A New Scenario for Pit and Dome Crater Formation on Jupiter's Moons Ganymede and Callisto

Ganymede and Callisto contain an immense variety of impact crater morphologies unparalleled elsewhere in the Solar System. Among these morphologies are the pit and dome craters, which are unique to these bodies. Building on mapping and modeling work from our group, we present a hypothesis for pit and dome crater formation that genetically links the two.

Hydrocode simulations show that impacts into these icy targets produce a large pool of meltwater in the crater center. This melt pool could roof over and self-seal the damaged region in the subsurface, leaving an isolated melt pocket that solidifies with time. Expansion during solidification stresses the surrounding ice. With sufficient stress, the seal on the melt pocket can break, leading to drainage and pit collapse. Some meltwater could remain below the pit, refreezing and expanding. The extra volume could be accommodated via bulging and cracks on the pit floor. Some refrozen material could also extrude through the cracks.

A remnant thermal anomaly in the now solidified ice results in lower material viscosities. Our previous finite element simulations show that pit craters can evolve into dome craters over time via topographic relaxation. This remnant heat enhances creep flow directly below the pit, causing a dome to emerge within the pit as the crater relaxes over time scales of order 10 Myr. Our simulations show that this process occurs for larger pit craters that penetrate to deeper, warmer material, because warmer ice surrounding the refrozen pocket can flow more readily than colder ice. For smaller pit craters, this does not occur, and they remain as pit craters. This finding is consistent with Voyager and Galileo observations that show a transition from pit craters to dome craters with increasing crater size. Observations also show fractures in the domes, as well as a higher albedo compared to the surrounding crater, which would be consistent with expansion and extrusion.