  Several distant icy worlds have atmospheres that are in vapor-pressure
equilibrium with their surface volatiles, including Pluto, Triton, and,
probably, several large KBOs near perihelion. Studies of the volatile and
thermal evolution of these have been limited by computational speed, especially
for models that treat surfaces that vary with both latitude and longitude. In
order to expedite such work, I present a new numerical model for the seasonal
behavior of Pluto and Triton which (i) uses initial conditions that improve
convergence, (ii) uses an expedient method for handling the transition between
global and non-global atmospheres, (iii) includes local conservation of energy
and global conservation of mass to partition energy between heating,
conduction, and sublimation or condensation, (iv) uses time-stepping algorithms
that ensure stability while allowing larger timesteps, and (v) can include
longitudinal variability. This model, called VT3D, has been used in Young
(2012), Young (2013), Olkin et al. (2015), Young and McKinnon (2013), and
French et al. (2015).
