Uranus as seen by NASA Voyager 2. Credit: NASA/JPL-Caltech
The Solar System has 8 known planets. Seven follow an orderly pattern. They spin in the same direction, point their north and south poles in more or less in the same direction, and have moon systems in which their major moons orbit in planes roughly aligned with the overall plane of the Solar System.
Then there is Uranus.
“Uranus is weird,” Christian Soto, an astronomer with the Space Telescope Science Institute at Johns Hopkins University, Baltimore, Maryland, said at the 246th meeting of the American Astronomical Society, 8-12 June, in Anchorage, Alaska. Weird enough, in fact, that when his team set out to test a seemingly simple hypothesis about its moons, they not only failed to confirm their hypothesis, but found the exact opposite.
Uranus is the 7th planet out from the Sun, and its weirdness is most dramatically evident from the fact that, compared to the rest of the Solar System, it is tilted 98 degrees on its side. That means its poles aim not toward the North Star and the Southern Cross – or any other constellations in Earth’s northern and southern skies – but toward constellations closer to our equator.
Jerry Oltion, an amateur astronomer and telescope maker in Eugene, Oregon, and long-term columnist for Sky and Telescope magazine, put this in perspective by using planetarium software to determine exactly where Uranus’s poles point. Its north pole, he told Cosmos, points towards a star named Sabik in the constellation Ophiuchus, just north of the ecliptic between Scorpius and Sagittarius. Its south pole points toward the border between Orion and Taurus. These are so far out of line with normal north and south poles, Oltion adds, that when NASA’s Voyager 2 made the first (and to date, only) flyby of Uranus in 1986, the planet’s south pole was pointed within a few degrees of directly at the Sun.
The orbits of Uranus’s major moons are similarly unusual, following a plane that, while closely aligned with the planet’s equator, is nearly perpendicular to the orbits of most other major bodies in the Solar System. And to top the weirdness off, planetary scientists have discovered that Uranus’s magnetic field is 59 degrees different from its spin axis. That’s roughly akin to Earth’s magnetic poles lying in Perth and northern Florida.
Soto’s goal was to determine how Uranus’s off-kilter magnetic field interacts with its 4 largest moons: Ariel, Umbriel, Titania, and Oberon.
These moons are all tide-locked to Uranus, meaning that, like Earth’s Moon, they always turn the same face toward their host planet. As a consequence, they also have permanent “leading” and “trailing” sides.
The leading side faces in the same direction as their orbits, while the trailing side faces away from their direction of motion.
That might make it sound like the leading side is the one most exposed to radiation from Uranus’s magnetic field, but the reverse is actually true. That’s because Uranus rotates once every 17 hours and 14 minutes, while none of these moons has an orbital period shorter than 60 hours. This means that Uranus’s magnetic field catches them from behind, sweeping most strongly across their trailing sides.
To determine the effect of this on the moons’ surfaces, Soto’s team used the Hubble Space Telescope to take ultraviolet images of their leading and trailing sides to look for signatures of radiation bombardment. “This is something we call radiolysis,” he said.
Millions of years of heavier bombardment on their trailing sides, he said, should have created more compounds like carbon dioxide, which absorb ultraviolet light. This would make their trailing sides comparatively dark when observed with Hubble’s ultraviolet camera.
But, he said, when the Hubble Space Telescope turned its sight on the moons of Uranus, it found nothing of the kind. For the innermost of the 4 moons, Ariel and Umbriel, there was little if any discernable difference between their leading and trailing sides. For the outer 2, especially the outermost, Oberon, it was the leading side that was darker.
It was, to put it mildly, startling.
One possibility is that Uranus’s magnetosphere (the zone containing radiation trapped by its magnetic field) might not be as active as previously thought. “Or,” Soto said, “it might be a lot more complicated than we thought.”
But that only explains the lack of darkening on the trailing sides of these moons. The darkening on their leading sides means that some other process is also occurring.
Most likely, Soto says, this is related a group of other Uranian moons, known as its irregular moons.
Irregular moons are moons whose orbits are in some way unusual. They might be far distant, highly tilted, extremely elliptical, or even rotating in the opposite direction of most other moons. Most likely, these moons are asteroids captured when they wandered too close to their planet. They tend to be much smaller than the major moons, though they can still be well over 100km in diameter.
The 4 largest moons of Uranus ordered by their distance from the planet. From left to right: Ariel, Umbriel, Titania and Oberon. Credit: CactiStaccingCrane, public domain (CC0 1.0)
Saturn has the most of them – 250, according to the latest tally on Wikipedia. Jupiter has 89. Uranus has only 10 known to date, but since it’s not as well-studied as Saturn and Jupiter, it could have many more, so far undiscovered.
“What we think is happening is that micrometeorites are constantly hitting the surfaces of Uranus’s irregular satellites,” Soto said. “[That] ejects dust into their orbits around the planet.” Over the course of millions of years, he said, “this dust moves inward toward Uranus and eventually crosses the orbits of the other moons. These moons sweep through the dust and pick it up, primarily on their leading hemispheres.”
“The best analogy we can come up with, he says, is driving fast on a highway, and bugs hitting your windshield. That’s what we’re seeing here.”
The reason the leading sides of the inner moons, Ariel and Umbriel, don’t get peppered with large amounts of this dust is probably because the outer ones, Oberon and Titania, shield them from it by scooping it up before it can migrate that far inward, Soto said. But why Oberon appears to be getting so much more than Titania is a mystery.
“That’s one of the weird findings we got,” Soto said. “We have all this dust, but for some reason Titania is safe and Oberon is getting all that dust.”
Next up, he said, are follow-up studies with the James Webb Space Telescope’s near infrared spectrograph, which will observe the same 4 moons in August, September, and January.
But the dream, he said, is a dedicated mission to Uranus, one of only 2 planets not to be visited in the last 39 years. “That’s what we’re hoping to get,” Soto said.
As recently as 2021, in fact, NASA was examining plans for a Uranus Orbiter and Probe, which could reach the giant planet by in the early 2040s. In the current NASA budget cycle, funding for such a mission looks unlikely, but Soto hasn’t given up on the prospect of a mission to revisit Uranus sometime in the future.
“It is very weird, so why not?” he said.