QUESTION:
From the Pluto pictures, what kind of materials may exist on the surface?

ANSWER from Marc Buie on May 26, 1996:
If all we had to study were the pictures from HST, all we could say is that
some stuff on the surface is bright and some stuff is dark.  The HST images
can tell us where stuff is but not what it's made of.  To figure out
composition, we need other types of observations.  To learn about
composition we usually turn to spectroscopy.  This is a tool that lets us
search for characteristic "fingerprints" that reveal the presence of
various molecules on the surface of Pluto.

Since 1976, we've known that there is solid methane (CH4) on Pluto.  More
recently we've discovered large amounts of solid nitrogen (N2) and solid
carbon monoxide (CO).  All of these materials would have to be relatively
fresh frost which also implies they are bright.  So, a reasonable assumption
is to say that the bright areas are rich in these frosts.  The composition
of the dark areas is not as easy to understand since much less light
is reflected from those areas.  However, we expect that the dark areas
contain organic residues that are left behind after solar UV radiation
rips apart methane molecules.