Nasa’s Voyager 2 spacecraft flew past Uranus in 1986 on its never-ending journey out into the depths of space – beaming back to Earth the only up-close images we have of the ice giant and its moons.
Now, another look at data gathered nearly 40 years ago suggests two of the planet’s moons may host active oceans beneath their frozen surfaces.
Astrobiologists believe moons such as Jupiter’s Europa or Saturn’s Enceladus could be strong candidates for supporting alien life, due to their subsurface oceans.
If liquid oceans are proven to exist below the surface of Uranus’s moons, they could be added to the list of potential hosts of life.
Scientists believe Miranda and Ariel, the smallest and second-smallest of Uranus’s five major moons, could be expelling vapour plumes – which on other moons in the solar system are thought to come from subsurface oceans.
Publishing the findings in the journal Geophysical Research Letters, the team led by the Johns Hopkins Applied Physics Laboratory (APL) said the moons “are adding plasma into the space environment through an unknown and mysterious mechanism” – which could be attributed to oceans.
“It isn’t uncommon that energetic particle measurements are a forerunner to discovering an ocean world,” said Ian Cohen, a space scientist at APL and the lead author of the new study.
The data suggesting the possibility of subsurface oceans comes from the particle and magnetic field data sent back by Voyager 2 all those years ago.
This type of data gave some of the first hints that Europa and Enceladus had oceans.
“We’ve been making this case for a few years now that energetic particle and electromagnetic field measurements are important not just for understanding the space environment but also for contributing to the grander planetary science investigation,” Cohen said.
“Turns out that can even be the case for data that are older than I am. It just goes to show how valuable it can be to go to a system and explore it first-hand”.
Voyager 2 is the only human spacecraft to visit Uranus – and growing interest in a return mission to Uranus and its neighbour Neptune led to several research teams taking another look at the old data.
Cohen and his colleagues looked again at data from the Low-Energy Charged Particle (LECP) instrument on Voyager 2, finding the spacecraft had observed trapped energy particles on its departure from Uranus.
“What was interesting was that these particles were so extremely confined near Uranus’ magnetic equator,” Cohen said.
Magnetic waves within the system would normally cause them to spread out in latitude, he explained, but these particles were all cramped near the equator between the moons Ariel and Miranda.
These were originally attributed to the spacecraft possibly flying through a stream of plasma “injected” from the planet’s magnetosphere. However Cohen pointed out: “An injection would normally have a much broader spread of particles than what was observed”.
Using the nearly 40 years of knowledge built up since the Voyager mission, the team tried to recreate the spacecraft’s observations, finding the explanation had to include both a strong, consistent source of particles and a specific mechanism to energise them.
After considering several possibilities, they concluded that the particles most likely came from nearby moons – whether Ariel, Miranda, or both.
“Right now, it’s about 50-50 whether it’s just one or the other,” Cohen said.
Scientists have already suspected that some of Uranus’ five largest moons – Ariel and Miranda included – could have subsurface oceans. Voyager 2 images of both moons show physical signs of geologic resurfacing, including possible eruptions of water that froze on the surface.
“The data are consistent with the very exciting potential of there being an active ocean moon there,” Cohen said. “We can always do more comprehensive modelling, but until we have new data, the conclusion will always be limited”.