Breaking the degeneracy of Eris' pole orientation

The most massive trans-Neptunian object, (136199) Eris, has one known satellite, Dysnomia. By observing changes in Dysnomia's orbit projected on the sky over multiple different epochs, we were able to break the mirror degeneracy and determine the pole orientation of Eris. Images of the Eris/Dysnomia system were obtained with WFC3/HST in January and February 2018 (program 15171). These images were taken through the F606W filter and visits were scheduled to capture Dysnomia at different orbital phases. We also folded in data from ACS/HRC/HST and NIRC2/Keck programs from 2005/2006 (Brown and Schaller, 2007) and WFC3/HST program 13668 from 2015. The 2005/2006 data were considered "Epoch 1" and the 2015 and 2018 data were combined into "Epoch 2." PSF-fitting produced relative astrometry of Dysnomia with respect to Eris and was used to fit Keplerian orbits to each epoch. Comparison of the orbit fits from these different epochs enabled the determination of Eris' rotation pole orientation, assuming Dysnomia orbits in Eris' equatorial plane. We report that Eris' obliquity is 78°, the sub-solar latitude in early 2018 was 42°, and the next period of mutual events will occur in 2239, all in agreement with Orbit 1 from Brown and Schaller (2007). With this viewing geometry, approximately 30% of the visible hemisphere of Eris is in constant sunlight. Assuming a peak-to-valley amplitude of 0.10 mag (Roe et al., 2008) due entirely to albedo variations, the hemispheres representing the maximum and minimum in the light curve have a 10% difference in albedo. Eris' high visible geometric albedo (0.96; Sicardy et al., 2011) and this small albedo difference together imply that Eris' surface lacks large-scale, low-albedo features like Cthulhu Macula on Pluto.