Astronomers Reveal High Concentrations of Ammonia on Charon

Charon’s Young Ammonia Crater. The informally named Organa crater (shown in green) is rich in frozen ammonia and – so far – appears to be unique on Pluto’s largest moon. Credits: NASA/JHUAPL/SwRI Enlarge

Astronomers have discovered a striking contrast between one of the fresh craters on Pluto’s largest moon Charon and a neighboring crater dotting the moon’s Pluto-facing hemisphere.

The crater, informally named Organa, caught scientists’ attention as they were studying the highest-resolution infrared compositional scan of Charon. Organa and portions of the surrounding material ejected from it show infrared absorption at wavelengths of about 2.2 microns, indicating that the crater is rich in frozen ammonia – and, from what scientists have seen so far, unique on Pluto’s largest moon. The infrared spectrum of nearby Skywalker crater, for example, is similar to the rest of Charon’s craters and surface, with features dominated by ordinary water ice.

Using telescopes, scientists first observed ammonia absorption on Charon in 2000, but the concentrations of ammonia around this crater are unprecedented.

“Why are these two similar-looking and similar-sized craters, so near to each other, so compositionally distinct?” asked Will Grundy, New Horizons Composition team lead from Lowell Observatory in Flagstaff, Arizona. “We have various ideas when it comes to the ammonia in Organa. The crater could be younger, or perhaps the impact that created it hit a pocket of ammonia-rich subsurface ice. Alternatively, maybe Organa’s impactor delivered its own ammonia.”

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