Jupiter is the largest planet in the solar system and is rightly named after the most important of all Roman gods.

Taken by the NASA/ESA Hubble Space Telescope – March 2019

With an equatorial diameter of 142 984 kilometres Jupiter is 1 321 times the size of Earth and accounts for 71% of the solar system’s planetary material. Like Saturn, the planet spins incredibly fast—more than twice as fast as Earth—with a period of rotation of just 9 hours 56 minutes. It rotates so quickly it bulges out at the equator (by about 7%), giving it a slightly squashed appearance. To the naked eye, the planet appears a brilliant white, sometimes beige, or light biscuit. In close up the characteristic horizontal bands stand out in orange, white and lesser streaks and whorls of yellow, red and brown. Its most obvious feature is the iconic Great Red Spot, a colossal storm that has been raging for at least 200 years. It is about one and a half times the diameter of Earth. Jupiter has a faint ring system—a dark ring of dust; a massive magnetic field; and may have as many as 67 moons.

Despite its size, Jupiter is relatively light with a density of 1.3g/cm3, only a little above water (1g/cm3). Earth is four times as dense. Jupiter’s volume, as mentioned, is equal to 1 321 Earths, yet its mass is 318 Earths.

The mass of the planets. Earth = 1

Jupiter is the largest of the four so-called “gas giants”. However, it is not all gas. Because of the intense gravitational forces and atmospheric pressures, many gases are in liquid, even solid form. Below the 1 000-kilometre-thick atmosphere is the outer shell where the planet’s hydrogen and helium have been crushed into a liquid for depths of at least 20 000 kilometres. This is despite its temperature of 2 000°C. [See note on Combined Gas Law below]. Deeper still the temperature reaches 5 000°C where the liquid hydrogen and helium act more like molten metal. Here the pressure is estimated at three million bars [the pressure on Earth at sea level is 1.13 bar]. Further down there is an icy mantle. Then at 60 000 kilometres depth it is thought the materials have become a solid core, about ten times the mass of Earth. The core consists of rock, metals and hydrogen compounds, and has a temperature of 35 700°C.


Jupiter’s atmosphere is a violent mix of hydrogen (96%) and helium (3%) with traces of methane, ammonia, water, hydrogen deuteride, and complex hydrogen compounds such as ethane, acetylene and propane. The white traces in its atmosphere are ammonia; the orange swirls ammonium hydrosulphide. Jupiter’s dense gas atmosphere of hydrogen is 64 000 kilometres deep.

The white bands in Jupiter’s atmosphere, as seen by telescope, are rising streams of cool air. These are called “zones”. The red-brown bands are descending warmer air, known as “belts”. This gives the planet its distinctive banded appearance. It radiates twice as much energy as it receives from the sun.

The Great Red Spot spins once every six days and towers some eight kilometres above the surrounding cloud cover. The red colour is perhaps from phosphorous dredged up from below.

Combined Gas Laws

The behaviour of Jupiter’s hydrogen and helium mix is a wonderful example of the Combined Gas Laws of physics.

Boyle’s Law reveals an inverse relationship between pressure and the volume of a gas: if the volume reduces, pressure increases and vice versa. Thus, Pressure times Volume equals a constant. PV = k. Robert Boyle (1627-1691).

Charles’s Law indicates that at constant pressure the volume of a fixed amount of gas is directly proportional to its temperature. As the volume increases, the temperature increases, and vice versa. Thus, Volume divided by Temperature also equals a constant. V/T = k (constant). Jacques Charles (1746-1823).

Combined we see that there is a direct relationship between Pressure, Temperature and Volume. When pressure rises temperature increases; when pressure rises, volume decreases; and so on.

As we descend into the atmosphere of Jupiter, the pressure increases, the temperature increases and the volume decreases: a gas becomes a liquid; a liquid becomes a solid. Meanwhile, the temperature has increased from about -110°C at cloud cover to 1 000°C at a depth of around 1 000 kilometres. Both Temperature and Pressure rise with depth to 2 000°C at the outer layer, to 5 000°C at the inner layer, and to over 30 000°C at the solid core.


So why all the pressure? Why does Jupiter bring its hydrogen to a molten liquid and then to a solid? The answer is gravity. The larger a planet (the larger any celestial body) the greater its gravity. There is a theory going around that Jupiter is really a failed star, that it is somewhat of a Brown Dwarf.

