Pluto’s Small Moons Are Unlike Any Other

The solar system past Neptune is filled with an uncounted number of small, unusual worlds, from barely visible specks of ice to sugar-coated snowmen to Pluto and its five known satellites. These trans-Neptunian objects (TNOs) are the icy leftovers of planet formation and provide a glimpse into the early composition and evolution of the solar system.

Recently, astronomers observed the Pluto system with the James Webb Space Telescope (JWST) and discovered that two of Pluto’s small moons, Nix and Hydra, have surface compositions unlike any TNO studied thus far.

The moons have abundant water like distant dwarf planet Haumea, ammonia like Pluto, and reddish material like Pluto’s major moon, Charon, explained Bryan Holler, a planetary scientist at the Space Telescope Science Institute in Baltimore, Md. This blend of surface chemistry has not been seen anywhere else.

“What is going on here? What is causing these objects to have these surface compositions that are unique in the outer solar system?” he asked. Answering these questions could reveal the mysterious and likely chaotic history of the Pluto system.

Outer Solar System Outliers

Nix and Hydra were discovered in 2005 as astronomers prepared for the launch of NASA’s New Horizons spacecraft, which flew through the Pluto system in 2015. Nix and Hydra, as well as the smaller moons Styx and Kerberos, orbit Pluto and its major moon, Charon.

Planetary scientists believe that a collision early in Pluto’s history resulted in its satellite system, in the same way that a collision formed Earth’s Moon, Holler explained. Spectroscopic data collected by New Horizons support that theory: Pluto and its moons share some similarities in their surface chemistry, such as being rich in water and ammonia.

But with limited flyby time and most of its attention on Pluto and Charon, New Horizons didn’t reveal much more about the minor moons beyond their shapes, orbits, and basic surface composition. After JWST launched in 2021, astronomers turned its infrared imaging capabilities to the system.

“The idea was to mostly look at Pluto and Charon, but it turns out that in these 90-second images, you actually also get Nix and Hydra,” Holler said, though Styx and Kerberos were too faint to see. “I consider this project bonus science,” he added.

These infrared observations allowed the researchers to better compare Nix’s and Hydra’s compositions to those of other TNOs. Previous research  found that most TNOs are largely unchanged since the birth of the solar system and are rich in either water and silicates, carbon dioxide, or reddish organic material. A fourth category describes objects that underwent a catastrophic collision that altered their original composition and are extremely water rich—objects like the dwarf planet Haumea.

Nix and Hydra don’t fit into any of these groups.

“Nix and Hydra actually look like the Haumea family at longer wavelengths,” Holler said, with abundant water on the surface. They also have ammonia like Pluto and Charon. “They diverge at the short wavelengths, though, and they become very red,” Holler said. “‘Red ice’ is how I describe them.”

This research was presented on 10 December at AGU’s Annual Meeting 2024 in Washington, D.C.

All Signs Point to a Collision

The source of Nix’s and Hydra’s red material is still unknown, Holler said. Charon got its red spots from methane shed by Pluto, but Nix and Hydra are too small to accumulate methane in the same way.

“There’s something else going on here,” he said. “Maybe that red color is related to the material that was present in the [postcollision] disk that formed the minor satellites. Even within the same small TNO system, we see two different origins for red material.”

This research “confirm[s] the uniqueness of the Pluto system from the compositional standpoint,” said Cristina Dalle Ore, an astronomer at the SETI Institute and NASA Ames Research Center in Mountain View, Calif., who was not involved with this research.

“Ammonia is an elusive and fragile molecule,” she added. “With the exception of a few objects where its presence has been suggested, ammonia is still not solidly confirmed on any TNO, a fact that makes the Pluto system stand out potentially for both its origin and evolution.”

The newly revealed chemical similarities between Nix, Hydra, and Charon are consistent with the leading theory that a collision created the Pluto-Charon system, according to Simon Porter, an astronomer at the Southwest Research Institute in Boulder, Colo., who was not involved with this research. Furthermore, dust from Nix and Hydra has likely been accumulating on Charon’s surface for billions of years, he said.

Anything new we learn about these tiny moons “helps to calibrate the giant impact models used to simulate the giant impact that created the Earth-Moon system, and how common that might be in the universe,” Porter said.

Nix and Hydra remain elusive targets for ground- and space-based telescopes, and another mission to Pluto, which would provide the highest-quality data, is not under consideration. Holler advised that new insights on Nyx, Hydra, and the other tiny Plutonian moons may depend on more “bonus science.”

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer

Citation: Cartier, K. M. S. (2024), Pluto’s small moons are unlike any other, Eos, 105, https://doi.org/10.1029/2024EO240527. Published on 10 December 2024.

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