Determining the amount of water that's locked up in Jupiter's atmosphere is essential for understanding the planet's history and its interior. Galileo's atmospheric probe--which also detected high winds even deep inside the atmosphere--tried to make such a measurement when it dove into Jupiter's clouds. But, curiously, it detected much less water than expected.

Scientists think that because Jupiter was formed from the same cloud of gas and dust as the sun, the planet's chemical composition should also be similar to the Sun's. But if the Galileo probe's water measurements accurately represented all of Jupiter's outer layers, then Jupiter must have a lot of carbon--in stark contrast to the sun's chemical composition. What's more likely, scientists say, is that the region that the probe explored just happened to have lacked water. Juno will be able to conclusively measure the amount of water throughout Jupiter--hopefully solving what has so far been a longstanding mystery about Jupiter's interior.