Smashed Icy Moons Cool Too Rapidly to Retain Oceans
Of the more than 450 confirmed moons orbiting the eight major planets of our solar system, only a handful have managed to captivate the scientific community in a truly profound way. What sets these particular worlds apart is not their size or proximity to Earth, but rather what may lie hidden beneath their frozen surfaces: vast subsurface liquid water oceans. Moons such as Jupiter's Europa and Ganymede, and Saturn's Enceladus, have become prime candidates in humanity's search for life beyond Earth — a quest that is now driving some of the most ambitious planetary science missions ever conceived. But a critical question has long eluded researchers: what fundamental processes are responsible for either creating or destroying these hidden oceans in the first place?
Now, an international team of scientists from NASA, the Southwest Research Institute (SwRI), and the Weizmann Institute of Science in Israel has made significant strides toward answering that question. Their findings, recently published in Nature Astronomy, employ sophisticated computer models to address a longstanding knowledge gap: specifically, how large-scale collisional impacts influence the formation and long-term evolution of subsurface oceans on icy moons throughout our solar system. The results are both surprising and consequential for how we understand the distribution of potentially habitable worlds.
The Moons Under the Microscope
The research team trained their focus on the smaller moons of Saturn and Uranus — two planetary systems that, despite being less glamorous than Jupiter's in the popular imagination, host some of the solar system's most intriguing icy bodies. Saturn's roster includes Enceladus, Titan, and Dione, all of which are considered candidates for harboring active or recently active subsurface oceans. Enceladus, in particular, has already demonstrated dramatic evidence of a subsurface ocean: NASA's Cassini spacecraft detected enormous plumes of water vapor and ice particles erupting from the moon's south polar region — a discovery that electrified the planetary science community.
The Uranian system presents a more enigmatic picture. Uranus hosts five major moons — Titania, Oberon, Ariel, Umbriel, and Miranda — all of which are still the subject of scientific debate. Researchers are uncertain whether these moons currently harbor active subsurface oceans or whether any oceans that once existed have long since frozen over entirely. The ambiguity stems in part from a remarkable lack of data: the only spacecraft ever to visit Uranus was NASA's Voyager 2, which conducted a historic flyby in January 1986, providing only a brief and incomplete glimpse of this remote system.
A central motivation of the new study was to investigate a deceptively simple but scientifically loaded question: are the smaller moons of Saturn and Uranus original moons that formed alongside their parent planets, or are they second-generation bodies — fragments assembled from the debris of a catastrophic giant impact on a larger progenitor body? The answer has enormous implications for understanding what conditions are necessary for subsurface oceans to take root and persist.
Collisions and Ocean Stagnation
Using advanced thermodynamic and geophysical computer models, the team simulated what happens when large-scale impacts strike small, frozen icy moons. Their findings challenge an intuitive assumption: that massive collisions might serve as a trigger for new ocean formation by delivering enormous amounts of heat energy to a frozen body. In reality, the models reveal a far less optimistic outcome.
The researchers found that large impacts do not meaningfully promote the formation of new ocean worlds. Instead, their principal effect is to temporarily influence the size and duration of an ocean that already exists. The key mechanism behind this finding lies in the composition of these icy bodies. When a large impactor strikes an icy moon, it generates tremendous heat, melting the ice into liquid water. However, the impact also violently mixes in silicate rock material. This denser rocky material rapidly sinks to the moon's interior, forming an insulating rocky mantle layer. Rather than trapping heat and sustaining liquid water, this configuration actually inhibits further ocean growth and accelerates freezing. The ocean, constrained and starved of additional heat input, quickly loses its thermal energy to space and solidifies.
"Imagine a moon that is small and frozen and isn't doing anything very interesting. If you throw something at it and cause a big collision, would that impart enough energy to cause an ocean to form? Our models indicated that is actually incredibly difficult. Most of the time a small moon experiencing a disruption may lose its ocean or prevent an ocean from forming in the first place."
This finding has profound implications for our understanding of ocean world diversity across the solar system. It suggests that the subsurface oceans we observe today are not easily born from chaos and destruction, but rather require very specific and sustained conditions — likely involving long-term radiogenic heating from rocky interiors and tidal forces exerted by a parent planet — to form and persist over geological timescales.
Why Ocean Worlds Matter: The Search for Life
The scientific fascination with subsurface oceans is ultimately rooted in the most profound question in all of science: are we alone in the universe? Liquid water is the universally recognized prerequisite for life as we know it. Water is the medium within which life's essential biochemical reactions — from cellular metabolism and enzyme function to DNA replication — take place. On Earth, water constitutes approximately 60 percent of an adult human body, though this proportion is significantly higher in many other organisms: jellyfish are composed of roughly 95 percent water, herbaceous plants of 90 to 95 percent, while marine mammals and insects typically range from 50 to 75 percent.
