Because it is squeezed and stretched by gravity, the body in the Solar System that JHQHUDWHV KHDW PRVW HI FLHQWO\ LV not the Sun but Jupiter’s moon, Io
On 8 March 1979, NASA’s Voyager 1 space probe, having streaked through the Jupiter system faster than a speeding bullet, turned its camera back the way it had come. Navigation engineer Linda Morabito became the first person to see, silhouetted against the black of space, the super-volcanoes (strictly speaking, geysers) of the moon Io. The most geologically active body in the Solar System is heated by nearby moons Europa and Ganymede, and Jupiter. The moons tug its orbit into an elongated ellipse. As it moves close to and far away from Jupiter, the gravitational stretching effect of the giant planet changes. Io is heated for exactly the reason a squash ball gets hot when squeezed repeatedly in your hand.