In 1995, the Galileo spacecraft sent a probe into the Jovian clouds to measure the chemical composition of the atmosphere. Since Jupiter formed out of the same cloud of gas and dust as the sun, scientists thought the Jovian atmosphere should be composed of a distribution of elements similar to the sun's. One common element is oxygen—which, on Jupiter, is mostly locked up in water molecules. But surprisingly, Galileo discovered much less water—and therefore much less oxygen—than expected. Some scientists think the spacecraft just happened to have entered a relatively dry spot in the atmosphere. But that's just one possible explanation out of many, and one of Juno's goals is to find out for sure just how much water is on Jupiter. The answer will help scientists piece together how Jupiter—and the solar system—formed.