The Atmosphere and Multipolar Magnetic Fields Of Uranus

Among the big planets in our solar system, Uranus is a unique planet. So, what is its atmosphere like, and what phenomena have we discovered that are directly linked to its atmosphere? Also, does Uranus really have a multipolar magnetic field? If so, how is that possible? Read on to know the answers.

Uranus was discovered in 1781 by an amateur astronomer named William Herschel. Astronomers using the first telescopes just assumed that it was a faint star.

However, Herschel recognized that Uranus wasn’t a star. He saw how Uranus looked under different magnifications of his homemade telescope and realized it didn’t behave the same way the stars did when magnification was changed. What convinced him was that Uranus did not have the highly elliptical orbit of a comet. It was much more circular and characteristic of planets.

Why Does Uranus Appear Blue?

When you look at Uranus, it appears as a featureless blue-tinted orb. It has an atmosphere that is mostly hydrogen and helium, just like the atmospheres of the gas giants Jupiter and Saturn. Uranus’s atmosphere is about 83% hydrogen, 15% helium, and 2% methane. It’s this 2% of methane that gives the planet its bluish hue.

Methane is also present in the atmosphere of Saturn’s moon Titan, but Titan isn’t blue, it’s orange. That’s because Titan’s atmosphere creates large haze particles that dominate the color. Although Uranus has a bit of haze, the haze particles are much smaller.

Learn more about Uranus, a water world on its side.

Wind Speeds and Storms on Uranus

Uranus’s atmosphere also has banded zonal winds like Jupiter and Saturn, but there aren’t as many bands, and they are much harder to see. It appears that the atmosphere of Uranus really has only three bands: a westward jet at the equator, and an eastward jet in each of the polar regions.

The wind speeds in these jets approach 900 kilometers per hour. That makes them somewhat stronger than Jupiter’s winds, but weaker than Saturn’s, and almost 2.5 times faster than the fastest wind speeds ever recorded on Earth.

We also see storms on Uranus, but way fewer than in the gas giants. For example, Voyager 2 saw only 10 cloud features across the whole planet during its flyby. Scientists were expecting to find more giant storms, like the Great Red Spot on Jupiter, and these types of storms do appear on Neptune, but none were seen by Voyager 2 on Uranus.

However, ground based telescopes on Earth and the Hubble Space Telescope have spent some time observing Uranus and have found some intermittent giant storms and bright clouds. So there is some activity going on in the atmosphere, although it seems to be less than at the other giant planets.

This is a transcript from the video series A Field Guide to the Planets. Watch it now, Wondrium.

Does It Rain Diamonds On Uranus?

As one descends through the atmosphere of Uranus, the fraction of icy compounds begins increasing as the fraction of hydrogen and helium decreases. At about 7,000 kilometers in depth, one is only about 30% of the way into this giant planet, reaching a location where the volatile ice materials take over as the main components.

Here, the water, ammonia, and methane experience pressures around 100,000 bars and temperatures around 2000 Kelvin. These conditions cause these materials to change into new phases and even break up and form new materials.

Since the deep interior can’t be probed directly, scientists use experiments and computer simulations that mimic the conditions inside Uranus to try and understand what is possible at these high pressures and temperatures.

On Earth, shiny gems are formed at high pressure. Deep into Uranus, diamond rain might form. Diamonds, for example, are a high-pressure phase of pure carbon. They could form if the methane molecules, which also contain carbon, are broken up. The carbon atoms could condense into crystals of diamonds that would rain through the icy layer.

At the very bottom of the icy layer, Uranus may even have an ocean of carbon under high pressure—which is basically liquid diamond—with floating chunks of solid diamond-bergs.

Superionic Water and Multipolar Magnetic Fields

Uranus is also a place where a lot of water is at high pressure and temperature. In the atmosphere, Uranus has water in its usual molecular form, with two hydrogens bonded to an oxygen. But as the pressure and temperature increase with depth, water would break up into ions, like OH and H, which themselves would have positive and negative ionic charges.

After going deeper, at about 1.5 million bars of pressure and a temperature of 4000 Kelvin, a new phase of water, called superionic water, can form.

In superionic water, the oxygen ions from water bond together making a crystal lattice. The hydrogen ions then move freely through the oxygen lattice.

The ionic and superionic phases of water turn out to be important for Uranus’s magnetic field because they are good electrical conductors. Fluid motions in the ionic ice layer can generate the currents that create a dynamo in the planet. Voyager 2 discovered that the magnetic field produced from this dynamo is unusual.

Before Voyager 2 measured the Uranian magnetic field, all observed planetary magnetic fields had looked like simple dipoles, with a north pole and south pole. But Voyager 2 discovered that Uranus’s magnetic field was much more complex, with many poles. That means, there were many locations where the magnetic field lines were plunging directly into, or directly out of, the planet. This is called a multipolar magnetic field.

Why would Uranus have such a multipolar field? One initial theory was that it might somehow be related to Uranus rotating on its side. That quickly turned out to be an unsatisfactory answer once Voyager visited Neptune and found that Neptune also has a multipolar field.

A second early theory was that Uranus might normally have a dipolar field, but maybe we arrived while Uranus was in the middle of a magnetic field reversal, where north and south polarities were changing places. Earth’s magnetic field has such reversals.

If it is assumed that Uranus’s field reverses in the same way that Earth’s field does, then there is only about a 0.2% chance that we would catch Uranus in a reversal. Not good odds, but not impossible. But remember, Neptune is also multipolar.

A better answer as to why Uranus and Neptune have multipolar magnetic fields may stem from the fact that both planets have ionic ice layers. 

The only way we would see multipolar fields at Uranus is if the source of those magnetic fields was fairly close to the surface. And it’s the ionic ice region, whose top is at about 30% depth in the planet, that has all the ingredients needed for creating a dynamo. The ionic ice region is shallow compared to the planet as a whole, allowing us to see multipolar magnetic fields from that dynamo at the surface. 

Learn more about how the solar system family is organized.

Common Questions about the Atmosphere and Multipolar Magnetic Field of Uranus

Uranus is bluish in color. Uranus’s atmosphere contains about 2% methane, which gives the planet its bluish hue. The methane molecules absorb light with longer, red frequencies and reflect the rest of the light, which is why we only see the bluish colors.

Uranus’s atmosphere has three banded zonal winds: a westward jet at the equator and an eastward jet in each of the polar regions. The wind speeds in these jets can reach up to 900 kilometers per hour, which is more than double the fastest wind speeds ever recorded on Earth.

Atmospheric pressure on Uranus can reach up to 100,000 bars and temperatures can reach around 2000 Kelvin. These conditions cause materials like water, ammonia, and methane to change into new phases and even break up and form new materials. We know that on Earth gems are formed at high pressure. So, deep inside Uranus’s atmosphere, carbon atoms could condense into crystals of diamonds that would rain through the icy layer.

Yes, Uranus does have a magnetic field. In fact, Voyager 2 discovered that Uranus’s magnetic field was quite complex, with many poles. There are many locations where the magnetic field lines are plunging directly into, or directly out of the planet. This is called a multipolar magnetic field.

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