Near-Infrared Surface Properties of the Two Intrinsically Brightest Minor Planets: (90377) Sedna and (90482) Orcus

We present low-resolution K-band spectra taken at the Gemini 8 m telescope of (90377) Sedna and (90482) Orcus (provisional designations 2003 VB12 and 2004 DW, respectively), currently the two minor planets with the greatest absolute magnitudes (i.e., the two most reflective minor planets). We place crude limits on the surface composition of these two bodies using a Hapke model for a wide variety of assumed albedos. The unusual minor planet Sedna was discovered on UT 2003 November 14 at roughly 90 AU, with 1.6 times the heliocentric and perihelion distances of any other bound minor planet. It is the first solar system object discovered between the Kuiper Belt and the Oort Cloud and may represent a transition population between the two. The reflectance spectrum of Sedna appears largely featureless at the current signal-to-noise ratio, suggesting a surface likely to be highly processed by cosmic rays. For large-grain models (100 μm to 1 cm) we find that Sedna cannot have more than 70% surface coverage of water ice and cannot have more than 60% surface coverage of methane ice, to 3 σ confidence. Minor planet Orcus shows strong water ice absorption corresponding to less than a 50% surface fraction for grain models 25 μm and larger. Orcus cannot have more than 30% of its surface covered by large (100 mm to 1 cm) methane grains, to 3 σ confidence.