For thousands of years, people have watched a bright wandering light in their sky. Only one other wanderer (Venus) was brighter, but it never appeared high in the night sky. This “wandering star” or planet was named after the ruler and most powerful of the Roman gods of mythology—Jupiter.

Jupiter is the fifth planet from the Sun and the largest planet in the solar system. In fact, it is more than twice as large as all the other planets combined.  If it were hollow, more than 1400 Earths could fit inside. However, Jupiter’s density is only a little more than that of water. It is a gas planet, not a rocky one like Earth. These types of planets do not have solid surfaces–their gaseous material becomes denser as the depth  increases. Since the mid-1600s, astronomers have  noted spots moving across Jupiter’s face as the planet rotates. The longest observed of these spots is the “Great Red Spot.” This gigantic red oval (about three times the size of Earth) was first reported in 1664 and still exists today. Astronomers have used these moving spots to roughly measure the planet’s rotation period. A Jupiter “day” is just under 10 hours.

Jupiter Statistics

 Mass (kg)


 Mass (Earth = 1)


 Equatorial radius (km)


 Equatorial radius (Earth = 1)


 Mean density (gm/cm^3)


 Mean distance from the Sun (km)


 Mean distance from the Sun (Earth = 1)


 Rotational period

9h 55 min 

 Length of a year

11.86 Earth Years

 Number of known satellites


 Mean cloud temperature


 Atmospheric pressure (bars)


 Atmospheric composition




Early Explorations

Five spacecraft from Earth have already made the journey to Jupiter. Pioneers 10 and 11, launched in 1972 and 1973. Voyagers 1 and 2 were launched on their tours of the outer planets in 1977. The Voyager missions were designed to study the planetary systems in greater detail than the Pioneers had done. The Voyager spacecraft were automated than the Pioneers. Voyager was able to maintain a fixed orientation, or attitude, in space. Photography of Jupiter began in January 1979, when images of the brightly banded planet already exceeded the best taken from Earth. Voyager 1 completed its Jupiter encounter in early April, after taking almost 19,000 pictures. Voyager 2 took more than 33,000 pictures of Jupiter and its five major satellites.

Although astronomers had studied Jupiter from Earth for several centuries, scientists were surprised by many of Voyager 1 and 2's findings. They now understand that important physical, geological, and atmospheric processes go on - in the planet, its satellites, and magnetosphere - that were new to observers.

The latest visitor to Jupiter, Ulysses, was launched in 1990 and arrived in early 1992. Ulysses’ prime mission was to study the poles of the Sun. The spacecraft used Jupiter’s gravity to swing out of the ecliptic plane so it could examine the polar regions of the Sun.

The Galileo spacecraft was designed to study Jupiter's atmosphere, satellites, and surrounding magnetosphere for two years. The spacecraft was named in honour of Galileo Galilei, the Italian Renaissance scientist who discovered Jupiter's major moons in 1610. The Galileo spacecraft was carried into space on October 18, 1989. Galileo has spent more than five years travelling to Jupiter, where it has become the first spacecraft to make direct measurements from an instrumented probe within Jupiter's atmosphere and has also become the first spacecraft to conduct long-term observations of Jupiter, its magnetosphere, and satellites from orbit around Jupiter. Galileo arrived at Jupiter, on December 7, 1995.

The next satellite flyby will occur on November 5, 2002 when Galileo flies by Amalthea.

The Atmosphere

Based on the data obtained from the Pioneer and Voyager missions, Jupiter’s atmosphere consists of about 82 percent hydrogen and 18 percent helium. If Jupiter had been between fifty and a hundred times more massive, it might have evolved into a star rather than a planet. Besides hydrogen and helium, small amounts of methane, ammonia, phosphorus, water vapour, and various hydrocarbons have been found in Jupiter’s atmosphere. Jupiter’s atmosphere displ ays alternating patterns of dark belts and light zones. The locations and sizes of the belts and zones change gradually with time. Within these belts and zones are clouds and storm systems that have raged for years.


Jupiter's atmosphere taken

by the Galileo spacecraft .    

                                                                 Ju piter's atmosphere taken by the Galileo spacecraft.

