2007A&A...466..395E

We report measurements of reflectances of 90377 Sedna at λ>2.5µm using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. Sedna orbits well beyond even the Kuiper Belt, with a perihelion distance of 76AU, and is therefore very faint as viewed from Earth, despite its relatively large size. Previously published near-infrared spectra show possible signatures of CH₄ and N₂ at ∼2.3 and ∼2.15µm, respectively. These and other ices also exhibit much stronger absorptions at λ>2.5µm, providing the motivation for the present work. We detected flux from Sedna at 3.6 and 4.5µm, but not at 5.8 or 8.0µm. The measured IRAC fluxes are converted to geometric albedos and combined with previous measurements of the visible and near-infrared spectra. Strong absorption at both 3.6 and 4.5µm (relative to the 2.0-2.5µm region) is readily apparent, confirming the presence of ices on the surface of Sedna. Spectral modeling of the full wavelength range (0.4-4.5 µm) provides further constraints. We find that CH₄ is required to fit the new data points, but that these new data points can not be adequately described with models containing CH₄ and N₂ as the only ices. We suggest that H₂O ice is also present. Several characteristics of the spectrum of Sedna suggest an absence of atmospheric volatile transport, in contrast to the large objects Eris and 2005 FY₉.