  An analytic expression for the variation in surface and sub-surface
temperature is developed for worlds whose surface pressures are nearly constant
with latitude and longitude and whose atmospheres are in vapor-pressure
equilibrium with the dominant surface volatiles. Such worlds include the
current Pluto and Triton, and other volatile-covered Kuiper Belt Objects during
some portion of their heliocentric orbit. The expressions also apply on worlds
with negligible horizontal heat flow, such as asteroids. Temperature variations
in volatile-covered or bare areas as a function of time is derived in terms of
three thermal parameters relating to (1) the thermal wave within the substrate,
(2) the energy needed to heat an isothermal volatile slab, and (3) the
buffering by the latent heat needed to change the atmospheric surface pressure.
For Pluto's current surface pressure (~17 microbar), atmospheric buffering
dominates over subsurface effects on diurnal timescales, and should keep the
surface pressure over a Pluto day constant to within 0.2%.
