Whereas Mercury, Venus, the Earth and Mars are called the terrestrial planets, because they are predominantly rocky, with high densities and small volumes, the next four, Jupiter, Saturn, Uranus and Neptune have low densities and large volumes and are known as the gas giants.
Jupiter lies 350 million miles beyond the orbit of Mars, but in the midst of this vast gap lies the Asteroid Belt. The Solar System has been described as the Sun, Jupiter and some rubble: Jupiter is by far the largest planet in the Solar System, 11 times larger in diameter than Earth. At its equator it is 88,846 miles across and has a volume more than 1,330 times, and a mass 317.828 times, that of Earth. It is sometimes called a brown dwarf or failed star by astronomers, as is Saturn, because although it never accrued sufficient material to become a nuclear fire, its deep interior may contain a small-scale furnace. Its mass, though only a tenth that needed to become a star, is greater than two-thirds the mass of all the known planets put together. Despite this it has an equatorial rotation period of only 9 hours 50 minutes 30.003 seconds (internally it is 5 minutes slower). This very rapid spin accounts for its slightly spheroid or flattened shape. It takes 11.86 years to orbit the Sun at a mean distance of 484 million miles (5.203 AU), travelling 30° per annum. It has an eccentricity of 0.0485, and is inclined at 1° 18' 17" to the Earth's ecliptic.
Jupiter is shrouded behind a cloud blanket of three layers consisting of red ammonia crystals, brown ammonia hydrosulphide, water ice and water droplets, but the surface is known to be perpetually stormy with perpetual lightning discharges, and this causes its radiant surface markings to be constantly changing. A Giant Red Spot 20,000 miles in length was discovered in 1664 by Robert Hooke, first becoming prominent in 1878 (though this vast whirlwind is itself constantly shifting and changing). It rotates anti-clockwise taking six days to complete each cycle. It has a spiral structure with a tilting centre, and is now believed to be a high-pressured weather system, sandwiched between two belts of wind moving in opposite directions at speeds of up to 300 mph.
Jupiter's chemical composition is almost identical to that of the Sun, with a ratio of helium to hydrogen of 24 %, compared to the Sun's ratio of 25%, and has a density only 1.3 times that of water. It emits powerful and deadly radiation; Jupiter has a magnetic field 10-20,000 times that of Earth, so large that Saturn can fall within its magnetosphere, and it was found in 1955 to be a powerful source of radio waves. Jupiter is not as bright as Venus is but with a maximum magnitude of -2.6 it is far brighter than the brightest star, Sirius. Unlike all the other planets it is a source of light as it has an ultraviolet glow.
Jupiter was first visited by a space probe in 1973 when Pioneer 10 voyaged there. Pioneer 11 returned images a year later. In 1977 a rare alignment of outer planets, which occurs only once in 176 years, made it possible to send a space probe past Jupiter, Saturn, Uranus and Neptune, and so Voyager 2 was launched 20 August 1977. It passed Jupiter in July 1979, adding much data to that obtained by Voyager 1 earlier in the year.
Voyager 2 discovered that Jupiter, like Saturn and Uranus, has some barely perceptible rings, formed as a result of collisions between its moons and space rubble left from the birth of the Solar System. These extend tenuously down to the cloud tops and comprise a flattened main ring, probably formed from the debris of its small inner moons Adrastea and Metis, material from Io, and remnants of meteoroids they have been bombarded by. There is also a cloud-like "halo" inside the orbit of the first ring. When the Galileo probe, launched in 1989, entered orbit around Jupiter on 7 December 1995 it spent 57 minutes travelling through 125 miles or so of Jupiter's cloud tops. It detected a third, gossamer ring made up of two bands, one embedded within the other, from the debris of Amalthea and Thebe.
It has also been found that Jupiter has polar lights, or aurora, as Earth does, though these are rather more brilliantly spectacular and a hundred times bigger. Though it has features resembling ice caps at the North and South Pole, this cannot be the case since Jupiter is believed to be a giant ball of gas without any solid surface. In 1991, as a result of new photographic techniques, it was found that they are caused, not by the Sun, as on Earth, but by volcanoes on Jupiter's nearest large moon, Io.
Galileo also showed that Jupiter's central regions were much hotter than previously thought, as surface winds were found to exceed 400 mph, driven by heat escaping from the interior.
