On June 22, 1978, James Christy was examining photographic plates from the U.S. Naval Observatory at Flagstaff when he observed an unusual protrusion on Pluto that altered its position in different images. This protrusion appeared to have a consistent periodicity, indicating the presence of a moon surrounding what was then known as the ninth planet in our Solar System. After examining archived plates dating back 15 years, Christy confirmed his discovery of Pluto's first moon, which he named Charon.
Charon's discovery was significant, as it allowed for a more precise determination of Pluto's size and mass, which had previously been estimated. Initially, Charon was merely considered an extension of Pluto, until the Hubble Space Telescope was able to distinguish the two celestial bodies in the early 1990s. However, the telescope could not provide detailed surface information.
A breakthrough occurred on July 14, 2015, when New Horizons became the first man-made object to fly over the Pluto system and capture close-up images of Charon. The moon exhibited a unique appearance, featuring a predominantly grayish surface with craters and rifts, along with a distinct reddish region at its pole. The cause of this unusual coloration remains unknown, but some theories suggest that it may be due to material condensed from Pluto's atmosphere.
In fact, temperatures at Charon's pole could be low enough for nitrogen and methane to condense after escaping from Pluto. When these ices interact with solar radiation, they acquire a reddish hue similar to the one observed on Charon's surface.
Credit: NASA, Johns Hopkins Univ./APL, Southwest Research Institute, U.S. Naval Observatory.
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