Researchers crack open secret of how cracks form on Pluto’s moon Charon

This time in 2015, a spacecraft about the size of a hatchback was 5 billion kilometres away on its closest approach to only ex-planet in our Solar System.

It seems even eight years later; scientists can still wring data out of the New Horizons’ flyby. New models have suggested that large cracks on Pluto’s moon, Charon, may have been caused by the internal oceans freezing and expanding – like a full water bottle put in the freezer.

“A combination of geological interpretations and thermal-orbital evolution models implies that Charon had a subsurface liquid ocean that eventually froze,” said Southwest Research Institute astrogeophysicist Dr Alyssa Rhoden.

“When an internal ocean freezes, it expands, creating large stresses in its icy shell and pressurizing the water below. We suspected this was the source of Charon’s large canyons and cryovolcanic flows.”

By the way – it’s worth noting that although Charon is known as Pluto’s largest moon, it’s very big comparably, around half the size of Pluto. Because it’s so close, and so big, the two are sometimes referred to as a ‘double dwarf planet system’, because both orbit each other.

But back to Charon’s scars – these far-reaching cracks on other worlds are called ‘chasmas’, and scientist already had a pretty good idea that oceans inside the moon were caused by expanding ice.

Charon chasmas. 2023
Charon with two chasmas highlighted. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

However, the new research has suggested that current models of Charon’s icy shell are far too thick to be cracked when frozen. And if the icy shell is too thin, it would have melted and not left those chasmas on the surface.

“Either Charon’s ice shell was less than 6 miles (10 km) thick when the flows occurred, as opposed to the more than 60 miles or 100 km indicated, or the surface was not in direct communication with the ocean as part of the eruptive process,” Rhoden said.

“If Charon’s ice shell had been thin enough to be fully cracked, it would imply substantially more ocean freezing than is indicated by the canyons identified on Charon’s encounter hemisphere.”

Read more: Pluto’s friend Charon has a red hat – now we think we know why

With just a 600-kilometre radius, a 100-kilometre shell of ice would be incredibly thick indeed.

However, the models in the new study have suggested that ocean freezing can still cause the deep fractures seen on the chasmas, but without freezing the inner layers of the ocean.

This – the team think – could have been what happened.

“Here, we identify the conditions in which a freezing ocean could create fractures that fully penetrate its ice shell, linking Charon’s surface with its ocean and facilitating ocean-sourced cryovolcanism,” they write in their new paper.

“We also find that ocean freezing can easily generate deep fractures that do not fully penetrate to the ocean, which may be the foundation of Charon’s canyons.”

As usual though, more research is needed to be properly sure. Rhoden suggests that if other large features – like cryovolcanos – could be found on the unimaged side of Charon, the research could be confirmed.

However, with New Horizons well on its way to leaving the Solar System, and with only vague plans to send another spacecraft, it might be a while away yet.

The research has been published in Icarus.

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