QUESTION:
What have we learned about the early formation of Pluto from these
pictures we have just gotten back

ANSWER from Marc Buie on May 26, 1996:
Just like the question about surface materials, the new HST pictures of
Pluto don't give us much information about the formation of Pluto.  The
pictures will be useful to help us understand other observations which in
turn tell us about Pluto's formation.

For example, one important clue to the formation is knowing the bulk
composition of Pluto.  We know the surface is covered with N2, CH4, and CO
but this only tells us what's on the top of the surface.  The Earth has
a molten iron core but we certainly didn't learn this by picking up and
studying the rocks on the surface.  By measuring the total mass of Pluto
and then combining that with its size, we measure the bulk density.  Pluto's
density implies that about half of the volume is water ice while the rest is
rock.  This compositional information has already played a critical role
in improving theories of what happens as planets are forming in the outer
solar system.