Spectroscopic analysis has revealed the existence of frozen nitrogen, methane, and carbon monoxide on the surface of Pluto. Such ices have a tendency to vaporize, producing an atmosphere. Since the proportion of any gas in such an atmosphere depends directly on how readily the corresponding ice vaporizes, astronomers have concluded that the components of Pluto' s atmosphere are nitrogen, carbon monoxide, and methane, in order of decreasing abundance.
The assumption that there is no frozen substance on the surface of Pluto that vaporizes more readily than methane but less readily than carbon monoxide is relied on by the astronomers' argument.