Jupiter was the king of the gods. The ancient Greeks called him Zeus. The Romans originally referred to him as Father Zeus - in Latin: Zeus Pater, which over time came to be pronounced Ju-piter. To early Anglos he was known as Jove. He was omnipotent. All other gods and mortals existed only by his consent.

Jupiter was the son of Saturn, who was king of the gods before him, and in the habit of swallowing his offspring to prevent them from usurping him. But Jupiter's mother, Rhea, hid him from Saturn, and took him into the wilderness where he was raised by nymphs. When he was grown he confronted Saturn and defeated him, taking over his throne, high among the clouds on Mount Olympus, as illustrated below in the 1811 painting of Jupiter and Thetis by Jean Auguste Dominique Ingres.

Jupiter had an voracious sexual appetite. Although married to his sister, Juno (Hera), he was constantly seducing both goddesses and mortals, often changing his form to hide his actions from his wife. He appeared to Europa as a bull, Leda as a swan, Leto as a partridge, Danae as a shower of gold, and Alcmene as her own husband. Women were powerless to refuse his advances.

The Temple of Jupiter was the largest and most important temple in ancient Rome. Its massive columns were seven feet in diameter and over sixty feet tall. Located high on a hill in what is now Lebanon, only six of the original 54 columns still stand, that once enclosed an area the size of a football field. Photo by Dominique G.

Click on photo to enlarge.

Like the god it was named for, the planet Jupiter is truly king of the planets, ruling the solar system from its throne high in the heavens, more than five times further from the Sun than Earth. With a diameter of 89,424 miles (143,884 kms), it is the largest of the planets, having a volume over 1,300 times greater than Earth. Outside the vast debris field of the Asteroid Belt, Jupiter is the first of the giant gas planets that reside in the outer solar system.

Jupiter is a failed star. It is composed of hydrogen and helium, as all stars are, including our Sun. Jupiter just never got quite big enough to ignite. Still, it generates more heat within itself than it receives from the Sun, and its large gravity well influences the entire solar system. Jupiter does, in fact, command a mini solar system all its own, with no less than 63 moons in orbit around it, three of which are larger than the Moon that orbits Earth. NASA's Hubble Space Telescope took the photo below of the mighty Jupiter in natural colour, with one of its moons, Europa, suspended in orbit beside it.

Despite its large size, Jupiter completes one full rotation about its axis in only 9.925 hours. This fast rotation generates a very strong magnetic field that extends billions of miles into space. The magnetic field in turn ionizes Jupiter's atmosphere, creating the most deadly radiation environment - for man and machine - of all the planets. Jupiter takes almost twelve Earth years to orbit the Sun.

Jupiter is a favourite object for backyard telescopes. On a clear night when the "seeing" is good, bands of multicoloured clouds girdling the planet are visible. And with a little patience, and at least an eight inch telescope, Jupiter's Great Red Spot can be seen. The Great Red Spot is an immense storm - the largest storm in the solar system, twice as wide as our entire planet Earth - that lifts material from deep within Jupiter's turbulent atmosphere to a height of eight kilometers above the surrounding clouds.

In April, 2006, the mighty planet spawned a second red spot. It was born back in the year 2000, when three small storms collided and merged into a large white spot, about half the size of the famous Great Red Spot. Scientists watched it over the ensuing five years, looking for changes, and in December, 2005, the spot turned brown. Then, just four months later, it turned red, and is now exactly the same colour as its older sibling.

Jupiter's Moons

On the evening of January 7, 1610, a free-thinking man by the name of Galileo Galilei looked through the small, crude telescope he'd just invented at the planet Jupiter, and discovered four tiny "stars" close beside it. It was the first time human eyes had ever seen the moons of Jupiter. Of course Galileo didn't know they were moons, at least not right away. The official teaching at the time was that Earth was the centre of the Universe, and everything in the sky moved around us. But Galileo noticed these tiny pin-pricks of light beside Jupiter changed positions from night to night, quite unlike all the other stars. The image below is a photo of the actual sketches Galileo took on subsequent nights in January, 1610, carefully marking the positions of the tiny mysterious stars. One can only imagine the thrill Galileo felt when his genius ultimately led him to the conclusion that the tiny stars were in fact moons orbiting another planet. It was the first visual proof that everything in the Universe did not revolve around Earth.

