The story behind Pluto's huge moon bodes well for distant ocean worlds

Unlike how scientists believe Earth's moon formed billions of years ago, Pluto and its biggest moon, Charon, didn't have a messy breakup.  

New computer simulations show the primitive dwarf planet and the object that struck it likely had an unforeseen kind of cosmic collision. Scientists usually classify planetary crashes as either hit-and-runs or graze-and-merges: One planet or rock swipes another and then keeps on trucking, or a thing smacks into another thing, and they mix together as one. 

But what a NASA postdoctoral fellow at the Southwestern Research Institute found was something quite different — a so-called "kiss-and-capture" scenario. 

When Pluto and Charon hit, they may have stuck together, rotating through space as one unit until they pushed against each other, according to a new study, sending the moon into a stable orbit. Neither would have lost too much of its original material in the process. 

The incident could have created enough heat for Pluto to form an underground ocean, Adeene Denton, the lead researcher, told Mashable. It's an intriguing implication, supporting existing predictions that Pluto is hiding water under its icy shell. These findings were published in the journal Nature Geoscience.

NASA's New Horizons spacecraft captured Pluto and its biggest moon, Charon, together. Credit: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute

Since New Horizons' close encounter with Pluto 10 years ago, experts have come to think of the dwarf planet as much more scientifically valuable. Rather than a cold, featureless ball on the fringes of the solar system, the spacecraft images revealed a geologically diverse world, with mountains, ice sheets, pits, cliffs, cracks, and valleys.

Charon, its biggest of five moons, was discovered in 1978 by the U.S. Naval Observatory. At about 750 miles wide, it's half the size of Pluto — extremely large for a moon.

In previous models, Charon formed in a similar fashion to Earth's moon: The theory goes that a Mars-sized planet whacked a primitive version of Earth like a paint ball, casting off a mixture of planetary guts. Rather than forming a disk of shattered debris, though, it morphed into two fluid blobs that yo-yoed material between them. The gravity of Earth eventually hurled the smaller blob onward, becoming Earth's sole stable moon. 

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But computer simulation advancements allowed Denton's team to include the structural strength of rock and ice, the primary materials of Pluto and other stuff in the Kuiper Belt, a disk beyond Neptune of comets and tiny ice worlds. That made all the difference, Denton said. The simulation showed that Pluto and its impactor didn't merge, lose a lot of material to the solar system, or become fluid blobs. 

Researchers now wonder if other objects in this brutally cold region have had kiss-and-capture collisions, too, based on the large sizes of orbiting moons and moonlets out there compared to other parts of the solar system. Given that kiss-and-captures could provide a way to add extra heat into the equation, that might also mean other distant objects have developed underground oceans over eons. 

"Eight out of 10 of the largest [Kuiper Belt objects] have a large mass fraction satellite like Charon," Denton said in an email. "This process might have operated all over the outer solar system early in its history."