Since Voyager 2’s 1986 flyby, scientists have speculated about Miranda’s potential as an “ocean world.” It could harbor a hidden ocean beneath its icy crust. Such a finding wouldn’t be entirely surprising, as moons like Europa and Ganymede have been shown to possess liquid water beneath frozen surfaces. But, until recently, confirming the existence of an ocean on Miranda was out of reach. A new study has taken steps to do just that.

Using geological mapping and stress modeling, researchers calculated Miranda’s ice shell and possible ocean thicknesses. Their findings indicate that Miranda’s crust could be as thin as 30 km, with a subsurface ocean extending to depths of up to 100 km — a discovery that stunned the team. “That result was a big surprise,” the researchers noted.

A bizarre geology

Miranda is the smallest of Uranus’s five round moons, yet it has an exceptionally complex surface. It is defined by intense tectonic activity and varied topography. In fact, it includes some of the most unusual landforms in the Solar System. Among the most remarkable features are three distinct coronae — Arden, Elsinore, and Inverness — oval-shaped regions created by upwellings of warmer material from below.

These vast, polygonal regions each display unique characteristics. Arden Corona, for example, consists of concentric grooves that appear to result from normal faults, while Elsinore Corona’s parallel ridges indicate tectonic folding and thrust faulting. Together, these features point to a dynamic geologic past, with processes like tectonic stretching and compression shaping the moon’s surface.

Caleb Strom, a graduate student at the University of North Dakota, revisited images gathered by Voyager 2 in 1986. With his colleagues, he mapped Miranda’s southern hemisphere, examining geological features in detail. The coronae appear to be relatively young geologically, potentially resulting from a resurfacing event within the last 100–500 million years.

They then tried to propose a model that would satisfy the observed features. They simulated the various forces acting on Miranda (like tidal stress) and geological processes, ultimately inferring that Miranda likely had a thin ice shell and a thick subsurface ocean at some point in its past.

A surprising find

The results suggest a likely scenario in which Miranda’s crust was thin enough to allow significant tidal flexing. This flexing would have been amplified if Miranda’s orbit experienced shifts in obliquity or eccentricity. Under these conditions, an ice shell around 30 km thick, combined with a deeper subsurface ocean, could have produced the stress needed for brittle fracturing and tectonic deformation on the surface.

The model implies that Miranda may once have harbored an ocean up to 100 km thick. Such a feature would categorize it as an “ocean world” alongside other icy moons like Enceladus and Europa.

“To find evidence of an ocean inside a small object like Miranda is incredibly surprising,” said Tom Nordheim, a planetary scientist at the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, and co-author of the study published in The Planetary Science Journal.

It helps build on the story that some of these moons at Uranus may be really interesting. There may be several ocean worlds around one of the most distant planets in our solar system, which is both exciting and bizarre.

How can such a cold body have a liquid ocean?

It may come as a shock to most people that several frozen moons in our solar system hold liquid water. In fact, frozen moons are the most likely places to host extraterrestrial life in our solar system.

Frozen moons like Miranda can sustain subsurface oceans due to a combination of tidal heating and insulating ice layers. While surface temperatures on these distant moons are far below freezing, the gravitational pull from a large host planet, like Uranus, exerts a squeezing and stretching force on the moon’s interior during its orbit. This tidal flexing creates frictional heat within the moon’s icy shell. This effect is intensified if the moon has a slightly eccentric (elliptical) orbit or an axial tilt.

The heat can be sufficient to keep water beneath the surface in a liquid state. Additionally, the thick outer layer of ice acts as an insulating blanket, trapping this internally generated heat and preventing the ocean from freezing solid. Also, chemical compounds like salts or ammonia in the water can further lower the freezing point, allowing a liquid ocean to persist even in the cold outer solar system.

Miranda wasn’t predicted to have an ocean, but then again, neither were bodies like Enceladus. Miranda was thought to be too small and inactive, but as this new study suggests, it may be a more interesting place than we thought.

The tiny moon and the ocean

The key to creating this ocean were tidal forces between Miranda and the nearby moons. It’s not just the gravitational pull, it’s also something called orbital resonance. In this configuration each moon’s period around a planet is an exact integer of the others’ periods. Jupiter’s moons, for instance, have a 2:1 resonance. For every two orbits Io makes around Jupiter, Europa makes exactly one, amplifying tidal forces that can sustain a liquid ocean below Europa’s surface.

Numerical models suggested that, at some point, Miranda was in resonance with some of its nearby moons. But, later, this orbital resonance started desynchronizing, and the moon’s insides got cooler and solidified. But the researchers believe it hasn’t solidified completely. If it had, it would have expanded and created more cracks than are visible on the surface.

However, all this suggests is that Miranda may have an ocean beneath its surface. For now, however, we don’t have the data to fully confirm this.

“We won’t know for sure that it even has an ocean until we go back and collect more data,” Nordheim said. “We’re squeezing the last bit of science we can from Voyager 2’s images. For now, we’re excited by the possibilities and eager to return to study Uranus and its potential ocean moons in depth.”

Journal Reference: Caleb Strom et al, Constraining Ocean and Ice Shell Thickness on Miranda from Surface Geological Structures and Stress Modeling, The Planetary Science Journal (2024). DOI: 10.3847/PSJ/ad77d7