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Uranus: Detection of X-rays and their Implications

Just as Earth’s atmosphere scatters light from the Sun, both Jupiter and Saturn scatter X-rays produced by our star. In a new study using data from the Chandra X-ray Observatory, we now learn that Uranus likewise scatters X-rays, but with an interesting twist. For there is a hint — and only a hint — that scattering is only one of the processes at work here, and that could produce insights into a system that thus far we have been able to study up close only once, through the flyby of Voyager 2. As the paper on this work notes: “These fluxes exceed expectations from scattered solar emission alone.” Just what is going on here will demand further work.

William Dunn (University College London) is lead author of the paper, which includes co-authors from an international team working with Chandra data from 2002 and 2017. In the image below, the X-ray data from Chandra is superimposed upon a 2004 observation of Uranus from the Keck telescope, which shows the planet at essentially the same orientation as the 2002 Chandra observations. Any image of Uranus reminds us that the ice giant, about four times Earth’s diameter, rotates on its side, with twin sets of rings around the equator.

Image: The first X-rays from Uranus have been captured by Chandra during observations obtained in 2002 and 2017, a discovery that may help scientists learn more about this ice giant planet. Researchers think most of the X-rays come from solar X-rays that scatter off the Uranus’s atmosphere as well as its ring system. Some of the X-rays may also be from auroras on Uranus, a phenomenon that has previously been observed at other wavelengths. This Uranus image is a composite of optical light from the Keck telescope in Hawaii (blue and white) and X-ray data from Chandra (pink). Credit: X-ray: NASA/CXO/University College London/W. Dunn et al; Optical: W.M. Keck Observatory.

Uranus’ orientation is significant given that its rotation axis is almost parallel to the plane of its orbit. Moreover, the tilt of the rotation axis differs from that of the planet’s magnetic field and is also offset from the planet’s center. That should play into the auroral activity observed on Uranus, a useful trait because exactly how the auroral process works has not been determined. On Earth, auroras are the result of electrons traveling along the planet’s magnetic field lines to the poles, and we likewise find auroral activity on Jupiter, where a similar effect is in play. But the Jovian auroras are also fired by positively charged atoms entering the polar regions.

Are the auroras of Uranus one source of the recently detected X-rays? The possibility remains in the mix pending further study. Given that X-rays are emitted on Earth and Jupiter as a result of auroral activity, the unusual axial and magnetic field tilt on Uranus may clarify our understanding of the process, while illuminating how other astrophysical objects emit X-rays.

Without further data, the authors can only speculate, but they see another possible cause in the rings of Uranus. The rings of Saturn are known to produce X-rays, and it is possible that the interactions of the Uranian rings with charged particles could cause an X-ray glow that is detectable.

As we learn more, it’s clear that Uranus is an interesting laboratory for X-ray work, with a highly variable magnetosphere interacting in complicated ways with the solar wind. That hint of processes beyond X-ray scattering, however, may point to an actual mechanism for their production or, as the authors note, may be no more than a statistical fluctuation in the Chandra data. How we develop this picture depends upon further Chandra work and possibly data from XMM-Newton, the European Space Agency’s X-ray observatory. The roadmap for that investigation is explained in the paper:

…an observation lasting a few XMM‐Newton orbits would be needed to provide a Uranus X‐ray spectrum that could be modeled. This would enable a deeper characterization of the spectrum from Uranus, to explore, for example, the presence of fluorescence line emissions from the rings. Further, and longer, observations with Chandra would help to produce a map of X‐ray emission across Uranus and to identify, with better signal‐to‐noise, the source locations for the X‐rays, constraining possible contributions from the rings and aurora. Such longer timescale observations would also permit exploration of whether the emissions vary in phase with rotation, potentially suggestive of auroral emissions rotating in and out of view.

The paper is Dunn et al., “A Low Signal Detection of X‐Rays From Uranus,” JGR Space Physics 31 March 2021 (Abstract / Full Text).

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{ 13 comments… add one }
  • Thomas Mazanec April 2, 2021, 14:43

    I wonder if Neptune also has X-Ray emissions.

    • Paul Gilster April 2, 2021, 15:33

      To my knowledge, Neptune is the only planet from which no X-rays have been detected.

      • ljk April 5, 2021, 9:24

        I am assuming that astronomers have looked for them? Why would Neptune be the only giant planet without them? Did Voyager 2 have any way of detecting x-rays?

