This NASA article claims that Titan, one of Saturn's moons, has a wet climate and occasionally, rainbows occur when sunlight refracts off these "water" droplets. However, these droplets are composed mainly of methane, rather than hydrogen and oxygen as on Earth.
The article claims that rainbows occur on Jupiter and some other planets or their moons, as well. If one were on one of these planets or moons and saw a rainbow, would one make a bracha, or is the concept of a rainbow defined only by how Earth forms rainbows?
Also, based on other MY questions, it seems that one can make a bracha on rainbow "prisms" according to some opinionsThis seems highly relevant. Consider editing in a link. – mevaqesh Nov 30 '16 at 05:43