Abundant water ice was formed in the solar nebula outward of about 4 astronomical units and, during the entire history of the solar system, has been carried into the inner solar system in icy and hydrated bodies. We do not know which sources of water dominated in the inner solar system, but micrometeorites derived from C-class asteroids may have been most important. Today, hydrous materials occur throughout the solar system from Jupiter inwards, at least. Significant quantities of liquid water are, however, probably present only on Earth and some icy moons of Jupiter and possibly Saturn.

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