Giant impacts may strip small icy moons of subsurface oceans

Saturn's moon Enceladus is one of many icy moons in the solar system believed to have a subsurface ocean. Southwest Research Institute scientists used simulations to understand how disruptive collisions might have affected ocean formation in such moons. A recent study published in Nature Astronomy concludes that larger moons may retain an ocean after a collision, but impacts do not seem to promote the formation of new oceans. The research combined a smoothed particle hydrodynamics (SPH) model with a thermal-structural evolution model to simulate the effects of collisions on icy moons. This simulation shows a disruptive collision between two small moons, where some material is dispersed into space while the rest consolidates into a second-generation moon. If the moon is large enough, it will retain a submerged ocean, but smaller moons may lose it.

• The study found that large-scale collisions affect only ocean thickness and longevity, but do not contribute to creating new oceans.
• Disruptive impacts are common, especially during the formative years of the solar system, and they can significantly alter the makeup of the moon.
• The research considered whether these kinds of impacts played a role in the formation of oceans. It found that most small moons experiencing a disruption may lose their ocean or prevent an ocean from forming in the first place.

The study found that overall, large-scale collisions affect only ocean thickness and longevity, but do not contribute to creating new oceans. Heat that fades too fast "That would seem to be counterintuitive. Introducing more energy to the system could melt the ice and create oceans," said SwRI's Dr. Raluca Rufu, another co-author. "But this energy dissipates very quickly, which actually works against forming oceans." Rhoden compares it to baking potatoes. "When you make a baked potato versus baking French fries, the smaller pieces heat up and cool down much faster than a whole potato. So, the same thing happens when you blow up a small moon. All the pieces lose their heat very quickly."

Reassembly leaves thicker ice. While disruptive collisions may not promote the formation of oceans, they do result in ice-rock differentiation. A moon may have chunks of rock and ice all mixed together, but in the aftermath of the collision, the ice briefly melts. Then heavier material sinks to the core before the water refreezes, forming a thicker coating of ice around it. "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." Smaller moons face long odds. Outer solar system moons span a wide range of physical and orbital characteristics, and the parameters that could have led to disruptive collisions are not well constrained. Therefore, more work is needed to determine whether any conditions can promote ocean formation in smaller reassembled moons. "For now, though, it seems unlikely that moons like Saturn's Enceladus or Dione—which are both thought to harbor oceans today—would have been disrupted and reassembled in the past 100 million years," Rhoden said. Dr. Marc Neveu of the University of Maryland and NASA's Goddard Space Flight Center led the research and was first author of the paper.

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