Voyager 2 data hid clues about Uranus’ mysterious magnetic field

The Voyager mission is a mine of data and information. The Grand Planetary Tour, which took Voyager 2 to visit all four giant planets in less than 12 years, laid the foundation for everything we know about the outer Solar System. While Jupiter and Saturn have since received their most profound visits, with probes such as Galileo, Cassini-Huygens or Juno, the ice giants still lack them. In that sense, reviewing data from almost forty years ago can still bring great surprises. The last one is the one he exposed a study published in Nature by a group from the California Institute of Technology, which has to do with the mysteries of the planetary magnetic field of Uranus.

The Uranus flyby

Voyager visited Uranus in early 1986, reaching the closest point on January 24 at a distance of 81,000 kilometers from the cloud tops (where the pressure is 1 bar). He discovered many things about that blue-green ball. For example, it discovered 2 of its 13 rings and 11 of its 28 icy satellites. Or numerous radio signals associated, perhaps, with lightning within clouds rich in methane and water. O measured the length of the day, 17 hours and 14 minutes, calculated from the rotation of the magnetic field. And precisely when it comes to the magnetic field, it turned out to be quite strange compared to what one would expect.

NASA/Voyager 2

A strange magnetic field

Earth, Jupiter and Saturn (the planets with a magnetic field that had been carefully studied until then) have a field aligned, more or less precisely, with the axis of rotation. Uranus’s axis of rotation is nearly horizontal (98° tilted perpendicular to the plane of the orbit), so researchers would have expected to find the magnetic field with a similar alignment. But no: Uranus’s magnetic field is completely misaligned with the axis, has an inclination of about 60° and is very weak. Furthermore, it does not originate in the center of the planet, but is off-center and forms at shallow depths in the southern hemisphere of the planet. Uranus’s magnetic field is therefore very asymmetric.

A radiation issue

From time to time, as it rotates, Uranus’s magnetosphere aligns with the solar wind, letting charged particles from the Sun pass through and forming radiation belts (like the Van Allen belt for Earth). The peculiar thing that was observed at the time of the flyby with the Voyager 2 instruments was that the charged particles contained in the radiation belts were much more intense than the models predicted. And furthermore, they were the only ones: the rest of the magnetosphere appeared strangely empty. In the gas giants this is not the case, because at least there are satellites orbiting within the magnetic field and producing other charged particles that add to those from the Sun. Perhaps then the satellites of Uranus were very unproductive?

Space Telescope Science Institute Public Outreach Office

By digging you learn

The new study then dug up the Voyager 2 data and found that it was “distorted” by space weather. Uranus’s magnetic field had changed in the days before the flyby, and if Voyager 2 had arrived a few days earlier, it would have observed a completely different magnetosphere. The origin of the change was in the solar wind. The interaction with the solar wind occurs at a distance from the planet equal to 23 times its radius, a limit called the “arc shock” of the magnetic field. However, if the flow of the solar wind is more intense than usual, the arc shock is compressed and moves closer to the planet: Uranus’s magnetosphere was more compressed than normal and the solar wind had transported many more particles than usual towards the radiation belts and had dragged those from the rest of the magnetosphere. Researchers say that the conditions observed by Voyager 2 are repeated in less than 5% of cases, so it is quite a coincidence that Voyager passed by just at that moment. At any other time, the solar wind would have been 20 times weaker and the magnetosphere much less compressed.

Photograph of the first telescope observation of Uranus.Space Telescope Science Institute Public Outreach Office; Science: NASA; THAT; CSA; STScI. Image processing: Joseph DePasquale (STScI)

Objectives of the Uranus Orbiter and Probe

The problem of the Voyager mission is common to all flyby missions, or even to observations made with space or ground observatories: the lack of continuity. Understanding the details of a complex system like a planet requires, in fact, continuity in observations over a long period of time that can provide high-resolution data on a global scale and for consecutive years. This continuity has been lacking until now in the case of the frost giants. It will also remain missing whether the Chinese Tianwen-4 mission includes (as it seems) a component towards Uranus, while the other will go towards Jupiter, since it will also be a flyby mission. Things could be different if NASA’s Uranus Orbiter and Probe mission (for now only a paper project) is launched, which will be dedicated precisely to the study of Uranus. If so, this mission will reach the Uranian system in 2044 and could help unravel the mysteries of the planet, including those related to its strange magnetic field.

Article originally published in WIRED Italy. Adapted by Andrea Baranenko.