Now, even the smallest of backyard telescopes allows anyone to relive Galileo's thrill of discovering the four largest moons of Jupiter, officially designated the Galilean Satellites, in his honour.

Io...

The closest Galilean moon to Jupiter is Io (eye-oh), named for one of the god's many mortal consorts. Poor tortured Io is torn between the gravity of Jupiter and the opposing pull of the other large moons. These intense tidal forces cause Io's solid surface to bulge in and out more than 330 feet (100 metres), creating enormous heat in the moon's interior, which when coupled with the effects of Jupiter's intense radiation, has resulted in Io being the most volcanically active body in the solar system, with dozens of mammoth volcanos covering the moon with fresh lava flows, and spewing plumes of volcanic gases hundreds of miles high.

The photo below of Io and its shadow sweeping across the face of Jupiter was taken by the Hubble Space Telescope.

The extraordinary photo below was taken by NASA's New Horizons spacecraft, as it swung by Jupiter in May, 2007, on its seven year journey to the dwarf planet Pluto. With unprecedented resolution and clarity, the cameras on board the spacecraft captured Io passing in front of Jupiter, showing an especially large active volcano on the night side of the moon. You can clearly see the red colour of fresh lava, and the blue colour of ionized volcanic gases. To see the full high resolution photo, which shows the volcano in amazing detail, just click on the photo.

Europa...

The 1968 classic film, 2001: A Space Odyssey, as well as the 1985 sequel, 2010: Odyssey Two, both proposed that primitive life forms were evolving in warm oceans sealed beneath the frozen crust of the Jovian moon Europa, a proposition that could very well be true. Europa's smooth uncratered surface suggests that warm non-volcanic liquids from the interior of the moon are leaking onto the surface and freezing. Europa is the only Galilean moon that is smaller than Earth's Moon, but not by much. Europa's diameter is 1,945 miles, compared to our Moon's diameter of 2,160 miles. The photo below was taken by NASA's Galileo spacecraft.

Ganymede...

With a diameter of 3,274 miles (5,268 kms), Ganymede is not only the largest moon in the solar system, it is also larger than the planet Mercury. Orbiting Jupiter at a distance of 668,750 miles (1,070,00 kms), it is much further from the planet's damaging radiation and tidal forces than Io and Europa. In mythology, Ganymede was a prince of Troy, famed for his beauty. Eos, goddess of the dawn, abducted him and brought him to Olympus, where he became Jupiter's cup-bearer. When Jupiter's wife, Juno (Hera), became jealous of his beauty, Jupiter placed him in the stars to keep him safe, where he became known as the constellation Aquarius. The photo of Ganymede below is by NASA's Galileo spacecraft.

Callisto...

Callisto is the furthest of the Galilean moons from Jupiter, orbiting at a nice safe distance of 1,176,875 miles (1,883,000 kms). With a diameter of 2,987 miles (4,806 kms), it is the third largest moon in the solar system, behind Ganymede and Saturn's moon, Titan. It is almost exactly the same size as the planet Mercury, and like Mercury, heavily cratered. It is considered to have the oldest, most heavily cratered surface in the solar system. Callisto is named for yet another of Jupiter's sexual conquests among mortal females. When Jupiter's wife discovered their tryst, she turned Callisto into a bear, and caused her to be killed by the huntress Diana. But Jupiter resurrected his lover and placed her in the stars, as the constellation Ursa Major, The Great Bear.

The remaining 59 of Jupiter's moons are much smaller, and are probably captured asteroids.

On March 4, 1979, as NASA's Voyager 1 spacecraft flew past Jupiter on its way to Saturn and beyond, it made an exciting discovery. It seemed that Saturn was not the only planet in the solar system with rings! Voyager discovered that Jupiter too, had a ring system, albeit nowhere near as large as impressive as the great rings of Saturn.

When NASA's Galileo spacecraft visited Jupiter seventeen years later, it took detailed photos of the rings. The image below was captured on November 9, 1996, as Galileo swung behind the giant planet into its shadow. The Sun, hidden behind Jupiter, back-lit the rings in a stunning display. Click on photo to enlarge.

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