Chapter 14. The Atmospheres of Uranus and Neptune

14.1 Introduction

Uranus and Neptune, as observed by Voyager 2

The goals of this module: After completing this exercise, you should be able to:

1. Describe the compositions of the atmospheres of Uranus and Neptune.
2. Explain why the two planets have different appearances.
3. Explain why their appearances have changed since first observed by Voyager 2.

In this module you will explore:

1. Why Uranus and Neptune have a distinctive bluish color.
2. How belts and zones form on Neptune.
3. Why Neptune's Great Dark Spot formed.

Why you are doing it: When Voyager 2 first visited Uranus and Neptune, astronomers noticed that while they are similar in many ways, their appearances were very different. Uranus had a near-featureless appearance while Neptune had bands running across it similar to Jupiter and Saturn. While astronomers have theories as to why these planets appear as they do, there are still some unanswered questions. Finally, since Voyager's visit, astronomers have since observed Uranus and Neptune with the Hubble Space Telescope and have found that their appearances have changed since they were first observed.

14.2 Background

Voyager 2 Spacecraft

In 1986 the Voyager 2 spacecraft flew by Uranus to give us our first up close look at this distant world. The spacecraft continued on and passed Neptune in 1989. Remarkable images were returned that gave us our first detailed views of these remote worlds.

Normally, the trip to these outer planets would take decades, but NASA scientists used a technique called "gravity assist" to cut the time to 9 years from Earth to Uranus with an additional 3 years to reach Neptune. The trick is to use a planet’s gravity to accelerate the spacecraft, which is sometimes referred to as the “slingshot effect.” The trajectory chosen for the Voyager 2 spacecraft boosted its velocity as it passed first Jupiter and then Saturn. Were it not for this skillful use of gravity, the Voyager 2 spacecraft would have reached Neptune in 2010!

14.3 Uranus and Neptune: The Blue-Green Planets

In the second half of the 1980's, the Voyager 2 space probe visited Uranus and Neptune, and was able to gather information about their atmospheres. Voyager 2 confirmed that, like Jupiter and Saturn, Uranus and Neptune's atmospheres are composed mostly of hydrogen and helium, the two most abundant elements in the universe and the main constituents of the Sun. The two medium sized planets differ from Jupiter and Saturn, though, in that they both contain higher concentrations of methane in their atmospheres: Uranus contains 2.3% methane, while Neptune contains 3% methane. What effect does this have on the appearances of these two planets?

The animation above depicts a planet with an atmosphere where you can vary the amount of methane and see the effect on in-coming photons. The different colors represents photons with different wavelengths (red is longer wavelength, blue is shorter wavelength). The animation begins with no methane (and assumes that no other elements or molecules have an effect on the color of the planet). Click on play to view the atmosphere with no methane present. Then, increase the percentage of methane in the atmosphere to 2.3% and note the appearance of the planet. Methane absorbs some of the photons and reflects others. This represents the case for Uranus. Next, increase the percentage to 3% and note the appearance of the planet. This represents the case for Neptune.

14.4 Uranus and Neptune Have the Same Surface Temperature

Uranus

While Uranus and Neptune appear similar in size and color, there are some obvious differences between the two. When Voyager 2 passed by these planets, it made a number of observations of what first appears to be twin planets. Two images taken by Voyager 2 are displayed above. As you can see, while Uranus is near featureless, Neptune has bands with varying shades of blue across it, as well as a large dark spot. How can we explain these differences?

First, let's investigate the temperatures of Uranus and Neptune. The surface temperature (or the temperature of the upper cloud layers) for both planets is 55 K. What is remarkable about this is that Uranus lies roughly 19 AU away from the Sun while Neptune lies roughly 30 AU from the Sun. Neptune is a little more than 50% farther away from the Sun than Uranus, and yet is the same temperature. At its distance, Neptune receives only 40% the amount of energy from the Sun that Uranus does, and yet is the same temperature. Why?

Neptune

The animation above shows what may, in part, be an explanation. As planets form, gravity pulls particles in to form the planet. The energy associated with their movement is then converted, in part, into heat, which is radiated away. The heat, though, has to move through the planet before it can reach the surface and escape into space. Therefore, a planet will take some time before it radiates away all of the energy it has generated from gravitational contraction. We know that Jupiter radiates more energy than it receives from the Sun because of this, and it could be that Neptune does as well. But if that is true, then why isn’t the temperature on Uranus higher? Both planets are nearly the same size, and therefore, it should take the same amount of time to radiate away this energy. Yet it seems as if Uranus has already done so, while Neptune has not. For whatever the reason, Neptune still has this extra energy that it is radiating away from its interior, causing it to be as hot as Uranus, even though it is farther away from the Sun.

