The Cold Finger

Outer solar system bodies — comets, Kuiper Belt objects, the icy moons — contain more CO relative to water than models of the mature protoplanetary disk predict. The standard disk model starts with a fully formed disk and evolves the chemistry: CO and water freeze out at their respective snowlines, pebbles drift inward, and the volatile budget is set by the equilibrium between freezing, sublimation, and transport. This predicts a CO/H₂O ratio that is too low to match observations.

Drążkowska (arXiv:2603.26876, March 2026) shows that including the disk buildup stage — the period when the disk is still forming from infalling cloud material — qualitatively changes the predicted volatile budget. During buildup, dust grains grow and begin drifting inward before the disk reaches steady state. The CO snowline acts as a cold finger: CO vapor from the inner disk diffuses outward and refreezes on drifting pebbles at the snowline. The pebbles carry this excess CO ice outward as they continue to grow and are incorporated into planetesimals.

The cold finger effect is a one-way pump. CO sublimates as pebbles drift inward past the snowline, diffuses as vapor, and refreezes on outward-facing pebble surfaces. The pebbles carry more CO ice than the local gas composition would predict because they accumulated it during their passage through the snowline region. The enrichment depends on the disk being actively built — in a mature disk at steady state, the cold finger saturates and the enrichment disappears.

The structural observation: the simplification of assuming a pre-formed disk — a standard starting condition in planet formation models — was suppressing a real chemical enrichment mechanism. The disk's formation phase is not merely a prelude to the interesting chemistry; it is when the interesting chemistry happens. By the time the disk reaches the steady state that models typically use as an initial condition, the volatile enrichment is already complete and frozen into the solid material. Starting the model too late erases the explanation.