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Chapter 14: Problem 9

Why are Uranus and Neptune distinctly blue-green in color, while Jupiter or Saturn are not?

Short Answer

Expert verified
Uranus and Neptune appear blue-green because their atmospheres contain methane which absorbs red light and reflects out blue and green light back into space. Saturn and Jupiter lack this quality of methane in their atmospheres and instead, other compounds along with atmospheric and cloud patterns, result in a different color scheme.

Step by step solution

01

Understand Light Absorption and Emission

The color of a planet as seen from space is influenced by the way sunlight interacts with its atmosphere. When sunlight, a form of white light containing all colors (wavelengths) of light, hits a planet's atmosphere, certain colors (wavelengths) of light may get absorbed while others are scattered back into space. The scattered light is the color we see when looking at the planet.
02

Investigate Planetary Atmosphere

Uranus and Neptune have similar atmospheres, comprising largely of hydrogen and helium, with a significant amount of methane. On the other hand, Saturn and Jupiter, while also predominantly composed of hydrogen and helium, do not have an appreciable amount of methane.
03

Effect of Methane on Color Perception

The presence of methane in the atmospheres of Uranus and Neptune plays a crucial role in their bluish-green color. Methane absorbs red light from the spectrum of sunlight hitting the planet and scatters the blue and green light back into space. This scattered blue and green light is what we see, making these planets appear blue-green.
04

Explain Jupiter and Saturn's Color

Saturn and Jupiter are not blue-green largely because their atmospheres lack appreciable amounts of methane that can impart a blue-green coloration. Instead, they have other compounds like ammonia clouds (which give a more white color) and possibly phosphine, which alongside storm patterns, impart shades of yellows, browns and oranges to these planets.

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Most popular questions from this chapter

Why do Uranus and Neptune have higher densities than Jupiter and Saturn?

The brightness of sunlight is inversely proportional to the square of the distance from the Sun. For example, at a distance of \(4 \mathrm{AU}\) from the Sun, sunlight is only \((1 / 4)^{2}=1 / 16=\) \(0.0625\) as bright as at \(1 \mathrm{AU}\). Compared with the brightness of sunlight on the Earth, what is its brightness (a) on Pluto at perihelion (29.649 AU from the Sun) and (b) on Pluto at aphelion (49.425 AU from the Sun)? (c) How much brighter is it on Pluto at perihelion compared with aphelion? (Even this brightness is quite low. Noon on Pluto is about as dim as it is on the Earth a half hour after sunset on a moonless night.)

Use the Stary Night Enthusiast \({ }^{\text {TM }}\) program to observe the five large satellites of Uranus. Open the Favourites pane and click on Guides \(>\) Atlas to display the entire celestial sphere. Open the Find pane and double- click the entry for Uranus to center this planet in the view. (Clicking once on the Space bar will speed up this centering). You can reduce the confusion in this view by removing the background stars by clicking on View \(>\) Stars \(>\) Stars and by ensuring that the celestial grid is removed by clicking on View > Celestial Grid. Using the controls at the right-hand end of the toolbar, zoom in to a field of view of about \(2^{\prime} \times 1^{\prime}\). In the toolbar, set the year to 1986 and the Time Flow Rate to 1 hour. Then click on the Run Time Forward button, the right-pointing triangle on the toolbar. You can scroll on and off the labels for the moons by clicking on Labels > Planets-Moons. (a) Describe how the satellites move, and relate your observations to Kepler's third law (see Sections 4-4 and 4-7). (b) Set the year to 2007 and again click on the Run Time Forward button. How do the orbits look different than in (a)? Explain any differences.

. Discuss some competing explanations of why Uranus and Neptune are substantially smaller than Jupiter and Saturn.

Some scientists are discussing the possibility of placing spacecraft in orbit about Uranus and Neptune. What kinds of data should be collected, and what questions would you like to see answered by these missions?
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