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Pioneer 10 and 11 were the first to see Jupiter's atmosphere up close. When they were on the opposite side of Jupiter from Earth, they sent back radio signals that traveled through Jupiter. Measuring the way the radio signals changed as they penetrated the Jovian atmosphere allowed scientists on Earth to determine the temperature at different depths.
They also discovered that the temperature at the poles is the same as at the equator. Because it gets more direct sunlight, the equator receives more heat from the sun than do the poles. But the fact that the temperatures of both places are the same means that something else must also be heating the atmosphere. And that other source is the heat emanating from Jupiter's interior, which turns out to be as important for warming the atmosphere as the sun.
Pioneer 10 confirmed scientists' hypothesis that Jupiter was mostly made of hydrogen and helium. Before the mission, no one knew if Jupiter had any helium at all.
Pioneer 11 took dazzling pictures of the Great Red Spot. The spacecraft traveled over Jupiter's north pole, providing a never-before-seen glimpse of the planet. Juno's polar orbit will also take it around Jupiter's poles as it probes the chemical composition of the atmosphere and takes dazzling pictures with its camera, JunoCam.