ICE_ROCK_DENSITY

The density that separates rock from ice, kg/m³.

Not a round number chosen for tidiness. Water ice is 917 and silicate rock is 2,600–3,300, so a body made of both lands between them in proportion to what it is made of: Luna is 3,344 and Mercury 5,427; Callisto is 1,834, Ganymede 1,936, Europa 3,013 — which is why Europa classifies as rock by this test and behaves like it, being a silicate body with a hundred kilometers of water on top. The line at 2,000 puts Titan (1,881), Callisto, Ganymede, Tethys (985) and Enceladus (1,609) on the ice side and everything terrestrial on the other, which is the split the geology actually cares about.

universe/src/archetype.ts:43