Uranus’s moon Miranda may harbor an ocean
Uranus is the sixth planet from the sun in our solar system, orbiting some 19 times farther away from the sun than Earth does. One of its moons, Miranda, is 1/7 the size of our moon and has a crazy-quilt surface of scarps and craters. On October 28, 2024, researchers from Johns Hopkins University said they’ve modeled how the interior structure of the moon could create the bizarre surface patterns. The model with the best fit required the existence of a vast ocean beneath Miranda’s ice some 100 to 500 million years ago. Co-author Tom Nordheim of Johns Hopkins said:
To find evidence of an ocean inside a small object like Miranda is incredibly surprising. It helps build on the story that some of these moons at Uranus may be really interesting … that there may be several ocean worlds around one of the most distant planets in our solar system, which is both exciting and bizarre.
The researchers published their peer-reviewed findings in The Planetary Science Journal on October 15, 2024.
Evidence for an ocean
The only part of Miranda we’ve seen is its southern hemisphere. Scientists believe its grooved terrain (pockmarked with craters) is a result of heating from the moon’s internal tidal forces. The team of scientists took another look at the Voyager 2 images and decided to try to work backward:
to uncover what the moon’s interior structure must have been to shape the moon’s geology in response to tidal forcing.
Lead author Caleb Strom, a graduate student at the University of North Dakota, worked with scientists from the Planetary Science Institute in Arizona. They mapped Miranda’s surface features and then used computer models to match the stress patterns to the moon.
What they found was the best match required a vast ocean under Miranda’s surface. The ocean would have to be no more than 19 miles (30 km) beneath the moon’s crust of ice. And the ocean itself would have to be at least 62 miles (100 km) deep. The little moon is only 292 miles (470 km) across. So the ocean would take up a big chunk of the moon’s interior. Strom said:
That result was a big surprise to the team.
A thin ocean might remain under ice on Uranus’s moon
Uranus has 28 known moons. Miranda and some of its neighboring moons tug on each other as they orbit. This would lead to deformations and friction that would warm the moons’ interiors. The scientists found that Miranda and its nearby moons once likely had what is called an orbital resonance. For example, one moon might make one orbit of Uranus in the same time it takes another moon to orbit twice.
But today, the moons no longer have this synchronicity. So that means their insides are cooling and freezing. But the scientists said Miranda is not completely cool yet. If it were, they would be able to see cracks on its surface from the expansion as the liquid turned to ice. And so Miranda may still have an ocean today. The remaining ocean would probably be quite smaller than what it would have been some 100 to 500 million years ago. Still, Strom said:
But the suggestion of an ocean inside one of the most distant moons in the solar system is remarkable.
Other moons in our solar system with possible oceans
One of the remarkable aspects of the discovery is that ocean moons in our solar system are locations where scientists believe life could exist. Currently, Jupiter’s Europa and Saturn’s Enceladus are the top contenders for icy worlds that harbor hidden oceans. On October 14, 2024, NASA launched the Europa Clipper mission toward Jupiter’s icy moon Enceladus. Scientists want to know more about the habitability – the ability for some form of live to exist – on this large moon.
There’s also ESA’s JUICE mission – JUICE stands for JUpiter Icy Moons Explorer – which launched in 2023 and will explore the icy Jovian moons when it arrives in 2031. And Saturn’s moon Enceladus is a top target for a future ESA mission. Co-author Alex Patthoff of the Planetary Science Institute compared the surprise of Miranda to the one scientists previously got from Enceladus. Patthoff said:
Few scientists expected Enceladus to be geologically active. However, it’s shooting water vapor and ice out of its southern hemisphere as we speak.
Perhaps one day Miranda and other moons of Uranus will get missions of their own.
Bottom line: A new study of Uranus’s moon Miranda looked at its crazy surface and used computer models to see how it might have gotten that way. The best fit for the jumbled terrain is an underground ocean.
Source: Constraining Ocean and Ice Shell Thickness on Miranda from Surface Geological Structures and Stress Modeling
Via Johns Hopkins