On 28 February 2003, Galileo's memory-bank and transmission of stored data to Earth
was switched off by NASA. It was programmedto crash into Jupiter's Equatorial Zone on
21 September 2003.
Pioneer and Voyager flybys have shown that Jupiter emits more heat than it receives from the Sun.
The sigil for Jupiter represents the semicircle of the soul rising above the cross of matter. Jupiter represents the stage in our development where our soul has triumphed over our experiences on Earth (the cross), and the understanding thus gained frees us from illusion.
Jupiter was named by the ancients in honour of the ruler of Olympus, the most powerful and the highest among the gods in the Roman pantheon, the seer, guardian of divine law, protector of justice and virtue. The name Jupiter corresponds to the idea of brilliance, celestial light.
He was identified with Zeus of the Greeks, the prodigious lover who pursued whatever goddess, nymph or mortal took his fancy with boundless energy and enthusiasm; and with the Scandinavian Thor. He is Father Abraham, he is a Brahman, he is Jehovah, and he is Meher Baba. To the Egyptians he was Ammon, to the Babylonians Bel or Moloch, to the Hittites Teshub, and to the Hindus he is Vishnu the Preserver. The Etruscan Jupiter, Tinia, warned and punished men with his three thunderbolts - appropriate considering the lightning storms on Jupiter's surface.
Jupiter, or Jove, the Greater Benefic, Lord of the Planets, represents the life-principle of expansion through growth, materially and by understanding. He indiscriminately amplifies everything he encounters. Jupiter is, after all, larger than all the other planets put together.
History, foreign countries and languages, travel, religion, philosophy and higher education are all referred to Jupiter. Its significators are joy, benevolence, philosophy, abundance, double standards and confidence; our need to reach beyond our immediate environment; optimism, faith, hope, good luck; our ability for progress and achievement. Also, the acquisition of those things that increase our sense of wellbeing, preserve us from injury or contribute to a widening of our understanding. Jupiter is a masculine planet related to left-side brain activity. It has planetary rulership of Thursday, Thor being the Nordic name for Jupiter. Jupiter is day ruler of Sagittarius and night ruler of Pisces. In esoteric astrology, Jupiter has rulership of Aquarius, and in the Greek pantheon Jupiter, their chief god, ruled Leo.
A Jovian influence in our chart tends to a relaxed liberation, tolerance, goodwill and moderation, but also a moral certainty and religious intolerance. A desire to play God encourages paternalism, benevolence from a position of authority, noblesse oblige. Jupiter is essentially expansive and optimistic, encouraging legislation, or, negatively, legal wrangling and disputes, national inflatus and imperialism.
Jupiter takes around 11.86 years to return to its starting point and is the basis of the Chinese 12-year astrological cycle. Each year is named after the natural zodiac, so that everybody's 13th year for example, which follows their first Jupiter return (one Jovian year), is their Aries year, regardless of Jupiter's position in the natal chart or by transit. At the time of a Jupiter return we may take important steps forward and improve our income, and be on the lookout for chances to put our ideas to those who can help us.
During each Jovian year Jupiter will rise and fall and the direction of its rising and setting will alter. For half of its cycle, as it moves through the upper part of the Zodiac (sidereal Pisces to Virgo), it will culminate relatively high in the sky (e.g. between 1999 and 2004). During this period it will come to opposition (with the Sun), and will be above the horizon for over twelve hours at a time. For the other half of the cycle (sidereal Libra to Aquarius) it lies closer to the horizon in the Southern sky, and is visible for shorter periods of time.
The ancients knew Jupiter and Saturn as the Chronocrators, or Lords of Time, because as the two slowest-moving planets known to them they enabled measurements of long spans of time. They are conjunct every 19.859 years, and each time Saturn enters the tail of Jupiter's magnetosphere. A series of successive conjunctions always fall in a sign of the same element, forming a sub-cycle. In 1980/81 they conjoined in Libra, an Air sign, after 200 years of Earth sign conjunctions.
