CONICET | Buscador de Institutos y Recursos Humanos

context: 2003 EL61 is the largest member of a group of trans-neptunian objects (TNOs) with similar orbits and spectral characterisitcs. The composition of these objects (almost 100% pure water ice) is very different from the typical composition of ´normal´ TNOs. The discovery of a carbon-depleted population in the trans-neptunian belt (TNb) and its possible relation with carbon-depleted comets is an interesting open question that needs to be further investigated. Deriving conclusions about the surface of 2003 EL61 can give useful constrains on the origin of this particular group of TNOs and on the outer Solar System´s history.aims: The aim of this paper is to study the composition of the surface of 2003 EL61.methods: 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 scattering models based on Hapke theory and constraints on the surface composition are derived.results: The observations confirm previous results: 2003 EL61 spectrum is neutral in color and presents very deep water ice absorption bands. Meanwhile, they provide us with some new facts about the surface of this object, no significant variations in the spectral slope (in the near-infrared) and in the depth of the water ice absorption bands at different rotational phases are evident within the S/N, suggesting that the surface of 2003 EL61 is homogenous. 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 big TNO, and constrain the presence of other minor constituents to a maximun of 7%.conclusions: The derived composition suggests that: a) cryovolcanism is unlikely the resurfacing process that keeps the surface of this TNO, and the other members of this population, covered mainly by water ice; b) the surface is older than 10^8 yr. This constrains the time scale of any catastrophic event, like the collision suggested to be the origin of this population, to at least 10^8 yr; c) the surface of 2003 EL61 is depleted of carbon chains what according with the orbital parameters of the population makes it a possible source of carbon-depleted Jupiter family comets.