Aqueous altered silicates at the surface of two Plutinos?

In April 2001 we obtained visible and near-infrared spectra, as well as photometric data, for two Trans-Neptunian Objects (TNOs), 2000 GN171 (now numbered 47932) and 2000 EB173 (now designated 38628 Huya), which belong to the dynamical class of Plutinos. These observations were made with the FORS1 and ISAAC instruments at the European Southern Observatory-Very Large Telescope (ESO-VLT). The detection of weak absorption features in the visible spectra of these two Trans-Neptunians has been reported elsewhere (Lazzarin et al. \cite{Lazzarin03}). In this paper, we discuss the interpretation of the features, which are different for the two objects, and we present some complementary observations in the near-infrared, as well as more recent (May 2002) visible spectra in which the features are absent. Although the visible colors of the two objects are comparable, the near-infrared spectra are different: (47932) 2000 GN171 shows a nearly flat spectrum, except in the H band where a broad absorption appears, whereas the spectrum of (38628) Huya (2000 EB173) has a red slope in the J band and some absorption beyond 2 micron. The features detected in the visible spectra of the two objects are tentatively attributed to the presence of iron oxides or phyllosilicates at the surfaces of the two objects. The differences between the April 2001 and May 2002 visible spectra are attributed to spatial variations at the surfaces of the objects. We briefly discuss possibilities for aqueous alteration in TNOs, after reviewing what we know about the presence of aqueously altered minerals in other small bodies of the solar system. Further studies monitoring the rotation of these two objects are highly desirable. Based on observations collected at the European Southern Observatory, Chile, programme 167.C-0340.