        Remember when we used to think only Saturn had rings?

  • ljk April 2, 2021, 15:31

    A mission to Uranus and Neptune could act as massive gravitational-wave detector

    By Paul Sutter 4 days ago

    https://www.space.com/ice-giant-missions-could-catch-gravitational-waves

  • Spaceman April 2, 2021, 15:37

    Paul:

    I am trying to understand the various processes presented here to explain the X-ray spectra. This work hints that there is more to X-ray spectra than scattering of solar X-rays, right? The two other possibilities mentioned are (i) auroras and (ii) X-rays from the planet’s ring systems. Each of these two sources of X-rays, correct me if I am wrong, can still be traced back to incoming X-rays from the Sun? Out of curiosity, would the ionizing radiation in the Uranus system be as harsh as the ionizing radiation around Jupiter and its moons?

    • Paul Gilster April 2, 2021, 20:20

      No, I think you’re right that both the aurora and ring system explanations for the tiny excess still involve radiation from the Sun. But there are other possibilities: Jupiter, for example, has an X-ray aurora thought to be produced by particles from the volcanoes on Io. The authors note this, and also say that the slight excess at Uranus might indicate ‘a larger X-ray albedo’ than either Jupiter or Saturn, or else “additional X-ray production processes at Uranus.’ Just what those might be I don’t know.

  • Jeff Wright April 4, 2021, 2:55

    Now, I have heard it said that ice giants might have diamonds. If so, some may be in orbit of one knocked on its side. Now, could you have cables orbiting an ice giant to bleed power off? Maybe from Saturn to harvest Titans high-test seas? Form plastic bag…cable drops near Saturn..its gravity pulls the cable down and that yanks bags of fuel off Titan. Repeat

    • ljk April 5, 2021, 9:32

      The terrestrial diamond market would never allow such extraterrestrial gems to saturate their market. The potential for so many diamonds would drop their prices to the level of toy jewelry.

      I am willing the bet the same will go for all those other precious and rare minerals that are to be found throughout the Sol system, especially in the Planetoid Belts.

      They will probably find their highest value as part of their usefulness for those who permanently inhabit space some day. They will either never be allowed on Earth or only in quantities too small to affect the markets.

      For most humans, it is always about the bottom line, even when it comes to the rest of the Universe.

      • Alex Tolley April 5, 2021, 18:43

        Indeed. Just look at the effort DeBeers made to devalue synthetic diamonds that were arguably indistinguishable from the mined diamond that they controlled.

        Synthetic diamond made by vapor deposition is more useful, as it could be made into surface coatings that are very resistant to wear and corrosion (but not fire).

  • ljk April 5, 2021, 9:48

    In a more truly civilized society with its priorities in place, we would not even need to make such a plea – especially over three decades later…

    https://www.discovery.com/space/it-s-time-to-return-to-the-land-of-the-ice-giants

  • ljk April 5, 2021, 9:54

    MARCH 31, 2021

    Two strange planets: Neptune and Uranus remain mysterious after new findings

    by Fe­lix Würsten, ETH Zurich

    Uranus and Neptune both have a completely skewed magnetic field, perhaps due to the planets’ special inner structures. But new experiments by ETH Zurich researchers now show that the mystery remains unsolved.

    The two large gas planets Uranus and Neptune have strange magnetic fields. These are each strongly tilted relative to the planet’s rotation axes and are significantly offset from the physical center of the planet.

    The reason for this has been a longstanding mystery in planetary sciences. Various theories assume that a unique inner structure of these planets could be responsible for this bizarre phenomenon. According to these theories, the skewed magnetic field is caused by circulations in a convective layer, which consists of an electrically conductive fluid. This convective layer in turn surrounds a stably layered, non-convective layer in which there is no circulation of the material due to its high viscosity and thus no contribution to the magnetic field.

    Full article here:

    https://phys.org/news/2021-03-strange-planets-neptune-uranus-mysterious.html

  • ljk April 9, 2021, 20:25

    Future Spacecraft Bound for Uranus Could Deploy Its Own Probe

    27 Mar 2021, 14:16 UTC · by Daniel Patrascu

    Most of the spacecraft humankind has sent into the depths of space use at least in part the energy of Sol, the star that is at the center of our solar system. The only problem is, the farther they move from it, the harder it is to harness this energy.

    https://www.autoevolution.com/news/future-spacecraft-bound-for-uranus-could-deploy-its-own-probe-158323.html

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