Giant impacts may strip small icy moons of their subsurface oceans, according to a study by scientists at the Southwest Research Institute (SwRI). The research, published in the journal Nature Astronomy, used computer simulations to examine how large collisions affect icy moons that are believed to have oceans beneath their frozen surfaces. The study found that such impacts can change the presence and stability of these oceans but do not appear to create new ones. Many icy moons in the outer solar system, such as some of Saturn’s and Uranus’ smaller moons, are considered "ocean worlds"—bodies that may contain large amounts of liquid water beneath their icy crusts. Scientists have debated whether these moons formed as single entities or as the result of massive collisions. In fact, it has been suggested that most or all of the smaller moons around Saturn and Uranus were once disrupted by large impacts. Dr. Alyssa Rhoden, a co-author of the study, noted that these collisions could have occurred as recently as 100 million years ago, meaning the ocean moons we see today may have reassembled from the debris of those collisions. To determine if reassembled moons could still form oceans, the research team used advanced computer models that simulate the physical and thermal evolution of moons. These simulations compared moons before and after an impact, as well as moons that were not struck. The study found that disruptive impacts change the moon's structure significantly, not just creating a crater. A disruptive impact was defined as one where the largest remaining piece of the moon was less than half the size of the original. The researchers also examined whether such impacts could lead to the formation of oceans. Dr. Raluca Rufu, another co-author, explained that while the energy from an impact could melt ice and create oceans, that energy dissipates quickly, which actually hinders ocean formation. Rhoden used an analogy, comparing it to baking potatoes: smaller pieces heat up and cool down much faster than a whole potato, just as small moon fragments lose heat rapidly after a collision. While disruptive collisions may not help form oceans, they can lead to a process called "ice-rock differentiation." After a collision, a moon with a mix of ice and rock might experience temporary melting, allowing heavier materials to sink toward the center before the water refreezes, creating a thicker layer of ice on the surface. Rufu explained that this process can result in a moon with a more substantial core and a thicker ice shell. However, smaller moons (those with a radius of less than 1,000 kilometers) that had subsurface oceans before an impact are more likely to lose them, while larger moons may retain their oceans. More research is needed to determine if any conditions could lead to the formation of oceans in smaller reassembled moons. Dr. Marc Neveu of the University of Maryland and NASA's Goddard Space Flight Center led the study and was the first author of the paper.