New research indicates that hidden oceans on icy moons around outer planets may boil due to tidal heating, explaining unusual surface features. The study, published in Nature Astronomy, focuses on smaller moons like Enceladus, Mimas, and Miranda. Lead author Max Rudolph from UC Davis highlights the processes shaping these worlds over millions of years.
Icy moons orbiting the outer planets of the solar system are encased in thick shells of ice, with some harboring vast subsurface oceans of liquid water. These environments are of interest to scientists because liquid water is key to life as known on Earth. A recent study in Nature Astronomy examines the dynamic processes beneath these frozen surfaces, proposing that tidal forces can lead to boiling in hidden oceans.
Tidal heating occurs as these moons orbit massive planets, with gravitational interactions from neighboring moons causing fluctuations in heat levels. When heating increases, the ice shell melts from below and thins; when it decreases, the shell thickens through refreezing. In prior research, the team found that thickening ice raises pressure, contributing to features like the "tiger stripes" fractures on Saturn's moon Enceladus.
This new work explores the reverse: thinning ice reduces internal pressure, potentially causing the underlying ocean to boil. On smaller moons such as Enceladus, Mimas of Saturn, and Miranda of Uranus, the pressure drop can reach the triple point, where ice, liquid water, and vapor coexist. For Miranda, this process may account for its enormous ridges and steep cliffs called coronae, as observed by the Voyager 2 spacecraft.
Mimas, less than 250 miles wide and known for its massive crater earning it the "Death Star" nickname, shows a subtle wobble suggesting a hidden ocean despite its inactive appearance. In contrast, larger moons like Titania experience ice cracking before boiling conditions arise, leading to cycles of thinning and thickening.
"Not all of these satellites are known to have oceans, but we know that some do," said Max Rudolph, associate professor of earth and planetary sciences at the University of California, Davis and lead author. "We're interested in the processes that shape their evolution over millions of years and this allows us to think about what the surface expression of an ocean world would be."
Coauthors include Michael Manga from UC Berkeley, Alyssa Rhoden from Southwest Research Institute in Boulder, and Matthew Walker from Planetary Science Institute in Tucson. The research was supported in part by NASA.