  2003 EL61 is the largest member of a group of TNOs with similar orbits and
'unique' spectra (neutral slope in the visible and the deepest water ice
absorption bands ever observed in the TNb). Studying the composition of the
surface of 2003 EL61 provides useful constrains on the origin of this
particular group of TNOs and on the outer Solar system's history.
  We present visible and near-infrared spectra of 2003 EL61 obtained with the
4.2m WHT and the 3.6m TNG at the Roque de los Muchachos Observatory (Canary
Islands, Spain). Near infrared spectra were obtained at different rotational
phases covering almost one complete rotational period. Spectra are fitted using
Hapke scattering models and constraints on the surface composition are derived.
  No significant variations in the spectral slope and in the depth of the water
ice absorption bands at different rotational phases are evident, suggesting
that the surface of 2003 EL61 is homogeneous. The scattering models show that a
1:1 intimate mixture of crystalline and amorphous water ice is the most
probable composition for the surface of this TNO, and constrain the presence of
other minor constituents to a maximum of 8%
  The derived composition suggests that: a) cryovolcanism is unlikely to be the
main resurfacing process responsible for the high presence of water ice on the
surface of these bodies; b) the surface is older than 10^8 yr. Any catastrophic
event, like the collision suggested to be the origin of this population, had to
happen at least 10^8 yr ago; c) the surface of 2003 EL61 is depleted of carbon
chains. According to the orbital parameters of this population, this makes it a
possible source of carbon-depleted Jupiter family comets.
