URANUS & NEPTUNE

Key Concepts

(1) Uranus and Neptune are nearly identical in their internal structure.

Uranus, shown below in an image taken by Voyager 2, was discovered serendipitously in March 1781 by William Herschel. Uranus was the first planet discovered using a telescope. Herschel, looking in the direction of the constellation Gemini, found what he described as ``a curious nebulous star, or perhaps a comet''. Watching it move day by day, he determined that it was a planet, with an orbit twice as large as that of Saturn.

The orbital period of Uranus, determined by Herschel, is 84 years. The rotation period, determined by Voyager 2, is 17 hours.

Neptune, seen below in a Voyager 2 image, was discovered in 1846, with the help of Newton's Law of Gravity and Newton's Laws of Motion. When the orbit of Uranus was plotted, it was discovered not to be a perfect ellipse. Even after accounting for the gravitational effects of Jupiter, Saturn, and the other planets, there was an unexplained perturbation to the orbit of Uranus. Two astronomers, Adams in England and Leverrier in France, independently came up with the hypothesis that Uranus was being tugged slightly by a planet lying OUTSIDE its orbit. Adams and Leverrier predicted where the hypothetical planet should be found. When a telescope was pointed in that direction, there was Neptune!

The orbital period of Neptune is 165 years; the rotation period is 16 hours. Thus, all the Jovian planets spin more rapidly than the Earth.

Although Venus and the Earth are sometimes called twin planets, it is Uranus and Neptune which are the pair of planets most similar to each other. Uranus is only 3% larger in radius, and 15% smaller in mass than Neptune. The insides of of the two planets are nearly identical, with a differentiated structure. From the outside in: Unlike Jupiter and Saturn, Uranus and Neptune have relatively little hydrogen and helium (Jupiter and Saturn have tremendously thick mantles of liquid hydrogen and helium). The central icy and rocky regions of Uranus and Neptune are not massive enough to attract and hold large quantities of hydrogen and helium.

Although the interiors of Uranus and Neptune are nearly identical, there is some difference in their atmospheres. Both Uranus and Neptune have a greenish or bluish color (the color pictures above are fairly accurate). This color results from the presence of methane in their atmospheres. In addition to absorbing infrared light (and thus acting as a greenhouse gas), methane also absorbs red light. When sunlight strikes Uranus or Neptune, the red light is absorbed, and the blue and green light is reflected.

The bland appearance of Uranus is probably related to its extreme seasons, as discussed in the next section.

(2) The rotation axis of Uranus is tilted by about 90 degrees, causing extreme seasonal variations in sunlight.

The Jovian planets have a wide range of axial tilts. The extreme tilt of the axis of Uranus has important implications for the weather on that planet. Consider how the seasons vary during the course of one uranian year: And so on...
Note that during summer and winter, the uneven heating of the two hemispheres will tend to make winds blow in the north-south direction, decreasing the importance of the east-west bands that exist on the other Jovian planets. Note, however, that the effects of uneven solar heating are reduced by the fact that Uranus is so far from the Sun. Sunlight on Uranus is 370 times weaker than sunlight on the Earth.

(3) Triton, the giant moon of Neptune, is a cold world with nitrogen geysers.

Uranus has 21 known moons, at the moment. (For the latest information about moons in the Solar System, try the JPL Solar System Dynamics web site.) None of the moons of Uranus, however, are large enough to qualify as a ``giant moon'' by our criterion; they are all smaller than Pluto. The five largest moons of Uranus are large enough to be spherical, differentiated bodies. All the uranian moons, like most moons in the outer solar system, are made of a mixture of ice and rock.

Neptune has only 8 moons, but one of them is the giant moon Triton .  Triton is ``Neptune's frosty moon'', since it is the coldest of the giant moons (being so far from the Sun) and has icecaps at its poles.

The surface temperature of Triton averages a VERY chilly 38 Kelvin (-390 Fahrenheit). The average density of Triton is 2100 kg/m3, similar to the densities of Callisto, Ganymede, and Titan. Like these moons, Triton probably consists of a thick icy mantle over a rocky core.

The image below, taken by the Voyager 2 spacecraft, shows the region near the south pole. The area close to the pole is covered with an icecap which consists of frozen methane (freezing point = 90 Kelvin) and frozen nitrogen (freezing point = 60 Kelvin). There are very few impact craters on the surface of Triton, indicating that the surface is young; it has been resurfaced recently by some sort of volcanic activity.

What sort of volcanic activity can take place at such numbingly cold temperatures? Well, when Voyager 2 swooped past Triton, it saw volcanos (or perhaps geysers is the more accurate term) going off near the south pole. The geysers send long plumes of gaseous nitrogen many kilometers above the surface. Thus, there are three bodies in the solar system which are known to be currently volcanically active:

Triton is in retrograde rotation around Neptune. That is, if you looked down from a vantage point over Neptune's north pole, you would see Neptune rotating in a counterclockwise direction, but you would see Triton revolving in a clockwise. This retrograde revolution, combined with the high inclination of Triton's orbit, is a clue that Triton is a captured body. Because Triton is revolving in the opposite direction to Neptune's rotation, tidal forces are driving it toward Neptune. (In the Earth-Moon system, the Moon is revolving in the same direction as the Earth's rotation, and hence is being driven away from the Earth.) In 100 million years or so, Triton will move inside the Roche Limit, and will be torn apart by tidal forces. When this happens, Neptune will acquire a spectacular ring system. The mass of Triton is many times the total mass of Saturn's rings, and thus will create a ring system that puts Saturn's current rings to shame.

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