After NASA’s New Horizons mission flew past Pluto in 2015, it turned around and took pictures of the dwarf planet’s back, revealing its moonlit dark sideMoonlight on Pluto has revealed some of the dwarf planet’s dark side. On its way past Pluto in 2015, NASA’s New Horizons spacecraft turned around and took pictures of this world’s back, and after a lengthy cleaning-up process, the images have revealed some of the first details we have ever seen of the side that wasn’t illuminated by the sun at the time.
Taking images of Pluto from beyond its orbit is difficult because at that position, it is backlit by the sun. “When you look back at Pluto’s dark side, you turn around and look almost right at the sun, and it’s pretty damn bright,” says Tod Lauer at the US National Optical Infrared Astronomy Research Laboratory in Arizona, who is part of the New Horizons team. “It’s like driving in a car with a dirty window, looking into the sun without a sun visor, trying to read a street sign.”
To make the pictures from New Horizons usable, Lauer and his colleagues had to rework the mission’s data processing procedure to eliminate the parts of the images that were overexposed by sunlight and therefore didn’t contain any useful data. Once those parts of the images were cleared away, the researchers could manipulate what remained to see the moonlit surface of Pluto.While Pluto’s major moon, Charon, is much smaller than Earth’s moon, it is shinier and closer to its host world than our moon is, so it provides about half as much light to the side of Pluto that’s facing away from the sun. Under this faint illumination, the researchers found one spot that was brighter than its surroundings, which is probably a deposit of nitrogen or methane ice.
They also found that the south pole appeared to be much less bright than the north pole. “You expect the poles should be more or less the same, and this difference is intriguing – it may indicate a seasonal thing,” says Lauer. New Horizons flew by Pluto at the end of the small world’s southern summer, so this may be a hint that bright ice deposits don’t survive that relatively warm period, or that haze particles from the tenuous atmosphere are deposited on the surface in the summer.