by a pre-Pleistocene impact into ice-free bedrock or a younger,
Pleistocene impact into locally thick ice, but based on our modeling we
conclude that latter scenario is possible.