Jupiter shares its orbital path with a number of asteroids known as the Trojans. These form two distinct groups, the first leads Jupiter at a distance of 60°, the others trail by the same amount, give or take one degree. From an Earth-centred chart they are always in sextile aspect to Jupiter (therefore in trine to each other) and can easily be calculated by looking exactly two signs either side of Jupiter at the same degree, or within 5° of orb, where they occupy about 1° of arc. The Leading Trojans can be seen as an advance guard, while the Trailing Trojans form the rear guard. Those that have been named (relatively few of the 700 or so of them), take their nomenclature from the heroes of the Trojan Wars, as told as Homer's The Iliad. An equivalent asteroid has been found for Mars, and the search continues for trojans for other planets.
They can be seen astrologically as disseminators of Jupiter's influence, and magnify and express its higher values and powers. They are strongest when forming conjunctions with the other planets, asteroids and sensitive chart points.
THE MOONS OF JUPITER
Jupiter has 63 known moons, though some are insignificant small lumps of rock, and only
the largest have names. Before the end of the 20th century, only 18 had been
discovered. 10 more of the smallest moonlets were then found during 2000, mostly
by University of Hawaii graduates using the Mauna Kea telescope, bringing the
total to 28, and the discovery rate continued over the next couple of
The 40th was found on Halloween 2002 by Scott S Sheppard (University of Hawaii)
and was announced on December 23rd. S/2002 J1 had an estimated diameter of only 3
or 4 kilometers and a 748-day retrograde orbit inclined 163 degrees to Jupiter's equator.
It marked the 23rd Jovian satellite discovered by Sheppard and his team.
They were also responsible for the first seven to be found in 2003, S/2003 J1
- S/2003 J7. Two of the new finds were in prograde orbits (traveling in the same direction as Jupiter's rotation), and five followed retrograde orbits; their revolution periods
ranging from 237 to 983 days. The faintest one had a magnitude of 23.4, and all
had diameters estimated at just a few kilometers. Eventually they would be named after "lovers of the Roman god Jupiter, or, in some cases, children and grandchildren of Jupiter's lovers,"
according to Brian Marsden, director of the Minor Planet Center. Hot on their
heels came five more, S/2003 J8 to S/2003 J12, all with retrograde orbits; their revolution periods
ranging from 533 to 767 days. The faintest one had a magnitude of 23.9, and all
have diameters of just 1 or 2 kilometers. A further six followed in February
was mountain-sized, roughly 2 kilometers across, travelling in a distant, "irregular" orbit
with large eccentricity and inclination. Three of them, designated S/2003 J13
- J15, were credited to Scott S. Sheppard and David C Jewitt (University of
Hawaii), along with Jan Klenya (Cambridge University). The other three, S/2003 J16
- J18, went to a group led by Brett Gladman (University of British Columbia).
S/2003 J19 and S/2003 J20 were spotted by Brett Gladman and Scott
S Sheppard's team respectively in April 2003, and S/2003 J21 by the
Brett Gladman team on May 29, bringing the total to 61.
Galileo Galilei (b. 16 February 1564 OS, 1528 hr LAT, 1500 GMT, Pisa; d. 8 January 1642, Arcetri) discovered the largest four moons, in Padua. At the time, no planets other than Earth were known or considered to have moons as the Earth was considered the centre of all celestial movement. After half an hour's routine observation of Jupiter on 7 January 1610 he "became aware of 3 little stars...lying near it", an hour and a half after sunset (1610 hr GMT). He soon found a fourth moon, the entire group comprising Io, Europa, Ganymede and Callisto (in order of increasing distance from Jupiter). These satellites are now known as the Galilean or Jovian Moons (he called them the Medicean Planets) and are visible through binoculars.
The discovery showed to Galilei that contrary to the old beliefs there were at least two centres of movement in the Universe (one being considered to be Earth), and was crucial in establishing that the Earth was not the centre of the solar system. Galileo believed that nature and revelation, both books of God, could not contradict each other. As he quoted the historian Cesare Baronius as saying, "The Bible tells us how to go to Heaven, not how the Heavens go."
The Catholic Church were not persuaded that the Sun was at the centre of the Solar System, and were moved to place Nicolaus Copernicus's pioneering work on the subject, De Revolutionibus ("On The Revolution", published in 1543), on the Holy Office's index of forbidden books. This so angered Galileo that he began to publicly mock the church. He later claimed that he was ordered only not to defend Copernicanism as undoubtedly true. Enemies claimed in 1616 that he had been specifically forbidden to speak of Copernicanism, and after many years of conflict, during which his own books were banned, the Inquisition dragged him to Rome on charges of heresy and disobedience.
