- Scientists have long thought Pluto’s moon Charon formed like Earth’s moon, from molten material blasted out in a huge collision.
- But a new study says the process might have worked differently for the Pluto-Charon system, which is much smaller and colder than Earth and its moon.
- The researchers have instead put forward a ‘kiss and capture’ mechanism, where Charon would have collided with and stuck to Pluto before eventually separating.
The University of Arizona published this original story on January 6, 2025. Edits by EarthSky.
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‘Kiss and capture’ could explain Pluto-Charon formation
For decades, scientists have theorized that Pluto’s moon Charon formed through a process similar to Earth’s moon. That is, two worlds smashed into each other, blasting out molten debris which eventually coalesced into a moon. But now, a new study from the University of Arizona has challenged this assumption. The revised version of events starts similarly, with a collision between two icy worlds billions of years ago. But rather than destroying each other, they may have spun together like a celestial snowman, before finally separating and remaining forever linked in orbit.
By taking into account the structural strength of icy worlds, the researchers say they’ve discovered an entirely new type of cosmic collision. They’re calling it the ‘kiss and capture’ mechanism.
They published the peer-reviewed study in the journal Nature Geoscience on January 6, 2025.
Pluto and Charon NOT like our Earth and moon
Scientists’ theory for how the Earth-moon system formed relies on the immense heat and huge masses involved. This would have made the colliding bodies behave like fluids, allowing our moon to form from ejected molten material. But this may not be true when applied to the smaller, colder Pluto-Charon system. The new study argues that scientists may have overlooked a crucial factor: the structural integrity of rock and ice.
Adeene Denton, the study’s lead author, explained:
Pluto and Charon are different – they’re smaller, colder and made primarily of rock and ice. When we accounted for the actual strength of these materials, we discovered something completely unexpected.
Hit and run, graze and merge … or kiss and capture?
The researchers ran impact simulations on the University of Arizona’s high-performance computing cluster. And they found that, instead of stretching like silly putty during the collision, Pluto and the proto-Charon may have become temporarily stuck together. They would have then rotated as a single snowman-shaped object before separating into the binary system we observe today. A binary system occurs when two celestial bodies orbit around a common center of mass, much like two figure skaters spinning while holding hands.
Denton said:
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.
Senior study author Erik Asphaug added:
The compelling thing about this study, is that the model parameters that work to capture Charon end up putting it in the right orbit. You get two things right for the price of one.
Phil Plait, the “Bad Astronomer”, uploaded this simulation of the “kiss and capture” interaction. Via Denton et al.
Effect on Pluto’s geology?
The study also suggests that both Pluto and Charon remained largely intact during their collision, with much of their original compositions preserved. According to Denton, this challenges previous models that suggested extensive deformation and mixing during the impact.
Additionally, the collision process would have deposited considerable internal heat into both bodies. This may provide a mechanism for Pluto to develop its suspected subsurface ocean, the origin of which has puzzled planetary scientists.
The research team is already planning follow-up studies to explore several key areas. The team wants to investigate how tidal forces influenced Pluto and Charon’s early evolution when they were much closer together, analyze how this formation scenario aligns with Pluto’s current geological features, and examine whether similar processes could explain the formation of other binary systems.
Denton outlined:
We’re particularly interested in understanding how this initial configuration affects Pluto’s geological evolution. The heat from the impact and subsequent tidal forces could have played a crucial role in shaping the features we see on Pluto’s surface today.
Bottom line: A new study says that Pluto and its largest moon, Charon, didn’t form in a destructive collision like Earth and its moon. Instead, they may have formed through a newly discovered “kiss and capture” mechanism.
Source: Capture of an ancient Charon around Pluto
Via University of Arizona