Uranus鈥檚 largest moons, Titania and Oberon, may be hiding buried oceans. Surface temperatures averaging around -200掳C mean the water-rich worlds are covered in ice, but radioactive elements deep inside these moons may keep some of their interior water melted.
The smaller moons that orbit closer to Uranus get most of their internal warmth from tidal heating, in which the gravity of the planet stretches and flexes each moon鈥檚 core, creating friction and heat. Earlier research has suggested that some of these moons may have subsurface oceans, but tidal heating wouldn鈥檛 be enough to melt the ice beneath the larger, more distant moons鈥� frozen surfaces.
at Arizona State University and at the University of California, Santa Cruz, calculated whether two of those distant moons, Titania and Oberon, could have oceans anyway, kept liquid by heat from the decay of radioactive elements in their cores. This also depends on how porous the ice shells of the moons are 鈥� a less porous shell is better at conducting heat and allowing it to escape away into space 鈥� and whether they contain ammonia, which lowers the melting temperature of a water-rich solution.
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The researchers found that if Titania had an ice shell porosity of more than 12 per cent, or more than 10 per cent ammonia in its sea by weight, heat from the radioactive decay in its core could have provided enough heat to maintain a liquid ocean more than a kilometre thick that would still be there today, billions of years after the moon formed.聽The numbers are similar for Oberon, as the two moons are roughly the same size.
Ammonia has been found on several other icy bodies in the solar system, and Earth鈥檚 moon has an average porosity of about 12 per cent extending down several kilometres, so these numbers are far from outrageous, says Nimmo. 鈥淚鈥檇 bet they do have oceans,鈥� he says. 鈥淚t would not be at all surprising.鈥�
Future missions to Uranus could potentially detect these oceans by how they interact with the planet鈥檚 magnetic field, and planetary scientists have already proposed several such missions to launch in the next decade or so.
Icarus
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