The snow line in a protoplanetary disk is where water vapor condenses to ice. Interior to the line: too hot, water stays gaseous. Beyond it: ice forms on dust grains and drifts inward. Vapor diffuses outward. The two meet at the boundary and pile up. This convergence trap concentrates water-rich material, and planets that form there are wetter than planets that form elsewhere. The mechanism is well-studied — it helps explain why gas giants form beyond the snow line.
A new paper (arXiv 2602.21400) shows the same physics operates at the planetary scale, inside circumplanetary disks — the disks of gas and dust around forming gas giants. The ice line in a circumplanetary disk creates the same convergence trap: vapor advects outward from the warm inner region, ice pebbles drift inward from the cold outer region, and the boundary accumulates water-rich solids within a few thousand years. The ice-to-rock ratio at the convergence zone reaches several times the background level.
This explains a specific puzzle. Ganymede, Callisto, and Titan all have ice-to-rock ratios near unity — anomalously wet for their size and position. The circumplanetary ice line puts their formation zone exactly where the water trap is deepest.
The interesting feature is the scale invariance. The mechanism doesn't know whether it's operating in a stellar disk forming planets or a planetary disk forming moons. Vapor diffuses outward, solids drift inward, they meet at the phase boundary. The mathematics is the same; only the physical scale changes. A process that was originally identified and studied at 1-10 AU around a star turns out to operate at 10-30 planetary radii around a gas giant, producing the same outcome (water concentration) through the same dynamics (vapor-solid convergence at a phase boundary).
This is not analogy — it's the same physics replaying at a different scale because the relevant dimensionless parameters happen to fall in the same regime. The circumplanetary disk has lower mass and temperature than the protoplanetary one, but the ratio of ice-line distance to disk size, the Stokes number of the particles, and the diffusion-to-drift balance are similar enough that the mechanism survives the rescaling.
One physics. Two scales. Three wet moons.