How can freezing subsurface water crack Triton’s outer ice layer?

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Moons of Neptune: Triton — Triton Image of Triton, Neptune's largest satellite, captured by a camera aboard Voyager 2, 1989.

The premise that freezing subsurface water cracks Triton’s outer ice layer is supported by scientific models of the moon's internal evolution.

Following its capture by Neptune, tidal interactions likely melted Triton's entire interior. As the moon subsequently cooled, a thick outer shell of water ice formed. Because water expands upon freezing, the gradual solidification of the interior liquid layer exerted significant pressure on this rigid outer crust. This expansion caused the surface to fracture, creating the network of intersecting cracks observed in Voyager imagery. These fractures likely allowed subsurface material to flow upward, filling the cracks and providing evidence of a water-ice interior, as no other candidate materials for Triton's composition expand in the same way when freezing.1