Charon: The Mega-Moon With a Mountain in a Moat

Pluto’s largest moon, Charon, is almost as enigmatic as Pluto. As NASA’s New Horizons spacecraft helps unveil the underworldly moon, most of its features are turning out to be more puzzling than expected.

Early images showed an unexpectedly dark polar cap, which scientists think could be the result of material migrating over from Pluto. And an image released July 15 revealed that the 1,200-kilometer-wide world appears to have a surprisingly young surface with tantalizing hints of geologic activity.

“We’re flabbergasted. We really are,” says New Horizons team member Bonnie Buratti of the Jet Propulsion Laboratory. “To tell you the truth, Charon looks more like I would have expected Pluto to look.”

The latest image release zooms in on a portion of Charon’s surface that lies kind of near the moon’s equator. Snapped by New Horizons as it approached the Pluto system on July 14, the image – unsurprisingly – has scientists completely baffled.

In it, Charon’s surface is smooth, with a smattering of craters and cracks – and there’s a bizarre mountain rising out of a depression that has been called a “mountain in a moat.”

“It looks like somebody just dropped a giant boulder on Charon,” Buratti says.

So far, scientists have no idea what’s going on with the Charonian peak. But, says team member Anne Verbiscer, it does look similar to a feature on another of the solar system’s most curious worlds: The two-toned moon Iapetus, which orbits Saturn. “I find both of them fascinating,” says Verbiscer, of the University of Virginia. She notes an intriguing and speculative connection between the mountain, icy volcanism, and the ground-based detection of a compound called ammonium hydrate.

“Ammonia hydrate acts like antifreeze when mixed with water ice, and can help with making cryovolcanic activities possible on bodies such as Charon and Iapetus,” Verbiscer says.

As for those craters and cracks? The cracks, Buratti says, are faults. She notes that similar-looking fractures have been seen in basins on Earth’s moon – where they’re the remains of lava tubes – as well as on Mercury, where they suggest that the planet is shrinking. But Buratti doesn’t think either of those explanations account for Charon’s wrinkles.  “We really have no idea what to think at this point,” she says.

Then there’s the relative sparseness of craters. Indeed, those missing craters seem to be one of the more perplexing observations the team is wrestling with. “There are fewer craters than we expected, and we don’t know why,” Buratti says.

Together, the cracks and relatively pit-free surface suggest Charon could have been a pretty active world, and nothing like the stagnant, battered chunk of rock many scientists had anticipated. “We expected it to be more of a dead object,” Buratti says. “But there’s some active process on the moon that happened in the past.”

Along with scrutinizing Charon, the team will be keenly studying Pluto’s surface for signs of activity as well. Already, the dwarf planet’s high-contrast, mountainous face is keeping a sleep-deprived New Horizons team busy. “Pluto did not disappoint,” said deputy project scientist Cathy Olkin. “And Charon did not disappoint, either.”