Most of Pluto’s appearance is driven by seasonal changes as the direction of its tilt and its distance from the Sun change over its 248-year orbit. It has was looks like young, yellowish methane-rich ice at the south pole (the one facing us), slightly bluer nitrogen ice at the mid-latitudes, and old, dark methane-rich ice at the equator. However, this pattern is broken by the bright ice of Tombaugh Regio, or “the Heart”.
We now know that the youngest, brightest part of Tombaugh Regio, Sputnik Planum, is a huge nitrogen glacier, also containing methane and, unlike the rest of Pluto, large amounts of carbon monoxide, suggesting a possible internal source in that region. The nitrogen glaciers seem to flow out from the middle of Sputnik Planum and into the surrounding highlands and mountains.
That weird “cantaloupe” terrain that covers the majority of the image does look a fair bit like like the “polygons” of Pluto’s Sputnik Planum. Perhaps this is some kind of common signature of nitrogen volcanoes.
That is the night side of Pluto backlit by the Sun. The ring of light is caused by sunlight scattering through a hydrocarbon haze high in the atmosphere–30 to 50 kilometers high–much higher than expected. This is despite the atmospheric pressure being much lower than expected. Pluto’s atmosphere has about 0.001% the pressure of Earth’s, and the latest measurements seem to support a particular theory that Pluto’s atmosphere freezes out very rapidly a couple decades after its closest approach to the Sun (which happened in 1989). Its atmospheric pressure dropped by half in just two years, so we may have gotten lucky to see it freezing now.