Rotational phase dependent J-H colour of the dwarf planet Eris

The largest bodies - or dwarf planets - constitute a different class among Kuiper belt objects and are characterised 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 colours, and variegations across the surface can cause broadband colours 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 colour 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, characterised 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 colour, in the current observing geometry of the system it can only partially explain the observed J-H variation.