March 17 () –
Uranus, the rare turquoise tilted world in the solar system, may have joined Jupiter, Saturn and Neptune as the host of at least one icy moon pumping particles into its planetary system.
In a new study led by the Johns Hopkins Applied Physics Laboratory (APL), researchers reanalyzed data of magnetic fields and energetic particles almost 40 years old taken by NASA’s Voyager 2 spacecraftthe only spacecraft so far to have gone to Uranus.
Their results, accepted for publication in the journal Geophysical Research Letters, suggest that one or two of Uranus’s 27 moons, Ariel and/or Miranda, are adding plasma to the space environment through an unknown and mysterious mechanism. One tantalizing explanation is that one or both moons have oceans beneath their icy surfaces. and they actively shed material, possibly through columns.
The team presented their results at the annual Lunar and Planetary Science Conference.
“It is not uncommon for energetic particle measurements to be a precursor to discovering an ocean world,” he said. it’s a statement Ian Cohen, APL rocket scientist and lead author of the new study.
For example, particle and magnetic field data provided some of the first clues that led to the identification of the two unambiguous oceanic moons in the solar system, Jupiter’s Europa and Saturn’s Enceladus. Those data provided the first compelling evidence that Europa and Enceladus were sources of particles and plasma, that they probably originated in salty liquid oceans below their icy surfaces.
“We have been arguing for a few years that measurements of energetic particles and electromagnetic fields are important not only for understanding the space environment but also for contributing to broader planetary science research,” Cohen said. “Turns out that may even be the case for data that is older than me. It just goes to show how valuable it can be to go into a system and explore it firsthand.”
The growing momentum for a return mission to Uranus and Neptune has spurred various research teams to dive back into previous flyby data, sometimes leading to new findings. Those influenced a panel of planetary scientists last spring to recommend a $4.2 billion flagship mission to Uranus as NASA’s next big planetary mission over the next decade, whose benefits APL’s Kathy Mandt recently detailed in the journal Science.
Fueling that push, Cohen and his colleagues dove into particle data from the APL-built Low Energy Charged Particle (LECP) instrument on Voyager 2, and found something peculiar: a trapped population of energetic particles that the spacecraft had observed while departing from Uranus.
“What was interesting was that these particles were extremely confined near Uranus’s magnetic equator,” Cohen said. Magnetic waves within the system would normally cause them to spread out in latitude, she explained, but these particles were all squeezed near the equator between the moons Ariel and Miranda.
Scientists originally attributed these features to the possibility that Voyager 2 flew through a haphazard stream of “injected” plasma from the distant tail of the planet’s magnetosphere. But that explanation doesn’t hold up, Cohen said. “An injection would normally have a much wider dispersion of particles than was observed.”
Using simple physical models and drawing on nearly 40 years of knowledge since then, the team attempted to recreate the Voyager 2 observations. They determined that the true explanation had to include both a strong and consistent source of particles and a specific mechanism for energizing them. After considering various possibilities, they concluded that the particles most likely came from a nearby moon.
The team suspects that the particles emanate from Ariel and/or Miranda through a vapor column similar to that seen on Enceladus or through sputtering, a process in which high-energy particles strike a surface and eject other particles into space. . “Right now, it’s about 50-50, either just one or the other,” Cohen said.
Regardless, the model suggests that the activation mechanism would be the same: A constant stream of particles flows from the moons into space, where they create electromagnetic waves. Those waves accelerate a small fraction of the particles to energies that the LECP could detect. This process, the team believes, kept the particles seen by LECP so tightly trapped.
However, with only a single observation of the region and no data on the composition of the plasma or measurements of the full range of electromagnetic waves within it, Cohen noted, there is no way to definitively determine the source of the particles.
However, scientists have already suspected that Uranus’s five largest moons, including Ariel and Miranda, may have underground oceans. Voyager 2 images of both moons show physical signs of geological revival, including possible eruptions of water that froze to the surface.
“The data is consistent with the exciting potential for an active oceanic moon there,” Cohen said. “We can always do a more complete model, but until we have new data, the conclusion will always be limited.”