The Jovian Planets: Uranus, and Neptune

Uranus has a radius of 25,500 km, and a mass of 14.5 Earth's. It has a density similar to Jupiter: 1.27 gm/cm3. It also spins rapidly, and has a "day" of 17 hours and 14 minutes. This is the internal rotation period, the cloud layers rotate in about 14 hours. It orbits the Sun at an average distance of 19.2 AU (2.8 billion km), and takes 84 years to complete one trip around the Sun. It was the first planet to be discovered since antiquity, when it was found by William Herschel in 1781. Its name was chosen as Uranus was the father of Cronus (Saturn), and thus grandfather of Zeus (Jupiter).

Comparison to Earth:

Uranus is very strange in that its rotational axis lies very close to its orbital plane. This means that the seasons on Uranus are very long, with 21 years that have near constant daylight ("summer"), and 21 years of almost perpetual darkness ("winter"). Here is a comparison of the obliquity of the planets in the solar system:

How Uranus got to this configuration is a mystery. Presumably, a large body must have crashed-into Uranus and flipped it over (note that it spins backwards compared to the other planets--except Venus). The disk of Uranus is almost completely featureless. There appears to be a high-altitude haze that blocks our view of the cloud deck. Saturn also has this haze layer, but it is not as thick as that of Uranus (Jupiter doesn't really have this structure). The cloud tops of Uranus are very cold, 49 K! Uranus has a structure similar to Jupiter and Saturn, a big ball of hydrogen and helium gas with a rocky core about equal to one half of an Earth mass:

The structure of Uranus is just like the other gas giants. The central pressure is high (8 Mbar), and with a high temperature: 5,000 K. But both Uranus and Neptune are somewhat different from Jupiter in Saturn--they have extensive "ice mantles". Again, this is not the ice you are familiar with---this ice is formed by high pressure (and the temperature is well above the freezing point of water on Earth): the density of this mantle is near 9 gm/cm3. It is still not completely clear whether Uranus even has a rocky core. As there are models that can explain the density that do not include such an feature. Here's the atmospheric profile:

Even though Uranus is mostly featureless, it has been possible to track clouds, and thus deduce the wind speeds in its atmosphere:

The wind speeds are much higher closer to the pole, than near the equator (note that 100 m/s = 360 km/hr = 250 mph). By false color stretching, we can extract some details, showing that there are bands/zones like the other Jovian planets:

And even spots:

If you notice, the last two pictures show radically different orientations of Uranus. In 1986, when Voyager 2 flew by Uranus, the south pole was pointed towards the Sun. But by the time of the HST picture in 2006, we were looking at the equator. Remember that Uranus has an orbital period of 84 years, thus the four seasons each last 21 years. It is 20 years between these two pictures, so our view of Uranus has changed rather dramatically! Here is a small table with times of the Uranian seasons:

Like Jupiter and Saturn, Uranus emits more heat than it recieves from the Sun, but just barely---about the same as that emitted by the Earth. There may be a layer inside Uranus that acts to trap the internal heat, but that remains unclear. Like the other Jovian planets, it has a magnetic field, but a very strange one: The field has a large tilt/angle with respect to the rotational poles (59 degrees), it is also not centered on the core of Uranus:

Like Saturn, Uranus has a ring system, but here the rings have much less material in them, and they are not quite as spectacular:

Uranus now has 27 named moons (from the works of Shakespeare and Pope), several of them quite interesting in their own rite, but none of them are very large (the largest, Oberon, has a radius of 761 km, for a comparison of the satellite systems of the Jovian planets, see the Figure 11.14 in the textbook also shown here:).

The Uranian moon systems is the least massive of the Jovian planets. This might be tied to the strange tilt of the spin axis. If something hit Uranus, then it would have scattered the moons into the solar system, and the small set of moons we see now might have mostly formed from the result of that collision. Here are images of the five biggest moons Oberon (ρ = 1.63 gm/cm3), Titania (ρ = 1.71 gm/cm3), Ariel (ρ = 1.66 gm/cm3), Umbriel (ρ = 1.39 gm/cm3), and Miranda (ρ = 1.20 gm/cm3):

Miranda has a dramatically fractured surface, as if it was stuck together from several large pieces. It is believed that there was enormous geological activity in Miranda's past, when it was stuck in a 3:1 resonance with Umbriel. This caused the orbit to get ellipitical, increasing the tidal heating due to the intense tidal forces exerted on it by Uranus. All of these moons are dominated by water ice, and it is possible that some of them have tenuous atmospheres created by geysers or cryovolcanoes. We had only a brief visit to Uranus 25 years ago, so much of the system remains a mystery.

Neptune

Neptune is much further from the Sun than Uranus (30 AU), and takes 165 years to complete one orbit. On July 12th, 2011, Neptune completed the first full orbit since its discovery in 1846. The story of the discovery of Neptune is rather interesting, and you can read more about it here (it is a story with a bit of intrigue/lying/cheating due to the intense British-French rivalry at the time). Summarizing, however, Uranus was not behaving itself. It was not moving as one would expect, and it was proposed that there must be something out there pulling on it. Calculations were made, and the hunt was on. It was eventually discovered by Galle in Germany. Neptune has 13 moons, many fewer than any of the other Jovian planets.

