It is known that the satellites orbiting Uranus have unusual characteristics: some of them are heavily cratered, others have tectonic features or a patchwork of ridges and rocks. With the help of the Hubble Space Telescope, scientists took a closer look at the surface of Uranus’ four largest moons and discovered something unexpected.
For the study, a team of astronomers was looking for signs of interaction between Uranus’ magnetic field and its four largest moons: Ariel, Umbriel, Titan, and Oberon. The moons, named after characters from William Shakespeare’s works, are tidal. This means that one side of the moon, the front, always faces the planet, while the other, the back, is always turned away from Uranus. Scientists have long assumed that the front side would be brighter and the back side darker. Instead, they have discovered the opposite, finding clear evidence that the front side of external satellites is darker than the back.
The findings, presented this week at the 246th meeting of the American Astronomical Society in Anchorage, Alaska, indicate that Uranus’ magnetosphere may not be interacting much with its large moons, despite previous evidence suggesting otherwise.
“Uranus is a strange planet, so it’s always been unknown how much the magnetic field actually interacts with its moons,” said Richard Cartwright, a researcher at the Johns Hopkins University Applied Physics Laboratory and principal investigator of the new study, in a statement.
The icy giant is indeed a certified weirdo. Uranus is tilted 98 degrees, making it the only planet in the solar system with an equator that is almost at right angles to its orbit. One day on Uranus lasts about 17 hours, which is how long it takes the planet to rotate on its axis. The planet makes one revolution around the Sun every 84 Earth years. “During the Voyager 2 flyby [in 1986], Uranus’ magnetosphere was tilted about 59 degrees from the plane of the satellites’ orbit,” Cartwright explained. “So there is an additional tilt to the magnetic field.”
Uranus and its magnetic field lines rotate faster than its satellites orbit the planet, causing the magnetic field lines to constantly sweep past the satellites. As a result, scientists believed that charged particles from the planet’s magnetic field, or magnetosphere, must hit the surface of the backs of the satellites. These charged particles would accumulate on the back of the moon, scattering the radiation and thereby making it darker on the side facing Uranus.
Using Hubble’s ultraviolet capabilities, the scientists behind the study found that the front and back hemispheres of Ariel and Umbriel are actually very similar in brightness. For Titania and Oberon, everything turned out to be the opposite of their expectations. The leading hemispheres of both outer satellites were darker and redder than their trailing hemispheres.
The team of scientists found an explanation for this strange phenomenon. Uranus’s irregular satellites, small distant bodies with eccentric orbits, constantly collide with micrometeorites and eject some of this material into orbit around the planet. Over millions of years, this material moves inward toward the orbits of Titan and Oberon.
As the satellites orbit Uranus, they kick up dust “much like insects hitting the windshield of your car as you drive down the highway,” the Space Telescope Science Institute said in a statement. All this accumulation could be the reason why Titan and Oberon look darker and redder. “So that supports another explanation,” Cartwright said. “It’s dust collection. I didn’t even expect to be interested in this hypothesis, but you know, data always surprises you.”
As for the other two satellites, Ariel and Umbriel, it is possible that Uranus’ magnetosphere does interact with them, but not in a way that results in a light and dark side. The recent discovery adds even more mysteries to Uranus and its system.
