Uranus is a strange world, knocked on its side and with a tilted magnetic field. Its moons might be even stranger.
Earlier this year, the US National Academies of Sciences, Engineering and Medicine recommended that the next flagship mission in planetary science – one that might cost maybe $4 billion – should go Uranus, with launch targeting the 2030s. Such a mission would be only the second to look into the Uranian system traveler 2‘s 1986 flyby; It would be the first spacecraft to make an extended stay in the neighborhood. And while the ice giant’s atmosphere and interior would be key priorities for the mission, there’s more to see.
“In terms of the scientific questions that we can answer with an orbiter and probe at Uranus, that list is really long,” Richard Cartwright, a planetary scientist at the SETI (Search for Extraterrestrial Intelligence) Institute in California, said during a presentation the annual meeting of the American Astronomical Society’s Department of Planetary Sciences, held earlier this month. “And I’m just thinking about the moons – specifically the big five.”
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Everything said, Uranus has 27 known moons. Closest to the planet itself are the inner ring moons, which Cartwright says are the most densely packed satellite system in the solar system and may exchange material with the rings. Farthest away, all over 2.7 million miles (4.3 million kilometers) from Uranus, are the irregular moons, which orbit backwards and are possibly captured asteroids. according to NASA.
“There are so many unanswered questions about the origins of the irregular satellites,” Cartwright said, noting that mission personnel could arrange for a spacecraft to fly past you as it approaches Uranus itself. “We don’t know much about these guys.”
But on the five remaining moons, dubbed the classic moons, a spacecraft could really shine. These are the large enough moons from which astronomers discovered them Earth until 1950.
Even the smallest and most recently discovered of these moons, Miranda, which is about 500 km wide, embodies the mysteries surrounding the moons of Uranus. “Miranda is really weird,” Cartwright said.
Images from Voyager 2’s flyby show geological features that are difficult to decipher, he noted. Miranda’s canyons are 12 times as deep as Earth’s Grand Canyon, according to NASA, and the lunar surface is unusually thick. Miranda is home to three major “corona” regions unlike anything scientists have seen before, and boasts volcanoes erupting slushy ice “lava.”
“There are craters that look like they’ve been filled in and then craters that don’t look like they’ve been filled in, and in many cases these craters are right next to each other,” Cartwright said. “So something really interesting happened in Miranda’s geological past, maybe multiple times.”
Miranda may be the strangest of the classic moons, but he’s in good company.
Ariel appears to have the freshest surfaces of the five classic moons. Umbriel has the oldest and darkest. Umbriel and the two largest, Titania and Oberon, were little known to Voyager 2, but all four of the largest moons may have buried oceans beneath their icy crusts and perhaps even spit plumes of water into space.
“We clearly need better coverage of these moons,” Cartwright said, noting in particular their northern hemispheres, which Voyager 2 couldn’t see at all. Also, Voyager 2 only saw a snapshot taken in spring in the southern hemisphere.
Much of the work Cartwright envisions on these worlds could be done with the instruments that a Uranus orbiter would carry independently, such as its cameras. But he also encouraged mission planners to add a dust analyzer that could identify compounds based on their weight, as a tool that would be particularly valuable for understanding Uranus’ moons.
“We could actually collect material ejected from the surfaces of these moons, dust grains, and then sweep it up with the dust analyzer and characterize the composition,” he said.
There is still time to propose instruments for the spacecraft. NASA said it could start studying early what a mission could look like in the coming year. However, Cartwright encouraged the scientists not to dally.
“It is important that we begin this mission as soon as possible so that we can reach the Jupiter gravity assist window, reach that window between 2030 and 2034 so we can reach Uranus faster before the system transitions back to southern spring in 2050” , said he said.
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