The rings of Uranus and Neptune can help map their interiors

Credit: NRAO/AUI/NSF/S. Dagnello

Mapping the interior of the ice giants is difficult to say the least. Not only are they far away and therefore more difficult to observe, but their constant ice cover makes it extremely difficult to detect what lies beneath. So scientists have to come up with more ingenious ways to see what’s inside. A team from the University of Idaho, Cal Tech, Reed College and the University of Arizona think they may have found a way to look at the structure of the rings of Neptune and Uranus.

However, this isn’t the first technique scientists have used. Previous attempts have attempted to use the general technique of photometry to detect: oscillations on the surface of the planet. Those vibrations can then be correlated to the density of certain parts of the planet’s interior. While the technique worked well for Jupiter, the photometric data we have so far from the ice giants has proved insufficient to determine the same density profiles.

An alternative is to use gravitational oscillations within the planet’s surface. In particular, there is a kind oscillation pattern known as a ‘normal mode’. This oscillation pattern occurs when all parts of a system begin to oscillate at the same sinusoidal frequency. And the gravitational effects of normal oscillations within the planet’s interior can be felt outside and reflected in the rings themselves.






The Trident mission, which would return to Neptune. Credit: Universe Today

It’s also not the first time patterns in a planet’s rings have been used to calculate internal density. Saturn has a better understood ring system than Uranus or Neptune, the two ice giants with known ring systems. Scientists have been conducting seismological analyzes on the Saturn ring system for years using data from Voyager and Cassini. The result is a better understanding of some of the normal modes of the planet’s interior and thus an estimate of the composition of the planet’s core and the rotational speed of most of its material.

Neptune and Uranus each have a range of different rings, although they haven’t been studied as well as Saturn’s. Some of those rings were rounded up by shepherd’s manes. But according to the new paper, the same density reflections of resonance waves visible in Saturn’s rings are likely also present in the ice giant’s ring systems.

In addition, the inner shepherd’s moons themselves can be affected by the same resonances. Some moons can even create their own resonances, such as one known as a Lindblad resonance. More typically seen at the galaxy scale, Lindblad resonances are known for driving density spiral waves, which cause the “arms” seen in many spiral galaxies. But on a much smaller scale, the same effect is happening on planetary ring systems, including those of Saturn, and most likely those of Neptune and Uranus.”






Planetary rings in the solar system. Credit: Universe Today

The problem with using these resonances reflected in the rings is one often faced by science – there isn’t enough data. So far, no probe has stayed long enough to map out the details needed to see the full scope of the ring system. The paper’s authors and numerous other researchers suggest it’s time to send a probe to the ice giants to effectively map the ring systems, moons and countless other recently discovered objects that are so elusive from Earth. . But for now, that mission is still on the drawing board, so we’ll have to wait to fully understand the interiors and ring system of these cold, barren worlds. When we finally send a probe that way, at least we’ll have the mathematical framework to shed light on these dark places.


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More information:
Joseph A. A’Hearn, Matthew M. Hedman, Christopher R. Mankovich, Hima Aramona, Mark S. Marley, Ring seismology of the ice giants Uranus and Neptune. arXiv:2206.05385v1 [astro-ph.EP]† arxiv.org/abs/2206.05385

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