Charon: A Dark Moon With a Dark Past

Something is rotten in the state of Charon, Pluto’s giant moon. OK maybe not rotten, exactly, but it’s just not…right.

Before the New Horizons spacecraft took a tour of the Pluto system in July, scientists guessed they might see a few interesting features on Charon — things like craters, and more craters, and maybe some notable landforms because Charon’s water ice surface is sturdy and capable of supporting dramatic landscapes. Several team members even dared to suggest that Charon would be surprisingly interesting — perhaps even more interesting than Pluto.

But overall, guesses seemed to converge on Charon being a dead, cratered husk.

“Craters were the number one thing people said we’d see on Charon,” said Amanda Zangari of the Southwest Research Institute, who conducted a survey of New Horizons team members before the flyby. But, she noted back in June, “I think we’re going to find that Charon is not just a dull, gray featureless thing.”

Turns out, Charon is totally not a dull, gray featureless thing.

It has curiously few craters, especially in its southern hemisphere, and a surface riven with massive canyons (some of which have cool informal names, like “Serenity Chasma”). It even has a mountain in a moat. Charon’s north pole is reddish — the same color red as the band near Pluto’s equator — and there’s a dark blotch on the hemisphere that faces away from Pluto.

In fact, the whole moon is dark, way darker than Pluto. Charon might not look especially shadowy in the image above, but that’s because of how those pixels were processed. If you look at the giant moon in relation to Pluto, its companion in binary planethood, you’ll see just how spooky it is. While Pluto is splattered with bright ices and colorful smears, Charon looms in the background like a scarred, charred world emerging from the darkness.

To me, it looks like half of Charon’s crust is missing, as if someone started peeling it like an orange and gave up after the southern hemisphere. The remaining northern rind is cratered, stained and old, while the southern pith is younger and smoother. The real story is probably much more interesting (yes, even more interesting than a giant peeling a moon). One early idea explaining the startling differences in terrain between Charon’s hemispheres invokes cryovolcanism, or the eruption of icy, frozen, maybe oozy materials. Icy satellites in the outer solar system appear to be particularly good at generating flavors of frozen “lavas”: Think geysers on Enceladus, plumes on Triton, and (possible) ice volcanoes on Titan.

Once upon a time, as the story might go here, a buried ocean sloshed inside Charon’s icy shell. At some point, as the moon froze, pressure started to build in the liquid water locked inside. As that pressure forced the water upward, it found a weak spot in Charon’s shell — and then all hell broke loose. Like water bursting through a crack in a dam, the ocean spilled onto Charon’s surface, coating part of it in smooth, young plains and erasing all the stuff that was there before. Younger crust, explains The Planetary Society’s Emily Lakdawalla, would be denser and thus sink below the older crust, explaining the elevation differences between Charon’s halves.

See? Icy volcanism at the solar system’s edge is way more interesting than imaginary cosmic giants, and perhaps not that uncommon on larger worlds in the Kuiper Belt.

If that did happen, it’s not the only violent event in Charon’s history. After all, the Pluto system was born from a giant impact — a collision that knocked proto-Charon into proto-Pluto and chucked out all kinds of icy shards that ended up forming the smaller moons. How the system evolved into the spectacularly diverse cast of characters we see today — paintballed Pluto, fractured Charon, super-dark Kerberos, odd-looking Nix, and so on — is still a mystery.

*Thank you to Emily Lakdawalla and Sarah Hörst for helpful discussions.*