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 more frozen nitrogen on the surface of Pluto than there is either frozen carbon monoxide or methane is required by the astronomers' argument.