Pluto and Charon May Have Formed in a “Kiss-and-Capture” – Sky & Telescope

We used to think that the crash that built Pluto and Charon totally obliterated the impactor and built two brand-new worlds out of two old ones. But new simulations recently published in Nature Geoscience by Adeene Denton (Southwest Research Institute) and coworkers suggest that the crash was much gentler. In fact, Pluto and Charon may have been strong enough to stay true to their original characters.

Pluto and Charon vs. Earth and the Moon

Since the mid-1970s, we’ve had an answer for how Earth and the Moon formed: In the “Giant Impact” scenario, a big asteroid crashed obliquely into the molten proto-Earth, splashing molten mostly-mantle material into orbit that eventually coalesced into the modern Moon.

Like the Earth-Moon pair, Pluto and Charon spin too quickly to have just formed as they are. And Charon is much too large for Pluto’s gravity simply to have captured it into orbit. It seems plausible that the mechanism that formed Earth and the Moon could also explain Pluto and Charon as well as other big binaries in the outer solar system.

Indeed, geophysicists have successfully simulated the formation of Pluto and Charon by smashing computer-modeled molten proto-worlds together. Only about a third of the mass of proto-Charon survives the crash; the rest splashes in all directions, escaping the system. Needless to say, a third of a formerly molten Charon would not bear much similarity to the original world.

But while working with Erik Asphaug at the Lunar and Planetary Institute, Denton and fellow postdoctoral researcher Alexandre Emsenhuber found that the Pluto-Charon formation story might not be the same as for Earth and the Moon.

At issue is the physical state of the worlds before and during impact. Earth would have been quite hot at the time of the giant impact — hot enough for much more of its interior to be molten than it is today. And the high speed of the collision would have melted or even vaporized a huge chunk of the outer part of the planet. Because the material is molten, computational modelers can make the simplifying assumption that individual fragments of material blowing out of the crash zone don’t stick to each other. In physical terms, the impact ejecta are cohesionless, without strength.

However, there’s no reason to assume that the small worlds beyond Neptune started out molten. While many of the larger ones may have developed (and may also have lost) subsurface oceans over time, they would have formed quite cold, with thick, rigid crusts of never-melted ice.

On top of that, impacts between worlds happen at much lower speeds in the outer solar system than in Earth’s neighborhood. New Horizons’ flyby of the snowman-shape Arrokoth revealed that small trans-Neptunian objects assembled gently through low-speed collisions of smaller components. These pieces stuck together while hardly changing their shapes.

So, the researchers wondered, what would happen if the computer simulations treated the oblique impact of two big worlds as two cold bodies made of solid material, rather than as liquid spheres?

Kiss-and-Capture

Work led by Emsenhuber, published in 2024, showed that impacts involving objects with masses more than a few percent of Earth’s could be treated as though they were liquid — even if both bodies started out solid — because of the enormous energy involved. But things changed dramatically at lower masses. For a world the size of Pluto, which has 0.2% the mass of Earth, strength would be very important.

The newest study, led by Denton, examined what incorporating the physics of material strength does for the formation of the Pluto-Charon system. “When we assume Pluto and Charon behave like geologic bodies rather than fluids, everything changes,” she says. “Pluto and Charon form like a super Arrokoth for a minute.”

In other words, on impact, the two worlds smush together, bounce slightly apart, and then re-smush. Eventually, they stick together like two snowballs in an Arrokoth-like contact binary.

What happens next in the simulation depends on proto-Pluto’s rotation: If it spins relatively slowly, the two bodies stay smushed together, but sufficiently fast rotation can tear the briefly merged bodies apart again.

This new scenario, which Denton and coworkers describe as “kiss-and-capture,” puts what’s left of proto-Charon into orbit around Pluto, with the original bodies’ cores mostly intact in the present ones. While the present-day surfaces of both worlds would reflect the impact’s dramatic geological effects, the Pluto and Charon would retain any bulk differences in their original chemical compositions.

The new scenario could explain a puzzle about Pluto’s present-day geologic activity. Denton explained that earlier work had suggested Pluto must have formed unusually warm, because otherwise “[its] surface would be covered in contractional features that we don’t see.” With kiss-and-capture, Pluto could still have started out cold and solid; post-impact resurfacing could have buried and obliterated the primordial contractional features.

“Heat from the impact, followed by tidal heating as Charon moves outward [in its orbit around Pluto], provides the extra heat that Pluto didn’t have from the start” to drive the modern geology.

Not all impacts produce kiss-and-capture; in fact, only some of them do. Impacts that occur with a higher angle between the two bodies’s trajectories produce “hit-and-run” scenarios, with no orbit capture. Low impact angles and/or low spin rates merge the two worlds into one. But, the researchers demonstrated, the right combination of medium impact angle and moderately fast spin rate reproduce not only the Pluto-Charon system but also other big, outer solar system binaries, such as Eris-Dysnomia and Orcus-Vanth.

What cool geology might we find if we visit Eris and Orcus? There’s only one way to find out.