The significance of this extends far beyond biology textbooks. If life can thrive in extreme aquatic environments on Earth — such as in hydrothermal vent communities on the deep ocean floor, where sunlight never penetrates and pressures are immense — then the subsurface oceans of distant moons become extraordinarily compelling targets for astrobiology. On worlds like Enceladus, where hydrothermal activity has been directly detected, the conditions may be tantalizingly similar to those very terrestrial environments where life flourishes in the absence of sunlight.
Known and Suspected Ocean Worlds in Our Solar System
- Europa (Jupiter) — Strong evidence for a deep, global saltwater ocean beneath an icy crust; considered one of the top candidates for extraterrestrial life.
- Ganymede (Jupiter) — The solar system's largest moon; magnetic field measurements suggest a subsurface ocean sandwiched between layers of ice.
- Callisto (Jupiter) — Possible subsurface ocean inferred from magnetic induction data collected by the Galileo spacecraft.
- Enceladus (Saturn) — Active geysers of water vapor confirm a global subsurface ocean; hydrothermal activity detected on the seafloor.
- Titan (Saturn) — Possible subsurface liquid water ocean; surface lakes of liquid methane and ethane make it uniquely complex.
- Mimas (Saturn) — Recent Cassini data suggests a surprisingly young, potentially active subsurface ocean despite its heavily cratered appearance.
- Dione (Saturn) — Modeled as a potential past or present ocean world; studied in this new research.
- Titania, Oberon, Ariel, Umbriel, Miranda (Uranus) — Debated as past or present ocean worlds; severely understudied due to a lack of dedicated missions.
- Triton (Neptune) — Captured Kuiper Belt Object with active geysers; may harbor a subsurface ocean.
- Pluto and Ceres — Dwarf planets with evidence suggesting subsurface liquid water reservoirs.
Upcoming Missions to Ocean Worlds
While the Uranian moons explored in this study remain largely mysterious — with no dedicated mission currently approved or planned for the near future — the broader field of ocean world exploration is entering a golden era. Two landmark missions will dramatically expand our understanding of these environments within the next decade.
NASA's Europa Clipper, the agency's largest planetary science mission to date, is currently en route to the Jovian system. After its launch in October 2024, the spacecraft is scheduled to arrive at Jupiter in April 2030, whereupon it will conduct approximately 50 close flybys of Europa. Equipped with a suite of nine scientific instruments, Europa Clipper's primary objective is to assess the habitability of Europa's subsurface ocean — measuring the thickness of its ice shell, characterizing its ocean's chemistry, and searching for plume activity similar to that observed at Enceladus. More information about this mission can be found at the Europa Clipper mission page.
Meanwhile, NASA's Dragonfly represents an entirely different — and breathtakingly bold — approach to ocean world exploration. Rather than orbiting from afar, Dragonfly is a rotorcraft lander — essentially a large quadcopter — designed to fly through the thick atmosphere of Saturn's moon Titan and land at multiple scientifically compelling sites. Its mission is to search for chemical signatures of prebiotic chemistry or biological processes in Titan's organic-rich environment. Dragonfly is scheduled to launch in July 2028 and is expected to arrive at Titan in 2034. Learn more about this revolutionary mission at the Dragonfly mission page hosted by Johns Hopkins APL.
Beyond these two flagship efforts, the European Space Agency's JUICE (Jupiter Icy Moons Explorer) mission is also en route to the Jovian system, with its primary focus on Ganymede — the only moon in the solar system known to generate its own magnetic field — along with detailed studies of Europa and Callisto. JUICE is expected to enter Ganymede's orbit in 2034, marking the first time any spacecraft has orbited a moon other than our own.
Looking Ahead: Unanswered Questions and Future Research
The new modeling work from NASA, SwRI, and the Weizmann Institute represents a meaningful step forward in unraveling the complex geological and thermal histories of icy moons. By demonstrating that catastrophic impacts are unlikely triggers for new ocean formation — and may in fact suppress ocean longevity — the study reshapes our probabilistic understanding of how common habitable subsurface environments might be throughout the outer solar system and, by extension, around other stars.
Yet the study also highlights how much remains unknown. The Uranian system in particular stands out as one of planetary science's most pressing blind spots. A dedicated Uranus orbiter and probe has been ranked as the top priority for large planetary science missions in the latest Planetary Science Decadal Survey (2023–2032) published by the National Academies of Sciences, Engineering, and Medicine. Such a mission could finally provide the high-resolution data needed to determine, definitively, whether worlds like Titania and Oberon are hiding oceans beneath their frozen shells — oceans that might, against all odds, be whispering the first faint signals of life beyond Earth.
As our models improve and our spacecraft venture deeper into the cold reaches of the outer solar system, the story of icy moon oceans — their birth, their survival, and their potential as cradles of life — is only just beginning to be told.