On 22 June 1633 at the convent of Santa Maria Sopre Minera, Rome, Galilei was forced by the Inquisition to "abjure, curse, and detest" his heresy that the Earth was not the fixed centre of the Universe, under threat of torture. As he got up from his knees, he is said to have muttered under his breath, "Eppur si muove!" ("And yet it moves!")
Io is subject to the gravitational pull of the other Galileans as well as Jupiter. It is most notable for at least eleven huge active sulphuric volcanoes, a feature it shares only with Earth, Venus, and possibly Neptune's moon Triton. These spew out molten sulphur and silicate rock from Io's interior, which is thought to be iron at its core, to heights of over 300 miles, and at speeds of around 2,000
mph. Projectiles of ash from Io can scorch through space at speeds of up to 670,000 mph,
even reaching Earth.
It is this volcanic activity that gives Io its dramatic ever-changing surface colouring of yellow, orange, red and brown. The three most active volcanoes are Pele, Loki and Prometheus. Io also features mountains of up to 52,000ft in height that periodically collapse under the force of gravity, producing massive landslides.
Io's volcanoes were first discovered by Linda Morabito (Linda Hyder), a navigation engineer at Voyager mission control in 1979, when she chanced upon what turned out to be the 175-mile high plume of Pele, making her the first person to witness activity on a non-Earthly surface. The Galileo spacecraft later discovered that Io's 100 or more volcanoes spew lava at a temperature greater than found in any planet in the Solar System, between 2,200° and 3,100° F.
Io's surface around these hotspots is by contrast -243°F. Pele has a red ring of sulphur, 800 miles in diameter, deposited by material from its plume. Loki is the most powerful volcano in the Solar System, hotter than all of Earth's volcanoes put together.
Io's surface is only thousands of years old, as its partial internal melting is continually resurfacing it. "Io makes Dante's Inferno seem like another day in paradise," said Galileo team member Dr Alfred McEwan of the University of Arizona, in Tucson. A doughnut shaped torus surrounds Jupiter along the orbit of Io, consisting of charged plasma particles producing powerful radio emissions. Io has a squashed shape, caused by the gravitational pulls of Jupiter and the three outer moons. Io is permanently linked to Jupiter by a flux tube carrying a current of 5 million amps, as Io generates 1 million megawatts of electricity. Jupiter's famous radio signals fluctuate and correspond to the 42-hour orbital period of Io. It is believed this is caused by the torus. When the European space probe Ulysses returned data in February 1992 it was found that the torus no longer formed a complete ring as its volcanic eruptions had temporarily subsided.
Europa is similar in size to Earth's Moon, and has a smooth yellow sulphur-dusted surface, no mountains, and a cracked cue-ball appearance, probably due to tidal stressing from tectonic plates, from Jupiter's gravitational squeezing. The depth of crust has yet to be accurately calculated. In 1995, Europa was found to contain a tenuous atmosphere of oxygen, in the form of gas, as breathed on Earth. This made Europa only the fourth moon in the Solar System known to have an atmosphere (the others are Io, Titan and Triton, but none has detectable quantities of molecular oxygen). It is also known to contain naturally produced hydrogen peroxide, which is quickly broken down into hydroxyl, as well as carbon dioxide, water ice, sulphur dioxide, and possibly salt molecules containing water.
After Galileo flew past the moon on 18-19 December 1996, on its first close approach to Europa (it returned on 20 February 1997), it was also thought it might have warm ice or liquid water (possibly an ocean up to 80-miles deep below its rocky crust). Ice flows and possible mobile "icebergs" were seen on the surface. One photograph shows a small, dark, smooth area that has been described as a "puddle". Europa may be geologically active as ice in its subsurface appears to have been cracked and churned in response to heat from below. These factors would make it a prime candidate for any extra-terrestrial life, as it has adequate heat, and the likelihood, due to the presence of water, of the organic compounds to be present, providing the stage for life to form within half a million years through vulcanism. Europa's surface has amazing scalloped features, unique to Europa, forming chains stretching for hundreds of miles. Scientists now take these to be strong evidence of the presence of an ocean when the flexi or cycloidal were formed from the daily rise and fall of the ice crust due to Jupiter's gravity.
