The Rings and Obliquity of Uranus

This week’s guess the planet image comes from Uranus, and shows the planet’s inner rings. Ring systems are present around all of the giant planets in the outer solar system. Those of Saturn are extremely obvious, and had been known about since the observations of Galileo. The other planets have much fainter ring systems, which were not detected until relatively modern times. The rings of Uranus were definitively detected by telescopic observations in 1977, prior to being confirmed by Voyager 2.

Interestingly the rings of Uranus may actually have been detected far earlier than this. William Herschel, who discovered Uranus in the 18^th century reported observations of rings as early as 1797. This observation was never confirmed, and so was largely disregarded by later astronomers. However more recent observations do seem to corroborate some of the properties that he described, suggesting that he could have observed a real feature of the distant planet.

 The rings of Uranus are quite faint, which is why this image, from the Voyager 2 spacecraft is quite a long exposure. They are believed to be quite young compared to the rings of Saturn. The rings are made up of particles of dust and ice, and different rings within the system have somewhat different compositions. Several are dusty, while others are made up of larger particles. Interactions with some of Uranus’ moons are believed to keep the rings fairly stable.  

The rings of Uranus highlight an interesting feature of this planet; Its obliquity, or axial tilt. All of the planets rotate around an axis, which defines their north and south poles, however this axis is rarely vertical with respect to the plane on which they orbit the sun. they are all tilted to a greater or lesser extent and this is why we experience seasons on Earth. A planet’s obliquity if rarely constant, but varies on million year timescales. These obliquity cycles cause dramatic shifts in climate, and so information about a planet’s past orientation is preserved in the geological record. Earth is currently tilted by approximately 23^o, while Mars has an axial tilt of around 25 degrees.

Uranus is tilted by 97.77°, putting its pole almost at right angles to those of the other planets. It is not entirely certain how Uranus ended up on its side with respect to the other planets, but it is hypothesised that it might be the result of impacts by one or more proto-planet sized bodies early in its history. As shown above the ring system of Uranus is also tilted, as are the orbits of the moons. This suggests that these features evolved after the event which caused the dramatic shift in axial tilt, as moons which predated this event would be expected to retain their original behaviour.

 Further reading

http://io9.gizmodo.com/5847495/how-uranus-got-knocked-on-its-side

Image credits

Voyager 2 image of the inner rings of Uranus (NASA/JPL) http://photojournal.jpl.nasa.gov/catalog/PIA00142

Hubble Space Telescope image of Uranus (NASA Marshall Space Flight Center)https://en.wikipedia.org/wiki/Uranus#/media/File:Uranusandrings.jpg