- Ariel, a moon of Uranus, may have a subsurface ocean, adding it to the list of potential ocean worlds in our solar system.
- Unexpected carbon dioxide and carbon monoxide ice deposits on the surface of Ariel – primarily on its trailing hemisphere, the side that faces opposite the direction the moon moves in orbit – are evidence for the ocean. These deposits hint at ongoing replenishment from its interior.
- Ariel shows signs of geological activity, maybe even ice volcanoes. Future missions are anticipated to explore further and confirm these findings.
Does Uranus’ moon Ariel have a hidden ocean?
The Webb space telescope has new observations of Ariel – a moon of the outer planet Uranus – suggesting that Ariel might harbor an ice-covered, subsurface ocean. Icy ocean worlds in our outer solar system are a big deal. Scientists are so fascinated by the possibility that they’re sending a space mission to another potential ocean moon, Jupiter’s moon Europa. Why? Because they think these ocean moons might be homes for life! On July 24, 2024, a research team led by the Johns Hopkins Applied Physics Laboratory (JHUAPL) in Laurel, Maryland, pointed to the surprising presence of carbon dioxide on the surface of Ariel. They said these deposits might be replenished by a liquid ocean hidden in Ariel’s interior.
The researchers published their new peer-reviewed study in The Astrophysical Journal Letters on July 24, 2024.
The mystery of Ariel’s carbon dioxide ice
The major clue to a subsurface ocean comes from Ariel’s frozen surface. Ariel’s surface is geologically the youngest of Uranus’ 28 known moons. It is largely covered by water, carbon dioxide and ammonia ices.
The carbon dioxide ice surprised astronomers. Ariel has no detectable atmosphere, so even in the extreme cold where Uranus orbits the sun, it should sublimate (turn to gas). But since there is a fair amount of it on the moon’s surface, something must be re-supplying it on an on-going basis. Also, most of the ice is on Ariel’s trailing hemisphere. That’s the hemisphere that always faces away from the moon’s direction of motion in its orbit.
Scientists have proposed two main theories over the years. One is that charged particles in Uranus’ magnetosphere or solar radiation interact with Ariel’s surface. This could create carbon dioxide through a process called radiolysis.
The other possibility is that primordial carbon dioxide is trapped by water ice in Ariel’s interior. Outgassing could then leave deposits of carbon dioxide ice on the surface.
Carbon monoxide that shouldn’t be there
The new observations from Webb suggest the second option is more likely. The researchers found Ariel has some of the richest carbon dioxide deposits in the entire solar system. On the moon’s trailing hemisphere, they are about 0.4 inches (10 mm) thick.
But Webb detected something else that has scientists puzzled: carbon monoxide. The scientists were not expecting to find it on Ariel. And like the carbon dioxide, it is primarily on Ariel’s trailing hemisphere. As lead author Richard Cartwright from JHUAPL noted:
It just shouldn’t be there. You’ve got to get down to 30 kelvins [-405 degrees Fahrenheit] before carbon monoxide’s stable. The carbon monoxide would also have to be actively replenished, no question.
The problem is that Ariel’s surface is an average of 65 degrees Fahrenheit warmer (36 C warmer) than that. So how can carbon monoxide be there? As Cartwright said, it would need to be replenished somehow.
The researchers said radiolysis could explain some of the replenishment. But it’s unlikely to account for all of it. Other research has suggested the process would be limited on Ariel. This is because the axis of the moon’s magnetic field and the orbital plane of Uranus’ moons are offset by about 58 degrees. Therefore, interactions with the magnetic field wouldn’t produce enough carbon dioxide or carbon monoxide ices to account for the amounts observed.
A subsurface ocean on Ariel?
So that leaves the gases originating from Ariel’s interior. The researchers said this is the most likely scenario to explain most of the icy deposits on the surface. In fact, the carbon dioxide and carbon monoxide may originate from a subsurface ocean.
The processes involved may be ancient, and the gases are simply still leaking to the surface. Or they may be on-going now. This means Ariel likely either had a hidden ocean in the past, or it still has one today.
Another tentative discovery – carbonate minerals – would also support the ocean explanation. The formation of those minerals requires the interaction of rock and liquid water. Cartwright said:
If our interpretation of that carbonate feature is correct, then that is a pretty big result because it means it had to form in the interior. That’s something we absolutely need to confirm, either through future observations, modeling or some combination of techniques.
Is Ariel an active world?
So far, only Voyager 2 has visited Ariel, the 4th-largest moon of Uranus, on January 24, 1986. The flyby was brief, but the spacecraft saw a world with many deep gashes and grooves on its surface. There were also other smooth areas that may have resulted from cryovolcanic (icy volcano) eruptions. All of these indicate Ariel was – and maybe still is – geologically active. There might even be plumes, reminiscent of those on Enceladus and perhaps Europa. A different study in 2023 suggested that Ariel or the moon Miranda, or even both, are releasing plasma particles into Uranus’ magnetosphere.
If that is the case for Miranda, then it, too, may have a subsurface ocean, the scientists said.
The scientists also noted that the canyons and grooves are primarily on the trailing side of Ariel. It’s possible the carbon dioxide and carbon monoxide are leaking out through them and onto the surface. They could provide an easy conduit for the gases to escape from inside Ariel. But researchers don’t know for sure, since Voyager 2 only saw some of Ariel’s surface, about 35%, during its flyby on January 24, 1986. There might be more canyons and grooves on the leading hemisphere as well. Cartwright said:
It’s a bit of a stretch because we just haven’t seen much of the moon’s surface. We’re just not going to know until we perform more dedicated observations.
Uranian system is compelling
Regardless, the findings show Ariel is a more complex world than we knew. This likely applies to Uranus and its other moons, too. As Ian Cohen at JHUAPL commented:
All these new insights underscore how compelling the Uranian system is. Whether it’s to unlock the keys to how the solar system formed, better understand the planet’s complex magnetosphere or determine whether these moons are potential ocean worlds, many of us in the planetary science community are really looking forward to a future mission to explore Uranus.
Bottom line: Researchers using NASA’s Webb space telescope have found Uranus’ 4th-largest moon, Ariel, may have a subsurface ocean, similar to Europa, Enceladus and others.
Source: JWST Reveals CO Ice, Concentrated CO2 Deposits, and Evidence for Carbonates Potentially Sourced from Ariel’s Interior
Via Johns Hopkins Applied Physics Laboratory
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