Senior author Barbara Romanowicz, a UC Berkeley professor of earth and planetary science, noted that the connections between the lower - mantle plumes and the volcanic hotspots are not direct because the tops of the plumes spread out like the delta of a river as they merge with the less
viscous upper mantle rock.
The researchers say this indicates an
upper crust made of a mixture of rock and ice — in which small craters could be carved — supported by a more
viscous mantle of ice lower down, which would allow the biggest impact basins to relax away with time, like silly putty returning to its original shape.