The Solar System:


"The Mutilation of Uranus by Saturn" by Giorgio Vasari (1511-1578 CE; Italian) and Cristofano Gherardi (1508-1556 CE; Italian), circa 1560. In Greek mythology, Uranus (or Ouranos, meaning "sky" or "heaven") was the son and husband of Gaia, Mother Earth. Uranus and Gaia were the parents of the first generation of Titans, and the ancestors of most of the Greek gods. Every night, Uranus covered the Earth in darkness and mated with Gaia, but he hated the children she bore to him. Uranus imprisoned Gaia's youngest children (the Titans) in the abyss of Tartarus, deep within the Earth, where they caused pain to Gaia. She constructed a flint-bladed sickle and asked her sons to castrate Uranus. Only Cronus (Kronos or Saturn), the youngest of the Titans, was willing. He ambushed his father and castrated him, casting the severed genitals into the sea, from which the goddess Aphrodite (Venus) was born.

Credit: Giorgio Vasari and Cristofano Gherardi, circa 1560; Palazzo Vecchio [link]

Featureless planet

In visible light, Uranus lacks an obvious pattern of distinctive multicolor bands as seen on Jupiter and Saturn. But some atmospheric structure (bands and clouds) become apparent when Uranus is viewed at other wavelengths, such as in the infrared and ultraviolet (see below).

Why so blue?

Methane in the atmosphere of Uranus absorbs red light, contributing to its blue color.

This is an image of the planet Uranus obtained by the Voyager 2 spacecraft in 1986 from a distance of approximately 7.8 million miles (12.7 million km).

Credit: NASA/JPL [link]

Uranus is on its side - can you guess why?

Yes, that's right - probably a collision with another planet-sized object early in the history of the Solar System! The impact of an Earth-sized object likely caused the almost 98 degree tilt of Uranus' rotation axis.

This collision also likely caused the formation of most of Uranus' 27 known moons, 18 of which orbit around the planet's equator, in orbits that match the large tilt of Uranus itself. The remaining moons, all of which are very small and orbit closer to the plane of the ecliptic, were likely captured objects.

Rotation axis tilts of the planets. The terrestrial and giant planets are to scale within each group, but the groups are not in scale with each other. Axis tilt data from the Planetary Fact Sheet at the NASA Space Science Data Coordinated Archive.

Credit: D. W. Hoard (2018), assembled from public domain images by NASA
  • Orbit period (length of a Uranus "year") = 84 years

  • Uranus' rotation axis stays fixed relative to the stars (like the rotation axes of Earth and other planets)

      • So each pole points at the Sun at one of the solstices.

      • The equator points at the Sun during equinoxes.

  • This means that the poles receive more sunlight during an orbit, but we find that the equatorial region is hotter.

      • We don’t know why! But there must be some kind of circulation of heat going on deeper in the atmosphere of Uranus.

Two images of Uranus taken by Voyager 2 at a distance of 9.1 million km. The picture on the left is a composite using images from the blue, green, and orange filters, processed to approximate Uranus as the human eye would see it. The atmosphere is very clear, the blue-green color coming from absorption of red light by methane. The image on the right was produced using ultraviolet, violet, and orange filters to exaggerate the contrast. The dark polar hood over the south pole of the planet is visible. The "donut" shapes in the image are camera blemishes, not features on Uranus.

Credit (image and some text): NASA/GSFC-NSSDC [link]

Uranus also has rings!

This Hubble Space Telescope infrared image from 1998 reveals Uranus surrounded by its four major rings and by 10 of its moons.

The colors in the image indicate altitude in the atmosphere of Uranus. Green and blue regions show where the atmosphere is clear and sunlight can penetrate deep into Uranus. In yellow and grey regions the sunlight reflects from a higher haze or cloud layer. Orange and red colors indicate very high clouds, similar to cirrus clouds on Earth.

The orange-colored clouds near the prominent bright band circle the planet at more than 300 mph (500 km/h). One of the clouds on the right-hand side is brighter than any other cloud ever seen on Uranus.

Credit (image and some text): Erich Karkoschka (University of Arizona) and NASA [link]

The rings of Uranus were discovered in 1977 when Uranus happened to pass in front of a background star (named SAO 158687). Uranus eclipsed the star during this event, blocking its light and causing a large dip in the "light curve" of the star. But unexpectedly, there were several smaller dips before and after the main eclipse event. These were caused by the rings of Uranus producing brief partial eclipses of the background star. The rings were subsequently confirmed in images obtained by Voyager 2 in 1986.

Animation demonstrating the discovery of the rings of Uranus.

Credit: Orion 8 (Wikimedia Commons), CC BY SA 3.0 [link]

Moons of Uranus

Montage of Voyager 2 images of Uranus' five largest moons. From left to right in order of increasing distance from Uranus are Miranda, Ariel, Umbriel, Titania, Oberon, and Miranda. By convention, the names of Uranus' moons are characters chosen from the works of William Shakespeare and Alexander Pope. These images show the correct relative sizes and brightnesses of the Moons - they are dark! Coverage is incomplete for Miranda and Ariel; gray circles depict missing areas.

Credit: NASA/JPL [link]

Rings and Moons

The rings of Uranus are shown here captured almost exactly edge-on to Earth. This false-color infrared image from 2007 was obtained by ESO's Very Large Telescope at Paranal, Chile. The image at left identifies several of the moons of Uranus. At right, contrast in the region around the planet has been enhanced to show a thin line, which is sunlight glinting off the ring edges and also reflected by dust clouds embedded within the system. Banding in the atmosphere of Uranus and a bright cloud feature near the planet's south pole are visible on the left side of the images.

Credit: ESO, CC BY 4.0 [link]

Miranda - broken moon

Uranus' icy moon Miranda is seen in this 1986 image from Voyager 2. Miranda's "jumbled" appearance is possibly the result of reassembling after being completely broken into pieces by a collision in the past.

Credit: NASA/JPL-Caltech [link]

The Verona Rupes cliff, visible on the edge of Miranda at the bottom of the previous image, is approximately 15 km tall. By comparison, the Grand Canyon on Earth is only 1.6 km deep and Mt. Everest is "only" 8.8 km tall!

Credit: NASA/JPL [link]


South pole circulation

  • Rages, K. A., Hammel, H. B., & Friedson, A. J., 2004, Icarus. 172, 548 - "Evidence for temporal change at Uranus' south pole” (2004Icar..172..548R)