The planet Uranus

The image on the right (Ref) shows the planet Uranus, the first planet discovered in modern times. It was found accidentally by William Herschel while he was searching the sky with a telescope in 1781. It had actually been seen many times before but dismissed as a star.

Uranus is largely hydrogen and helium, but (like Neptune) contains higher proportions of heavy elements than Jupiter or Saturn, and is covered with clouds. Our only direct spacecraft observation of Uranus came from Voyager 2 in 1986.

General Features

Uranus is the 3rd of the Gas Giant planets, and the first planet discovered in "modern" times (1781). It is barely visible from the Earth without a telescope, which explains why it was not known as a planet to the ancients, and why it had been observed various times after the telescope had been invented without the observers realizing that it was a planet and not a star. Documented sitings go back to at least 1690 when Flamsteed catalogued it as a star.

The density is about 1.2 g/cc, implying that it is mostly hydrogen and helium. The mass is about 15 times that of the Earth, which makes it the 4th most massive planet. But its radius of about 4 times that of the Earth makes it the 3rd largest planet, since Neptune has a smaller radius but larger mass (because Neptune's density is higher). The rotation axis is unusual in that it lies only 8 degrees out of the plane of the orbit. Thus, at times the rings and the orbits of the moons appear like a "bulls-eye" when viewed from the Earth. The average rotational period is a little over 17 hours, and its orbital period is 84 years, at a mean separation of 19.2 A. U. from the Sun.

The rings are less extensive than those of Saturn, and may be rock rather than ice. There are 5 large moons and 10 small ones.

The Surface and the Interior

Uranus has a relatively featureless appearance at visible wavelengths. Even from Voyager 2 at a distance of 80,000 km there were few distinguishable features. This is believed to be due to Uranus being further from the Sun than Jupiter and Saturn, which means its temperature is lower (only 58 degrees Kelvin in the upper atmosphere). This decreases the liklihood of chemical reactions making the colorful compounds that give the surface features on Jupiter and Saturn. In addition, the upper atmosphere is thought to have a high-level petrochemical haze that obscures features lower in the atmosphere.

The blue color is because of methane gas in the atmosphere, which absorbs red and orange light strongly, leaving more blue light to be scattered to the observer. The clouds are thought to be mostly methane ice, with a temperature at the cloud tops of about -221 degrees Celsius.

Voyager 2 confirmed the suspicion that Uranus had a magnetic field. The field is about 50 times stronger than that of the Earth and is tilted about 60 degrees with respect to the rotation axis. As a result, the magnetic field moves like a corkscrew as Uranus rotates, as illustrated in the following movie (5 MB). One hypothesis for this behavior of the magnetic field is that it originates in a thin conducting shell outside the core of the planet rather than deep in the core as for the Earth or Jupiter. The pressure would not be high enough for the relevant conducting material to be metallic hydrogen. A mixture of water, methane, and ammonia under sufficient pressure could provide the requisite electrical conductor.

The magnetosphere contains belts of charged particles similar to those of the Earth. The rings and most of the moons orbit within the magnetososphere and thus are protected from the Solar wind.

The Rings

The rings of Uranus (and some of its moons) are shown in the adjacent Hubble Space Telescope image. The rings were discovered from the Earth in 1977 when Uranus occulted (passed in front of) a star and it was noticed that there were dips in the brightness of the star before and after it passed behind the body of Uranus.

This data suggested that Uranus was surrounded by at least 5 rings. Four more rings were suggested by subsequent occultation measurements from the Earth, and 2 additional ones were found by Voyager 2, bringing the total to 11 (the adjacent image shows only some of the brighter rings.

Most of the rings are not quite circular, and most are not exactly in the plane of the equator. The rings vary in brightness with angle around the moon, apparently because they vary in width with angle. The rings are very narrow (some only a few kilometers across) and no material can be detected in the regions between the rings. It is speculated that this stability of the narrow rings may be due to small "shepherding satellites", as discussed in conjunction with Saturn's rings.

The brightest ring is termed the Epsilon ring. Voyager 2 found two small shepherd moons for it, one just inside and one just outside (adjacent image (Ref); the white dots inside the yellow circles). They appear to be about 20-30 km in diameter, and have been named Ophelia and Cordelia.

The Moons

The following image (Ref) shows the 5 largest satellites of Uranus:Miranda, Ariel, Umbriel, Titania, and Oberon (here is further discussion about Oberon).There are 10 additional small satellites.

The satellites of Uranus

The most interesting moon geologically is Miranda, shown on the left. Even though it is only 500 km in diameter, it shows surface geological features that are as varied as any site in the Solar System.

It is not clear why Miranda has been so active geologically. Some theories invoke tidal heating effects earlier in its history, or a collision that tore it apart and allowed it to coelesce again. None are very conclusive.