The Great Red Spot

The Great Red Spot One of these giant storms, called the “Great Red Spot,” has lasted over 300 years. This spot rotates once counter-cloc kwise every 6 days. The Great Red Spot is the primary feature of the planet.  It is a complex storm (larger than twice the size of ea rth) moving in a counter-clockwise direction.  The reddish colour is a puzzle to scientists, but several chemicals, including phosphorus , have been proposed. In fact, the colours and mechanisms driving the appearance of the entire atmosphere are not well understood. These mysteries cannot by solved be taking pictures. Direct measurements from within the atmosphere are necessary—measurements like those made by the Galileo probe. Jupiter is swept by about a dozen prevailing winds, reaching 150 meters per second at the equator. On Earth, winds are driven by the large difference in temperature, more than 40 degrees Celsius, between the poles and the equator. But, Jupiter’s pole and equator share the same temperature, –121 degrees Celsius

The Interior: Jupiter from Core to Cloud Top

Jupiter’s core is estimated to be about one-and-a-half times Earth’s diameter, yet ten to thirty times more massive. The core’s temperature is estimated to be 30,000 degrees Celsius. This high temperature is the result of a pressure of as much as a hundred million atmospheres.  Surrounding this core is a 40,000-kilometer deep sea of liquid metallic hydrogen. Unknown on Earth, liquid metallic hydrogen forms under the extreme pressures that exist on Jupiter. At this depth, the pressure is more than three million atmospheres. Hydrogen molecules are so tightly packed that they break up and become electrically conductive. Scientists believe it is this electrically conductive liquid that causes Jupiter’s intense magnetic field. Next there is a 21,000-kilometer thick layer of hydrogen and helium. This layer gradually changes from liquid to gas as the pressure falls into the range of tens of atmospheres. Finally, in the uppermost regions of the atmosphere, the temperatures and pressures are low enough to allow clouds to form.

The Ring

One of the surprising discoveries made by Voyager 1 in March 1979 was the detection of an extremely faint ring around Jupiter. This simple ring system is composed of an inner halo, a main ring and a Gossamer ring. To the Voyager spacecraft, the Gossamer ring appeared to be a single ring, but Galileo imagery provided the unexpected discovery that Gossamer is really two rings. One ring is embedded within the other. The rings are composed of dust particles which are particles created by meteoroids crashing into the nearby moons. Jupiter joins Saturn, Uranus, and Neptune as a ringed planet - although each ring system is unique and distinct from the others.

 Rings of Jupiter








92,000 km

30,500 km

20,000 km




122,500 km

6,440 km

< 30 km

1 x 10^13 kg


Inner Gossamer

128,940 km

52,060 km




Outer Gossamer

181,000 km

40,000 km





The Magnetosphere

One of the by-products of Jupiter’s ocean of liquid metallic hydrogen is a magnetic field stronger than that of any other planet. Jupiter’s magnetic field has the opposite sense of Earth’s. A compass would point south rather than north. The region of space dominated by a planet’s magnetic field is called a “magnetosphere.” Jupiter’s magnetosphere is moulded by the solar wind into a teardrop shape—its point directed away from the Sun. If Jupiter’s magnetosphere were visible from Earth, it would be several times larger than the full Moon in the night sky. The environment near Jupiter contains high levels of radioactive particles, which are trapped by this magnetic field.  These radioactive belts are the most powerful in the solar system and would be instantly fatal to unprotected humans.

A  portion of the front-side of the
magnetosphere of Jupiter.

                                                                                                                                                              A portion of the magnetosphere of Jupiter.

The Satellites, Jupiter’s Moons

Thirteen of Jupiter’s 16 known moons were discovered from Earth. The other three were first seen by Voyager. The four largest moons—Io, Europa, Ganymede, and Callisto—were observed by Galileo Galilei of Italy. Nearly four centuries ago he turned his homemade telescope towards the heavens and discovered three points of light, which at first he thought to be stars, hugging the planet Jupiter. These stars were arranged in a straight line with Jupiter. Sparking his interest, Galileo observed the stars and found that they moved the wrong way. Four days later another star appeared. After observing the stars over the next few weeks, Galileo concluded that they were not stars but planetary bodies in orbit around Jupiter. These four stars have come to be know as the Galilean satellites.

Jupiter and its four moons, the Galilean satellites .



