A good on-line resource for information and images of Uranus is NASA's Space Science Data Center Uranus page.
Thursday April 19
We have now reached the limit of classical astronomy. That is, we have now discussed all the planets that one can see with the naked eye. The remaining planets of the Solar system are all faint enough that one needs a telescope to see them. Thus they were not discovered until modern times.
Uranus and Neptune are both large planets, with radii about four times that of Earth. Pluto is very small, and not at all like the Jovian planets. Also, we have sent a probe to both Uranus and Neptune. Voyager 2 flew by Uranus in 1986 and Neptune in 1989.
Uranus was discovered in 1781 by William Herschel. Herschel was doing a survey of bright stars looking for evidence of stellar parallax. He didn't find any parallaxes (the first parallax measured for any star was about 50 years after this). But he DID discover a faint, blue resolved object. It was not a star, and Herschel was a good enough observer to realize it was not a star. It was a new planet.
It is a cautionary tale to note that there are at least 17 surviving examples of star charts made by astronomers before 1781 that show Uranus on them. But in each case, it was mis-identified as a blue star. More than two centuries later, our best current ground-based technology still doesn't give us a very clear look.
Uranus is about four times the radius of Earth, and about 15 times the mass of Earth. It is so far from the Sun that its orbital period is about 84 years. But the most remarkable gross physical property of Uranus is that it is tipped over on its side. The inclination of its rotational axis to its orbital plane is about 98 degress (compared to 23.5 degrees for the Earth). This means that for about a quarter of each orbit a given pole is pointing more or less straight at the Sun. So Uranus probably has some pretty peculiar seasons.
Why is Uranus tipped over on its side? We don't really know, of course. But the currently favored answer (this shouldn't surprize you, if you've been paying attention) is that something really big slammed into it a long time ago. In this case "really big" means something about Earth-sized.
We can tell from its spectrum that the atmosphere of Uranus is similar to that of both Jupiter and Saturn:
Unlike Jupiter and Saturn, there is no obvious cloud structure to the atmosphere of Uranus. This is because the atmosphere is simply too cold for the sort of strong turbulence that causes the cloud structure we see on Jupiter and Saturn. But there is a belt/zone circulation pattern that becomes apparent when the Voyager images are heavily color-enhanced. And there is even some weak evidence for clouds. The cloud is that small streak at about 1 to 2 o'clock.
The fact that the belt/zone circulation pattern exists at all on Uranus is very interesting. Because it means that the primary driver of this sort of circulation is the planetary rotation rather than the nature of the Solar heating. This must be so because of the strong tilt of Uranus's rotation axis.
The interior of Uranus is a bit more complicated than that of Jupiter or Saturn. The density is considerably higher (about 1.3 g/cc on average). The mass is much lower, so the central density must be lower. This only makes sense if the composition of Uranus includes a much higher fraction of heavy elements than that of Saturn or Jupiter.
There is an outer layer of liquid H2 and He. Inside this, there is a mantle that is a mixture of volatiles ("icy material") and refractory elements (rocky material). And there is a solid core of heavy elements (rocky material plus iron). The core is solid, and the internal pressure is nowhere high enough to produce liquid metallic H2.
One of the really big surprizes of the Voyager 2 fly-by was that, despite this, Uranus has a substantial magnetic field (about 0.75 the strength of Earth's). And the really odd thing is that the magnetic field axis does not go through the center of the planet. It is both tilted (by about 60 degrees), and off-set (by about 0.3 planetary radii).
Now, one reason humans put these remote planetary probes out there is to surprize ourselves. And it sure worked this time. But about 15 years have passed since the Voyager 2 fly-by, and we've had a chance to puzzle this one out. The solution appears to be that the magnetic field isn't generated in the core. It's generated by liquid (and conductive -- don't use an electric razor in the bathtub!) H2O and NH3 in the mantle. That's why the field axis is offset.
Because there is a magnetic field, it turns out we can see evidence for aurorae in the Sun-facing polar regions. The magnetic field also allowed us to measure a good rotation period for Uranus (given the lack of visible atmospheric structure, the faintness of the planet, and the fact that it's been nearly pole-on to us for the last 30 years made that difficult to do otherwise). The rotation period ois about 17.25 hours. This fits in well with the observe oblateness, and composition.
