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Here's How Pluto and Charon Became a Bizarre Double Planet
On July 14th, 2015, the New Horizons probe made history by accomplishing the first flyby of Pluto and its largest satellite, Charon. The stunning images this mission took of these icy worlds have helped scientists address some of the key questions about Pluto and its massive moon, which have been shrouded in mystery for decades (owing to their great distance from Earth). One of the biggest mysteries that scientists have contemplated since Charon was first discovered in 1978 is how it came together with Pluto in the first place.
For decades, astronomers suspected that Pluto and Charon formed through a process similar to Earth and the Moon. This theory, known as the Giant Impact Hypothesis, states that roughly 4.5 billion years ago, primordial Earth was struck by a Mars-sized body named Theia. In a new study, a team of researchers from the University of Arizona challenged this assumption and offered an alternate theory known as “kiss and capture.” Their findings could help scientists better understand how planetary bodies in the outer Solar System form and evolve.
The study was led by Adeene Denton, a NASA postdoctoral fellow at the University of Arizona’s Lunar and Planetary Laboratory and the Southwest Research Institute (SwRI). She was joined by Erik Asphaug, a Planetary Science Professor in the School of Earth and Space Exploration (SESE) and the Lunar and Planetary Laboratory (LPL) at the University of Arizona; Robert Melikyan, an LPL Graduate Student, and Alexandre Emsenhuber, a Postdoctoral Researcher from the Space Research and Planetary Science (SRPS) at the University of Bern. The paper that describes their findings, “Capture of an Ancient Charon around Pluto,” was published in the journal Nature Geoscience.
Previously, scientists believed that Pluto and Charon formed from a massive collision, similar to the Giant Impact Hypothesis. According to this theory, a Mars-sized planet named Theia collided with a primordial Earth roughly 4.5 billion years ago. This impact turned both bodies into molten debris that eventually coalesced to form the Earth and Moon, eventually settling into the Earth-Moon system. According to the team’s study, this theory does not fit when it comes to Pluto and Charon because it fails to take into account the structural strength of cold, icy worlds.
Using the University of Arizona’s high-performance computing cluster, the team conducted advanced impact simulations. This showed that when Pluto and a proto-Charon collided, they became temporarily stuck together and formed a single snowman-shaped object – not unlike Arrokoth, the first Kuiper Belt Object (KBO) that New Horizons surveyed on December 31st, 2018. Over time, they separated to become the binary system we observe there today. Said Denton in a U of A News story:
“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. 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.”
Their results also suggest that Pluto and Charon remained largely intact during their collision and retained much of their original composition. This challenges previous models that suggest that colliding bodies will exchange material during the impact. This is based on studies of the Apollo moonrocks, which indicated that the Earth and Moon are similar in composition, a finding that led scientists to conclude that the Earth-Moon system formed together. What’s more, their research offers a potential explanation for how Pluto may have developed an internal ocean.
View from the surface of Pluto, showing its large moon Charon in the distance. Credit: New York Times
The collision process, they state, combined with the tidal friction caused by the separation of Pluto and Charon, would have caused considerable internal heating for both bodies. This could have provided the necessary mechanism for creating a subsurface ocean, contrary to a previous theory where scientists have argued that Pluto formed during the very early Solar System when there were far more radioactive elements. However, scientists have expressed doubts about this theory because of the timing constraints it imposes.
Denton and her colleagues are now planning follow-up studies to explore several related questions about this system of icy bodies. This includes how tidal forces influenced Pluto and Charon’s early evolution when they were much closer together, how this formation scenario aligns with Pluto’s current geological features, and whether similar processes could explain the formation of other binary systems. Said Denton:
“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.”
Further Reading: University of Arizona, Nature
The post Here's How Pluto and Charon Became a Bizarre Double Planet appeared first on Universe Today.
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