The giant planets II: Ice giants - Uranus (yes, it's the humor lecture) and Neptune, and ring systems


(Additional info.)


The planets to scale from The RASC Calgary Centre

History:

These planets are not observable by naked eye observations. Thus, they were unknown to the astronomers of classical antiquity and the middle ages.

Uranus was discovered in 1781 by William Herschel (British musician, astronomer, pioneer in spectroscopy and discoverer of infrared radiation). He named the planet "Georgium Sidus" after King George III. Received a Knighthood and pension for his efforts. Johann Bode (German astronomer and director of the Berlin Observatory) recommended continuation of Greco-Roman mythology and was ultimately followed. Uranus was father of Saturn.


Galileo's notes recording Neptune from Jane Houston Jones
Neptune was discovered by prediction. In 1846, observations by Urbain Le Verrier revealed that the orbit of Uranus was perturbed by something massive further out. John C. Adams (British astronomer and mathematician, not American president) and Le Verrier (French) calculated the position of the perturbing mass and planet was quickly discovered. (Although disputed, Le Verrier got there first, publishing his results two days before Adams). It turns out it had been previously observed, including by Galileo (right) but not recognized for what it was.

The only spacecraft to visit Uranus and Neptune was Voyager 2. It was a flyby, not an orbiter, so the images, especially of its moons, are not as good as we have for other planets. No additional missions are currently planned.

Comparisons and contrasts


Density: Uranus and Neptune are twins in radius (25,360 km and 24,620 km respectively) but have very different densities:

This indicates either structural differences or compositional differences. Current models suggest that Neptune is compositionally similar to Uranus but with a higher proportion of heavy material.

Composition and differentiation: First, we notice that Uranus and Neptune are distinctly different from the gas giants Jupiter and Saturn compositionally. If they weren't, we would expect them to be less dense than Saturn. Instead they are at least slightly denser than Jupiter. We conclude that they have a higher proportion of heavier elements. Ices and rock are likely candidates. Neither planet likely has the metallic H layer like Jupiter and Saturn, because pressures are insufficient in the outer hydrogen-rich regions. The relatively high density of Uranus indicates that 75-90% of its total mass is heavier than He. Thus, it has a very different composition from the Sun or Jupiter and Saturn. Most recent modeling of the internal structure of Uranus suggests it likely consists of three layers:

Alas, the conclusions that there is a rocky/metallic core inside these planets is based on comparison with Jupiter and Saturn and not actually supported (yet) by positive evidence. An alternative model proposes a mixed inner region of ices and silicates, like a giant Callisto.

Why no metallic hydrogen? Insufficient pressure to form it.

Effective temperature: Neptune is almost twice as far from Sun, yet their effective cloud-top temperatures are strangely similar:

These observations make it seem as if Uranus lacks an internal source of heat, whereas Neptune has a significant heat source. Indeed, like Jupiter and Saturn, Neptune seems to be radiating 2.5 times as much internal heat as it receives from the sun. Presumably primordial heat. Rock and ices retain their heat very well and, as we see from Neptune's density, it probably has a higher proportion of these materials.

This difference in these otherwise similar planets is a real enigma. Possible scenarios to explain it include peculiarities of:


Magnetic fields

When Voyager 2 flew past Uranus, we got our first glimpse of its magnetic field. These were like slightly smaller versions of Saturn's, roughly 50 times the strength of Earth's. Unlike other planets, where magnetic dipole axes align reasonably well with rotational axes (if not perfectly), that of Uranus was offset by 60 deg. Moreover, the center of the field was significantly offset from the center of the planet, causing its northern hemisphere to experience stronger magnetic flux than its southern hemisphere. First speculation was that Uranus had been caught in the middle of magnetic polarity reversal, like are known to occur on Earth. But then Voyager 2 found Neptune to be similarly strange, with a 47 deg. offset. Strange.

To make things more complex, what electrically conductive fluid supported the fields? To our best knowledge, these planets lack:

One hint comes from the odd alignment of the fields, suggesting that they arise not in the center but from some intermediate layer of the planets. Current thinking: The field arises from the ionization of the "ices," themselves. Recall, on worlds like Europa and Ganymede, magnetic fields arise from layers of saline water. The presence of dissolved salt ions is what give rise to electrical currents, not the water itself. But at the pressures of the ice giants' fluid volatile ice layers, water and ammonia ionize readily into: Indeed, this is the tame part. Redmer et al., 2011 speculate that more extreme and exotic ions are formed from water and ammonia at these depths.

The magnetospheres of both planets are remarkable only in their offset from the axis of rotation. As a result, they corkscrew around as the planet rotates, carrying their radiation belts with them. As a result, the moons of Uranus are regularly exposed to ionized particles that seem to have interacted with them extensively, giving their surfaces a darker color than one would expect from ice.

Atmospheric dynamics: Through a telescope Uranus and Neptune appear greenish and are free of cloud structures. Voyager saw almost no cloud structure on Uranus. Neptune shows some white clouds (this is at visible wavelengths!). This is caused by the fact that sunlight penetrates deeply and is scattered by methane (CH4) as it travels back to the surface, giving these planets their blue-green color.

