Giant Collisions May Destroy, Not Create, Oceans Inside Icy Moons

Beneath the frozen shells of many moons circling Saturn and Uranus, scientists suspect vast oceans of liquid water may be hiding — worlds that could rank among the most promising places to search for life beyond Earth. But a new computational study suggests a sobering twist in the story of how these hidden seas come to be, and how easily they might be lost. According to research led by Dr. Marc Neveu of the University of Maryland and NASA’s Goddard Space Flight Center, with contributions from Southwest Research Institute scientists Dr. Alyssa Rhoden and Dr. Raluca Rufu, catastrophic collisions that shatter icy moons appear to do more harm than good when it comes to generating subsurface oceans. The work, published in the journal Nature Astronomy, indicates that while disruptive impacts can reshape a moon’s interior in profound ways, they rarely conjure new oceans — and often destroy the ones already there.

The motivation for the study stems from a growing realization among planetary scientists that many of the smaller moons in the outer Solar System may not be the originals. Several lines of evidence have led researchers to question whether the moons we observe today around Saturn and Uranus, particularly those with radii below 1,000 kilometers, are primordial bodies that formed alongside their parent planets. Instead, a compelling hypothesis holds that most or all of these smaller moons were blown apart in large collisions at some point in their history, with the moons now visible to spacecraft reassembling later from the orbiting debris of their shattered predecessors. For Saturn’s system, some models have proposed disruptive collisions occurring remarkably recently — even within the last 100 million years. That possibility raises an immediate and consequential question: if the candidate ocean moons we see today are, in effect, second-generation bodies stitched together from collisional wreckage, can they still host oceans at all?

To tackle this problem, the research team assembled a sophisticated two-part modeling framework. The first component was a smoothed particle hydrodynamics, or SPH, model, a computational technique widely used in astrophysics to simulate the behavior of fluids and deformable materials under extreme conditions. SPH allowed the team to render catastrophic collisions in fine detail, tracking how a moon’s icy and rocky components scatter, mix, and reaccumulate when struck by an impactor of comparable or substantial size. The second component was a thermal-structural evolution model, which simulated how a moon’s interior evolves over time following such an event — how heat generated by the impact redistributes, how melted material refreezes, and how the boundary between ice and rock migrates as the body cools. By coupling these tools, the scientists could compare three scenarios side by side: a moon before an impact, the same moon after a disruption, and an otherwise identical moon that never experienced a collision at all.

The team’s definition of a disruptive impact was deliberately stringent. As Rhoden explained, the threshold required that the largest surviving fragment be less than half the size of the original target moon. In other words, these were not glancing blows or crater-forming events, but true catastrophic disruptions — collisions energetic enough to break a moon apart entirely, dispersing some fraction of its material into space while the remainder gravitationally reconsolidates into what is essentially a brand-new moon built from the debris. Such events, the researchers note, are far from exotic. Disruptive impacts were common throughout the Solar System’s formative era, and they would fundamentally rework a moon’s interior structure rather than merely scarring its surface.

The intuitive expectation might be that pumping enormous quantities of energy into a frozen moon would be a recipe for ocean creation. After all, a sufficiently violent impact should melt ice, and melted ice is, by definition, liquid water. Rhoden framed the question directly: imagine a small, frozen, geologically quiet moon. If something slams into it with enough force to cause a major collision, would that impart enough energy to generate an ocean? The models delivered a decisive answer — it is actually incredibly difficult. In most simulated scenarios, a small moon that experiences a disruption either loses an existing ocean or fails to form one in the first place. The counterintuitive explanation lies in the physics of heat loss at small scales. Rufu noted that although introducing energy into the system could in principle melt ice and create oceans, that energy dissipates very quickly, which works against ocean formation rather than for it.

Rhoden offered an everyday analogy to illustrate the effect: baking a whole potato versus baking French fries. The smaller pieces heat up and cool down far faster than an intact potato. The same principle applies to a moon blown apart by an impact. Once the body is fragmented into countless pieces of ice and rock, each fragment radiates its heat into space rapidly, and any meltwater generated during the collision freezes again before it can accumulate into a lasting ocean. The very act of breaking the moon apart, which concentrates energy in the moment, undermines the long-term thermal conditions needed to sustain liquid water. The net result, the study found, is that large-scale collisions affect the thickness and longevity of existing oceans but do not contribute meaningfully to creating new ones.

Disruptive collisions do, however, leave a lasting structural imprint on the moons that survive them. In the aftermath of a catastrophic impact, the reassembling body briefly experiences widespread melting, which triggers a process known as ice-rock differentiation. A disrupted moon may begin as a jumbled mixture of rock and ice, but when the ice melts, the denser rocky material sinks toward the center while the lighter water rises. As the water refreezes, it forms a thicker coating of ice around a more substantial core. As Rufu described it, when everything consolidates after a collision, the end product is a moon with a larger core and a thicker ice shell than it had before. This differentiation matters for future exploration, because the thickness of a moon’s ice shell and the structure of its interior strongly influence how scientists interpret gravity measurements, surface features, and potential plume activity observed by spacecraft.

The fate of an existing ocean, meanwhile, depends heavily on the moon’s size. The simulations showed that when moons possessed subsurface oceans prior to impact, larger bodies — those with radii of 1,000 kilometers or more — might retain their oceans through a disruption and reassembly. Smaller moons, by contrast, tend to lose their oceans entirely. This size-dependent outcome carries significant implications for how scientists interpret the moon systems of Saturn and Uranus. If a small moon is suspected of harboring an ocean today, and if it also shows signs of having been disrupted and reassembled, the new results suggest the two histories may be difficult to reconcile — unless the disruption occurred long enough ago, or under conditions the models have not yet fully explored.

That caveat points to the remaining uncertainties in the study. Outer Solar System moons span a wide range of physical and orbital characteristics, and the parameters governing disruptive collisions — the frequency, scale, and timing of such events — are not well constrained by current observations. The authors emphasize that more work is needed to determine whether any set of conditions could promote ocean formation in smaller reassembled moons. The modeling framework itself, combining SPH collision simulations with long-term thermal evolution, offers a template for such follow-up studies, and future refinements could probe a broader range of impact scenarios, moon compositions, and orbital environments.

For now, the findings offer a measure of reassurance for scientists excited about some of the most famous ocean world candidates. Rhoden noted that it seems unlikely that moons like Saturn’s Enceladus or Dione — both thought to harbor oceans today — would have been disrupted and reassembled within the past 100 million years. Enceladus, in particular, has captivated the planetary science community since NASA’s Cassini spacecraft observed plumes of water vapor and icy particles erupting from its south polar region, providing direct evidence of a subsurface sea in contact with a rocky core. If those oceans are to be taken as long-lived and potentially habitable environments, the new simulations suggest their host moons must have avoided catastrophic disruption over geologically recent timescales — a conclusion that, if confirmed, strengthens the case that these small, icy worlds are genuine ocean worlds rather than transient products of violent collisions.

Subject of Research: The effects of disruptive impacts on the formation and longevity of subsurface oceans in icy moons of the outer Solar System

Article Title: SwRI simulations indicate large impacts may limit ocean formation in icy moons

Article References: SwRI simulations indicate large impacts may limit ocean formation in icy moons. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: icy moons, subsurface oceans, disruptive impacts, Saturn, Uranus, Enceladus, smoothed particle hydrodynamics, ice-rock differentiation, Nature Astronomy, Southwest Research Institute, ocean worlds, planetary science