14.5 Circulation of Neptune's Atmosphere

How does the fact that Neptune is radiating away internal heat affect its atmosphere? Take a look at the figure below:

Summary

Because Neptune is radiating heat from its interior, convection occurs in its atmosphere. Convection is a large-scale circulation of a fluid (liquid or gas) due to a temperature difference. As energy from the interior of Neptune heats the bottom layer of atmosphere, it rises upward, while cooler atmospheric gases descend to take its place. This causes a circulation of gases in the atmosphere, which, along with Neptune's rapid rotation rate of around 16 hours, causes bands of color parallel to the equator that flow in opposite directions across Neptune's surface. The darker bands of blue represent the Belts where the cooler gases are sinking into the lower atmosphere, and the lighter bands of blue represent the Zoneswhere hotter gases are rising into the upper atmosphere. Uranus does not display any color variations (any belt or zonal flow) because it is not releasing any internal energy, which would drive convection, causing these features.

14.6 Neptune's Great Dark Spot

In addition to belts and zones, astronomers also found other interesting features on the surface of Neptune caused by convection in its atmosphere. In the image on the right, taken by Voyager 2, you can clearly see what astronomers call the Great Dark Spot of Neptune. Similar to the Great Red Spot of Jupiter, the Great Dark Spot is a huge storm roughly the size of the entire Earth! Just as on Earth, storms on the Jovian planets are caused by changes in atmospheric conditions, as warm gases meet cooler gases. Along the boundaries between belts and zones, storms such as the Great Dark Spot periodically form.

The flow of atmosphere in the belt regions is in the opposite direction of the flow in the zonal regions. When storms form along their boundaries, instead of moving along with one or the other regions, they tend to simply rotate. When Voyager 2 viewed the Great Dark Spot in the southern hemisphere in 1989, it observed the Great Dark Spot rotating counterclockwise in Neptune's southern hemisphere. Unlike the Great Red Spot on Jupiter, which has been observed for over 400 years, the Great Dark Spot has since disappeared. Recently, astronomers have observed a new dark spot on Neptune in 1995, but this one is located in Neptune’s northern hemisphere.

In addition, astronomers have observed white clouds in the upper atmosphere of Neptune. Astronomers believe that these clouds are composed of methane ice crystals, which form as winds bring methane gas up into the cool upper atmosphere, where it then crystallizes into methane ice clouds. When Voyager 2 observed Uranus it didn’t find any evidence of these methane ice clouds, most likely because they form at lower elevations on Uranus, and Uranus doesn't have a convective atmosphere like Neptune to circulate them up to where we can easily see them.

14.7 A New Look for Uranus

Since Voyager's visit to Uranus in 1986, astronomers have noticed some changes taking place. To understand why, we need to first understand Uranus' seasons.

Click on play to start the animation.
Like the other planets, Uranus rotates on its axis as it revolves around the Sun, but unlike the other planets, Uranus' axis is tilted 98 degrees relative to its orbit around the Sun. This means that during its solstices, one or other of Uranus' poles is pointed almost directly at the Sun. With an orbital period of 84 years, Uranus experiences extreme seasonal variations over long time periods.

When Voyager 2 visited Uranus in 1986, it was experiencing a northern winter solstice and a southern summer solstice; its south pole was pointed almost directly at the Sun. Because of this, the southern hemisphere was in constant sunlight while the northern hemisphere was in constant darkness. The images we received from Voyager 2 were solely of its southern hemisphere.

Since then, the Hubble Space Telescope has observed Uranus as well. The above images compare Voyager's image of Uranus taken in 1986 with Hubble's image of Uranus taken in 2004 (three years before its Equinox in 2007). As you can see, the surface of Uranus has changed somewhat (some differences are due to the fact that the later image is a combined ultraviolet, visible, and infrared image taken by Hubble to enhance cloud features). Astronomers believe that as Uranus passes out of its northern winter into northern spring, the northern hemisphere begins to receive more sunlight. This additional energy may be driving atmospheric storms.

14.8 Quick Check Quiz

Indepth Activity: The Atmospheres of Uranus and Neptune