Io has been described as looking like a giant pizza (due to its bright reddish-orange and white markings). Active volcanism on Io was the greatest unexpected discovery at Jupiter. It was the first time active volcanoes had been seen on another body in the solar system. It appears that activity on Io affects the entire Jovian system. Io appears to be the primary source of matter that pervades the Jovian magnetosphere -- the region of space that surrounds the planet, primarily influenced by the planet's strong magnetic field. Sulphur, oxygen, and sodium, apparently erupted by Io's volcanoes and sputtered off the surface by impact of high-energy particles, were detected at the outer edge of the magnetosphere.  Approximately one third of the surface is covered with bright white sulphuric snow. Io is perturbed in its orbit by Europa and Ganymede, two other large satellites nearby, then pulled back again into its regular orbit by Jupiter. Io acts as an electrical generator as it moves through Jupiter's magnetic field, developing 400,000 volts across its diameter and generating an electric current of 3 million amperes that flows along the magnetic field to the planet's ionosphere.

Io in eclipse, taken by the Galileo spacecraft .        

                                                                                                  The possible interior of Io .


Europa is named after the beautiful Phoenician princess who, according to Greek mythology, Zeus saw gathering flowers and immediately fell in love with.

If Io is a pizza, then Europa, the next satellite out from Jupiter, is a cracked hard-boiled egg. It has a bright white surface, crisscrossed with dark fissures. It has neither mountains nor valleys, craters nor volcanoes. Recent observations from Earth indicate the moon may have a thin atmosphere of oxygen and sodium. Some scientists think that a giant water ocean may lie beneath a layer of ice that has cracked and refrozen at temperatures of about –146 degrees Celsius. If so, it would be the only place we know of in our solar system besides Earth with a significant supply of liquid water.

Europa chaos regions taken by the
Galileo spacecraft.

                                                                   A model of the interior of Europa.






The third Galilean satellite, Ganymede, is the largest moon in the solar system. If Ganymede orbited the Sun instead of Jupiter it could be classified as a planet. It has a variety of geological formations, including craters and basins, grooves, and rough mountainous areas. About half the surface is covered with water ice and half with dark rock. These heavily cratered dark regions are thought to be ancient. The newer, lighter regions give evidence of tectonic activity that may have broken up the icy crust. Ganymede has no known atmosphere but a thin layer of ozone has been detected surrounding Ganymede by the Hubble Space Telescope.




Image of Ganymede taken by Voyager
showing the ancient "dark terrain".

                                                                            The possible interior of Ganymede.



Callisto i s the second largest moon of Jupiter, the third largest in the solar system, and is about the same size as Mercury. It orbits just beyond Jupiter's main radiation belt. Callisto is the most heavily cratered satellite in the solar syste m . Although the exact rate of impact crat er formation is not known, scientists estimate that it would require sever al billion years to ac cumulate the number of craters found on Callisto. Therefore, the moon must have been inactive at least that long, a fine record of the past. Callisto’s crust is very ancient and dates back 4 billi on years, just shortly after the solar system was formed.

                                                                                                                      The possible interior of Callisto .

  The Four Galilean satellites, arranged by size.

  Four Galilean satellites, arranged by size.                                 

                                                                                               The possible interiors of  the Galilean moons.


All the other satellites are comparatively minor objects, up to 170 km across. Eight are in inclined orbits far from the planet, and four are close to the planet, inside Io’s orbit. The 16 moons are Metis, Adrastea, Amalthea, Thebe, Io, Europa, Ganymede, Callisto, Leda, Himalia, Lysithea, Elara, Ananke, Carme, Pasiphae, and Sinope.













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Jupiter in comparison with other planets

The planets of the outer solar system are Jupiter, Saturn, Uranus, Neptune and Pluto:

The nine bodies conventionally referred to as planets are often further classified in several ways:

  • by composition:
    • terrestrial or rocky planets: Mercury, Venus, Earth, and Mars:
      • The terrestrial planets are composed primarily of rock and metal and have relatively high densities, slow rotation, solid surfaces, no rings and few satellites.
    • jovian or gas planets: Jupiter, Saturn, Uranus, and Neptune:
      • The gas planets are composed primarily of hydrogen and helium and generally have low densities, rapid rotation, deep atmospheres, rings and lots of satellites.
    • Pluto .
  • by size:
    • small planets: Mercury, Venus, Earth, Mars and Pluto.
      • The small planets have diameters less than 13000 km.
    • giant planets: Jupiter, Saturn, Uranus and Neptune.
      • The giant planets have diameters greater than 48000 km. 
  • by position relative to the Sun:
    • inner planets: Mercury, Venus, Earth and Mars.
    • outer planets: Jupiter, Saturn, Uranus, Neptune and Pluto.
    • The asteroid belt between Mars and Jupiter forms the boundary between the inner solar system and the outer solar system.
  • by position relative to Earth :
    • inferior planets: Mercury and Venus.
      • closer to the Sun than Earth.
      • The inferior planets show phases like the Moon's when viewed from Earth.
    • Earth .
    • superior planets: Mars thru Pluto.
      • farther from the Sun than Earth.
      • The superior planets always appear full or nearly so.
  • by history:
    • classical planets: Mercury, Venus, Mars, Jupiter, and Saturn.
      • known since prehistorical times
      • visible to the unaided eye
    • modern planets: Uranus, Neptune, Pluto.
      • discovered in modern times
      • visible only with telescopes
    • Earth .