Uranus has a far lower fraction of its mass in H2 and He than do Saturn and Jupiter. Thus it must have formed more slowly than they did. And, given its place in the Solar System, that much makes sense. There was less material in the Solar Nebula that far out in the Solar System, and Uranus moves more slowly in its orbit. But the problem is models indicate it would form TOO slowly. That is, if you try to make a planet that big, that far out in the Solar Nebula, the Nebula disperses before you have much of a planet. Now, we know Uranus is there, so it MUST be possible for it to form. Current models for its formation (and that of Neptune as well) argue that it was more likely to have formed much closer to the Sun -- somewhere between the orbits of Jupiter and Saturn, and then got gravitationally scattered out to its present orbit by Jupiter.
Tuesday April 24
Uranus has a ring system. Like Jupiter's, it is faint and the rings are narrow. The rings were detected in 1977 by stellar occultation. A bright star was going to pass behind Uranus. As this star was monitored before the occulation, its brightness dropped and came back up several times. It was being partly occulted by the rings. We couldn't actually image the rings until the Voyager 2 fly-by. There are ten known major rings, but like Saturn, they are actually composed of many ringlets. The ring material is actually quite dark. It shows up best when it is back-lit. This indicates the rings are composed of small particles. The particles have a very low albedo (about 0.04 - roughly that of soot). They are probably rock/ice particles in which the methane ice (CH4) has been photoreduced a amorphous carbon (that is, soot) by Solar UV radiation.
It is impossible for the rings to be stable structures in and of themselves. Something must be maintaining them unless we got very lucky to catch them before they dissipated. And we think we know what maintains them. Shepherd moons. These are small satellites in orbits that cause them to pull material into orbits between them, and stabilize that material. We see them in some cases. We just suspect they must be there in others.
There are 5 large moons, and at last count 16 tiny ones (including several Shepherd Moons). The five large ones (all known before the Voyager 2 fly-by are named after sprites from The Tempest:
The other tiny moons are very dark, much like the ring particles. The interpretation is the same - that the surfaces of the the small moons are coated with amorphous carbon from the photoreduction of methane.The five large moons are all differentiated bodies, with rocky interiors and icy mantles. Oberon and Umbriel are both dark, and heavily cratered. Titania is of interest because, even though it is heavily cratered, it has no large craters. This would seem to imply that it was resurfaced just after the phase of really heavy bombardment. Lots of objects have struck it since then, but no really large ones.
Ariel has a relatively smooth and bright surface. It also has many very large cracks or channels on its surface. These all indicate that the surface is comparatively young. There may be an orbital resonance at work here that puts sufficient energy into Ariel that it can be resurfaced periodically.
Miranda is too weird for words. It has been seriously messed with. It has many seeminly impossible geologic features. Including the cheveron, a cliff that is at least a dozen miles high, and a stunning collection of grooves and cracks. Early interpretations of Miranda focused on the idea that something smashed it to rubble soon after it formed, but did not hit it hard enough to disperse it. Thus it reassembled itself due to self gravity. A more recent model is that these features are due to slow surface convection. But we really don't understand this one.
A good on-line resource for information and images of Neptune is NASA's Space Science Data Center Neptune page.
After the discovery of Uranus, astronomers worked out its orbit. Over the next 50 years, the orbital parameters became well established. This led to the realization that there was a problem. Given Newton's laws of motion and of gravity, Uranus's orbit did not make sense unless there was another planet realization that there was a problem. Given Newton's laws of motion and of gravity, Uranus's orbit did not make sense unless there was another planet further out.
The first calculation demonstrating this was done by an Englishman, John Couch Adams, in 1845. He predicted the location of a new planet, outside the orbit of Uranus, and he forwarded his results to Sir George Airy, the Astronomer Royal. And Airy basically ignored him. About a year later, a Frenchman, Urbain Leverrier, made the same calculation, and came to the same conclusion. He sent his results to Johann Galle, an astronomer he knew at the Berlin Observatory. Galle went out, and looked that night, and found Neptune exactly where both Adams and Leverrier had predicted.
