ANALYSIS: Not a Dwarf: Is Pluto a Binary Planet?
"Once you create an ocean on an icy body, it's hard to get rid of it," said Barr. That's because as the ocean freezes, the remaining liquid portion gets enriched with salts and ammonia -- which serve as antifreeze.
Next comes the part where that ocean could have created icy tectonic plates on Pluto's surface.
"One thing that we know is the angular momentum will be conserved as the system evolved," said Barr.
With that fact, they simulated a bunch of scenarios based on where Charon's orbit was right after the collision -- since nobody actually knows where Charon started. Then in each scenario they saw Charon's orbit gradually migrate outward -- just like the moon's orbit did around Earth.
ANALYSIS: Pluto's 'Thick' Air Isn't Going Anywhere
When Pluto and Charon were closer and still hot from their collision, they pulled more forcefully on each other and were more egg-shaped as a result. But as Charon moved away, Pluto became more spherical. To change shape, the icy surface would have had to crack and create faults -- telltale signs of tectonics.