The dwarf planet Quaoar has a ring that is too big for its metaphorical fingers. While all other rings in the solar system lie within or near a mathematically determined distance of their parent bodies, Quaoar’s ring is much farther out.

“For Quaoar, for the ring to be outside this limit is very, very strange,” says astronomer Bruno Morgado of the Federal University of Rio de Janeiro. The finding may force a rethink of the rules governing planetary rings, Morgado and colleagues say in a study published February 8 in Nature.

Quaoar is an icy body about half the size of Pluto that’s located in the Kuiper Belt at the solar system’s edge (SN: 8/23/22). At such a great distance from Earth, it’s hard to get a clear picture of the world.

So Morgado and colleagues watched Quaoar block the light from a distant star, a phenomenon called a stellar occultation. The timing of the star winking in and out of view can reveal details about Quaoar, like its size and whether it has an atmosphere.

The researchers took data from occultations from 2018 to 2020, observed from all over the world, including Namibia, Australia and Grenada, as well as space. There was no sign that Quaoar had an atmosphere. But surprisingly, there was a ring. The finding makes Quaoar just the third dwarf planet or asteroid in the solar system known to have a ring, after the asteroid Chariklo and the dwarf planet Haumea (SN: 3/26/14; SN: 10/11/17).

Even more surprisingly, “the ring is not where we expect,” Morgado says.

Known rings around other objects lie within or near what’s called the Roche limit, an invisible line where the gravitational force of the main body peters out. Inside the limit, that force can rip a moon to shreds, turning it into a ring. Outside, the gravity between smaller particles is stronger than that from the main body, and rings will coalesce into one or several moons.

“We always think of [the Roche limit] as straightforward,” Morgado says. “One side is a moon forming, the other side is a ring stable. And now this limit is not a limit.”

For Quaoar’s far-out ring, there are a few possible explanations, Morgado says. Maybe the observers caught the ring at just the right moment, right before it turns into a moon. But that lucky timing seems unlikely, he notes.

Maybe Quaoar’s known moon, Weywot, or some other unseen moon contributes gravity that holds the ring stable somehow. Or maybe the ring’s particles are colliding in such a way that they avoid sticking together and clumping into moons.

The particles would have to be particularly bouncy for that to work, “like a ring of those bouncy balls from toy stores,” says planetary scientist David Jewitt of UCLA, who was not involved in the new work.

The observation is solid, says Jewitt, who helped discover the first objects in the Kuiper Belt in the 1990s. But there’s no way to know yet which of the explanations is correct, if any, in part because there are no theoretical predictions for such far-out rings to compare with Quaoar’s situation.

That’s par for the course when it comes to the Kuiper Belt. “Everything in the Kuiper Belt, basically, has been discovered, not predicted,” Jewitt says. “It’s the opposite of the classical model of science where people predict things and then confirm or reject them. People discover stuff by surprise, and everyone scrambles to explain it.”

More observations of Quaoar, or more discoveries of seemingly misplaced rings elsewhere in the solar system, could help reveal what’s going on.

“I have no doubt that in the near future a lot of people will start working with Quaoar to try to get this answer,” Morgado says.

Rhoden et al. revisited New Horizons data to explore the source of cryovolcanic flows and an obvious belt of fractures on Charon. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute.

“A combination of geological interpretations and thermal-orbital evolution models implies that Charon had a subsurface liquid ocean that eventually froze,” said Dr. Alyssa Rhoden, a researcher at the Southwest Research Institute.

“When an internal ocean freezes, it expands, creating large stresses in its icy shell and pressurizing the water below.”

“We suspected this was the source of Charon’s large canyons and cryovolcanic flows.”

New ice forming on the inner layer of the existing ice shell can also stress the surface structure.

To better understand the evolution of the moon’s interior and surface, Dr. Rhoden and colleagues modeled how fractures formed in Charon’s ice shell as the ocean beneath it froze.

They modeled oceans of water, ammonia or a mixture of the two based on questions about the makeup.

Ammonia can act as antifreeze and prolong the life of the ocean; however, results did not differ substantially.

When fractures penetrate the entire ice shell and tap the subsurface ocean, the liquid, pressurized by the increase in volume of the newly frozen ice, can be pushed through the fractures to erupt onto the surface.

Models sought to identify the conditions that could create fractures that fully penetrate Charon’s icy shell, linking its surface and subsurface water to allow ocean-sourced cryovolcanism.

However, based on current models of Charon’s interior evolution, ice shells were far too thick to be fully cracked by the stresses associated with ocean freezing.

