Jupiter has thick, gaseous atmospheres and low densities. It has no solid rock surface. The temperature ranges from about -190°F (-124°C) for the visible surface of the atmosphere, to 9°F (-13°C) at lower cloud levels. localized regions reach as high as 40°F (4°C) at still lower cloud levels near the equator. Jupiter radiates about four times as much heat energy as it receives from the sun, suggesting an internal heat source. This energy is thought to be due in part to a slow contraction of the planet. At least 17 natural satellites are known to orbit Jupiter. They are conveniently divided into three groups. The four largest—Io, Europa, Ganymede, and Callisto—were discovered by Galileo in 1610, shortly after he invented the telescope, and are known as the Galilean satellites. A second group is comprised of the four innermost satellites. The final group consists of the eight satellites with orbits outside that of Callisto. The seventeenth satellite, 1999J1, was discovered in 2000. Eleven small previously unknown satellites were reported in 2001. If these sightings should be confirmed, it would raise the number of Jovian moons to 28. … The spot and other markings of the atmosphere also provide evidence for Jupiter’s rapid rotation, which has a period of about 9 hr 55 min.