How to get a green gas giant?

You need methane, ammonia and atmospheric temperatures of $∼$150 K.

The color of a giant planet depends on the type of clouds dominating its upper atmosphere. These in turn depend on the temperature of the planet, as different molecules reach their gaseous phases at different temperatures. There are a few realistic potential cloud types on gas giants:

• Methane, dominant at $∼$100 K and producing a cyan-aquamarine color
• Ammonia, peaking around $∼$150 K and giving teal hues
• Water, found in atmospheres of $∼$250-300 K and producing deep blues
• Alkali metals, gaseous from $∼$500 K on up and giving the planet a reddish color

Take Uranus as an example. Though most of its atmosphere is in fact hydrogen and helium, methane clouds at the topmost layers are responsible for its green-blue appearance. If you're looking for a greenish planet, the best you're going to get is a layer of ammonia-methane clouds at the top, like Uranus and Neptune.

The teal-ish colors of Uranus and Neptune, on the other hand, are found at lower temperatures. This is because volatiles like ammonia and methane are only found farther out from a star, beyond what we call the ice line. You'll need to place this planet fairly far away from its parent star, lest these compounds be unavailable.

Chlorine won't work

Folks have mentioned chlorine; I agree with them that it won't work. Putting aside the issue of how you could get enough of it to occur naturally, chlorine's boiling point is about $∼$240 K, which is higher than the temperatures of giant planets where ammonia and methane dominate. An atmosphere with a substantial layer of chlorine-rich clouds would likely be hotter than that, in the realm where water-rich clouds could also form, making chlorine potentially not the dominant cloud constituent. Furthermore, an atmosphere rich in methane and ammonia could only exist at temperatures below chlorine's boiling point, making them incompatible.

posted almost 5 years ago

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