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Everybody, even those who are complete beginners in the field of Astronomy, know Io, the cryovolcanic moon of Jupiter that looks like a burned pizza with skin cancer. Volcanoes constantly spit out sulphur in various forms and compounds, which gives Io its characteric coloring that consists mainly of yellow, red and orange fields with occasional black or green patches. The pattern can change completely within 2 weeks!

Now, for my SF novel, I want to create a similar celestial body, with one condition: the planet is going to be blue. Light or dark blue with occasional white, black or green patches, with severe cryovolcanism just like Io has.

My questions are:

  • What chemical elements or compounds could lead to such a coloring of the planet or moon?
  • How could they be created naturally and occur as a part of a cryovolcanic system like Io's?

EDIT: The compounds which color the moon blue should be solid at a temperature of -200 degrees.

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    $\begingroup$ Just as a note, Io is not cryovolcanic, it is conventionally volcanic. $\endgroup$ – Alex S Apr 18 '16 at 16:57
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As anyone who's studied inorganic chemistry knows, many salts of copper in the +2 oxidization state have colors ranging from green to a nearly black dark blue, including some remarkably intense shades of sky blue like those of copper(II) sulfate and copper(II) nitrate:


Images from Wikimedia Commons. Left image by user Stephanb at the German Wikipedia, used under the CC-By-SA 3.0 license; right image created and released into the public domain by user Benjah-bmm27 at Wikimedia Commons.

One important detail to note is that the colored forms of most of these salts are hydrated, i.e. they include some water molecules as part of the crystal lattice. Removing this water of crystallization will typically destroy the coordination complex, and hence the color. Thus, in addition to copper, your blue planet also needs to retain at least some water (although it need not be present in liquid form). In a pinch, other small polar ligands like ammonia may also serve the same role (either alone or mixed with water, as in e.g. tetraamminecopper(II) sulfate), although the exact hues will be different.

Ps. Some iron salts in the +2 oxidation state, such as iron(II) sulfate, also have a blue-green color, but iron compounds can take on many other colors as well, depending on the oxidation state and on the other compounds present. Unless you tweak the surrounding chemistry just right, an abundance of iron is far more likely to give your planet the rust-red hue of iron(III) oxide, just like on Mars, than the deep blue shade of iron(II,III) hexacyanoferrate (a.k.a. Prussian blue).

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TL;DR: Add methane, remove organonitrogens and other yellow substances.

Both Neptune and Uranus get their blue hue from methane in their atmospheres. So you can start out with a moon like Enceladus, which is white but has cryovulcanos and then add methane. Or you start out with Titan, which has cryovulcanos belching methane, and remove the organonitrogen haze.

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    $\begingroup$ not sure it works, The methane on neptune is a gas, pure solid methane is pretty colourless, and as "snow" would be white. If you irradiate methane for a while and you get red tholins, which is the reason that so many kbos are red. $\endgroup$ – James K Apr 18 '16 at 18:58
  • $\begingroup$ @JamesKilfiger: Oh yes, I should have remembered methane hydrates for instance, are white $\endgroup$ – Abulafia Apr 19 '16 at 11:14
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It's not easy being blue. You only have to look around to see how little blue there is in nature.

Oxygen is blue, and freezes at -218*C, but pure oxygen is rather too reactive to consider existing in in substantial amounts. It would react with almost anything it found.

Many of the "ices" would either be white, due to dispersion, or tinted with red, due to the formation of thorins. Methane would look red. On the other hand, most rocks are grey.

If you are willing to imagine rather far-fetched geology then there are a few pigments that could occur: Ultramarine does form naturally as a metamorphic rock. If you could imagine the right conditions existing in your volcanoes you could dye your methane snow with ultramarine. The elements for Prussian blue (iron, carbon, nitrogen) are also available in the universe. However Earth's geology seems not to allow its synthesis naturally.

Oceans look blue, and a nitrogen ocean could cover the planet to give a blue colour to much of the planet, with the continents being some other colour.

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    $\begingroup$ Ha! Try looking around nature on a day with little cloud cover. $\endgroup$ – Xandar The Zenon Apr 19 '16 at 4:27
  • $\begingroup$ @XandarTheZenon I presume you are referring to the sky. This question concerns the ground. $\endgroup$ – ApproachingDarknessFish Apr 19 '16 at 7:36

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