The icy volcanism and crack belt on the surface of Pluto’s large moon Charon may have resulted from a subterranean frozen ocean breaking through a thin icy crust. The new models suggest that when Charon’s inner ocean froze, it may have formed deep, long depressions along its middle, but this could mean that the outer crust was thinner than currently thought at some point in the moon’s history. The models also suggest that cryovolcano eruptions involving ice, water and other materials are less likely in Charon’s northern hemisphere.

Charon’s icy geological features came as a shock to scientists when NASA’s New Horizons spacecraft visited the Pluto-Charon system in 2015, which previously thought of Charon as a motionless ball of ice. Since then, a research team led by researchers at the Southwest Research Institute (SwRI) has been examining New Horizons data in an attempt to discover the cause of these cold geological features.

Team member and SwRI researcher Alyssa Rhoden is an expert in the geophysics of icy moons, particularly involving their own oceans.

“A combination of geological interpretations and thermal orbital evolution models imply that Charon has an underground liquid ocean that eventually freezes,” he said in a statement. “When the interior ocean freezes, it expands, creating great tensions in the ice sheet and creating pressure in the water below. We suspected Charon was the source of its great canyons and cold volcanic flows.”

To better understand the evolution of the moon’s surface and interior, Rodin modeled how cracks form in Charon’s icy crust as the ocean below freezes. The oceans considered in the models consisted of water, ammonia, and their combinations. While ammonia can act as an antifreeze and high concentrations can help preserve the life of liquid oceans, Roden found that different ocean compositions did not have a significant effect on the results.

The frozen ocean pushes through Charon’s outer crust, causing rifts that pierce the entire crust. As the volume of the ocean increases, it creates pressure on the surface of the liquid, causing it to erupt through the cracks onto the surface of the Charon.

The team looked for conditions that would allow cracks to fully penetrate Charon’s ice crust to connect surface and groundwater to enable oceanic cryovolcanism. This showed that existing theories about the evolution of Pluto’s moon could be wrong. This is because these theories suggest that Charon’s icy crusts were too thick to separate completely under the stress of freezing the ocean.

“Either Charon’s ice crust was less than 6 miles (10 km) away when the flows occurred, as opposed to the 60 miles or more than 100 km stated, or the surface did not have direct contact with the ocean as part of the eruption process,” he said. “If Charon’s ice crust were thin enough to separate completely, that would mean much more ocean freezing than would indicate by the canyons that emerged in Charon’s encounter hemisphere,” Rodin said.

These canyons run along the spherical tectonic ridge belt that runs along the face of Charon, separating the northern and southern geologic regions of the Moon. The team’s model suggests that the canyons may have formed from cracks in the moon’s icy crust that do not reach the ocean;

The idea that Charon’s cryovolcanism originated in a frozen ocean could be confirmed if larger extended features are observed in the lunar hemisphere in a future mission. These features, unseen by New Horizons, support the idea that Charon’s ocean is thicker and its crust thinner than expected.