  X-Shooter is the first second-generation instrument for the ESO-VLT. It as a
spectrograph covering the 300 - 2480 nm spectral range at once with a high
resolving power. These properties enticed us to observe (136199) Eris during
the science verification of the instrument. The target has numerous absorption
features in the optical and near-infrared domain which has been observed by
different authors, showing differences in their positions and strengths.
  We attempt at constraining the existence of super-volatiles, e.g., CH4, CO
and N2, and in particular try to understand the physical-chemical state of the
ices on Eris' surface.
  We observed Eris in the 300-2480 nm range and compared the newly obtained
spectra with those available in the literature. We identified several
absorption features, measuring their positions and depth and compare them with
those of reflectance of pure methane ice obtained from the optical constants of
this ice at 30 K to study shifts in their positions and find a possible
explanation for their origin.
  We identify several absorption bands in the spectrum all consistent with the
presence of CH4 ice. We do not identify bands related with N2 or CO. We
measured the central wavelengths of the bands and find variable shifts, with
respect to the spectrum of pure CH4 at 30 K. Conclusions. Based on these
wavelength shifts we confirm the presence of a dilution of CH4 in other ice on
the surface of Eris and the presence of pure CH4 spatially segregated. The
comparison of the centers and shapes of these bands with previous works suggest
that the surface is heterogeneous. The absence of the 2160 nm band of N2 can be
explained if the surface temperature is below 35.6 K, the transition
temperature between the alpha and beta phases of this ice. Our results,
including the reanalysis of data published elsewhere, point to an heterogeneous
surface on Eris.