In the early days, Jupiter cannibalised a number of moons through its sheer pulling power. Indeed, it has been a significant influence in the shaping of our solar system because of its enormous gravity. For example, it is theorised that both Uranus and Neptune were flung out to the far reaches of the solar system by Jupiter’s gravitational influence. The asteroid belt may also be a collection depot created by Jupiter trapping billions of pieces of material that might otherwise make landfall on Earth. It is thought Jupiter also deflects comets arriving in the solar system, sucks them in and shatters them. The planet’s gravity even makes the sun wobble.

But is Jupiter really our guardian angel? There is a contrary view. Kevin Grazier, a physicist who used to work at Nasa, wonders if Jupiter is just as likely to take “pot shots” at Earth, sending asteroids and comets our way.  Jupiter’s gravity slows down the passages of asteroids and comets barrelling through the inner solar system, making it more likely that material from these small bodies would accumulate on Earth and other locations. Grazier’s study, therefore, argues that Jupiter’s role in the solar system is less of a shield, and more as a bringer of water and other “life-enabling volatiles” to the terrestrial planets; and that Saturn has a far bigger role to play when deflecting asteroids and comets.

In any event, there are over a billion asteroids in the asteroid belt and Jupiter prevents them from coalescing. Does Jupiter also pull them out of the asteroid belt and fling them our way?


Jupiter follows a slightly elliptical orbit, ranging from 741.6 million kilometres from the sun at perihelion to 817.4 million kilometres at aphelion—a difference of some 76 million kilometres. Only Saturn, Mars, and Mercury (in that order) have more pronounced elliptical orbits.

The intensity of sunlight on Jupiter is only 1/25th of that on Earth, as it is five times the distance, at a mean 778.4 million kilometres away from the sun. It is also in a very lonely isolated place. Its nearest neighbour is Mars, orbiting 550 million kilometres closer to the sun; then Saturn is a further 650 million kilometres out from the sun. Jupiter’s orbital speed is a pedestrian 13 kilometres a second. Indeed, orbital speeds decrease consistently for each planet the more distant they are from the sun. It takes Jupiter almost twelve earth years to complete one circuit. If you were able to live on the planet you could look forward to a birthday every 10 457 Jovian days, give or take.

Magnetic field

Jupiter has the most powerful magnetic field in the solar system. It is a hundred times greater than Earth’s and extends 20 000 times further. The axis joining the magnetic poles is at 11° to the spin axis.

All the gas giants have magnetic fields, though they do not have the same dynamo generator of an iron liquid shell surrounding sold iron core (as is the case with Earth). Instead, their magnetic fields are caused by a metallic hydrogen core reacting in a very fast rotation period. As mentioned Jupiter rotates at just under ten hours. This generates a speed at the equator of around 14 300 kilometres an hour.

In turn, Jupiter’s magnetic field generates a huge magnetosphere. This is a region in space that will react and manipulate charged particles. It is so powerful that it can deflect solar flares as far as three million kilometres away. The magnetosphere extends some seven million kilometres towards the sun, where it is compressed by the solar wind. On the dark side, it is thought Jupiter’s magnetosphere forms a tail of as long as 600 million kilometres, almost as far as Saturn’s orbit, at 650 million kilometres distant.

Charged particles, which are not deflected become trapped in the magnetosphere and form intense radiation belts. These are similar to Earth’s Van Allen belts but are some one thousand times stronger. Not surprisingly the magnetic field affects Jupiter’s moons.

The moons of Jupiter

Jupiter has a very large family of moons— 63 at the last count. Two-thirds of them were discovered since January 2000. They fall into three categories: the four inner moons; the four large Galilean moons; and the rest, the small outer moons. The inner and Galilean moons, as well as the planet itself, orbit in an anticlockwise motion, as viewed from above the north pole (in the same direction as Jupiter’s spin). Most of the outer moons, however, travel in the opposite direction suggesting they were from an asteroid that disintegrated after it was captured by Jupiter’s gravitational field.

The four Galilean moons are Io (the nearest), Europa, Ganymede, and Callisto (the furthest, way out at 1.88 million kilometres distant). They were formed at the same time as the planet.

Io is geologically active. Jupiter’s gravitational forces tug at Io creating heat through tidal forces and have turned her innards molten. The interior feeds up to 400 geysers of superheated sulphur dioxide. It is the most active body in the solar system and is estimated to discharge 10 000 million tonnes of sulphurous lava onto the surface each year. It brushes against the magnetic field of Jupiter, which causes electrical storms on the planet.