Neptune has 17.2 Earth masses, so it is more massive than Uranus. Note, however, that it is smaller in size than Uranus, having a radius of 24,700 km. Thus, it must have a higher density: 1.63 gm/cm3. Neptune has a rotation period of 16 hours. Strangely, Neptune's atmosphere appears to be much more active than that of Uranus (maybe the haze layer "freezes out" at this distance, where the cloud-top temperature is colder than that of Uranus), and has several large storms similar to those on Jupiter:

Close-up on high latitude, high velocity clouds:

Additional activity:

The "great dark spot", and the "small dark spot", and "scooter" (the white blob between the two spots--it moved around very quickly).

So, as you have noticed, the images we have presented of Uranus and Neptune show them to have a blue-green color. This is real. As we have discussed, the color of an object indicates that it is good at reflecting that color. In the case of Uranus and Neptune, their color derives from the most important absorber in their atmospheres---methane. Here is the spectrum of methane1:

While Jupiter and Saturn certainly have lots of methane, they are warm enough that there are clouds due to ammonia, water, and sulfur molecules. But at Uranus and Neptune it is too cold at the cloud layer to have such molecules--methane dominates the atmosphere at these levels. Thus, it is what shapes the spectra of these two planets. Recently, astronomers have created a new class of planet for Uranus and Neptune because they actually are somewhat different than Jupiter and Saturn. While we still call them "Jovian" planets, the preferred terminology is "ice giants", as inside of Uranus and Neptune there is a large region of water, methane and ammonia and is a strange form of "ice":

Here's a close-up of Neptune's internal structure:

Neptune is more massive than Uranus, and this increases its density and that means that it is slightly smaller than Uranus. Here's a graphic showing the sizes of the Jovian planets (compared to Earth):

Neptune generates much more internal heat than Uranus, as it emits 2.6 times as much energy as it recieves from the Sun. This might explain why Neptune appears to have much activity in its atmosphere. Neptune only receives 40% of the sunlight that Uranus gets, but the atmosphres of both have similar temperatures due to the internal heat emitted by Neptune. "Neptune is the farthest planet from the Sun, yet its internal energy is sufficient to drive the fastest planetary winds seen in the Solar System. Several possible explanations have been suggested, including radiogenic heating from the planet's core, conversion of methane under high pressure into hydrogen, diamond and longer hydrocarbons (the hydrogen and diamond would then rise and sink, respectively, releasing gravitational potential energy), and convection in the lower atmosphere that causes gravity waves to break above the tropopause"2. Neptune has a very similar magnetic field to Uranus in that it is tilted off of the rotational axis (by 47 degrees), and also offset from the center, but even more so than Uranus: it is located one half of a radius from the center3:

Neptune also has a very tenuous ring system like the other gas giants:

Neptune has one very large moon (of the thirteen known) named Triton, that is 2,700 km in diameter, and has a density of ρ = 2.06 gm/cm3:

Triton is the only large moon in the solar system that orbits its planet "backwards" (retrograde). It also has a large inclination with respect to the equator of Neptune (157 degrees). Because of this, and the expectation that large moons form in the left over accretion disk that forms the parent planet (and thus form near the equator of the planet), it is believed that Triton is a captured "Kuiper belt" object (next class) like Pluto. The combination of this orbital tilt, along with Neptune's own axial tilt (obliquity), Triton has seasons like those of Uranus, only longer! The capture of Triton probably disrupted the orbits of any small moons that were orbiting Neptune at the time--this explains the paucity of moons around Neptune. It also explains why Neptune's second biggest moon, Nereid (ρ = 1.5 gm/cm3), has the most eccentric orbit of any moon in the solar system (e = 0.75!), varying in distance from Neptune by a factor of 7! The rest of the moons are small chunks of rock (the largest of these has a mass of 0.0025 of Triton), several are associated with the rings of Neptune.

Triton has a tenuous atmosphere that is mostly composed of nitrogen. A surprising discovery were these black plumes seen in the photo below that appear to be liquid nitrogen geysers. The plumes seem to be blowing in a wind from the point of origin:

"Triton has a tenuous nitrogen atmosphere, with trace amounts of carbon monoxide and small amounts of methane near the surface. Like Pluto's atmosphere (next class), the atmosphere of Triton is believed to have resulted from evaporation of nitrogen from the moon's surface. The surface temperature is at least 35.6 K (-237.6 C) because Triton's nitrogen ice is in the warmer, hexagonal crystalline state, and the phase transition between hexagonal and cubic nitrogen ice occurs at that temperature. An upper limit in the low 40s (K) can be set from vapor pressure equilibrium with nitrogen gas in Triton's atmosphere. This temperature range is colder than Pluto's average equilibrium temperature of 44 K (-229 C). Triton's surface atmospheric pressure is only about 0.017 millibar.

Turbulence at Triton's surface creates a troposphere (a "weather region") rising to an altitude of 8 km. Streaks on Triton's surface left by geyser plumes suggest that the troposphere is driven by seasonal winds capable of moving material of over a micrometer in size. Unlike other atmospheres, Triton's has no stratosphere, and instead consists of a thermosphere from 8 to 950 km above the surface, and an exosphere above that. The temperature of Triton's upper atmosphere, at 95 kelvins, is higher than the temperature at the surface due to heat deposited from space. A haze permeates most of Triton's troposphere, believed to be composed largely of hydrocarbons and nitriles created by the action of sunlight on methane. Triton's atmosphere also possesses clouds of condensed nitrogen that lie between 1 and 3 km from the surface."3

Triton haze layer: