New research suggests that Pluto may have acquired its most massive moon, Charon, through an ancient grazing impact, which the science team refers to as a “kiss and capture”.
The study uses computer models to suggest a possible new method by which large bodies in the Kuiper Belt could come into orbit of one another. It was led by C. Adeene Denton, a NASA postdoctoral fellow at the Southwest Research Institute, and published today in the journal Nature Geoscience.
A reassessment of strength
Pluto and Charon hold a unique place in the hearts and minds of scientists and the public. Discovered in 1930, Pluto was identified as the ninth planet until reclassified as a dwarf planet in 2006 following the landmark discovery of several similarly sized trans-Neptunian Objects or TNOs, which orbit beyond Neptune in the distant reaches of the outer solar system.
One of Pluto’s most unusual aspects is its huge moon Charon, which is roughly 12 percent as massive as Pluto. That might not seem like a lot, but for comparison, our Moon — which is also considered somewhat large compared to its host planet — is only about 1.2 percent as massive as the Earth. In fact, Charon is so large compared to its host world that it and Pluto actually orbit a common center of mass (or “barycenter”) that is outside the surface of Pluto itself. This peculiar mass ratio was part of the inspiration for Denton and her team’s research.
Charon also has an abnormally circular orbit of Pluto, with its orbital eccentricity (a value between 0 and 1, where 0 is a perfect circle and 1 is an open-ended parabola) only being about 0.000161.
Scientists have thought that this system arose in a way similar to the Earth and Moon, with a massive impactor striking the young Pluto in the Solar System’s ancient past, thereby casting off a large field of ice and rock debris which subsequently coalesced into the proto-Charon. The problem with this theory is that the estimated velocities and known masses of the bodies don’t quite add up in simulations. “Pluto isn’t massive enough to capture Charon through a normal mechanism,” says Denton of the old models.
But Denton and her team’s code more accurately models the strength of the materials in the bodies. and consider how this element could affect the dynamics of the collision and what came after. “Previous simulations had Pluto and Charon hitting each other and assumed that both bodies are basically fluids,” says Denton. She compares the way these old models described the Pluto-Charon collision as being similar to the way hot wax flows around inside a lava lamp, with the angular momentum of the impact easily imparted to the vaguely fluid bodies.
“Implementing strength means adding another layer that says, ‘Okay, I want you to behave like you’re made of rock and ice, instead of behaving like a fluid.’ And we can do that because we have laboratory measurements of how strong rock and ice are,” says Denton. “Strength is just the amount of force you can apply to a material before it starts to deform, so by adding in a strength model, we allow Pluto and Charon to maintain a level of structural integrity that’s realistic for geological bodies.”
Once this information is plugged in, the results suggest that the proto-Charon may have collided with Pluto sometime in the ancient past, then briefly merged with the larger parent body, before being cast off and pulled into its current highly circular orbit by angular forces imparted by the collision. Denton and her team call this method “kiss-and-capture,” as it involves a brief period in which the two bodies were in contact. Other trans-Neptunian worlds can be currently seen in a similar state, called a contact binary, like 486958 Arrokoth.
A heated kiss?
This research and the results it provides are exciting in part because they point to a path forward for future study of the Pluto-Charon system and other bodies throughout the Kuiper Belt. The trans-Neptunian object 90482 Orcus (which many scientists consider to be a dwarf planet) and its moon Vanth, for example, share a similar mass ratio (Vanth is 14 percent as massive as Orcus). And a number of other TNOs have moons of great size in comparison to their parent worlds.
Denton says her team’s findings could help explain why Pluto and Charon have surprisingly amounts of geological activity despite their tiny stature and their immense distance from the Sun.
“One of the interesting things about the collision we modeled is that it imparts a lot of heat to the system,” says Denton. “It bumps the internal heat up by around 100 [degrees Celsius (180 degrees Fahrenheit)],” she says. “For ice, it could make a big difference.”
With this in mind, it seems likely that there are still plenty of big discoveries left in store regarding these small but mighty worlds of the outer solar system.
