The Galileo atmospheric probe measured and analyzed many aspects of Jupiter, including its temperature, pressure, chemical composition, clouds, lightning, and the sunlight and heat that radiate from inside the planet. Galileo found that Jupiter has three times more heavy noble gases--argon, krypton, and xenon--than the sun. This suggests that Jupiter likely first formed as an Earth-sized ball of rock and ice that later attracted gas. But a more detailed understanding of this scenario will have to wait until Juno gathers some crucial data: the amount of oxygen in Jupiter.