Neptune: A Planet Full of Surprises

FROM THE LECTURE SERIES: A Field Guide to the Planets

By Sabine Stanley, Ph.D., Johns Hopkins University

The Voyager 2 flyby of Neptune in 1989 drove home the point that even similar planets can harbor big surprises. For example, instead of the weirdly calm atmosphere of Uranus, Neptune turns out to have some of the fastest winds in the solar system.

An image of Neptune showing the dark spots of giant hurricanes.
Neptune’s localized storms are the fastest storms ever recorded in our solar system. (Image: NASA images/Shutterstock)

The Storms of Neptune

Neptune’s storms really brought into focus the striking differences in the ice giants’ atmospheres. Neptune is the coldest planet, in terms of light from the Sun, yet it has a very dynamic atmosphere, with storms that come and go. The largest storms are giant hurricanes that appear as darker spots and typically have accompanying bright white clouds. Voyager 2 saw an impressive dark spot in the southern hemisphere that was named the Great Dark Spot because of its similarity to Jupiter’s Great Red Spot.

The Great Dark Spot is about the size of Earth and has wind speeds approaching 2400 kilometers per hour. That’s almost 10 times faster than category 5 hurricane winds here on Earth. But the Great Dark Spot wasn’t the only storm seen by Voyager 2. Farther south was a smaller storm dubbed the Small Dark Spot.

Learn more about how our Sun defines our solar system.

The Differences Between Neptune’s Great Dark Spot and Jupiter’s Great Red Spot

Although the extent and fury of the Great Dark Spot are similar to that of Jupiter’s Great Red Spot, there are also some important differences. The Great Dark Spot is dark because of the local absence of clouds. It’s essentially a hole in the methane cloud deck, whereas Jupiter’s Great Red Spot was full of clouds.

The Great Dark Spot also didn’t last very long! After its 1989 observation by Voyager, the region was next looked at in 1994 by Hubble, and by then, both the Great and Small Dark Spots had disappeared. However, in 2016, a similar Great Dark Spot appeared in the northern hemisphere.

The Process behind the Formation of Dark Spots

An image highlighting the dark spots of Neptune.
Giant hurricanes are visible in the form of dark spots with bright white clouds.(Image: NASA/Voyager 2 Team/Public domain)

Although the storms themselves appear as dark holes, they are typically accompanied by bright white methane clouds. These clouds form because the storm itself is a high- pressure area. As the storm moves through a region, the atmosphere in that region sometimes has to go over the storm to higher altitudes. These higher altitudes are colder, causing the methane in the atmosphere to freeze into ice crystals and form the clouds.

This is sort of similar to how water ice clouds form when they pass over mountains here on Earth. Neptune’s white methane clouds also appear in other regions around Neptune. For example, Voyager 2 spotted a grouping of white clouds between the Great and Small Dark Spots. The white cloud spot moved faster around the planet than the Great Dark Spot and so was affectionately named ‘Scooter’ for its blazing speed.

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

Zonal Jets of Neptune

In addition to the storms and clouds, Neptune also has zonal jets—winds that travel eastward or westward in bands around the planet. The directions of these jet streams are very similar to jets on Uranus. That is, there is a large westward jet at the equator and an eastward jet at each pole.

Differences Between the Atmospheric Behavior at Uranus and Neptune

Neptune’s swirling, localized storms are the fastest ever recorded in the solar system. Neptune also has fast zonal jets, at speeds similar to on Saturn. So why is there such a difference between the atmospheric behavior at Uranus and Neptune? The composition of the atmospheres is very similar. That’s why their colors are similar.

Neptune’s atmosphere is about 79% hydrogen and 18% helium. The remaining 3% is methane, and this is where Neptune gets its blueish color, just like Uranus. If you compare Uranus’s and Neptune’s colors, Neptune looks a bit bluer, whereas Uranus is a bit grayer or greener.

Learn more about mighty Jupiter, the ruling gas giant.

The Reason Behind the Differences in the Weather of Neptune and Uranus

The planets are about the same size. Their rotation rates are similar. So what’s causing such an extreme difference in the weather? The most likely culprit is the amount of heating each atmosphere receives from the planet’s interior. Voyager 2 measured the heat coming out of Uranus and Neptune.

Neptune has a large internal heat source, similar to Saturn and Jupiter. This internal heating can help provide energy to the winds and storms we see in Neptune’s atmosphere. The higher internal heat flow at Neptune also means that the temperatures in Neptune’s atmosphere are quite similar to the temperatures in Uranus’s atmosphere, even though Neptune is about 50% farther from the Sun than Uranus.

Why Do Uranus and Neptune Have Different Internal Heat Flows?

Neptune only receives a little over as much solar energy as Uranus does. But this begs the question: Why do Uranus and Neptune have such different internal heat flows? We really don’t know why Uranus would have such extremely low heat flow.

