Pluto: The 'Other' Red Planet (NASA)
July 3, 2015
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 Plutos atmosphere and on its surface.
Plutos 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 Plutos 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 Plutos reddening process occurs even on the night side where theres 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.
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more: https://www.nasa.gov/nh/pluto-the-other-red-planet (pix, animations, previous press releases -- just keep scrolling down)
The term 'tholin' was introduced by Carl Sagan while studying the atmospheric photochemistry of Titan and similar bodies.