Europa is an ice moon. It has an oxygen atmosphere. The interior and crust rotate at different speeds, suggesting that the surface may be floating on liquid. It is further suggested this liquid is warm ocean water, between 80 to 170 kilometres thick, and containing more liquid than Earth’s oceans combined.

With a diameter of 5 262 kilometres, Ganymede is bigger than Mercury, and the largest moon in the solar system. The moon has a metal and rock interior under an icy crust. The surface is sheered up and broken and looks a bit like the Antarctic. It also has a thin oxygen atmosphere, with a faint trace of hydrogen.

Callisto is the only moon that orbits outside of Jupiter’s deadly radiation belts. It is the third largest moon in the solar system after Ganymede and Titan (a moon of Saturn). It is almost the same size as planet Mercury. It has a core of rock and ice and may also have a liquid ocean beneath the crust. It has a very thin atmosphere of carbon dioxide, with a trace of oxygen. The largest surface feature on Callisto is the Valhalla Basin, a large impact crater of 600 kilometres in diameter.

These four moons are the first bodies discovered to be orbiting a planet in our solar system. They are called the Galilean Moons after the astronomer who publicised their existence. Europa, Ganymede and Calisto are rocky, frozen worlds with an abundance of water ice on their surfaces. Perhaps one day, the heat of an ageing expanding sun will reach out and warm these ocean worlds sufficiently to bring them to life?

Nearer to the planet than Io are the four inner moons: Metis, Adrastea, Amalthea and Thebe. The rest—some 50 odd—are way beyond Callisto’s orbital distance of 1.88 million kilometres. The nearest of them, Thermisto, is 7.5 million kilometres from Jupiter.

The moons of Jupiter 
Inner moonsDiameterPlanetary distance
Metis40 kms127 960 kms
Adrastea26 kms128 980 kms
Amalthea262 kms181 300 kms
Thebe110 kms221 900 kms
Galilean moons
Io3 643 kms421 600 kms
Europa3 122 kms670 900 kms
Ganymede5 262 kms1.07 million kms
Calisto4 821 kms1.88 million kms
Outer moons
Thermisto8 kms7.5 million kms
[54 others]

The first mission to Jupiter was Nasa’s Pioneer 10. Launched in March 1972 this was a successful fly-by. It was also the first satellite to fly through the asteroid belt. It fell silent in its 30th anniversary year (2002). This was followed a year later by Pioneer 11. Also successful, it paid Saturn a visit, passing through its outer rings in September 1979. This was the year Jupiter’s rings were discovered.

Two US Voyager probes flew by Jupiter and Saturn: Voyager 1 in 1979 and 1980; and Voyager 2 in 1979 and 1981. Voyager 2 went on to fly past Uranus in 1986 and Neptune in 1990. As of February 2020, these two satellites are still in operation and are now in interstellar space. At about 18.5 billion kilometres they are the most distant man-made objects in space. Signals take a round trip of 34 hours.

In October 1989 Galileo was launched as an orbiter with a probe. On its way to Jupiter, it flew past Venus, taking infra-red images of the planet’s atmosphere, and was the first spacecraft to visit two asteroids before finally arriving in orbit around Jupiter in December 1995. After a successful mission, it was intentionally sent into Jupiter’s atmosphere in 2003 to prevent an unwanted impact with Europa.

The Ulysses left in October 1990 on the first ever mission to the sun, using Jupiter to help achieve solar polar orbit. It studied the sun’s north and south poles and collected data on the solar wind and interstellar dust. The mission ended in 2009.

Cassini-Huygens was launched in October 1997 on a mission to Saturn. In 2000 it took a six-month orbit around Jupiter to pick up speed for its onward journey. After capturing detailed images of Jupiter and its larger moons it continued on to Saturn. The mission ended in September 2017.

The Juno probe was launched atop an Atlas 5 rocket on 5th August 2011. It cruised beyond Mars and made orbit around Jupiter in July 2016. It measured the planet’s properties, mapped Jupiter’s magnetic and gravity fields, and explored the planet’s magnetosphere. It was the first solar-powered mission to venture this far from the Sun. Sunlight is minuscule as far out as Jupiter. Space missions would normally resort to a plutonium battery. But Juno instead uses three wings coated with 18 000 solar cells. Juno was the second in Nasa’s so-called New Frontiers class missions.

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By Nigel Benetton, science fiction author of Red Moon and The Sands of Rotar.

Last updated: Friday, 14th February 2020