Maybe the planet formed much slower than the other planets; and hence, it was cooler from the start. Or, maybe Uranus experienced a significant loss of heat early on, perhaps due to a catastrophic impact—Uranus is tilted on its side after all. Perhaps, such an impact somehow excavated heat from the interior.

The Seasons of Neptune

Neptune’s axis tilt is similar to Earth’s and so the planet experiences seasons. Unfortunately, for Neptune watchers, a year on Neptune is 165 Earth years long. That means that all the data we have about Neptune’s atmosphere, whether from Voyager 2 or Earth-based telescopes, is less than a quarter of a year. So the only seasonal change we’ve seen on Neptune so far is a southern hemisphere change from spring to summer.

The Density of Neptune

Neptune and its moons in the dark background of space.
Neptune is the densest of all four giant planets. (Image: NASA, ESA, E Karkoschka, and H. Hammel/Public domain)

Neptune is the densest of all four giant planets, denser even than Jupiter. As we go deeper into Neptune, we transition from the atmospheric layer, which is hydrogen and helium-rich, to the ice layer, rich in water, ammonia, and methane. Uranus and Neptune are quite similar in size and mass, with Neptune being about 18% more in mass, yet 2% smaller in diameter. That means Neptune has a slightly higher proportion of ice and rocks, the heavier materials, than Uranus.

That means Neptune also has similar phase transitions as Uranus, like the diamond rain and the superionic water, but they may occur at slightly shallower depths in Neptune compared to Uranus. For Neptune, at a depth of about 5000 km below the clouds, the ice experiences pressures of about 250,000 bars and temperatures over 2000°. Here the ice is a fluid soup of ions. This ionic ice layer is where Neptune’s magnetic field is generated by a dynamo.

The Magnetic Field of Neptune

Scientists were particularly eager to see Neptune’s magnetic field data from the Voyager flyby. A mere three years earlier, Uranus’s magnetic field had been discovered to be multipolar, unlike the dipolar fields of the other planets. Explanations for Uranus’ magnetic field would be tested by observations at Neptune. As data was returned in 1989, it became obvious that Neptune’s magnetic field was multipolar as well.

That doesn’t mean that the magnetic fields of the two planets look the same. In fact, they look quite different, with the multitude of poles sticking out of the planets at different locations. But the key point is that both planets’ magnetic fields are not dominated by their dipolar components.

The Implications of the Multipolar Fields

So what did this mean for explaining these anomalous multipolar fields? Well, first it meant that theories about the Uranus field being a result of the planet’s sideways rotation were no longer compelling. It also meant that the theory that we happened to have caught Uranus and Neptune each in the middle of a reversal also seemed incredibly unlikely. The chances of catching two planets in the process of a reversal seemed, well, astronomical.

So what to do? We need new high-pressure experiments, and state-of-the-art computer simulations, and more data on the interior properties and magnetic fields of both planets. But the fact that Uranus and Neptune have such similar interior structure and composition suggests that their multipolar magnetic fields may be related to something distinctive about their interiors compared to the other planets.

Could Ionic Ice of the Ice Giants Be the Reason behind the Multipolar Fields?

For both Uranus and Neptune, the dynamo is generated in an ionic ice layer, not in an iron core or a metallic hydrogen layer. So, could the distinctive ionic ice of the ice giants be the cause?

On a basic level, all dynamos should be fundamentally the same. The dynamo only cares that there is a conductor capable of generating electrical currents faster than they can be dissipated.

And the ionic layer in Uranus and Neptune is conductive enough to do this. But if the convective flows in Uranus and Neptune are more vigorous and smaller in length scale than the other planets, then they could generate magnetic fields with smaller length scales, and that can lead to a multipolar field.

The challenge with that explanation is that Uranus has an extremely low internal heat flow, yet is somehow supposed to have more vigorous convection, which seems to be a contradiction. Still, one possible difference is that the ionic ice layer is a thin outer shell in Uranus and Neptune. Below the ionic layer, there may be a layer of superionic water that is solid.

If so, the dynamo may not be able to operate down there because superionic water is a crystalline lattice that would prevent movement. This restriction would confine the dynamo to the thin outer layer that’s ionic.

Computer simulations have shown that dynamos operating in thin shells can more easily produce multipolar magnetic fields. So this may be the explanation for the multipolar fields of Uranus and Neptune.

Common Questions about Neptune: A Planet Full of Surprises

Q: Is Neptune all water?

No, Neptune is formed by a solid planet core surrounded by a mixture of thick water, ammonia, and methane. For this reason, it is also known as an ice giant.

Q: What makes Neptune special?

Despite being similar to Uranus, Neptune surprisingly has some of the fastest winds in the solar system. Neptune also has an irregular satellite called Triton. Neptune is also confirmed to have rings.

Q: Is Neptune a gas giant?

Neptune is a solid planet core surrounded by a mixture of thick water, ammonia, and methane. It is also known as an ice giant.

Q: Why is Neptune so hot?

Neptune is so hot because of its axial tilt, which has exposed Neptune’s south pole to the Sun for roughly 40 Earth years

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