  Since the last Pluto volatile transport models were published (Hansen and
Paige 1996), we have (i) new stellar occultation data from 2002 and 2006-2012
that have roughly twice the pressure as the discovery occultation of 1988, (ii)
new information about the surface properties of Pluto, (iii) a spacecraft due
to arrive at Pluto in 2015, and (iv) a new volatile transport model that is
rapid enough to allow a large parameter-space search. Such a parameter-space
search coarsely constrained by occultation results reveals three broad
solutions: a high-thermal inertia, large volatile inventory solution with
permanent northern volatiles (PNV); a lower thermal-inertia, smaller volatile
inventory solution with exchanges between hemispheres, and a pressure plateau
beyond 2015 (exchange with pressure plateau, EPP); and solutions with still
smaller volatile inventories, with an early collapse of the atmosphere prior to
2015 (exchange with early collapse, EEC). PNV is favored by stellar occultation
data, but EEC cannot yet be definitively ruled out without more atmospheric
modeling or additional occultation observations and analysis.
