Jakarta –
The icy volcanism of Pluto’s massive moon Charon and its belts of surface rifts may have been caused by frozen subsurface oceans exploding through thin ice shells.
Recent modeling suggests that when Charon’s internal ocean freezes, it may form a deep, elongated depression along its center, but this may imply that its outer shell was thinner than currently thought at some point in the moon’s history. The models also show that a cryovolcano erupting with ice, water and other material is less likely to occur in Charon’s northern hemisphere.
The geological features of Charon’s ice surprised scientists when NASA’s New Horizons spacecraft visited the Pluto and Charon systems in 2015. Previously, scientists believed that Charon was an inert ball of ice. Since then a science team led by Southwest Research Institute (SwRI) researchers have been poring over New Horizons data, trying to find the cause of this cold geological feature.
“A combination of geological interpretations and thermal-orbital evolution models implies that Charon had a subsurface liquid ocean that eventually solidified,” said ice satellite geophysicist and SWRI researcher Alyssa Rhoden, quoted from Space.com, Tuesday (14/2/2023).
“When the internal ocean freezes, it expands, creating immense pressure in its icy shell and exerting pressure on the water below. We suspect this is the source of the great Charon canyon and cryovolcanic flows.”
Rhoden modeled how cracks formed in Charon’s icy shell when the ocean beneath froze, to better understand the evolution of this moon’s surface and interior. The oceans considered in the model consist of water, ammonia, and a combination of the two. While ammonia can act as an antifreeze and high concentrations can help keep the oceans liquid, Rhoden found the different compositions of the oceans didn’t have a substantial effect on the outcome.
When the frozen ocean exerted pressure on Charon’s outer shell, it caused a crack to run through the entire shell. As the ocean’s volume increases, it puts pressure on the liquid above it causing it to erupt through cracks onto the surface of Charon.
The team sought conditions that would allow fractures to fully penetrate Charon’s icy shell to connect surface and subsurface water to allow cryovolcanism to originate from the ocean. This reveals that current theories surrounding the evolution of Pluto’s moon could be wrong, as these theories suggest that Charon’s icy shell is too thick to fully crack under the stresses associated with the freezing of the oceans.
“The Charon ice shell was less than 10 km thick when the flow occurred, as opposed to indications of more than 100 km, or the surface was not in direct contact with the ocean as part of the eruption process,” said Rhode.
“If Charon’s ice shell was thin enough to fully fracture, it would imply more freezing of the oceans than is implied by the canyons identified in Charon’s confluence hemispheres,” he said.
This gorge runs along a global tectonic belt of mountains across the surface of Charon, separating the north and south geological regions of the moon. The team’s model suggests that the canyon may have started from a crack in the moon’s icy shell that didn’t reach its ocean, meaning it formed after the cryovolcanism-causing crack and as Charon’s shell thickened.
The idea that Charon’s cryovolcanism originated in a frozen ocean could be confirmed if future missions find additional, more extensive features on the lunar hemisphere. These features, invisible to New Horizons, would support the idea that Charon’s oceans are thicker than previously thought and its shells are thinner.
“Ocean freezing also predicts a sequence of geological activity, where ocean-sourced cryovolcanism stops before tension-created tectonism,” said Rhoden.
“A more detailed analysis of Charon’s geological record could help determine whether such a scenario is feasible,” he concluded.
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(rns/fay)
