The Cold and Icy Heart of Pluto

Abstract:

The locations of large deposits of frozen volatiles on planetary 
surfaces are largely coincident with areas receiving the minimum 
annual influx of solar energy. Thus we have the familiar polar caps of 
Earth and Mars, but cold equatorial regions for planets with 
obliquities between 54 and 126 degrees. Furthermore, for 
tilts between 45-66 degrees and 114-135 degrees the minimum incident 
energy occurs neither at the pole nor the equator. We find that the 
annual average insolation is always symmetric about Pluto's equator 
and is fully independent of the relative locations of the planet's 
pericenter and equinoxes. Remarkably, this symmetry holds for 
arbitrary orbital eccentricities and obliquities, and so we provide a 
short proof in the margin of this abstract. 

The current obliquity of Pluto is 119 degrees, giving it minima in 
average annual insolation at +/- 27 degrees latitude, with ~1.5% more 
flux to the equator and ~15% more to the poles. But the obliquity of 
Pluto also varies sinusoidally from 102-126 degrees and so, over the 
past million years, Pluto's annual equatorial and polar fluxes have changed 
by +15% and -13%, respectively. Interestingly, the energy flux 
received by latitudes between 25-35 degrees remains nearly constant 
over the presumably billions of years since Pluto acquired its current 
orbit and spin properties. Thus these latitudes are continuously cold 
and should be favored for the long-term deposition of volatile ices; 
the bright heart of Pluto, Sputnik Planum, extends not coincidentally 
across these latitudes. 

Reflected light and emitted thermal radiation from Charon increases 
annual insolation to one side of Pluto by of order 0.02%. Although 
small, the bulk of the energy is delivered at night to Pluto's cold 
equatorial regions. Furthermore, Charon's thermal IR is delivered very 
efficiently to icy deposits. Over billions of years, ices have 
preferentially formed and survived in the anti-Charon hemisphere.