Ignoring all the political squabbling for credit, this was a smashing triumph for Newtonian physics. By taking the laws of physics, and comparing them with measurement, people had come up with a startling prediction about the Universe which turned out to be true. This is science at its best.
And the funny thing is that Neptune actually shows up on Galileo's star charts. He had observed it close in the sky to Jupiter in the early 1600s, but had marked it down as a blue star. Of course, Galileo ended up in enough trouble without claiming the discovery of a new planet, so perhaps it was best for him that he didn't realize it was a resolved disk he saw.
Neptune is about the same size and mass as Uranus (about four times the size, and 15 times the mass of Earth). And it is the same color: bright blue. This is due to methane in Neptune (and Uranus's) atmosphere. Methane has strong molecular absorption bands in the red part of the spectrum. So a cold atmosphere, rich in methane, will look blue because that is the part of the spectrum not absorbed by the atmosphere.
We got our only really good look at Neptune during the Voyager 2 fly-by in 1989. And we discovered a number of surprizing things about the Neptune atmosphere during that fly-by. First of all, the belt-zone circulation structure is much more obvious on Neptune than on Uranus (the image is a montage of images, showing one of the Neptunian poles) even though Neptune is further from the Sun.
Also, large (and, it turns out, transient) storms are common on Neptune. We see these storms as dark cyclonic spots, and often see associated clouds. The clouds are methane ice crystals, rather than the water aerosols that make Earth's clouds.
Neptune's interior is essentially like that of Uranus. There is a roughly Earth-sized solid core of heavy elements. This is surrounded by a mantle of a mixture of icy and rocky material. Above that, there is an envelope of liquid hydrogen. And, like Uranus, Voyager 2 discovered that Neptune has a magnetic field that is both tilted by a large angle to the rotation axis, and off-set from the planetary center. In both planets, we believe the B-field arises due to liquid, conducting H2O and NH3 in the mantle. Thus, the field is off-set from the planetary center because it arises in the mantle, not the core.
Like the other Jovian planets, Neptune has a ring system. The rings were suspected before the Voyager encounter due to earlier stellar occultation measurements. But there was a perplexing issue: Ring occultations were not always observed. It was as if the rings were just ring segments, rather than complete rings.
It turns out the rings are complete, but they are not smooth. They have strong "azimuthal" variation (that is, variation as one goes around the ring). The rings are much denser in some places than others. This is a pretty weird result. If nothing were maintaining the rings in such a state, the variations would smooth out on something like an orbital timescale. That's days to weeks.
Voyager gave us enough information to work out the probably solution: Shepherd moons. We detect these moons in some cases, and suspect them in the rest. The moons act to pull the material into orbits that are defined by the moons, and thus can be stable for long periods.
The rings of Neptune are dark, much like those of Uranus. And we expect the same reason: The rings are composed of bits of icy material on which the methane has been photoreduced to amorphous carbon. The source material is supposed to be from collisions between Neptune's satellites and small, outer Solar system bodies.
Neptune has 13 known moons. Eleven of these are small objects, six discovered by Voyager and five by HST. The other two were previously known. One of these, Nereid, is also quite small, and on a highly elliptical orbit. Other than that, we know little about it.
The one large moon is Triton. Triton is on a nearly cirular orbit, much closer to Neptune than Nereid. But it is in a retrograde orbit. This strongly suggests that some sort of collision/capture process was responsible for Neptune's moon system.
Although Triton is not a very large moon (its radius is less than that of Earth's Moon, and its mass is *much* less), it is so cold (T ~ 100 K) that it is able to retain a thin atmosphere. The atmosphere is mostly N2 and CH4, and the pressure is only about 0.00001 that of Earth's atmosphere.
Spectroscopy reveals that the surface composition of Triton is mainly N2 ice, with various other ices mixed in. The surface has relatively few craters. From this we estimate that the average age of the surface is only a perhaps 100 Myr. There is evidence for surface buckling and cracking as well. And there are dark patches that are believed to be due to N2 geysers. Solar heating brings the surface N2 to vaporization temperature, and the geyser erupts, with other material forming the resulting dark ejecta.
