Evrim Yazgin
Cosmos science journalist
Pluto and Charon sitting in a tree,
K-I-S-S-I-N-G!
New research challenges decades of assumptions about how Pluto and its largest moon Charon formed on the fringes of our solar system.
The new theory could shed light on how other planetary bodies form and evolve.
Scientists have long believed that Pluto’s unusually large moon formed in a process similar to Earth’s moon – a massive collision between two planetary bodies. Like Pluto, Earth’s moon is large suggesting that it formed through a collision, unlike other moons in the solar system which were likely drawn to the gravity of much larger host planets.
The story goes that a Mars-sized object named Theia smashed into Earth about 4.5 billion years ago. While remnants of Theia may remain in Earth’s mantle, crust and even oceans, a chunk is also believed to have formed our moon.
After the Earth-Theia collision, extreme heat and tidal forces would have meant the planetary bodies deformed and stretched like fluids.
This “giant-impact” hypothesis works well to explain the Earth-moon system.
“Pluto and Charon are different – they’re smaller, colder and made primarily of rock and ice,” says Adeene Denton, a NASA postdoctoral fellow and study lead on the research published in the journal Nature Geoscience.
“When we accounted for the actual strength of these materials, we discovered something completely unexpected,” Denton adds.
Denton’s team used the high-performance computing cluster at the University of Arizona, USA, to simulate the collision event which could have formed the Pluto and proto-Charon system.
They found that, instead of stretching like putty during the collision, the two icy bodies temporarily stuck together to form a snowman-like structure. They then separated into the binary system we see today.
“Most planetary collision scenarios are classified as ‘hit and run’ or ‘graze and merge’,” says Denton. “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.”
“The compelling thing about this study, is that the model parameters that work to capture Charon, end up putting it in the right orbit,” adds senior author Erik Asphaug, a professor at the University of Arizona’s Lunar and Planetary Laboratory. “You get two things right for the price of one.”
Unlike the violent birth of the Earth-moon system, the new simulation suggests that Pluto and Charon remained mostly intact during their collision. Most of their original composition would have been preserved.
“We’re particularly interested in understanding how this initial configuration affects Pluto’s geological evolution,” says Denton.
“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.”
This includes the formation of a subsurface ocean on Pluto.
