A combination of geological interpretations and thermal-orbital evolution models implies that Pluto’s large moon, Charon, had a subsurface water ocean that eventually froze. Ocean freezing causes great tensile stresses in the upper part of the ice sheet. It pressurizes the ocean below, perhaps forming Charon’s great canyons and putative cryovolcanic flows.

In a new study, scientists at the Southwest Research Institute have modeled the formation of fractures in Charon’s ice sheet as the ocean below freezes to investigate the evolution of Charon’s interior and surface.

Using the data from NASA’s New Horizons spacecraft, which encountered the Pluto-Charon system in 2015, scientists created a model to investigate the source of cryovolcanic flows and a clear belt of fractures on Pluto’s large moon Charon. The model suggested that when the moon’s internal ocean froze, it may have formed the deep, elongated depressions along its periphery, but less likely led to the eruption of cryovolcanoes containing ice, water and other materials in the Northern Hemisphere.

SwRI’s Dr. Alyssa Rhoden, a specialist in the geophysics of icy satellites, said: “When an internal ocean freezes, it expands, creating great stresses in the icy shell and pressurizing the water below. We suspected that this was the source of Charon’s great canyons and cryovolcanic flows.”

The fluid, compressed by the increased volume of the newly formed ice, can be pushed through the fractures to erupt on the surface as fractures pierce the entire ice sheet and drain the deep ocean. The goal of the models was to determine the conditions that could lead to fractures that completely breach Charon’s icy shell and connect the surface and subsurface waters to allow for cryovolcanoes from the ocean.

However, the ice sheets on Charon were far too thick to be completely cracked by the forces caused by the freezing of the ocean, according to current models of Charon’s internal evolution.

Another crucial factor is when the ocean freezes. Tidal warming only occurred during the solar system’s first million years because the synchronous and circular orbits of Pluto and Charon stabilized very early.

Rhoden said, “Either Charon’s ice cover was less than 10 km thick when the flow occurred, as opposed to the stated more than 60 miles or 100 km, or the surface was not directly connected to the ocean as part of the eruption process. If Charon’s ice cover had been thin enough to burst completely, it would mean that the ocean would freeze significantly more than indicated by the canyons identified in Charon’s meeting hemisphere.

These canyons may have been created by cracks in the ice sheet along the ridges that make up the global tectonic belt that runs across Charon’s face and separates the moon’s northern and southern geological regions. If further major expansion features were identified in the hemisphere that are not imaged by New Horizons, or if compositional analysis could prove that Charon’s cryovolcanism originated in the ocean, this would support the idea that the ocean was significantly thicker than expected .

Rhoden said, “Ocean freezing also predicts a sequence of geological activity, with ocean-induced cryovolcanism ceasing before stress-induced tectonism. A more detailed analysis of Charon’s geological record could help determine whether such a scenario is viable.”

Magazine reference:

  1. Alyssa Rose Rhoden, Maxwell L. Rudolph, Michael Manga. The challenges of expelling Charon’s cryovolcanism from an icy ocean. Icarus, 2023; 392: 115391 DOI: 10.1016/j.icarus.2022.115391