I want to get a deeper look at Uranus. I know, I know. It is dark, full of noxious gases, and is expensive and difficult to get near. Oh. In case you didn’t know, I am speaking of the 7th planet in our solar system.
Now that we have gotten past the schoolyard snickers, lets realize that thanks to the bastardized language we call English, we have three pronunciations that have become rather accepted. We have “You-RAN-us,” the infamous “your-ANUS,” and the most correct due to its connection to the original Greek, “Your-uh-nus.” The name comes from the Greek god of the Sky. It was he that sired the first of the Titans with his wife and mother, Gaia (aka Mother Nature). The planet is interesting in a number of ways.
Uranus was the first planet discovered in recorded history. All the inner six planets, including the Earth, were discovered in prehistory. Uranus was accidentally discovered by William Hershel in March of 1781. He may not have actually been the first to have seen it but was the first to record it and later determine it wasn’t a star. He actually thought it was a comet at first. Considering the telescopes of the day and the fact no new planet had been discovered in historical times would lead anyone to think it something other than a planet. But indeed that is what he found. It was the first planet to be discovered with a telescope and the last to be discovered simply by naked eye astronomy.
Only one spacecraft has ever been to Uranus and that was the famous Voyager 2. This spacecraft, beloved by all planetary scientists, flew past Jupiter in 1979, Saturn in 1981, Uranus in 1986, and Neptune in 1989. It is still out there but with fading power cells and with nothing out there, the imaging cameras have been long turned off but Voyager 2 is still returning magnetic field data and is about to, or has already, crossed into interstellar space. It will never come home and carries with it a gold record, “The Sounds of the Earth.” In the highly unlikely case it is ever picked up by ET, it is expected ET will be able to figure out how to play it. Since the quick flyby of Voyager 2, Uranus has been studied mostly by the Keck telescope and the Hubble telescope. Neither of these telescopes was around back in Voyager’s day.
Uranus is way out in the outer solar system. It is far away, cold, dark, and these worlds, along with its fellow ice giant planet, Neptune, are still quite a mystery. Speaking of ice giants, this is a relatively new category of planet. When I was young, the four outer worlds (ignoring the dwarf planet, Pluto here), Jupiter, Saturn, Uranus, and Neptune were all considered jovian planets, otherwise known as the gas giants. These are worlds composed mostly of gases but it became clear even before Voyager 2, that there was something different about the outer two planets. The most obvious is that our ice giant planets are bluish in color and quite smaller than either Jupiter or Saturn.
When Voyager 2 shot past Uranus it was somewhat of a disappointment to the planetary science community. It was, in visible light, about as interesting as a cue ball. There was almost nothing in the cloud deck of interest. Also, it was just a few days before the Space Shuttle Challenger was destroyed so NASA had bigger issues to contend with than a grand presentation of the latest images of the first ice giant to be explored.
Size comparison and featureless Uranus from Voyager 2
A few years later, Voyager flew by Neptune. The science community expected another featureless blue planet but instead found storms raging all over and lots of dark spots along with fast moving wisps of white clouds moving at incredible speeds. Neptune’s winds are the fastest known in the solar system.
With the masses of the planets better known with the flybys, it became clear that these worlds are not much like Jupiter or Saturn as once thought. In short, although they too are composed of gases, there is a lot less hydrogen and helium and a lot more methane, water, and ammonia in their atmospheres. Furthermore, there didn’t seem to be the strange liquid metallic hydrogen cores like in the jovian worlds. Also, the ice giants have a weaker and stranger magnetic field which may be highly indicative of the idea that the interiors of the ice giants are quite different.
Uranus and Neptune are more massive than the earth (about 15 times whereas Jupiter is 300+ times more massive and Saturn is 95 times more massive than the earth). They are expected to have a hot rocky core and a deep, thick mantle of slushy ices. When I say “ices,” I have to explain what this really means. Don’t think of a frozen slush drink from Seven-Eleven. These ices are not cold. They are quite hot by terrestrial standards, a few thousand degrees Kelvin perhaps. The immense pressure due to the gravity of these planets compresses water and ammonia into liquids and at higher pressures, they become solid which we commonly call ices. Understand that water ice we experience is only one of 15 distinct types, or phases, of solid water. Ammonia also probably experiences similar properties at high pressures. With a high pressure crushing down on these gases and turning them to liquids and solids, the temperature becomes rather high.
These slushy ices are so hot that some of the gas molecule’s outer electrons are stripped away and can move around in almost the same way a metal’s free electrons move around. Since they are partially melted and with a hot interior and very cold exterior (the outer cloud decks are quite chilly in the outer solar system), the ices convect like water in a hot pot on the stove. Moving free electrons sets up a magnetic field. But if the interiors are slushy, then some parts of the interior may move more than others. This might explain the strange magnetic fields.
Uranus in particular seems to be an odd ball. Unlike all the other planet’s that have a magnetic field, the field is not aligned with the center of the planet. If you draw a line through the north and south poles of the magnetic field on Uranus, it misses the geometric center of the planet by a few thousand kilometers, or about a third of the radius of the planet. Its magnetic poles are sticking out of the planet at the mid latitudes! But the biggest and most mysterious feature of Uranus is that the planet is tipped over on its side!
