Pluto’s Color Variations Finally Make Sense

Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all

Why are some areas redder than others? It’s what happens to methane ice in the Sun.

“This is in a real sense the capstone of the initial missions to explore the planets. Pluto, its moons and this part of the solar system are such mysteries that New Horizons will rewrite all of the textbooks.” –Alan Stern

When viewed in enhanced color, the icy, outer world of Pluto looks very different from its more uniform, subdued hues.

*Image credit: NASA/JUAPL/SwRI, of a natural color composite of Pluto and Charon from the Long Range Reconnaissance Imager (LORRI) instrument and the Ralph instrument.*

In natural color, Pluto is reddish while Charon is a dull grey.

*Image credit: NASA/JHUAPL/SwRI, of Pluto and Charon in enhanced color thanks to observations from the spacecraft’s Ralph/Multispectral Visual Imaging Camera (MVIC).*

But the enhancements shows the presence of reddening in various regions all across the planet.

*Image credit: NASA/JHUAPL/SwRI, of the barely-perceptible variations in color along the plains region. The “bumps” are likely due to migrating water-ice hills that float atop the nitrogen sea.*

The hilly plains in Pluto’s heart-shaped area are mostly uniform, as a flat surface of nitrogen ice is disrupted by water-ice mountains and evaporative pits.

*Image credit: NASA/JHUAPL/SwRI, of the color variations that appear just outside of Pluto’s uniform “heart” region.*

The more mountainous regions on the outskirts of the heart show subtle variations in both terrain and color.

*Image credit: NASA/JHUAPL/SwRI, of the interface between the heart-shaped region (lower right) and the mountainous terrain (upper left) that show severe color differences as well.*

Huge variations are showcased at the interface of these two regions, as icy mountains transition into a smooth, nitrogen sea.

*Image credit: NASA/JHUAPL/SwRI, of the color variations revealed in the complexity-laden craters of Pluto’s mountains.*

But the biggest variations can be seen where craters exist along mountain ranges. While nitrogen and water can both sublimate in the sunlight, the methane ices react differently.

*Image credit: NASA/JHUAPL/SwRI, of the color variations along Pluto’s equator, shown at the top of the full-size image of Pluto from New Horizons.*

The ultraviolet sunlight ionizes methane, setting off a chain of events that creates tholins — red-colored hydrocarbon compounds — that get deposited at various locations.

*Image credit: NASA/JHUAPL/SwRI, of a darker (redder) region just below the right-hand side of Pluto’s “heart”.*

It’s only where fresh, white methane snow covers the tholin-rich regions that a white color reappears.

*Image credit NASA/JHUAPL/SwRI, of methane snow atop tholin-rich regions.*

The next time we visit Pluto, this world’s colorscape will appear very different.

Mostly Mute Monday tells the story of a single astronomical object or phenomenon in pictures and visuals, with no more than 200 words of text.

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Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all