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Jupiter, the king of the planets, is aptly named for the king of the gods. A brilliant, white wanderer through the skies, Jupiter has been praised with countless superlatives since ancient times. The Red Giant, as it is commonly referred to, is the largest in the solar system with a diameter of more than 88,000 miles, more than 11 times the size of Earth.
Although it is made up of hydrogen and other light elements, it is also the heaviest, with 318 times Earth's mass—more than the sum of all the other planets combined. Requiring only 10 hours to make one complete rotation, Jupiter is the fastest-rotating planet, as well. It spins on its axis so quickly that it bulges at the equator and looks flat at the poles when seen through a telescope.
One day on the Gas Giant might only be 10 hours, but a full year on the planet is much longer than on Earth. Jupiter orbits the Sun at a distance of about 484 million miles. It takes about 12 Earth years to complete one Jovian year (Jove is another name for the god Jupiter).
It towers over the other planets in terms of pure size. It is also the most colorful with belts and bands and spots. Most importantly, however, the largest planet in our solar system is accompanied by the greatest number of satellites. Scientists argue anywhere between 13 and 16 total satellites orbit Jupiter due to the planet’s massive amount of gravitational force. These objects were the primary discovery that opened the door for gravitational studies and further research into the planet itself.
Galileo Makes a Discovery
Very little was known about Jupiter until Galileo looked at it through his small telescope in the 1600s. He discovered four bright, swiftly moving satellites around the giant planet. These satellites are now called the Galilean moons. They were important in helping to develop our understanding of celestial mechanics. The Galilean moons are named Io, Europa, Ganymede, and Callisto. Each one is a bit bigger than our Moon, but that's where the similarities end. The ones closest to Jupiter (Io and Europa) are dense, rocky worlds, like Mercury, Venus, Earth, and Mars. The ones farther out (Ganymede and Callisto) are made of lighter elements, like Jupiter and Saturn. They seem to form a kind of miniature Solar System.
Io is the reddest, most geologically active body in the Sun's neighborhood. Its surface is covered with hot sulfur compounds which its many volcanoes spew into space. When photographed, it looks like a strange (and overbaked) pizza! Since Io is so close to Jupiter, the volcanoes must be heated up by the tidal forces created inside the satellite by the gravity of Io's giant neighbor. In the few months that elapsed between the Voyagers, Io’s surface changed. Io may be the most active solid body in all the solar system.
Europa doesn't have volcanoes. It seems, instead, to be an ice-covered rock. Scientists think that the ice on Europa's surface might have melted and refrozen, since it is very smooth. With Voyager 2 came the discovery that this moon may be the flattest member of the sun's family. Europa has few craters but many curious wrinkles. These show up as dark lines on the surface, but there is little vertical relief. They're almost as flat as if they were painted.
Ganymede is Jupiter's largest moon and the second-largest in the solar system, after Saturn's Titan. The most beautiful of mortals, Ganymede was taken to Mount Olympus to serve the gods. His namesake may end up serving as another planetary laboratory for plate tectonics. Ganymede's surface is pocked with recent craters, scoured with ridges and grooves, and spotted with large light and dark areas, which are not yet understood. Ganymede is a real ball of ice, but has lots of craters, as well as flat, smooth areas. If it melted and re-froze like Europa, it must have done so a long time ago.
Callisto, named for an Arcadian nymph, is probably the most heavily cratered body in the solar system and farthest moon from Jupiter. Callisto looks like a shattered ice ball, with zillions of criss-crossing craters, bull's eyes, and lines. The pictures taken of Callisto remind me of a rural road sign full of bullet holes. It must have been bombarded long, long ago, when Jupiter was just forming. The impacts were frozen into a record for us to see. Therefore, some say Callisto may preserve a billion-year record of the last stages in the accretion of the planets from the debris that formed the solar system.
These four Jovian satellites are great teachers. They show us how surfaces can be changed by internal activity as well as by outside forces. Studying these four strange worlds will help us understand the rest of our Solar System.
Decades after Galileo’s original discovery, such superlatives have been all but exhausted as astronomers and laymen alike have seen, close up, not only Jupiter itself but the five inner satellites. Though the man in the street might well argue that the spectacular photographs sent back by the Voyager program justify the few dollars they cost each US citizen, planetary scientists expect far more. They now have a new astrophysical laboratory that may tell us more about meteorological and geological processes than we could get by studying conditions here on Earth. What we see of Jupiter—either from Earth or by spacecraft camera—is not a solid surface but the top of a swirling cloud cover. If Jupiter has a solid surface at all, it lies deep beneath the clouds.
We knew quite a bit about Jupiter before the space age. But flybys by Voyagers 1 and 2, and several aircrafts since, have given us so much information that, in comparison, we were very ignorant. We now know that Jupiter is a planet of weather, on the grandest of scales. Jupiter is a stormy planet—some of the tornado-like monster storms are thousands of miles across, with winds well over hundreds of miles per hour. Some of these storms last for decades, or even for centuries. The rapid rotation of the planet, combined with Jupiter's internal heating system, cause the miserable weather.
Because of Jupiter's fast rotation, the gases at its equator are traveling at about 46,000 kilometers an hour. The clouds do not rotate in unison, as a solid body would: Each latitude rotates at a slightly varied speed. This shearing tears the clouds and atmospheric features as they slide past one another. The Great Red Spot, discovered over 300 years ago, was shown in the Voyager movies to be a huge, swirling cyclone. We're not sure why it's red, but some scientists think that the color is due to a chemical reaction. The visibility of the cloud features and the speed and scale of their activity make Jupiter's atmosphere a testing ground for meteorologists theories. Although it would be unfair to suggest that the Voyager mission will soon provide better weather forecasts on Earth, the studies will enhance our knowledge of how planetary atmospheres work. Studying Jupiter's weather will help scientists understand Earth's weather better, too.
Not far above Jupiter's cloud tops, a surprise waited for Voyager's cameras: a fine, thin ring made of small bits of orbiting debris. Jupiter now joins Saturn and Uranus in the proud ownership of a ring. It seems that all of the large, gassy planets have rings.
More recently, a complex orbiter and probe satellite mission, named in honor of Galileo, launched in 1989 in an attempt to further study the surface of Jupiter and its atmosphere. Galileo, the spacecraft, not the man, became the first craft to orbit Jupiter. While in orbit, it launched the first probe into the Red Giant, directly measuring the planet’s atmospheric composition and recording ammonia clouds that might’ve formed from the lower depths of the atmosphere itself. Galileo also collected further data on the Galilean moons. Io’s volcano and plasma interactions with Jupiter were confirmed, while the theory of liquid ocean under Europa’s icy surface was proven. Although we thought we had learned ample information about the largest planet in our solar system from Voyagers 1 and 2, Galileo proved that there is always more to learn when it comes to space.
The planet Jupiter—along with its retinue—has certainly lived up to its eponym's mythological reputation as the source of human happiness, if you happen to be a planetologist. In fact, Jupiter was almost more than just a planet. If it hadn't quickly lost so much of its original mass (about 80 times more than it has today), it would have been a small star! The night sky as seen from Earth would certainly look very different than it does today. Our night-lights would not only be the Moon and stars, but a bright red second Sun as well. Can you imagine that?