Context. X-Shooter is the first second-generation instrument for the
ESO-Very Large Telescope. It is a spectrograph covering the entire
300-2480 nm spectral range at once with a high resolving power. These
properties enticed us to observe the well-known trans-Neptunian object
(136199) Eris during the science verification of the instrument. The
target has numerous absorption features in the optical and near-infrared
domain that have been observed by different authors, showing differences
in these features' positions and strengths. Aims: Besides testing
the capabilities of X-Shooter to observe minor bodies, we attempt to
constrain the existence of super-volatiles, e.g., CH4, CO and
N2, and in particular we try to understand the
physical-chemical state of the ices on Eris' surface. Methods: 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, measured their positions and depth, and compare
them with those of the reflectance of pure methane ice obtained from the
optical constants of this ice at 30 K to study shifts in these features'
positions and find a possible explanation for their origin.
Results: We identify several absorption bands in the spectrum that are
all consistent with the presence of CH4 ice. We do not
identify bands related to N2 or CO. We measured the central
wavelengths of the bands and compared to those measured in the spectrum
of pure CH4 at 30 K finding variable spectral shifts.
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 that is spatially segregated. The
comparison of the centers and shapes of these bands with previous works
suggests 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 a heterogeneous surface on Eris.
Observations made during X-Shooter Science Verification, program
60.A-9400(A), PIs: Alvarez-Candal and Mason.