Because it is less cratered, it may be less than a hundred million years old. Europa, or at least its surface, is very slowly spinning as it orbits Jupiter, instead of being "locked" by Jupiter's gravity as expected. It takes 10,000 years or so to complete one rotation. Its orbital period is 3.55 days, and it has a temperature of -145°C, so any surface water would be frozen. Its surface was thrillingly found in the seventies to be covered with smooth white and brown-tinted ice.
In 1968, Arthur C Clarke (b. 16 December 1917, Minehead, Somerset), in his novel 2001: A Space Odyssey dreamt up a fictional moon of Jupiter. Below a frozen ice layer an ocean, kept liquid by the heat from volcanoes on the seabed, hosted an ecosystem of fish-like creatures, seaweed and microbes. We now know of life forms on the Pacific ocean floor clustered around volcanic vents (known as black smokers): luminescent eyeless fishes, giant worms, feeding off nutrients from volcanic lava and dissolved gases, all indifferent to the existence of the Sun. It is not impossible that Europa may actually be as Clarke imagined his fictional moon.
Ganymede, the largest and heaviest known moon in the Solar System, is slightly larger than Mercury, 3273.5 miles in diameter, and 2.017 times heavier than our Moon. It is covered with unexplained strange geological groove formations. Its day is equal to 7.16 of our own. Galileo flew past Ganymede at 0629 UT on 27 June 1996, at a distance of 517 miles, and again on 6 September 1996, and sent back a wealth of images and data for analysis, including evidence of "ice-quakes", suggesting an unstable crust.
Audio recordings made from data collected by Galileo show that Ganymede has a self-generated magnetic shield, or magnetosphere, which protects it from the magnetosphere of Jupiter itself, in which it orbits. This is so far unique in our Solar System. The moon's interior probably has a three-layer structure, a metallic core 250-800 miles in, and a rocky silicate mantle, both enclosed by an ice shell about 500 miles thick. Ice volcanoes caused the smooth stripes on its surface, with troughs up to 600 miles long, probably created 1 billion years ago. The surface contains calderas, depressions like volcanic craters, caused by the collapse of underground lava reservoirs. The oldest part of Ganymede's surface is called Galileo Regio and consists of ice stained by meteoric dust. Galileo detected organic molecules containing carbon and nitrogen on both Ganymede and Callisto, indicators of possible life forms. A weak atmosphere of oxygen was also found there, and in May 2000 data was collected while Galileo was 508 miles from the surface that indicated the presence of water. The American Geophysical Union in San Francisco CA announced on 17 December 2000 that a saltwater ocean had been detected 120 miles below the active ice surface, with a probable temperature of -56°C.
Callisto, the outermost moon, with an orbital period of 16.69 days, was revealed by the Voyager probes as a 3,000-mile wide, intensely large-cratered world, having been bombarded for hundreds of millions of years by the countless smaller bodies it had gathered up in its ancient mass. It was thought to be a rocky core with a five-hundred mile deep crust enveloped in ice, but data collected by Galileo led to the discovery in 1998 that, like Europa, a liquid salt water ocean five to ten feet deep must be present just below its surface. This was deduced from the way currents of electricity are conducted as the moons pass through Jupiter's magnetic field. Tidal forces, unlike Europa, do not heat Callisto so it is not understood how the ocean remains liquid. Internal heating, or the presence of "antifreeze" such as salt or ammonia, providing nursery conditions for possible microscopic life forms, could explain it. Three of the four Galilean moons are therefore now thought to contain water.
Callisto has two huge impact basins of which the largest, Valhalla, is 1,500 miles wide, consisting of concentric rings caused by an asteroid collision in early solar system history, four billion years ago. The height of the rings has since been smoothed over by plastic flow, which has also obliterated Callisto's smaller craters.
All four satellites are larger than the Moon apart from Europa, making them more like small planets than moons. They are locked in orbits facing Jupiter and in mutual relationship to each other; they all have synchronous rotation and circular or near circular orbits in Jupiter's equatorial plane.
This is also true of the innermost four - Metis (J16)(23km)(discovered 1980),
Thebe (J14)(48km)(1980), Adrastea (1979) and Amalthea (J5)(85km)(1892). These are far smaller,
Adrastea measuring an egg-shaped 5 x 4 x 3 miles. Both Adrastea and Metis move in the region of Jupiter's orange ring system.