The timing of the ocean freeze is also important. The synchronous and circular orbits of Pluto and Charon stabilized relatively early, so tidal heating only occurred during the first million years.

“Either Charon’s ice shell was less than 10 km (6 miles) thick when the flows occurred, as opposed to the more than 100 km (60 miles) indicated, or the surface was not in direct communication with the ocean as part of the eruptive process,” Dr. Rhoden said.

“If Charon’s ice shell had been thin enough to be fully cracked, it would imply substantially more ocean freezing than is indicated by the canyons identified on Charon’s encounter hemisphere.”

Fractures in the ice shell may be the initiation points of these canyons along the global tectonic belt of ridges that traverse the face of Charon, separating the northern and southern geological regions of the moon.

If additional large extensional features were identified on the hemisphere not imaged by NASA’s New Horizons spacecraft, or compositional analysis could prove that Charon’s cryovolcanism originated from the ocean, it would support the idea that its ocean was substantially thicker than expected.

“Ocean freezing also predicts a sequence of geologic activity, in which ocean-sourced cryovolcanism ceases before strain-created tectonism,” Dr. Rhoden said.

“A more detailed analysis of Charon’s geologic record could help determine whether such a scenario is viable.”

The study was published in the journal Icarus.

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Alyssa Rose Rhoden et al. 2023. The challenges of driving Charon’s cryovolcanism from a freezing ocean. Icarus 392: 115391; doi: 10.1016/j.icarus.2022.115391

Mimas’ heavily cratered surface suggests a cold history, but its librations rule out a homogeneous interior. Rather, Mimas must have a rocky interior and outer hydrosphere, which could include a liquid ocean or be fully frozen with a non-hydrostatic core. Image credit: NASA / JPL-Caltech / Space Science Institute.

Mimas is the innermost, and smallest (radius = 198.2 km, or 123 miles), regular moon of Saturn.

The moon’s surface is heavily cratered, and it is easily identified by the large Herschel impact basin.

Tectonic activity on Mimas is sparse, and there is no evidence of past or present volcanism.

“In the waning days of NASA’s Cassini mission to Saturn, the spacecraft identified a curious libration, or oscillation, in Mimas’ rotation, which often points to a geologically active body able to support an internal ocean,” said Dr. Alyssa Rhoden, a researcher at Southwest Research Institute.

“Mimas seemed like an unlikely candidate, with its icy, heavily cratered surface marked by one giant impact crater that makes the small moon look much like the Death Star from Star Wars.”

“If Mimas has an ocean, it represents a new class of small, ‘stealth’ ocean worlds with surfaces that do not betray the ocean’s existence.”

Dr. Rhoden and Purdue University graduate student Adeene Denton wanted to better understand how a heavily cratered moon like Mimas could possess an internal ocean.

They modeled the formation of the Hershel impact basin using iSALE-2D simulation software.

The models showed that Mimas’ ice shell had to be at least 55 km (34 miles) thick at the time of the Herschel-forming impact.

In contrast, observations of Mimas and models of its internal heating limit the present-day ice shell thickness to less than 30 km (19 miles) thick, if it currently harbors an ocean.

These results imply that a present-day ocean within Mimas must have been warming and expanding since the basin formed.

It is also possible that Mimas was entirely frozen both at the time of the Herschel impact and at present.

However, the authors found that including an interior ocean in impact models helped produce the shape of the basin.

“We found that Herschel could not have formed in an ice shell at the present-day thickness without obliterating the ice shell at the impact site,” said Denton, who is now a postdoctoral researcher at the University of Arizona.

“If Mimas has an ocean today, the ice shell has been thinning since the formation of Herschel, which could also explain the lack of fractures on Mimas.”

“If Mimas is an emerging ocean world, that places important constraints on the formation, evolution and habitability of all of the mid-sized moons of Saturn.”

“Although our results support a present-day ocean within Mimas, it is challenging to reconcile the moon’s orbital and geologic characteristics with our current understanding of its thermal-orbital evolution,” Dr. Rhoden said.

“Evaluating Mimas’ status as an ocean moon would benchmark models of its formation and evolution.”

“This would help us better understand Saturn’s rings and mid-sized moons as well as the prevalence of potentially habitable ocean moons, particularly at Uranus.”

“Mimas is a compelling target for continued investigation.”

The results were published in the journal Geophysical Research Letters.

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C.A. Denton & A.R. Rhoden. Tracking the Evolution of an Ocean Within Mimas Using the Herschel Impact Basin. Geophysical Research Letters, published online December 26, 2022; doi: 10.1029/2022GL100516