  Herschel PACS and SPIRE images have been obtained of NGC 6720 (the Ring
Nebula). This is an evolved planetary nebula with a central star that is
currently on the cooling track, due to which the outer parts of the nebula are
recombining. From the PACS and SPIRE images we conclude that there is a
striking resemblance between the dust distribution and the H2 emission, which
appears to be observational evidence that H2 forms on grain surfaces. We have
developed a photoionization model of the nebula with the Cloudy code which we
used to determine the physical conditions of the dust and investigate possible
formation scenarios for the H2. We conclude that the most plausible scenario is
that the H2 resides in high density knots which were formed after the
recombination of the gas started when the central star entered the cooling
track. Hydrodynamical instabilities due to the unusually low temperature of the
recombining gas are proposed as a mechanism for forming the knots. H2 formation
in the knots is expected to be substantial after the central star underwent a
strong drop in luminosity about one to two thousand years ago, and may still be
ongoing at this moment, depending on the density of the knots and the
properties of the grains in the knots.
