Scientists assumed Pluto was a dead rock just like Earth's Moon. Instead, NASA found a bizarrely active world where towering mountains of solid ice float on churning nitrogen glaciers.
The Moon is a classic geologically dead body. Covered in a powdery layer of shattered rock and dust called regolith, its surface has been locked in time for billions of years. With no atmosphere, no weather, and a cooled interior, the only thing that changes the lunar landscape is the occasional impact of a meteor. Every crater from the last three billion years remains exactly where it formed.
Despite surface temperatures hovering around -390°F (-234°C), Pluto is a dynamic world constantly resurfacing itself. The strongest contrast is a vast, heart-shaped glacier of frozen nitrogen known as Sputnik Planitia. This region is nearly devoid of impact craters. Heat slowly radiating from Pluto's interior warms the bottom of the nitrogen ice layer, causing it to bubble up and sink back down in giant, polygonal convection cells—much like a slow, freezing lava lamp. This continuous churning paves over new craters, keeping the surface looking brand new.
Bordering these smooth nitrogen plains are jagged mountain ranges, such as the Norgay Montes, that rise up to 20,000 feet high. While the Moon's mountains are made of silicate rock pushed up by ancient impacts, Pluto's ranges are blocks of solid water ice. At Pluto's extreme temperatures, water ice becomes as hard as granite, allowing these colossal blocks to ride atop the denser, softer nitrogen below.
Unlike the monochromatic gray of the Moon, Pluto is highly varied in color. Large stretches of its surface are stained a deep reddish-brown by tholins—complex organic molecules formed when ultraviolet sunlight breaks down the methane and nitrogen in Pluto's thin, hazy atmosphere.