Methane absorption bands are seen in spectra, much stronger than on Jupiter and Saturn. This indicates that the proportion of methane is higher on Neptune and Uranus and is consistent with the higher proportion of ices in the total masses we have already discussed.

Uranus and Neptune are deficient in H and He relative to Jupiter and Saturn. Where the pressure is about 1 bar (i.e. like sea level) the temperature is about 73 K (-200 C). This is the temperature of liquid nitrogen, and is much colder than on either Jupiter or Saturn. Ammonia and water are frozen solid at this temperature and pressure of the visible atmosphere. Hence, no ammonia or water clouds. Such visible clouds as are seen are made of methane crystals.



Uranus from Wikipedia

Uranus

Uranus is pretty nondescript. It is light greenish, with few markings in the atmosphere. The atmosphere is seen as a murky haze. It is composed of H, He (12- 15%) and methane. The atmosphere superrotates, i.e. it rotates faster than planetary interior (in 14-17 hours as opposed to 17.2 hrs).

Axial inclination One of the most striking aspects of Uranus is that its rotation axis is tilted by 97.9 deg. In other words, it rotates almost on its side. Uranus rotates on its axis once every 17.2 hours, and orbits the Sun every 83.75 years. Uranus has at least 27 satellites of which five are proper spherical worlds.

Weather Voyager showed that Uranus has a thin haze forming a "collar" on its sunlit pole indicating that some sort of photochemistry is going on. IR spectra showed that the ratio of H/He is even more like the Solar value (0.15), since He does not precipitate out into metallic H as on Jupiter and Saturn.

Weather is difficult to observe, because the atmosphere is clear and free of clouds. Also, the inclination of its axis makes study difficult. (Axial inclination = 98 deg., so for 42 years one pole received all the solar radiation.) Uranus recently passed its equinox (December 2007). The next one won't be until 2049.


Uranus from Wikipedia
We would expect under these conditions that Uranus might have one big Hadley cell, but Voyager found that the rotation dominates the circulation. It is banded like the other giant planets. Uranus' weather and climate patterns are very curious. The winds decrease toward the equator, and are fastest at high latitudes, near the poles.

Even more curious, although only one pole is heated by the Sun, the temperature is the same at both poles! The heat must be redistributed from the sunlit pole to the rest of the planet, probably by some internal process.


Dark spot on Uranus from Space.com
Seasons: Voyager 2 encountered Uranus close to southern hemisphere midsummer. In the fifteen years since then, Uranus has passed its spring/fall equinox (2007). As the seasons have changed, so has its weather: The short version: Near the equinox, then both hemispheres are illuminated by the Sun every day, Uranus starts to look like Neptune, where this happens year-round.


Winds of Uranus from Wikipedia
Nevertheless, averaged annually, Uranus' poles receive more solar energy than it equator. As a possible consequence, its winds are fastest near the poles.

Rings: Uranus' ring system is second only to Saturn's in size and complexity. Ring particles appear to be icy, as in Saturn, but are dark in color because of "space weathering" by direct exposure to the solar wind as Uranus's magnetic field corkscrews around.

Giant collisions?

Uranus could not have originally formed with its extreme axial tilt, so it has long been assumed that early in its history it suffered a giant collision. Until recently, models of the physics of this collision have been inadequate because they failed to explain the prograde orbits of its moons and rings in Uranus' current equatorial plane. Morbidelli et al., 2012 suggested that for the moons to have formed in their current orbits, Uranus would have needed to have undergone a series of such collisions.


The Neptune system from NASA - Astronomy Picture of the Day

Neptune

Neptune has a more well-defined cloud structure, and it is easier to study its atmospheric flow patterns and its weather. Its axis is tilted at 29.6 deg., similar to Earth, but its seasons are 165 times longer. Voyager observed:

Atmospheric composition: Neptune shows, in addition to H2 (molecular hydrogen), He, and CH4 (methane) in its atmosphere, also measurable N2, CO (carbon monoxide), and HCN (hydrogen cyanide). Neptune's atmospheric profile is similar to that of Uranus.

Continued observation by the Hubble Space Telescope shows that the Great Dark spot has disappeared since Voyager's visit, but that similar cyclones continue to form.

Wind speeds: Here we have a difference. Neptune's equatorial regions experience its highest wind speeds, up to -400 m s-1. This pattern resembles that of Saturn except that the winds are slower than overall rotation, the opposite of Saturn.

Rings: Voyager 2 revealed that Neptune has a thin, dark ring system.

Formation: We expect the ice giants to resemble the gas giants because they probably formed in a similar manner. Why are they smaller than Jupiter and Saturn?

But careful! Much research finds the Solar System's current distribution of mass and angular momentum hard to reconcile with models of its formation. Why is Neptune, which is larger than Uranus, twice as far from the Sun, for instance? Researchers such as Tsiganis, et al., 2005 propose ways in which the ice giants could have formed much closer to the sun, then migrated outward. More to come on this subject.


Key concepts and vocabulary:

Additional reading: