A recent study concludes that the moon of Uranus, Ariel, probably transported a deep and hidden ocean and that its orbit was once stretched to divide the surface.
The analysis connects a scheme of fractures and smooth plains with water under the ice and orbital tightness which loaded the crust until its rupture.
Modeling indicates an orbital eccentricity close to 0.04 and a layer of water close to 100 miles deep. This combination can apply enough stress to break the ice and surface the parts of the moon in a geologically recent period.
Ariel’s ocean under an icy shell
The images of traveling 2 show a world of valleys crossing and long chasms that cross older craters. These cuts are grabens, crust blocks that fall between parallel faults.
The nearly smooth plains nearby have probably formed by cryovolcanism, where water sludge and other volatile substances broke out and spread in the low areas.
The mixture of very old and very young terrains refers to a long, intermittent geological history. Only the southern hemisphere was photographed in detail, so that the global scheme remains incomplete.
An eccentricity value close to 0.04 means that Ariel’s orbit was slightly outside a circle. This small discrepancy can considerably increase the bending of the moon during each trip around Uranus.
When the researchers compared the constraint cards to the orientations of faults and ridges, the planned hot spots have lined up on the main belts of fractures.
Constraints strong enough to break the ice appeared in regions which seem disturbed in the mosaics of travel.
The same set of tests favored a layer of water with a thickness of the order of 100 miles under a relatively thin shell of ice. With ice that thinks above the liquid, fractures can spread and reach the surface.
Tides that warm Ariel
The tidal stress is the push and the traction caused by the modification of the gravitational forces through a body. As Eeriel got closer and moved away slightly during each orbit, the strength of this attraction changed over time and the place.
Repeated flexions transform orbital energy into internal heat. This heat can melt certain parts of the interior, thin the ice shell and maintain an underground oceanic liquid sheltered from the sun.
If the flexion fits diminished as the orbit circulates, the water layer can cool and freeze from the top and the bottom. This change can leave behind a fossil motif of fractures and smooth areas.
When the Ariel ocean frozen
It is difficult to estimate the era at which the Ariel ocean existed because its surface shows both ancient and relatively young regions. Fractures, plains and cryovolcanic flows overlap to blur the chronology.
The researchers suggest that the most recent resurfacing has probably occurred in recent billions of years, for a period of strong heating of the tides, when Ariel’s orbit was much more elliptical.
If this chronology is correct, the Ariel ocean may have gradually frozen as its orbit stabilized and heat production decreased.
The remains of this process could explain why ammonia and smooth plains still appear on the surface today, preserving the last traces of internal activity before the moon becomes geologically calm.
Signs in Ariel’s ice
The near infrared spectra show ammonia compounds on the surface of Ariel. The ammonia deteriorates quickly under the bombing of loaded particles, its presence therefore indicates a new contribution coming from below.
The ammonia also lowers the freezing point of water, which allows the liquid to persist at cooler temperatures. This chemistry supports the idea that Ariel’s interior once moved the matter to the surface.
Any persistent ocean would probably be a mixture of water and salts with dissolved ammonia. Such a solution would modify the way heat moves and the way the cracks develop in the ice.
Explore the Ariel Ocean
The decennial planetary study recommended an orbiter and a Uranus probe as a top priority for the next major mission.
A dedicated visit could map the invisible northern hemispheres and check whether the fractures and the crests planned appear where the models await them.
“We find evidence that the Uranus system could house twin ocean worlds,” said Tom Nordheim of the applied physics laboratory at Johns Hopkins University (Jhuapl), stressing why it is important to look more closely.
“But anyway, we need an ocean to be able to create the fractures we observe on the surface of Ariel,” said Dr. Alex Patthoff of the Planetary Science Institute (PSI). The argument in favor of an ocean is also based on the adjustment of the observed cracks.
A mission would not be content to take better photos. It could measure gravity and magnetism to probe the internal structure, sample the composition of the high resolution surface and observe the bending of the crust during each orbit.
Models like these do more than explain old images. They transform the surface scars into a recording of the basement, which we can check with new data.
The study is published in Icare.
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