APOD: Two Hours Before Neptune (2015 Feb 15)

[APOD Robot]

Two Hours Before Neptune

Explanation: Two hours before closest approach to Neptune in 1989, the Voyager 2 robot spacecraft snapped this picture. Clearly visible for the first time were long light-colored cirrus-type clouds floating high in Neptune's atmosphere. Shadows of these clouds can even be seen on lower cloud decks. Most of Neptune's atmosphere is made of hydrogen and helium, which is invisible. Neptune's blue color therefore comes from smaller amounts of atmospheric methane, which preferentially absorbs red light. Neptune has the fastest winds in the Solar System, with gusts reaching 2000 kilometers per hour. Speculation holds that diamonds may be created in the dense hot conditions that exist under the cloud tops of Uranus and Neptune. Twenty-six years later, NASA's New Horizons is poised to be the first spacecraft to zoom past Pluto this July.

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[Ann]

Speaking about atmospheric methane, why is it that no one is surprised that there is methane in the atmosphere of Neptune, but very many people were excited when elevated levels of methane were found in the atmosphere of Mars?

Is the methane in the atmosphere of Neptune a humdrum phenomenon because Neptune is so far away from the Sun that its methane isn't broken down by radiation from the Sun?

Ann

[geckzilla]

Speaking about atmospheric methane, why is it that no one is surprised that there is methane in the atmosphere of Neptune, but very many people were excited when elevated levels of methane were found in the atmosphere of Mars?

The methane of Mars has some chance of being produced by some kind of life while the methane of Neptune and other gas giants is not. I think you already knew that though so I'm not sure what you are confused about.

[Ann]

Well, I am confused. Where does the methane in the atmosphere of Neptune come from? How is it replenished? Or if it is not replenished because it is not broken down, why is it not broken down?

Ann

[Chris Peterson]

Well, I am confused. Where does the methane in the atmosphere of Neptune come from? How is it replenished? Or if it is not replenished because it is not broken down, why is it not broken down?

Neptune has a methane cycle. The solar UV is certainly strong enough to break down the methane in the upper atmosphere. The result is heavy hydrocarbon particles which sink, and when it gets warmer they evaporate, mix with hydrogen and produce methane again, which is buoyant and rises to the upper atmosphere.

Of course, such a cycle can only occur on a gas giant. On a terrestrial planet, you need some other mechanism to replenish atmospheric methane, and there aren't too many mechanisms known.

[Ann]

Thanks, Chris.

Ann