Of these, Edward Barnard at the Lick Observatory found all apart from Amalthea on the Voyager images, making Amalthea the last satellite to be discovered by direct visual means.
Amalthea orbits close to the Roche limit, the point beyond which a planet would be disrupted by gravitational pull, and it is in fact elongated into an ellipsoidal shape, its long axis pointing towards Jupiter. Amalthea and Jupiter are the only bodies in the Solar System, other than the Sun and Moon, which can form an eclipse, due to their relative sizes and distance.
The Galileo probe returned high quality images of Thebe, Amalthea and Metis in January 2000. Thebe has an enormous impact crater, known as Zethus, 40km in diameter, and Amalthea was seen to have a prominent white region encompassing its South Pole, which could be a collection of sulphur-rich material collected over aeons from Io.
These eight satellites and Jupiter's system of rings all fall within Jupiter's magnetosphere. Bursts of trapped dust particles are sometimes released from the magnetosphere in the form of cosmic rays, which travel near the speed of light and have been detected near Mercury, more than 435 million miles from Jupiter. It is know known that these primarily emanate from Jupiter's moon Io. They can cause mutations in living organisms by altering or destroying genes, and although Earth is protected from these, it has been shown that some computer systems have been affected by extremely random changes within their microcircuits. It is not impossible that life is indirectly influenced by the presence of cosmic rays.
The second group of Jupiter's moons all have a mean orbital distance of 6.83 million miles, but their orbits alter from one revolution to another due to solar perturbations. All were discovered by photographic means and were known by number only for many years, although unofficial names were used. Himalia, for example, was known as Hestia, and Sinope was called Hades.
Official names were finally ratified by the International Astronomical Union. Those ending with the letter "a" have direct orbital motion, while those ending in "e" are retrograde. They are
Leda (discovered by Charles Kowal, at Palomar, from a photograph taken at Hale Observatories, 10 September 1974);
Himalia (CD Perrine, Lick Observatory, 3 November 1904); Lysithea (SB Nicholson, Mount Wilson CA, 6 July 1938); and
Elara (CD Perrine, Lick, plate taken 2 January 1905 but not studied until February). These have more eccentric orbits and are inclined to the first group by angles of 26.7 to 29°. Leda is one of the faintest known objects in the Solar System and may be less than 6.5 miles in diameter, smaller than Phobos and Deimos.
The remaining named satellites, Ananke (Nicholson, 29 September 1951),
Carme (Nicholson, 30 July 1938), Pasiphaë (PJ Melotte, Greenwich, 27 January 1908) and
Sinope (Nicholson, 21 July 1914), make up the final group, thought to be asteroidal in origin. These move in retrograde orbit, inclined to the first group at angles of between 147° and 163°, at a mean distance of over 13 million miles, all strongly perturbed by the Sun.
Sinope is the greatest distance from its parent of any known satellite, at a mean distance of 14,700,000 miles from Jupiter.
Io was one of the loves of Zeus (Jupiter), one of Juno's priestesses, to whom Zeus came in the form of a cloud. Zeus seduced Europa when he took the form of a bull. Ganymede was distinguished among mortals for his good looks and was abducted by Zeus to Olympus where he became cupbearer of the gods. Callisto was a nymph sworn to chastity by Artemis, but who succumbed to Zeus after he came to her disguised as Artemis. Metis was his first wife. Adrastea was one of the nymphs who mothered the young Zeus, and the nymph Amalthea also suckled him. Thebe married Zethus, a son of Zeus. Leda was a conquest of Zeus, who approached her in the form of a dazzling swan while she was married to Tyndareus, while Carme was another mistress.
The mythology of Jupiter's moons augments the Jovian association of abundance, indulgence and excess. Several of the moons are named from lovers of Jupiter in the myths, who suffered for their desires and producing startling and unique children. Hermes (Mercury) figures in several of these myths, suggesting utilisation of the intellect, and strong intuitive faculties. The moon symbolism shows up the Jovian associations with religion and dogma, infidelity, double standards and decadence. Violations of the purported principles of Jupiter reflect the syndrome of "do as I say and not as I do".
Last updated 21 April 2005