Giant collisions between icy moons can change the course of subsurface oceans already present, but they cannot create new oceans in frozen worlds, according to a study published in Nature Astronomy. The research suggests that the outcome depends heavily on a moon's size: impacts can expand oceans in larger bodies, while disruptive collisions may remove the conditions that allow oceans to survive in smaller ones.
The paper, titled 'The Role of Disruptive Impacts on Ocean Generation and Longevity in Icy Moons,' was led by Dr. Marc Neveu of the University of Maryland and NASA's Goddard Space Flight Center. It examined whether violent impacts would help or harm the ability of icy moons around Saturn, Uranus and Neptune to retain liquid water beneath their ice shells.
Such moons are important targets in the search for extraterrestrial life because satellites including Enceladus, Dione and Miranda often harbour liquid oceans beneath icy crusts. Their orbits around giant planets are closely packed, and catastrophic collisions can occur from time to time. The study combined simulations of collision physics with models of long-term thermal evolution to assess the effect on hidden oceans.
Researchers tested moons with a radius of approximately 500 kilometers and moons with a radius of approximately 1,000 kilometers. For larger moons, with a radius of 1,000 km or more, an impact can turn collision energy into heat that expands an existing ocean and keeps it present for approximately two billion years.
Smaller moons respond differently. Before an impact, their undifferentiated crust of mixed rock and ice can act as insulation, helping retain heat inside the body. An impact can drive differentiation between rock and ice, removing that insulating layer. The collision energy also dissipates very quickly, which works against sustaining an ocean.
The researchers defined a disruptive impact as one in which the largest surviving fragment is less than half the size of the original target. They found that, most of the time, a small moon undergoing such a collision may lose its ocean or fail to develop one in the first place.
The study found no route by which a major impact could melt enough ice to start an ocean on a world that lacked one. Neither heat from the collision nor tidal heating after it was sufficient to produce an ocean from scratch. Impacts therefore affect the lifespan of pre-existing oceans rather than initiating them.
The findings could alter expectations for smaller moons in the Saturn and Uranus systems. Most or all moons with a radius less than 1,000 km around those planets are suggested to have been disrupted by major collisions. For Saturn's moons, disruptive collisions have been proposed even within the last 100 million years.
Even so, the study says it seems unlikely that moons such as Enceladus or Dione were disrupted and reassembled in the past 100 million years. Both are thought likely to have subsurface oceans now. The timing and frequency of collisions across these moon systems, and the balance between impact effects and tidal heating on individual worlds, remain uncertain.
The researchers plan to test their models against observations of Enceladus, Mimas, Tethys, Dione and Rhea at Saturn; Miranda, Ariel, Umbriel, Titania and Oberon at Uranus; and Triton at Neptune. Rhea is covered with many craters that appear unusually smooth and softened, as if they were partly melted. It is still unknown whether Rhea itself experienced a disruptive collision, or whether its crater patterns reflect impact-driven ocean thickening or other processes.
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