* Astronomy

Title: Methane and Nitrogen Abundances On Pluto and Eris
Authors: S.C. Tegler, D.M. Cornelison, W.M. Grundy, W. Romanishin, M.R. Abernathy, M.J. Bovyn, J.A. Burt, D.E. Evans, C.K. Maleszewski, Z. Thompson, F. Vilas

We present spectra of Eris from the MMT 6.5 meter telescope and Red Channel Spectrograph (5700-9800 angstroms; 5 angstroms per pix) on Mt. Hopkins, AZ, and of Pluto from the Steward Observatory 2.3 meter telescope and Boller and Chivens spectrograph (7100-9400 angstroms; 2 angstroms per pix) on Kitt Peak, AZ. In addition, we present laboratory transmission spectra of methane-nitrogen and methane-argon ice mixtures. By anchoring our analysis in methane and nitrogen solubilities in one another as expressed in the phase diagram of Prokhvatilov and Yantsevich (1983), and comparing methane bands in our Eris and Pluto spectra and methane bands in our laboratory spectra of methane and nitrogen ice mixtures, we find Eris' bulk methane and nitrogen abundances are about 10% and about 90%, and Pluto's bulk methane and nitrogen abundances are about 3% and about 97%. Such abundances for Pluto are consistent with values reported in the literature. It appears that the bulk volatile composition of Eris is similar to the bulk volatile composition of Pluto. Both objects appear to be dominated by nitrogen ice. Our analysis also suggests, unlike previous work reported in the literature, that the methane and nitrogen stoichiometry is constant with depth into the surface of Eris. Finally, we point out that our Eris spectrum is also consistent with a laboratory ice mixture consisting of 40% methane and 60% argon. Although we cannot rule out an argon rich surface, it seems more likely that nitrogen is the dominant species on Eris because the nitrogen ice 2.15 micron band is seen in spectra of Pluto and Triton.

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Eris was discovered by the team of Mike Brown, Chad Trujillo, and David Rabinowitz on January 5, 2005, from images taken on October 21, 2003. The discovery was announced on July 29, 2005, the same day as Makemake and two days after Haumea.
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Strange weather on the icy dwarf planet Eris could be causing changes that scientists are now seeing at the methane-ice surface of this distant object in our solar system.
Eris is the largest known solar-system object beyond the orbit of Neptune. It is larger than Pluto, with a diameter of ranging somewhere between about 2,400 km and 3,000 km.
A team of researchers examined data on Eris collected from the MMT Observatory in Arizona. They specifically looked at concentrations of methane ice based on light-reflection and absorption information.
Their results show possibly nitrogen ice mixed in with the methane ice covering Eris' surface. And the relative amount of nitrogen ice increases with depth into the ice.

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Title: Polarimetry of the dwarf planet (136199) Eris
Authors: I. Belskaya, S. Bagnulo, K. Muinonen, M.A. Barucci, G.P. Tozzi, S. Fornasier, and L. Kolokolova

Context. Study the surface properties of the transneptunian population of Solar-system bodies
Aims. We investigate the surface characteristics of the large dwarf planet (136199) Eris.
Methods. With the FORS1 instrument of the ESO VLT, we have obtained Bessell broadband R linear polarimetry and broadband V and I photometry of Eris. We have modelled the observations in terms of the coherent-backscattering mechanism to constrain the surface properties of the object.
Results. Polarimetric observations of Eris show a small negative linear polarisation without opposition surge in the phase angle range of 0.15 0.5#. The photometric data allow us to suppose a brightness opposition peak at phase angles below 0.2 0.3#. The data obtained suggest possible similarity to the polarimetric and photometric phase curves of Pluto. The measured absolute magnitude and broadband colours of Eris are HV=-1.15 mag, V R = 0.41 mag, and V I = 0.75 mag.
Conclusions. The observational data with theoretical modelling are in agreement with the surface of Eris being covered by rather large inhomogeneous particles.

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