Blue and Green with Envy – Into the West

In this week’s discussion topic, I attempt to answer the question “Why are Uranus and Neptune distinctly bluer than Jupiter and Saturn?”

On Uranus and Neptune, the methane absorbs red, orange and yellow light, reflecting back the blue. In contrast, Jupiter and Saturn have only minor trace amounts of methane and quite a bit more hydrogen and ammonia.

This view of Uranus was recorded by Voyager 2 on Jan 25, 1986, as the spacecraft left the planet behind and set forth on the cruise to Neptune Even at this extreme angle, Uranus retains the pale blue-green color seen by ground-based astronomers and recorded by Voyager during its historic encounter. This color results from the presence of methane in Uranus’ atmosphere; the gas absorbs red wavelengths of light, leaving the predominant hue seen here. Image Credit: NASA/JPL

This Voyager 2 high resolution color image provides obvious evidence of vertical relief in Neptune’s bright cloud streaks. These clouds were observed at a latitude of 29 degrees north near Neptune’s east terminator, the “line” on a planet where daylight meets darkness.
The resolution of this image is 6.8 miles per pixel. The width of the cloud streaks range from 31 to 124 miles, and their shadow widths range from 18 to 31 miles. Cloud heights appear to be around 31 miles.
Image Credit: NASA/JPL

The colors observed in the outer planets are due in large part to the chemical components in their global cloud decks. Chemicals that contribute to color are known as chromophores. The two gas giants, Jupiter and Saturn, appear yellow, orange, brown and red – depending on different chromophores in the bands, belts and spots. Ammonia crystals and water ice can be found in Jupiter’s cloud tops, but convection dredges up other elements and compounds, including phosphorus, sulfur and hydrocarbons. Saturn’s clouds are similar to Jupiter with one exception – they contain phosphine. (Singh, 2015)

Outer Planet	Atmosphere Composition
Jupiter	89.8% hydrogen / 10.2% helium / traces of methane, ammonia, water vapor, other gases
Saturn	96.3% hydrogen / 3.3% helium / 0.4% methane, ammonia, water vapor, other gases
Uranus	82.5% hydrogen / 15.2% helium / 2.3% methane
Neptune	79% hydrogen / 18% helium / 3% methane

(Comins, 2015, pp. 198, 208, 221)

The two ice giants, Neptune and Uranus, contain one particular hydrocarbon which plays a crucial role in giving them their blue appearances. Methane absorbs red light (600 billionths of a meter wavelengths), so sunlight falling on Neptune and Uranus only reflects back the bluer light. The more azure blue-green color of Uranus hints at an additional as-yet-unidentified chromophore in Neptune’s atmosphere responsible for its deep blue hue. (Singh, 2015)

“The increasing amounts of methane are largely responsible for the outer Jovian planets’ blue coloration. Methane absorbs long-wavelength red light quite efficiently, so sunlight reflected from the planets’ atmospheres is deficient in red and yellow photons and appears blue-green or blue. As the concentration of methane increases, the reflected light should appear bluer. This is just the trend observed: Uranus, with less methane, looks blue-green, while Neptune, with more, looks distinctly blue.” (Reeves, 1998)

Now that I know that the two ice giant planets in our solar system appear blue due to the absorption of red and yellow light by methane, I can return to the other blue ball, Earth, and wonder why the sky is blue. Despite the proliferation of cattle in Western Kansas, our atmosphere has little to no methane. Rather, our air is composed of 78% nitrogen, 21% oxygen, 0.035% carbon dioxide and about 1% water.

The blue of our sky is caused by scattering, specifically Rayleigh scattering. Longer wavelength light – red, orange and yellow – pass straight through the Earth’s atmosphere. The shorter wavelength light – especially blue – is absorbed by gas molecules and is then radiated in different directions, scattering it all around the sky. (“Why is the Sky Blue?,” 2013)

Whichever way you look, the scattered blue like reaches you, from everywhere overhead, making the sky appear blue.

I am quite contented to enjoy our blue heavens here on Earth and not be envious of greener pastures on azure Uranus or colder bluer Neptune.

***

References

Comins, N. F. (2015). Chapter 7 The Outer Planets and Their Moons. In Discovering the essential universe (6th ed., pp. 191-232). New York, NY: W.H. Freeman and Company.

Neptune – THE SOLAR SYSTEM. (n.d.). Retrieved from https://solar-sys.weebly.com/neptune.html

Reeves, A. (1998). Astronomy Today. Retrieved from http://lifeng.lamost.org/courses/astrotoday/CHAISSON/NAV/FRAMESET/FRAME13/IDX13-04.HTM

Singh, K. (2015, May 23). Why is Neptune Blue? Retrieved from http://sen.com/features/why-is-neptune-blue

Uranus – Solar System. (n.d.). Retrieved from http://solarsystemparas.weebly.com/uranus.html

Why is the Sky Blue? (2013). Retrieved from http://www.sciencemadesimple.com/sky_blue.html

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