Scientists Unveil How Massive Impacts Reshape Icy Moons and Their Hidden Oceans

Extreme impacts in the distant Solar System may have dramatically influenced the internal makeup of icy satellites. New computational models suggest these colossal collisions rarely generate new subsurface oceans, instead often causing smaller moons to lose any existing liquid water trapped beneath their icy exteriors.

Numerous moons orbiting Saturn and Uranus are thought to be ocean worlds due to the potential presence of vast liquid water reservoirs beneath their frozen crusts. These concealed oceans intrigue researchers interested in understanding how water theaters can form and persist well beyond the Sun’s warmth.

However, the evolution of smaller moons appears more complex. Researchers hypothesize that several icy moons with radii less than 1,000 kilometers might have suffered catastrophic collisions that shattered them before gravitational forces caused the debris to reassemble. This recent research published in Nature Astronomy investigates whether such destructive encounters could foster ocean formation or influence their longevity.

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New Oceans Are Unlikely to Emerge From Collisions

A team from the Southwest Research Institute employed advanced simulation techniques to explore the outcomes of high-energy impacts on frozen moons. The approach integrated collision dynamics with models tracking thermal and interior structural evolution over time.

Could these collisions generate sufficient heat to melt subsurface ice layers and spawn new oceans? The results showed this is far more challenging than anticipated, as impact-generated heat dissipates rapidly. Dr. Alyssa Rhoden, a co-author from SwRI’s Solar System Science and Exploration Division, shared insights into the motivation behind studying this process.

“Imagine a moon that is small and frozen and isn’t doing anything very interesting. If you throw something at it and cause a big collision, would that impart enough energy to cause an ocean to form?” she said. The answer from the simulations was mostly negative. “Our models indicated that is actually incredibly difficult. Most of the time a small moon experiencing a disruption may lose its ocean or prevent an ocean from forming in the first place.”

The study highlighted that these impacts primarily influence the thickness and durability of existing subsurface oceans, rather than prompting the creation of new liquid layers beneath the ice.

Internal Structures of Moons Remodeled by Collisions

Although collisions do not spark ocean formation, they can significantly reshape an icy moon’s interior. When a moon is disrupted and later reassembled, its constituent materials may reorganize. This process entails brief melting of ice, enabling denser rock to migrate inward while water refreezes externally, resulting in a moon characterized by a bigger rocky core enveloped by a thicker ice shell. Dr. Raluca Rufu, another researcher from SwRI involved in the project, clarified how added collision energy impacts liquid water retention.

“Introducing more energy to the system could melt the ice and create oceans,” she said. “But this energy dissipates very quickly, which actually works against forming oceans.”

Dr. Rufu likened the process to warming small food portions rather than a large dish; smaller fragments cool much faster. Similarly, broken moon pieces lose heat quickly after collisional disruption, hampering the persistence of subsurface water.

Smaller Moons Struggle to Retain Subsurface Oceans

Simulation outcomes imply that a moon’s radius strongly governs its ability to preserve liquid water after a major disruption. Larger moons may retain the necessary internal conditions for oceans, whereas smaller bodies are prone to losing their hidden waters.

“When everything consolidates after a collision, you will end up with a more substantial core and a thicker layer of ice,” Rufu said. “When moons had subsurface oceans prior to impact, larger moons—with a radius of 1,000 kilometers or more—might keep it, but smaller moons tend to lose them.”

Based on Nature Astronomy, the precise environmental factors causing disruptive impacts among moons in the outer Solar System remain uncertain. Further research is essential to explore scenarios that might enable smaller reformed moons to develop or maintain subsurface oceans.

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