Throughout the history of the planets, the outer solar system has proven to be a dangerous place with many collisions smashing small bodies apart — but new simulations have shown that violent smashes do not necessarily prevent the formation of potentially habitable sub-surface oceans. This is true for large, icy moons, at least.
"These simulations were pretty much the biggest collisions we could come up with," said the University of Maryland’s Marc Neveu, who led the research, in a statement. "If those don't make a difference, it's unlikely smaller ones would either."
The planets of the outer solar system are replete with frozen moons that could potentially harbor oceans of liquid water deep beneath their surface. However, we know those moons have been peppered with impacts over the course of their 4.5-billion-year history, with some collisions large enough to smash the moons apart entirely. We can see this in Saturn's moon Iapetus, which appears to be the relic of a moon that has been smashed apart. In some cases, the damaged moon can reform through its own gravity pulling its parts and pieces back together again. For instance, Uranus' moon Miranda has a patchwork quality to its surface, giving the impression of it having been broken apart and then reformed.
Neveu's team, which also includes Raluca Rufu, Alyssa Rhoden and Kevin Walsh from the South-west Research Institute in Boulder, Colorado and Yuval Steinberg from Israel's Weizmann Institute, have now conducted detailed simulations in their quest to learn whether such titanic collisions affect the likelihood of an icy moon supporting an interior ocean.
They found that if an icy moon is large enough, not even the largest collisions can prevent it from having a subsurface ocean.
"What we found was that those big collisions don't really matter as far as oceans are concerned," said Neveu. "If there was an ocean before, there's likely to be an ocean after and vice versa."
Neveu and his colleagues combined two different types of simulation, both revolving around moons of two different sizes, 620 miles (1,000 kilometers) and 1,240 miles (2,000 kilometers) wide. One type of simulation focused on what happens to the moons when a giant impact breaks one apart, describing how it would shatter, melt and then reform through the mutual gravity of itself and its partner. The second type of simulation modeled the interior of each size of moon over 4.5 billion years, showing how heat leaks out from their core to keep a layer of liquid warm.
The simulations showed that moons smaller than 620 miles across were unlikely to retain an ocean after a cataclysmic impact. A small moon born with an ocean can retain enough heat for an ocean because it has not fully differentiated — or, in other words, the dense material hasn't all sunk into the core and the moon's outer layers are still an insulating jumble of ice and rock. After an impact disrupts the moon, however, the rock is able to sink down into the core, and the outer layer no longer insulates as well as it did, so heat leaks out into space and the ice remains frozen and does not form an ocean.
For larger moons, however, the ocean can actually deepen after the moon coalesces back together again following an impact.
"In larger moons, the energy of the crash converts into extra heat that can actually thicken up an existing ocean for a couple of billion years," said Neveu.
And in both cases, if the moon did not have an ocean to begin with, the simulations showed that a collision could not create one.
Casting his eye around the solar system, Saturn’s moon Rhea catches Neveu's attention as potentially having reformed following an impact. The second largest moon of Saturn, Rhea is 949 miles (1,528 kilometers) across and its craters appear softened as though they have melted. Neveu likens it to how a snowman will appear deformed once the sun's heat begins to melt it.
"Rhea's craters look smoothed out like that, but it wasn't the sun," he said. "The heat came from below. A long-ago collision that boosted an interior ocean could help explain why the moon appears that way."
Neveu's impact simulations are not the entire story. Tidal heating from a giant planet can keep an ocean liquid even in small moons, as we see in the case of Saturn's 310-mile-wide (500-kilometer-wide) moon Enceladus, and so small moons that have been disrupted through a collision will not necessarily freeze solid if they are close enough to their parent planet to experience strong gravitational tidal forces that keep their interiors warm.
Ultimately, Neveu hopes that simulations will become so detailed that we'll be able to replay the evolution of icy moons in planetary systems such as our own.
"Down the line, I would love to press play on a moon system and watch the moons move, how they smash into each other and how their interiors change as a result," he said. While Jupiter's Galilean moons, being almost planet-sized, are relatively impervious to impacts, "Maybe we can reconstruct what happened around Saturn, Uranus and Neptune."
The results were published on 20 August in Nature Astronomy.
