Uranus is a strange, upside-down world with an unbalanced magnetic field. Its moons can be even stranger.
Earlier this year, the U.S. National Academies of Sciences, Engineering and Medicine advised that the next flagship planetary science mission – one costing perhaps $4 billion – should go to Uranuswith a launch targeting the 2030s. Such a mission would be the second to peer into the Uranian system, after only Traveler 2the 1986 overview; it would be the first spacecraft to make an extended stay in the neighborhood. And while the atmosphere and interior of the icy giant would be key priorities for the mission, there’s more to the picture.
“When it comes to scientific questions that we can address with an orbiter and a probe to Uranus, that list is really long,” said Richard Cartwright, planetary scientist at the SETI (Search for Extraterrestrial Intelligence) Institute in California, during a presentation. at the annual meeting of the American Astronomical Society’s Division of Planetary Sciences held earlier this month. “And I’m just thinking of the moons – specifically the big five.”
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In all, Uranus has 27 known moons. Closest to the planet itself are the inner ring moons, which Cartwright says are the densest satellite system in the solar system and can exchange material with the rings. Farthest, all beyond 2.7 million miles (4.3 million kilometers) from Uranus, are the irregular moons, which orbit backwards and may be captured asteroids, according to NASA.
“There are so many open questions about the origins of irregular satellites,” Cartwright said, noting that mission personnel could have a spacecraft fly past one as it approaches. Uranus itself. “We don’t know much about these guys.”
But the remaining five moons, dubbed the classic moons, are where a spacecraft could really shine. These are the moons large enough that astronomers have spotted them since Earth around 1950.
Even the smallest and last spotted of these moons, Miranda, which is about 500 km wide, epitomizes the mysteries surrounding the Uranian moons. “Miranda is really weird,” Cartwright said.
Images from the Voyager 2 flyby show geological features that are difficult to decipher, he noted. Miranda sports canyons 12 times deeper than Earth’s Grand Canyon, according to NASA, and the moon’s surface is unusually thick. Miranda is home to three large “corona” regions unlike anything scientists have seen, and it boasts volcanoes that erupt melting ice “lava.”
“There are craters that look like they’ve been filled by something, and then craters that don’t look like they’ve been filled by something, and in many cases those craters are right next to each other,” Cartwright said. . . “So something really interesting has happened in Miranda’s geologic past, maybe multiple times.”
Miranda may be the weirdest of the classic moons, but she’s in good company.
Ariel seems to have the coolest surfaces of the five classic moons. Umbriel has the oldest and darkest. Umbriel and the two largest, Titania and Oberon, Voyager 2 are barely glimpsed, but the four largest moons could have oceans buried under their icy crusts, possibly even spouting plumes of water into space.
“Obviously we need better coverage of these moons,” Cartwright said, pointing in particular to their northern hemispheres, which Voyager 2 couldn’t see at all. Also, Voyager 2 only saw one snapshot, one taken when the Southern Hemisphere was in spring.
Much of the work Cartwright envisions on these worlds could be done with the instruments that a Uranus orbiter would carry anyway, such as its cameras. But he also encouraged mission planners to consider adding a dust analyzer that could identify compounds based on their weight as an instrument that would be particularly useful for understanding Uranus’ moons.
“We could actually collect materials that are ejected from the surface of these moons, grains of dust, and then sweep them 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 might look like in the coming year. However, Cartwright encouraged scientists not to hang around.
“It’s important that we start this mission as soon as possible so that we can get that window of Jupiter’s gravity assist, reach that window between 2030 and 2034 so that we can reach Uranus more quickly, before the system returns. in the southern spring in 2050,” he said. said.
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