Ariel Voyager 2 – Seen by NASA
The surface of Uranus’s lunar surface is coated with a significant amount of carbon dioxide ice, especially in its „retrograde hemisphere,” which always faces away from the direction of the Moon’s orbital motion. This fact comes as a surprise, because even in the coldest parts of the Uranian system – 20 times farther from the Sun than Earth – carbon dioxide immediately turns to gas and is lost to space.
Scientists theorize that something on the surface of Ariel is supplying carbon dioxide. Some support the idea that interactions between the moon’s surface and charged particles in Uranus’ magnetosphere produce carbon dioxide through a process called radiolysis, in which molecules are broken up by ionizing radiation.
But a new study published July 24 in The Astrophysical Journal Letters tips the scales in favor of an alternative theory — that carbon dioxide and other molecules are coming from within the aerial, perhaps even from a surface liquid ocean.
A research team led by Richard Cartwright of the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, made the findings by collecting chemical spectra of the moon using NASA’s James Webb Space Telescope and comparing them to spectra of simulated chemical compounds in the laboratory. Ariel contains some of the most carbon dioxide-rich deposits in the Solar System, adding up to 10 millimeters (0.4 in) or more in thickness to the Moon’s trailing hemisphere. Among those deposits was another disturbing discovery: the first clear signals of carbon monoxide.
„It shouldn’t be there. You need to drop 30 Kelvin [minus 405 degrees Fahrenheit] Carbon monoxide is stable,” Cartwright said. Ariel’s surface temperature, meanwhile, averages 65 F warmer. „Carbon monoxide needs to be seriously replenished, there’s no question.”
Radiolysis may still account for some of the filling, he said. Laboratory experiments show that both carbon dioxide and carbon monoxide can be produced. Therefore, radioactivity can provide a recombination source and calculate the enriched abundance of the two molecules in the trailing hemisphere of Ariel.
But there are many questions about the Uranian magnetosphere and the extent of its interaction with the planet’s moons. Even during Voyager 2’s flyby of Uranus nearly 40 years ago, scientists suspected that such interactions might be minimal because Uranus’s magnetic field axis and the orbital planes of its moons are offset by about 58 degrees from each other. Recent models have supported that prediction.
Instead, much of the carbon oxides may have come (or are still occurring) from chemical processes in the water ocean beneath Ariel’s icy surface, escaping through cracks in the moon’s icy exterior or even through exploding craters.
What’s more, the new spectral observations indicate that Ariel’s surface may contain carbonate minerals — salts that can only form through contact of liquid water with rocks.
„If our interpretation of that carbonate feature is correct, that’s a big decision because it means it must have formed indoors,” Cartwright said. „This is something we need to confirm with certainty through future observations, modeling, or some combination of techniques.”
Ariel’s surface is covered with gas-like valleys, transverse craters, and soft spots thought to be caused by cryovolcanic seepage, leading researchers to suspect that the moon may have already existed or may still be active. A 2023 study led by APL’s Ian Cohen suggests that Ariel and/or its sister moon Miranda may release material into Uranus’ magnetosphere, including plumes.
„All of these new insights underscore just how compelling the Uranian system is,” Cohen said. „Many of us in the planetary science community look forward to future missions exploring Uranus, whether unlocking keys to how the Solar System formed, better understanding the planet’s complex magnetosphere, or determining whether these moons are potential ocean worlds.”
In 2023, with its Planetary Science and Astrophysics Decadal Survey, the planetary science community prioritized the first dedicated mission to Uranus, raising hopes that a scientific expedition to the turquoise ice giant is on the horizon.
Cartwright sees this as an opportunity to gather valuable data about the Solar System’s ice giants and their potential ocean-bearing moons, both of which have applications for worlds discovered in other star systems.
But it’s also an opportunity to get definitive answers that are only possible by being on the computer. For example, most of Ariel’s observed craters—suspicious openings in its interior—are on its back. An alternative explanation for why carbon dioxide and carbon monoxide are somehow leaking through those craters could be an alternative explanation for why they are so abundant at the rear of the aerial.
„It’s a bit of a stretch because we haven’t seen much of the moon’s surface,” Cartwright cautioned. Voyager 2 captured only about 35% of Ariel’s surface during its brief journey. „We’re not going to know until we do more dedicated observations,” he said.
JWST reveals evidence for CO ice, enriched CO2 deposits, and carbonates from inside ArielThe Astrophysical Journal Letters (Open Access)
Astronomy