All the other planets have their north and south poles tipped relative to the plane of their orbit about the sun about 24 degrees at most, or way less. Uranus’ angle is 98 degrees. The planet is rolling about its side every 10 or so hours as it revolves about the sun. It takes it 84 earth years to complete one Uranus year. In that time, it experiences seasons like the earth but these are extreme seasons. For 21 years, the southern hemisphere, not just the southern pole will be in near complete darkness while the northern hemisphere is nearly in full daylight. Talk about a real long cold winter for the south! Later in the Uranus year, the planet moves so that the whole planet is exposed to the sun much like earth is in the spring and fall. This is about where it is now. You can continue the path to find eventually the northern hemisphere and pole will go into 21 years of darkness.
Another odd feature of Uranus is that it emits about as much energy as it gets from the sun. This, at first, seems quite normal. But let’s look at it a bit more. Jupiter, Saturn, and Neptune actually emit more light in the infrared than they receive in visible light. This indicates that these three planets have a large interior source of heat that is being generated and working its way out the deep interior. Uranus though, has such a weak internal heat source, that so far we see only about the same amount of energy leaving it (in the form of infrared light) as it gets from the sun.
Out where Uranus is, the sunlight is rather feeble. It is only about 1/600th of what we get on the Earth. At Neptune, it is only about 1/900th. Because of the feeble sun, the lack of weather seen on the Voyager 2 flyby of Uranus wasn’t terribly surprising. It was thought by some that in the cold outer solar system, weather would be weak because of the weak sunlight. After all, Saturn’s weather is a bit muted compared to Jupiter so the trend wasn’t unexpected at Uranus. But then suddenly, at Neptune, the weather is again violent and rich. The idea now, and many thought of this in the days of Voyager 2, is that with Neptune’s really hot interior and very, very cold cloud tops, the heat coming out of Neptune drives the weather into a frenzy like it does at Jupiter and Saturn.
However, we are now seeing more action on Uranus. The Hubble and Keck telescopes are seeing more of the classic bands, dark spots, and high thin, wispy clouds on Uranus now that we are imaging it at equinox. The planet is seeing more sunlight over the whole body of it and maybe this is powering the newly discovered weather. Considering it has a much weaker internal heat source, it makes sense that Uranus’ weather would have a much stronger solar component to it. Maybe the fact it is tipped over so much also has something to do with it.
Enhanced image of Uranus showing high clouds and banding
The bluish color of these two worlds is due to methane. Methane is common in Jupiter, Saturn, and their moons but much more of it is found in the far outer solar system. Methane, in large concentrations, absorbs red light leaving more of the blues and greens to be reflected. The upper atmospheres have a haze of methane. But sometimes some clouds of ammonia or water get pushed above the haze so that the red part of sunlight is not absorbed by the methane haze. With all the visible light now being reflected, these high wispy clouds appear white as they would on Jupiter, Saturn, Mars, Venus, or the Earth for that matter.
I think there is a lot to be learned about both planets but Uranus has some deeper mysteries to be unlocked. Why and how did it get knocked over? What is with the strange magnetic field and the near lack of an internal heat source? Are these two factors related? What is the profile of the temperature and pressure going down the atmosphere? How is the climate controlled considering the extreme differences in solar radiation compared to other worlds of the solar system? The fact that planet seems to go from Neptune like violent weather to much more quiescent periods is incredible to behold and understand and it is all occurring on the same planet! Is there a liquid water ocean deep down at a temperature that isn’t too hot? If so, could there be life?
Here is the short, short of it. To understand more of atmospheric science means we have to study more than just the Earth. Imagine you are a biologist but could only study grass, humans, and bacteria. And for the latter you have one hour with a crappy microscope. You would start to get an idea about complex life but would be missing massive amounts of unifying principles and the data to support or refute them. The same is true with planetary and atmospheric science for the ice giants. We know little about them and they represent an entire kingdom, speaking metaphorically with biology, of which we have some tantalizing information and raising many, many more questions.
Presently I am reading a series of science papers about the atmospheres and weather of these distant planets. Almost all it is based on observations of one spacecraft and from two telescopes. It is interesting and now that Uranus is starting to exhibit weather more like Neptune, and consequently a bit more like Jupiter and Saturn, maybe there are some underlying principles to discover. I have no doubt of this. Physics is physics and fluid dynamics obeys the same physics everywhere. Folks have been modeling these atmospheres and people are getting closer to simulating more and more features of these worlds.
But we don’t know everything yet. But we are learning more and more and each bit of data reinforces or refutes basic planetary/atmospheric concepts that are applicable here on earth. The more we know from all the planetary laboratories in the solar system, from Venus to Neptune, the more deeply we will understand the Earth.
If you get a chance, take a good hard long look at Uranus. You may find it to be more interesting than you once considered. Oh, borrow a telescope or view the images on the internet. Call it your planetary porn. 😉