Solar System Moons May Hide Subsurface Oceans Beneath Frozen Crusts

The Expanding Core Group and Unexpected Additions of Hidden Oceans

Liquid water is no longer treated as an exclusive feature of Earth or bodies located close to the Sun. Several moons in the outer solar system maintain underground reservoirs kept liquid through combinations of tidal flexing, radioactive decay, salts, and ammonia. While Europa, Ganymede, Callisto, Enceladus, and Titan have long formed the core group of ocean worlds, new analyses keep expanding the roster.

Other worlds also keep the boundaries open. Similarly, the five large moons of Uranus have climbed candidate lists.

Europa’s Ice Shell Thickness and the Challenge of Deep Ocean Ascent

Jupiter’s moon Europa features a global subsurface ocean holding roughly twice as much water as all of Earth’s oceans combined. However, physical barriers complicate how scientists might study that water. During a close flyby on 29 September 2022, NASA’s Juno spacecraft used its Microwave Radiometer instrument to measure temperatures across half of Europa’s surface. An analysis led by Steve Levin estimated an average 29 kilometres of cold, conductive ice in the observed region—a frozen barrier taller than three Mount Everests stacked end to end.

Reaching that deep ocean via narrow fractures, or dikes, faces severe physical hurdles. Rapid heat loss causes rising water to churn against freezing walls, forming microscopic ice crystals known as frazil ice that clog pathways within hours.

“There’s all this speculation about how that water can come from deep underground and make its way all the way up without freezing en route. That’s really what we think we disproved.”

Lujendra Ojha, planetary scientist at Rutgers University, via Open Access Government

Fresh Data Challenges Long-Standing Assumptions at Titan and Ceres

Saturn’s largest moon, Titan, may also harbor a very different interior than previously suspected. Researchers at NASA’s Jet Propulsion Laboratory analyzed Cassini spacecraft data by measuring the timing between Saturn’s gravitational peak and the rising of Titan’s surface. They detected a 15-hour lag, which indicates an interior composed of slushy ice and pockets of liquid water rather than a uniform global ocean. Lead author Flavio Petricca noted that Titan’s ocean may have frozen in the past and is currently melting, or its hydrosphere is evolving toward complete freezing.

Meanwhile, dwarf planet Ceres has emerged as an ancient ocean world candidate. Purdue University researchers analyzing data from NASA’s Dawn mission discovered that Ceres contains a crust composed of about 90% ice near its surface, preserved by rocky impurities that prevent the ice from flowing like a glacier.

Upcoming Missions and the Search for Extraterrestrial Habitability

These evolving models provide critical context for flagship planetary missions currently underway.