  Kuiper belt object (KBO) Eris is exceptionally bright with a greater visible
geometric albedo than any other known KBO. Its infrared reflectance spectrum is
dominated by methane, which should form tholins that darken the surface on
timescales much shorter than the age of the Solar System. Thus one or more
ongoing processes probably maintain its brightness. Eris is predicted to have a
primarily nitrogen atmosphere that is in vapor pressure equilibrium with
nitrogen-ice and is collisional (not ballistic). Eris's eccentric orbit is
expected to result in two atmospheric regimes: (1) a period near perihelion
when the atmosphere is global (analogous to the atmospheres of Mars, Triton,
and Pluto) and (2) a period near aphelion when only a local atmosphere exists
near the warmest region (analogous to the atmosphere of Io). A numerical model
developed to simulate Eris's thermal and volatile evolution in the local
atmosphere regime is presented. The model conserves energy, mass, and momentum
while maintaining vapor pressure equilibrium. It is adaptable to other local,
collisional, sublimation atmospheres, which in addition to Io and Eris, may
occur on several volatile-bearing KBOs. The model was applied for a limiting
case where Eris is fixed at aphelion and has an initial nitrogen-ice mass
everywhere equal to the precipitable column of nitrogen in Pluto's atmosphere
during the New Horizons encounter (the resultant mass if the Pluto atmosphere
collapsed uniformly onto the surface). The model results indicate that (1)
transport of nitrogen in the local, collisional, sublimation atmosphere regime
is significant, (2) changes of Eris's albedo or color from nitrogen transport
may be observable, and (3) uniform collapse of a global, nitrogen atmosphere
likely cannot explain Eris's anomalous albedo in the present epoch. Seasonal
volatile transport remains a plausible hypothesis to explain...
