Two of Pluto’s midsize moons may be made of the guts of its largest moon, Charon, new research suggests.
New observations with the James Webb Space Telescope (JWST) show that the two moons, Nix and Hydra, are more similar to the interior of Charon than to other objects in the surrounding Kuiper Belt. The findings hint that the midsize satellites may be composed of the bits of Charon’s interior that were ejected during the rough collision that formed Pluto and Charon.
According to previous simulations, Pluto and Charon formed by a “kiss-and-capture” process. The pair’s progenitors engaged in a rendezvous that stripped off the outer layers of proto-Charon to create a disk of icy debris. That debris later came together to form at least four smaller moons: Nix, Hydra, Kerberos and Styx, according to this hypothesis.
JWST has provided the opportunity to investigate that idea. Earlier this year, researchers used the space telescope to classify the colors of trans-Neptunian objects (TNOs) — objects beyond the orbit of the eighth planet — in the outer solar system. But the new research shows that Nix and Hydra are not well matched to any of those classifications.
According to Brian Holler, a planetary scientist at the Space Telescope Science Institute, the mismatch is caused primarily by reddish material on the surface of the moons, likely something containing carbon. Holler presented his new findings at the “Progress in Understanding Pluto: 10 Years After Flyby” conference in July in Laurel, Maryland.
“This surface type appears to be largely unique in the trans-Neptunian region,” he told Space.com by email.
In fact, Charon’s ancient interior may be falling back onto the surface of Charon today. As Nix and Hydra are hit by small meteorites, debris from their surfaces may be flung into space due to their small mass and low gravity. The ejecta can then be captured by Charon’s gravitational pull and then fall to its surface.
“We may be looking at the original material of the proto-Charon,” Holler said in his presentation in July. Ejecta from Nix and Hydra may fall back onto the surface to create a centimeters-thick layer of dust over time. Because Charon is a geologically dead world, that dust remains on the surface, creating a layer similar to the crust and upper mantle that were stripped off in the collision that birthed it.
“We might be seeing Charon today as it was [before the collision],” Holler said.
A well-timed glimpse
When NASA’s New Horizons spacecraft flew by the Pluto system in 2015, it caught a tantalizing glimpse of Nix and Hydra. That included compositional data for the pair. Enhanced color images of Nix revealed a red-tinted bull’s-eye pattern. Both moons are irregularly shaped.
JWST’s observations didn’t necessarily reveal significant detail about the satellites, Holler said. Instead, they provided the ability to directly compare them with other TNOs. The space telescope was focused on studying Pluto and Charon. “The detections of Nix and Hydra are completely serendipitous,” he said.
The previous study that used JWST to classify TNOs filled a gap between the visible and near-infrared wavelengths captured by New Horizons. This allowed a direct comparison between Pluto’s midsize moons and other outer solar system objects “that would otherwise not have been possible,” Holler said.
Nix and Hydra themselves are likely to be relatively unaltered since their birth. They are too small to host active geologic processes, so the only changes in their surface likely would have come from impacts and space weathering.
“Overall, I believe the surfaces of Nix and Hydra are as close to unaltered as you can get,” Holler said.
Holler hopes to use JWST to make more in-depth observations of the pair.
“The obvious next step after an imaging appetizer is to order a spectroscopic entree,” Holler said. Such observations would pry into the composition of the moons and may answer questions about the ammonia-rich material on the surface. Ammonia is efficiently destroyed by solar radiation and cosmic rays, so either something is replenishing the molecule on Nix or there is some sort of balance in its destruction and re-formation, Holler said.
By looking at similar systems, researchers may get a better understanding of what’s going on in the two minor moons. If other TNOs have undergone similar impacts that created their own minor satellites and those compositions could be studied, it could provide even more insight into Pluto’s small-but-mysterious moons.