One of Uranus’s mid-sized moons may have once been home to a subsurface ocean that was more than 100 miles (160 kilometers) deep, and possibly deeper than the deepest point in Earth’s own oceans—an extreme environment that could hold clues to how and where life has taken root elsewhere in the universe.
The study, which appeared in the journal Icarus, suggests that an earlier phase of tidal heating is behind broad-scale melting within this icy moon.
For scale, that depth is about 40 times the average depth of the Pacific Ocean. But because Ariel’s gravity is so low, the pressure at the bottom of such an ocean would be tens of megapascals—much lower than that at the bottom of Earth’s deep sea trenches—allowing salty water and exotic ices to coexist in intricate layers.
A Hidden Ocean Powered by Tides on Ariel
The team’s simulations suggest that Ariel had a time where its orbit would have been more elongated than it is now. In that arrangement, Uranus’s gravitational tugs would have regularly flexed the moon—an internal kneading that generates heat, which is known as tidal dissipation. That energy could have melted much of Ariel’s icy shell, the lead author Caleb Strom and his team noted.
The investigators suggest that the tidal acceleration probably continued as Ariel was caught in some sort of orbital resonance with another Uranian moon. After that resonance was broken, Ariel’s orbit would have circularized, tidal heating slackened and its ocean started to refreeze. Antifreeze compounds like ammonia—which have been spotted on Ariel’s surface—could depress the freezing point, allowing pockets of liquid or slushy brine to persist long after peak heating has faded.
Hinting at this thermal history are the marks scrawled across Ariel’s face. Images from NASA’s Voyager 2 flyby showed wide canyons, long fissures that slashed through the old terrain and large surfaces that appeared relatively smooth—evidence of resurfacing. Such tectonic and cryovolcanic-looking features would be consistent with an interior that heated up, expanded, then forced fresh new material to the surface.
Clues On The Surface And In Its Chemistry
Ammonia-containing ices and carbonaceous material have been reported from Ariel’s surface spectroscopic observations. The molecules decompose relatively quickly under radiation and the hailstorm-like bombardment of micrometeorites, so their presence suggests that they are currently being delivered from within or have arrived very recently. That fits with the notion of active—or at least geologically young—exchange between the surface and subsurface.
One possibility is cryovolcanism: seeing a slurry of water, ammonia and other volatiles spew out and freeze as they flow—the equivalent of molten lava on rocky worlds. But as the study authors note, cryovolcanism on far-off icy moons is still controversial; some planetary scientists contend that volume changes from an ocean freezing up could also crack crusts and resurface plains without actual eruptions. Either way, Ariel’s geology seems to require internal flow that is difficult to explain with straightforward, stagnant models.
Ariel in the Family of Possible Ocean Worlds
If Ariel did have a deep ocean, it would add to an expanding list of potential ocean worlds in the far reaches of the Sun’s realm, which include Europa around Jupiter and Enceladus around Saturn.
The same group of researchers had previously modeled Miranda, another moon of Uranus, and found a similar matched history of tidal heating—suggesting that Uranus may harbor a duo of former or still ocean worlds.
Now, interest in the Uranian system has jumped, due to new observations by the James Webb Space Telescope and fresh analysis of Voyager-era data, as well as the discovery of an additional outer moon that brought the planet’s moon count to 29. These discoveries transform Uranus from a dull, static orb to a dynamic world of ice, rock and chemistry that—even when initiated by mere subtle gravitational kneading—can unleash profound change in the interiors of planets.
What a Mission to Uranus and Ariel Would Discover
It will probably take a dedicated spacecraft to confirm that Ariel has an ocean history. The “Uranus Orbiter and Probe” was cited by the National Academy of Sciences’ Planetary Science Decadal Survey as the next major outer planet mission. With the right payload—a magnetometer to search for induced magnetic fields from a conductive ocean, precision gravity science to map interior structure, high-definition cameras to monitor tectonic activity and infrared spectrometers to inventory surface chemistry—researchers might be able to directly test Ariel’s tidal past.
Co-author Tom Nordheim of Johns Hopkins University Applied Physics Laboratory has stressed that the next key is to go back to Uranus with 21st-century instruments. If Ariel does still possess salty pockets tucked under ice—or unambiguous remnants of a once-great sea—it would expand the list of known ways in which oceans come into being and survive. That in turn focuses the hunt still further for habitable homes in the cold, dim reaches of the solar system.