Uranus is an oddity in the solar system for several reasons, but mainly because of its 28 perpendicular rotations relative to the other planets in the solar system. Researchers at the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, recently reexamined energetic particle and magnetic field data collected nearly 40 years ago by NASA’s Voyager 2 spacecraft.

During its three-day flyby of Uranus in 1986, Voyager 2 captured the only in-situ observations of the planet and its system. A new analysis of these three-decade-old observations has revealed a mysterious source of energetic ions in the planet’s magnetosphere. Their results suggest that one or two of Uranus’ 27 moons – Ariel and/or Miranda – add plasma to the space environment through an unknown and mysterious mechanism.

The intriguing possibility that one or both moons have oceans beneath their icy surfaces and are actively ejecting material, perhaps through plumes, is one explanation.

Ian Cohen, a space scientist at APL and the lead author of the new study, said: “It is not uncommon for energetic particle measurements to be a precursor to the discovery of an ocean world.”

After diving back into the particle data from the APL-built Low-Energy Charged Particle (LECP) instrument on Voyager 2, the research team discovered something peculiar: a trapped population of energetic particles that the spacecraft had observed as it left Uranus.

The interesting fact about these particles: they were extremely confined near the magnetic equator of Uranus.

Cohen said, “Magnetic waves within the system would normally cause them to spread across latitude, he explained, but these particles were all close to the equator between the moons Ariel and Miranda.”

Scientists originally attributed these features to Voyager 2 which may have flown through an accidental stream of plasma “injected” from the far tail of the planet’s magnetosphere. But that explanation does not hold. An injection would normally have a much greater spread of particles than what was observed.”

It started to look like a whodunit. The researchers tried to reproduce the Voyager 2 observations using simple physical models and building on knowledge more than 40 years later. They concluded that the real explanation requires both a powerful, reliable source of particles and some mechanism to activate them. After weighing many alternatives, they concluded that the particles most likely came from a nearby moon.

The team believes Ariel and/or Miranda are responsible for the particles, either through a vapor plume similar to Enceladus’s or through a process known as sputtering, in which high-energy particles hit a surface and send more particles into space.

The activation mechanism would therefore likely be the same according to modeling: the moons continuously release particles into space, where they produce electromagnetic waves. Those waves accelerate some small parts of the particles to the energy that LECP can detect. The team theorizes that this procedure caused the particles detected by LECP to be so tightly confined.

Cohen said, “However, with just a single observation of the region and no plasma composition data or measurements of the full range of electromagnetic waves within it, there is no way to definitively determine the source of the particles.”

Still, scientists have already suspected that Uranus’ five largest moons — including Ariel and Miranda — may have subsurface oceans. Voyager 2 images of both moons show physical signs of geological resurfacing, including possible eruptions of water that froze on the surface.

Magazine reference:

  1. Ian James Cohen, Drew L. Turner, Peter Kollmann, et al. A localized and surprising source of energetic ions in the Uranian magnetosphere 1 between Miranda and Ariel. ESS Open Archive. 13 March 2023. DOI: 10.22541/essoar.167874177.75849866/v1