A heart of nitrogen ice. Pluto is smaller than the Moon, and a year there lasts 248 of ours. New Horizons flew past in July 2015, nine and a half years after leaving the Earth, and found its bright heart: a plain of nitrogen ice about 1,000 km across.
New Horizons’ map from its flyby in July 2015, in black and white. It saw one side closely and only the north well: the south was in a winter night lasting decades, so it is shown plain grey.
USGS Astrogeology, New Horizons global mosaic, July 2017, public domain.