Evidence is mounting that Uranus moon Ariel may be the latest world found to have, or once have had, a subsurface ocean, scientists have revealed at a meeting in Idaho in the US today.
If so, that brings to at least 10 the number of such worlds identified as such since NASA announced its astrobiology quest to “follow the water,” more than two decades ago. With the addition of Ariel, moons of all four giant planets (Jupiter, Saturn, Uranus, and Neptune), plus the dwarf planets Pluto and Ceres, are now on the list of “water worlds” potentially capable of hosting life as we know it, at least at some point in their evolution.
Ariel (diameter 1,158km) is Uranus’s fourth-largest moon. The possibility that it might host an ocean beneath its icy crust has been suspected for some time, but the latest evidence comes from spectroscopic images of its surface taken by the James Webb Space Telescope (JWST)—images that, according to study team leader Richard Cartwright of Johns Hopkins Applied Physics Laboratory, reveal signatures of carbon dioxide (CO2), carbon monoxide (CO), and possibly carbonate salts on its surface.
All are signs that something interesting is going on below the ice, and an ocean is the most likely explanation.
On Earth, CO2 and CO are gases. But in the cold depths of the outer Solar System, they can be ices, with CO2 having a freezing point of -78°C and CO having a freezing point of -205°C.
At an average surface temperature of -213°C, Ariel would appear to be cold enough to support both. But there’s one problem: even at these temperatures, both ices, especially CO, would evaporate into space in a process known as sublimation. The fact that they are still there means they are being replenished from somewhere.
One possibility is that they are being created by a process called radiolysis, Cartwright said today at a meeting of the American Astronomical Society’s Division for Planetary Sciences in Boise, Idaho. That could occur if high-energy particles trapped in Uranus’s magnetic field smash into water ice contaminated with any of a number of carbon-containing molecules often found in the outer Solar System. “We certainly cannot rule that out,” he says.
It’s not clear, however, that Uranus’s magnetic field is strong enough to do this, especially because the JWST data show that the CO2 ice isn’t just a microscopically thin frost, but could be at least 1cm thick in places on one side of the world. That’s a lot of CO2.
Also important is the failure of the JWST data to reveal another molecule, hydrogen peroxide (H2O2), easily formed by radiolysis under Ariel-surface conditions.
It’s possible, Cartwright says, that its absence is because the surface coating of CO2 is somehow “gobbling up” all or most of the incoming radiation, preventing it from reaching the underlying water ice. But it’s also possible that the magnetosphere of Uranus is simply not strong enough to produce significant radiolysis. If so, the CO2 and CO have to have come from the interior.
Added to this are the carbonates, which could only have been formed is by slow chemical processes in a subsurface ocean from which they subsequently escaped onto the surface. “You likely need to have water in contact with rock to make most carbonates,” Cartwright says.
But even though signatures that appear to be carbonates appear in the JWST spectra, it’s still too early to declare them with too much certainty, Cartwright says. That’s because it’s not impossible that under Ariel’s surface conditions, CO2 ice might also be producing similar spectroscopic signatures.
“The difficulty is we don’t really understand the spectral properties of CO2 ice to a degree I’m confident in,” Cartwright says.
Meanwhile, there are hints that Ariel may not be the only one of Uranus’s moons to have an ocean. Spectral analysis of three of the planet’s other largest moons—Titania (1,577km), Umbriel (1,169km), and Oberon (946km)—are tantalizingly similar.
“We don’t see any good evidence for hydrogen peroxide [there], either,” Cartwright said at the DPS meeting, “but we certainly see CO2, and we also see a little bit of carbon monoxide. So perhaps it’s coming from their interiors as well.”
All of this means that the options for where we might find extraterrestrial life, if it exists, are becoming ever broader.
Each time we find a new ocean world, Cartwright says, “you’ve got more potentially habitable environments where maybe you have the right energy inputs, the right nutrients, the right shieldings from things like radiation, and the right internal temperatures to actually have life form and flourish.”
Read more: Uranus moon eclipse
