We often speak about the water on the icy moons in the outer solar system, such as Jupiter's Europa. But what is not is well-known is Jupiter itself has quite a bit of water in it. The upper atmosphere is actually seeded with water from the Shoemaker-Levy 9 comet impact of 1994. The water was discovered in 2013 after the Herschel Space Observatory found water concentrated in areas close to where the comet fragments
slammed into the atmosphere
Water in other parts of the atmosphere may have come about during Jupiter's formation, when icy planetesimals were abundant in the solar system. Looking at water and other elements within Jupiter will give us a sense of what the solar system used to look like, because Jupiter -- unlike our own planet -- is very close in composition to what it was when it was formed. (Earth gained a new atmosphere through plants and volcanic eruptions, among other factors.
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Image: Jupiter's water distribution in the stratosphere, mapped by the Herschel Space Observatory. White and cyan show high concentrations, and blue is lower concentrations. The map is overlaid on a visible-image picture of Jupiter taken by the Hubble Space Telescope.