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Recent studies indicate that billions of years ago, Pluto might have entraped its largest moon, Charon, through a fleeting icy “kiss.” This hypothesis could clarify how the dwarf planet (indeed, we also wish Pluto retained its planetary status) managed to capture a moon nearly half its own size.
The researchers behind this study propose that two icy celestial bodies in the Kuiper Belt, a region of frozen entities located at the far reaches of the solar system, collided billions of years ago. Rather than completely annihilating each other, these entities fused together as a rotating “cosmic snowman.” They split apart relatively fast but continued to be gravitationally linked, leading to the Pluto/Charon system we observe today.
This “kiss and capture” concept introduces a novel theory of moon acquisition and cosmic interaction. It might also aid scientists in further examining the structural integrity of cold, icy worlds within the Kuiper Belt.
“We’ve discovered that if we treat Pluto and Charon as entities with material resilience, Pluto can indeed capture Charon following a substantial impact,” stated Adeene Denton, the lead researcher and a lunar and planetary scientist at the University of Arizona, while speaking to Space.com. “The mechanism behind this collisional acquisition is termed ‘kiss-and-capture’ because Pluto and Charon temporarily merge, the ‘kiss’ aspect, before separating to form two distinct entities.”
The majority of planetary collision theories are categorized as “hit and run” or “graze and merge,” which makes this “kiss and capture” model notably different.
“We were definitely astonished by the ‘kiss’ element of kiss-and-capture,” Denton continued. “There hasn’t previously been an impact of this type where the two bodies only briefly unite before re-separating!”
The team’s findings were published on Monday (Jan. 6) in the journal Nature Geoscience.
Pluto won Charon over with a 10 hour kiss
The complexity of Pluto’s relationship with Charon has puzzled scientists due to the relatively minor size and mass differences between the two icy entities.
“Charon is massive in relation to Pluto, to the extent that they form a binary system,” Denton clarified. “It is half the size of Pluto and 12% of its mass, making it significantly more comparable to Earth’s moon than any other moon in the solar system.”
In comparison, our moon is merely a quarter the size of Earth, while the largest moon in the solar system, Ganymede, is approximately 1/28 the size of its parent planet, Jupiter.
The University of Arizona researcher, who is also a NASA postdoctoral fellow, added that acquiring such a relatively large moon through a “normal” method is challenging. (“Normal” refers to the gravitational capture of moons like Mars’ moons Phobos and Deimos and the moons of the giant planets Jupiter and Saturn.)
This implies that the dominant explanation for the formation of the Pluto and Charon system relies on the idea of collisional capture, akin to the theory that a considerable body crashed into Earth, ejecting material that eventually formed our moon.
“An impactful event strikes Pluto, resulting in Charon, but similar to the Earth-moon system, we do not completely understand the mechanics or conditions that lead to this,” Denton commented. “It’s a significant question since many other large Kuiper Belt Objects also possess sizable moons, suggesting that this phenomenon might occur with some regularity in the Kuiper Belt, yet we remain uncertain about its mechanisms or reasons.”
During a conventional “collision capture,” a significant impact occurs, leading both bodies to stretch and deform in a fluid-like manner. This mechanism effectively illustrates the formation of the Earth/moon system due to the intense heating caused by the collision and the larger mass of the involved bodies, leading them to behave fluidly.
When examining Pluto and Charon through a collision capture lens, an additional factor must be considered: the structural resilience of the colder icy and rocky bodies. This factor has often been overlooked in past evaluations concerning the collisional genesis of Charon.
To incorporate this into their simulations, the team utilized the University of Arizona’s high-performance computing cluster. Upon including the material strength in their simulations, the researchers observed totally unexpected results.
“Since both entities possess material strength, Charon did not penetrate Pluto deeply enough to merge; this doesn’t hold when the entities exhibit fluid characteristics,” Denton outlined. “Under equivalent impact conditions, assuming Pluto and Charon lack strength, they do merge into a single entity and Charon gets absorbed. However, with material strength, both Pluto and Charon maintain structural integrity during their brief union.”
Due to Charon’s inability to sink into Pluto in this scenario, it remained outside the so-called “co-rotation radius” of both entities. Consequently, it could not rotate as quickly as Pluto, preventing the two bodies from remaining merged. As they separated following this icy kiss, the researchers theorize that Pluto would have pulled Charon into a closer, higher, circular orbit from which the moon would subsequently migrate outward.
“The ‘kiss’ in this kiss-and-capture, essentially the merger, is extremely brief when viewed from a geological perspective, lasting approximately 10 to 15 hours before both bodies part ways,” Denton explained. “Charon then initiates its slow outward journey toward its current location.”
The team hypothesizes that the initial impact occurred early in the history of the solar system, likely tens of millions of years after its formation, which would place it billions of years ago.
“Standard large collisions often result in clear mergers, where bodies fuse, or both bodies remain distinct,” Denton remarked. “Thus, this scenario was quite novel for us and raised several intriguing geological questions we wish to explore further, since the viability of kiss-and-capture is contingent on Pluto’s thermal state, which can likewise be linked to its current geological conditions for verification.
“I aim to establish how the original Pluto-Charon impact might influence whether and how both develop oceans.”
Denton indicated that the team has two paths to pursue in enhancing this research.
“The initial path involves examining how this theory may apply to other sizable Kuiper Belt Objects with substantial moons, such as Eris and Dysnomia, Orcus and Vanth, and others,” Denton elaborated. “Our preliminary analysis indicates that kiss-and-capture could also be responsible for these other systems; however, given their varied compositions and masses, it is crucial to understand how kiss-and-capture may have functioned throughout the Kuiper Belt.”
The second path the team intends to follow involves investigating the long-term tidal evolution of Charon to substantiate their formation hypothesis.
“To confirm that this process formed Pluto and Charon, we must ensure that Charon migrates to its current position, located approximately 8 times the width of Pluto away,” Denton asserted. “However, that process spans significantly longer timeframes than the original collision, making our models inadequate for tracking it.
“We plan to delve deeper into this aspect in the future to clarify the conditions that not only reproduce Pluto and Charon as separate entities but also accurately position Charon where it exists today.”
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