Scientists simulate cosmic crashes to test whether icy moons gain or lose their ability to sustain life

Scientists simulate cosmic crashes to test whether icy moons gain or lose their ability to sustain life. A University of Maryland-led study published in Nature Astronomy found that even the biggest collisions don't fundamentally change whether these icy worlds can hold an ocean. The study's lead author, Marc Neveu, explained that the simulations showed that if a moon had an ocean before a collision, it is likely to have one after, regardless of the impact's size. The findings surprised the researchers, as they expected smaller collisions to have a more significant effect.

The study combined two types of simulations: one recreating the violent physics of a cosmic crash and the other simulating the slow burn of a moon's interior over billions of years. The researchers followed two sizes of moons, roughly 500 and 1,000 kilometers in radius, as they were struck by smaller space rocks, blown apart and reassembled. The team then fast-forwarded through 4.5 billion years of each simulated moon's afterlife and found that moon sizes played a bigger role than expected in determining a collision's aftereffects. In larger moons, the energy of the crash converted into extra heat that could actually thicken up an existing ocean for a couple of billion years. In smaller moons, the story flipped: the impact shook everything apart, and when the moon re-forms, the rock sinks to the center and the ice floats to the top. It gets harder for the smaller moon to keep an ocean because the blanket's gone, but in neither case did a collision create an ocean that would've otherwise stayed frozen.

The findings apply to a whole family of real worlds that NASA and other space agencies plan to investigate, including Saturn's mid-sized moons Mimas, Enceladus, Tethys, Dione and Rhea; the moons of Uranus, including Miranda, Ariel, Umbriel, Titania and Oberon; and Neptune's large moon Triton. One of Saturn's moons, Rhea, already caught the team's attention as a possible test case because its ancient craters look strangely smooth and softened—almost as if warmed from within. "It's like building a snowman in the winter, and then a week later, there's been warming and sunlight so the snowman is melting away," Neveu explained. "Rhea's craters look smoothed out like that, but it wasn't the sun. The heat came from below. A long-ago collision that boosted an interior ocean could help explain why the moon appears that way." The team's results offer new insight as NASA and the planetary science community weigh future missions to these icy worlds. Knowing which moons are most likely to harbor water could help decide where to send spacecraft and what to look for, from the gravitational fingerprint of a hidden ocean to salty deposits and icy "cryovolcanoes" on the surface. It even shapes how sensitive life-detecting instruments need to be. "If there's only a handful of microbes, you won't be designing the same kind of search mission as you would if the ocean were full of whales," Neveu said. Still, he cautioned that a moon's collision history is just one factor shaping its potential to host life. Others, like the heat generated by tides, matter at least as much. For Neveu, the study is one piece of a much bigger puzzle that includes how tides heat these planetary oceans and how shifting moon orbits set them on collision courses in the first place. "Down the line, I would love to press play on a moon system and watch how the moons move, how they smash into each other and how their interiors change as a result," Neveu said.

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