  We describe optical spectroscopic observations of the icy dwarf planet Eris
with the 6.5 meter MMT telescope and the Red Channel Spectrograph. We report a
correlation, that is at the edge of statistical significance, between blue
shift and albedo at maximum absorption for five methane ice bands. We interpret
the correlation as an increasing dilution of methane ice with another ice
component, probably nitrogen, with increasing depth into the surface.
  We suggest a mechanism to explain the apparent increase in nitrogen with
depth. Specifically, if we are seeing Eris 50 degrees from pole-on (Brown and
Schaller, 2008), the pole we are seeing now at aphelion was in winter darkness
at perihelion. Near perihelion, sublimation could have built up atmospheric
pressure on the sunlit (summer) hemisphere sufficient to drive winds toward the
dark (winter) hemisphere, where the winds would condense. Because nitrogen is
more volatile and scarcer than methane, it sublimated from the sunlit
hemisphere relatively early in the season, so the early summer atmosphere was
nitrogen rich, and so was the ice deposited on the winter pole. Later in the
season, much of the nitrogen was exhausted from the summer pole, but there was
plenty of methane, which continued to sublimate. At this point, the atmosphere
was more depleted in nitrogen, as was the ice freezing out on top of the
earlier deposited nitrogen rich ice.
