Pluto: The “other” red planet

What color is Pluto? The answer, revealed in the first maps made from New Horizons data, turns out to be shades of reddish brown. Although this is reminiscent of Mars, the cause is almost certainly very different. On Mars, the coloring agent is iron oxide, commonly known as rust. On the dwarf planet Pluto, the reddish color is likely caused by hydrocarbon molecules that are formed when cosmic rays and solar ultraviolet light interact with methane in Pluto’s atmosphere and on its surface.

“Pluto’s reddish color has been known for decades, but New Horizons is now allowing us to correlate the color of different places on the surface with their geology and soon with their compositions,” said New Horizons Principal Investigator Alan Stern of the Southwest Research Institute in Boulder, Colorado. “This will make it possible to build sophisticated computer models to understand how Pluto has evolved to its current appearance.”

Experts have long thought that reddish substances are generated as a particular color of ultraviolet light from the Sun, called Lyman-alpha, strikes molecules of the gas methane in Pluto’s atmosphere, powering chemical reactions that create complex compounds called tholins. The tholins drop to the ground to form a reddish “gunk.” Recent measurements with New Horizons’ Alice instrument reveal that a diffuse Lyman-alpha glow falling on Pluto from all directions in interplanetary space is strong enough to produce almost as much tholin as the direct rays of the Sun. “This means Pluto’s reddening process occurs even on the night side where there’s no sunlight and in the depths of winter when the Sun remains below the horizon for decades at a time,” said New Horizons co-investigator Michael Summers of George Mason University in Fairfax, Virginia.

Tholins have been found on other bodies in the outer solar system, including Titan and Triton, the largest moons of Saturn and Neptune, respectively, and made in laboratory experiments that simulate the atmospheres of those bodies.

The mission’s first map of Pluto is in approximate true color — that is, the color you would see if you were riding on New Horizons. At left is a map of Pluto’s northern hemisphere composed using high-resolution black-and-white images from New Horizons LORRI instrument. At right is a map of Pluto’s colors created using data from the Ralph instrument. In the center is the combined map, produced by merging the LORRI and Ralph data.

“Now the unique colors and characteristics of its varied terrains are coming into view,” said Simon Porter, a member of the New Horizons Geology and Geophysics team. Added Alex Parker, a member of the New Horizons Composition team, “Pluto’s largest dark spot is clearly more red than the majority of the surface, while the brightest area appears closer to neutral gray.”

Scientists hope to learn more about the cause of Pluto’s reddish tint as New Horizons closes in for its July 14 flyby.