Cracks on Pluto’s moon Charon hint at a frozen subsurface ocean

A team of researchers at Southwest Research Institute (SwRI) has proposed that cracks found on the surface of Pluto’s biggest moon, Charon, may have resulted from a frozen ocean beneath its surface.

These interesting findings are based on the data NASA’s New Horizon spacecraft collected in 2015 during its visit to Pluto. The researchers at SwRI created a new model using information from New Horizon and studied the possibility of cryovolcanic activity (eruption of ice and water from volcanoes) on Charon. 

They also looked for conditions on the moon that have caused large fractures on its surface. The findings come as a shock to many scientists who previously considered Charon an ice ball and believed in theories suggesting the moon was nothing but a lump of thick ice shells.

Did Charon have water and ocean?

The researchers developed new ocean models for Pluto's moon using water, ammonia, and their mixture. The previous models of Charon’s interiors suggest that ice shells on the moon couldn’t be cracked due to the freezing of any sub-surfaced ocean. 

This is because the ice shells were too thick (about 60 miles) to get fractured. In contrast, the new model argues that ice shells were only 6 miles thick when cryovolcanoes used to erupt. There was possibly water and ocean beneath the moon’s surface.

It is possible the surface of the moon was not in contact with the ocean at that time. “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,” said Alyssa Rose Rhoden, Principal scientist at SwRI and first author of the study. 

However, the researchers argue that the cryovolcanic flows wouldn’t have lasted long because the conditions on Charon only supported the loss of heat. Even the radioactivity on Charon then was not good enough to produce any heating effect.   

“Due to the low radiogenic heating within Charon and the loss of tidal heating early in its history, a thin ice shell should have been short-lived, implying that ocean-sourced cryovolcanic flows would have ceased relatively early in Charon's history, consistent with interpretations of its surface geology,” the authors note. 

Ocean freezing may have led to the formation of deep cracks that didn’t fully penetrate but formed canyons on Pluto's moon. The cracks may have originated first in the ice shells along the tectonic belt of ridges that divide the planet’s northern and southern regions. 

Rhoden and her team suggest that further research is required to confirm these new exciting findings about Charon.

The study is published in the journal Icarus.

Study Abstract: 

A combination of geological interpretations and thermal-orbital evolution models imply that Pluto's large moon, Charon, had a subsurface water (and possibly ammonia) ocean that eventually froze. Ocean freezing generates large tensile stresses in the upper part of the ice shell and pressurizes the ocean below, perhaps leading to the formation of Charon's large canyons and putative cryovolcanic flows. 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. We find that current models of Charon's interior evolution predict ice shells that are far too thick to be fully cracked by the stresses associated with ocean freezing. Either Charon's ice shell was <10 km thick when the flows occurred (as opposed to >100 km) or the surface was not in direct communication with the ocean as part of the eruptive process. 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, Serenity and Mandjet Chasma. Due to the low radiogenic heating within Charon and the loss of tidal heating early in its history, a thin ice shell should have been short-lived, implying that ocean-sourced cryovolcanic flows would have ceased relatively early in Charon's history, consistent with interpretations of its surface geology. An additional (and perhaps implausibly large) heat source would be required to generate the substantially larger ocean implied by through-going fractures. 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.