Rotational Phase Dependent J − H Colour of the Dwarf Planet Eris* *Visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract 80HQTR19D0030 with the National Aeronautics and Space Administration.

Abstract

The largest bodies—or dwarf planets—constitute a different class among Kuiper Belt objects and are characterized by bright surfaces and volatile compositions remarkably different from that of smaller trans-Neptunian objects. These compositional differences are also reflected in the visible and near-infrared colors, and variegations across the surface can cause broadband colors to vary with rotational phase. Here we present near-infrared J and H-band observations of the dwarf planet (136199) Eris obtained with the GuideDog camera of the Infrared Telescope Facility. These measurements show that—as suspected from previous J − H measurements—the J − H color of Eris indeed varies with rotational phase. This suggests notable surface heterogenity in chemical composition and/or other material properties despite the otherwise quite homogeneous, high albedo surface, characterized by a very low amplitude visible range light curve. While variations in the grain size of the dominant CH4 may in general be responsible for notable changes in the J − H color, in the current observing geometry of the system it can only partially explain the observed J − H variation.