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#Science

Even from 11 million miles, new colour images of Pluto and Charon are stunning

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New Horizons may be having technology that is nearly nine years old, yet it is delivering results that are not only scientifically important, but also awe-inspiring.

Images grabbed by New Horizons’ LORRI instrument between June 23 and June 29, 2015 from a distance of 11 million miles (18 million kilometers) reveal color surface features of Pluto and its largest moon Charon.

“It’s a bit unusual to see so much surface detail at this distance,” said New Horizons co-investigator William McKinnon of the Geology and Geophysics Investigation Team, Washington University in Saint Louis. “What’s especially noteworthy is the level of detail in both bodies. It’s certainly whetting our appetite for what’s to come.”

Pluto Charon colour

Six high-resolution black-and-white images from New Horizons’ LORRI instrument were combined with color data from the Ralph instrument to produce the movie [embedded above].

Thanks to the detailed images, mission control scientists have been able to establish the colour of Pluto. Based on the first maps made from New Horizons data, it turns out that Pluto has 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, 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 (CH4) 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, George Mason University, 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, 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.