Researchers have discovered that Pluto’s moon Charon likely experienced a significant despinning
event early in its history, with its rotation slowing from approximately 14.3 hours to its current 153.3-hour tidally locked state. The findings, published in Nature Communications, provide new geological evidence for the moon’s cold, rigid evolution.
Tectonic Evidence in Oz Terra
Planetary scientists have theorized that rotating celestial bodies undergo a process called despinning, where tidal forces gradually slow their rotation and alter their physical shape. While this has been a staple of orbital mechanics, clear geological evidence on the surfaces of moons has remained elusive. A new study led by Hanzhang Chen, a researcher affiliated with the University of California, Los Angeles and ETH Zurich, identifies such a record within the mountain ranges of Oz Terra, a region in Charon’s northern hemisphere.
By analyzing high-resolution data from NASA’s 2015 New Horizons flyby, the research team identified large, arcuate mountain ranges that contradict earlier theories of global expansion. While previous studies suggested these features were caused by cryovolcanism or crustal extension, the new analysis points to compression. Charon exhibits a topographic dichotomy of rugged northern highlands and smoother southern plains,
Chen noted, explaining that the asymmetric slopes of these ridges are consistent with thrust-fault mountain belts rather than extensional troughs.
Modeling a Cold Start for Charon
The team’s modeling suggests that as Charon’s rotation slowed, the crust near the equator shortened by approximately 1%. This process created a distinct pattern of north-south trending compressional faults near the equator and east-west extensional features at the poles. According to ETH Zurich’s reporting on the findings, these patterns provide observational evidence that planetary despinning leaves a lasting tectonic signature.
The study also challenges assumptions regarding the moon’s thermal history. The modeling suggests that Charon possessed an ice shell between 30 and 36 kilometers thick at the time of formation. This supports a cold start
hypothesis, indicating that Charon likely formed in a relatively cold state.
“Our work suggests that Charon’s surface presents an example that records the planetary despinning history, which predates the proposed global extension and cryovolcanism on Charon. The coevolution of despinning and global contraction favors a cold start for Charon, offering insights into the early thermal evolution of icy satellites in the outer Solar System.”
Hanzhang Chen et al., via Nature Communications
The Kiss and Capture
Formation Theory
The mechanisms behind Charon’s initial orbit have been a subject of intense academic debate. Led by Adeene Denton of the University of Arizona, this research proposes a kiss and capture
scenario.
Unlike traditional models that assume colliding planetary bodies behave like fluids, the kiss and capture
theory accounts for the structural strength of rock and ice. In these simulations, Pluto and the proto-Charon collided and remained briefly joined in a snowman
shape for 10 to 15 hours before separating, yet remained gravitationally bound. Most planetary collision scenarios are classified as ‘hit and run’ or ‘graze and merge.’ What we’ve discovered is something entirely different—a ‘kiss and capture’ scenario where the bodies collide, stick together briefly and then separate while remaining gravitationally bound,
said Denton.
Comparing the Dynamics of the Pluto System
The behavior of Charon stands in contrast to Pluto’s four smaller moons—Styx, Nix, Kerberos, and Hydra. While Charon is tidally locked, rotating once per orbit, the smaller moons exhibit chaotic, high-speed rotations. As noted by planetary scientist Mark Showalter, these smaller bodies behave like spinning tops,
with Hydra completing 89 rotations during a single orbit of Pluto. This disparity highlights the complexity of the Pluto-Charon system, where gravitational tidal forces have effectively stabilized the largest moon while leaving the smaller satellites in a state of rapid, non-synchronous rotation.
Future research is expected to further integrate these findings. Scientists are now looking to connect the initial kiss and capture
impact, the subsequent tidal despinning recorded in Oz Terra, and the current, stable orbit of the binary system to create a unified history of the outer solar system’s most enigmatic dwarf planet.
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