Here are some related tid-bits I found in reading "When the Earth Had Two Moons" (Asphaug, 2019) [my notes]
@20K, things stick together even ions. Mantles form around cold silicate accretions of water, methane, ammonia, CO2, CO, and others.
Adding UV will make simple molecules become complex.
Most common compound is CO. Others are: CO2, CH4, NH3, HCN, and other CHON stuff. The result gives free oxygen to make oxides like H2O (2nd most common compound).
Comets are dust and ice…They include “super volatiles” – CO, CO2, U2, CH4, O2 vaporizing @ extremely low temperatures…They came apart, producing water vapor and silicate dust.
Deep inside these orbits, amorphous solids can now get warm enough to crystallize, which is exothermic, causing wholesale decomposition of the nucleus.
The first 100 million years was frantic. Then came 4.5 Gyrs of cold storage…The outer surface layer materials became highly processed under this rotisserie, with ice-cemented carbonaceous lag on top of primeval interior. Baked caramelized ice cream is one analogy, where the best stuff is the inner ice cream.