Sublimation is the world rule by which a body loses a resource from its surface as vapour. It applies to a body with too little gravity to hold its vapour (under 500) that is warmer than the temperature at which one of its resources sublimes. Some of that resource moves from the surface to the body's trail, which follows it on the same orbit. A heavier body holds on instead, and its ice can melt; see Melting. A comet on a stretched orbit sublimates only near its star.
See Glossary for sublimation and gravity.