[small worlds]

his composite shows Earth and the remaining 11 large solar system objects at a scale of 100 km/pixel.


The Biggest

                Distance   Radius    Mass 
 Name       Orbits (000 km)    (km)     (kg) 
 ---------  ------- --------  -------  -------  
 Sun                           697000  1.99e30  
 Jupiter    Sun       778000    71492  1.90e27  
 Saturn     Sun      1429000    60268  5.69e26  
 Uranus     Sun      2870990    25559  8.69e25  
 Neptune    Sun      4504300    24764  1.02e26  
 Earth      Sun       149600     6378  5.98e24  
 Venus      Sun       108200     6052  4.87e24  
 Mars       Sun       227940     3398  6.42e23  
 Ganymede   Jupiter     1070     2631  1.48e23  
 Titan      Saturn      1222     2575  1.35e23  
 Mercury    Sun        57910     2439  3.30e23  
 Callisto   Jupiter     1883     2400  1.08e23  
 Io         Jupiter      422     1815  8.93e22  
 Moon       Earth        384     1738  7.35e22  
 Europa     Jupiter      671     1569  4.80e22  
 Triton     Neptune      355     1353  2.14e22  
 Pluto      Sun      5913520     1160  1.32e22  

The Brightest

There are 12 major bodies brighter than magnitude 6 (as viewed from Earth). All of these can be seen with the unaided eye or with binoculars.

                   Distance   Radius 
 Name       Orbits (000 km)    (km)     
---------  ------- --------  -------   
Sun        ?              0   697000 
Moon       Earth        384     1738 
 Venus      Sun       108200     6052    
 Jupiter    Sun       778000    71492    
Mars       Sun       227940     3398  
Mercury    Sun        57910     2439  
Saturn     Sun      1429000    60268   
 Ganymede   Jupiter     1070     2631      
 Io         Jupiter      422     1815    
 Europa     Jupiter      671     1569    
Uranus     Sun      2870990    25559   
Callisto   Jupiter     1883     2400   

The Densest

There are 11 major bodies whose density is greater than 3 g/cm3:

            Radius    Mass   Density 
Name        (km)     (kg)   (g/cm3) 
--------- -------  -------  ---- 
Earth        6378  5.97e24  5.52 
 Mercury      2439  3.30e23  5.42  
 Venus        6052  4.87e24  5.26  
 Adrastea       10  1.91e16  4.5  
 Mars         3398  6.42e23  3.94  
 Io           1815  8.93e22  3.53  
 Moon         1738  7.35e22  3.34  
 Elara          38  7.77e17  3.3  
 Sinope         18  7.77e16  3.1  
Lysithea       18  7.77e16  3.1 
Europa       1569  4.80e22  3.01 

The Best Prospects for Life

Name  Why 
---------  ------- 
Mars       most Earth-like; more so in the past; 
Europa     may have liquid water 
Enceladus  may have liquid water 
Titan      complex chemistry and liquids likely 
Io         complex chemistry, warmer than most 
Jupiter    long shot: warm, plenty of organic material 



Studying Jupiter may help us to understand how the solar system, and our own planet, formed and evolved. The flyby missions of the Pioneers, Voyagers, and Ulysses gave us quick glimpses of this exciting world. The orbiter will spend almost 2 years studying the planet, its satellites, and the vast magnetosphere up close.

The composition of Jupiter’s atmosphere may tell us about the original star stuff from which all the planets formed. There are many unanswered questions about Jupiter that Galileo will try to answer.

Learning more about Jupiter’s atmosphere will advance our understanding of the nature of all

planetary atmospheres, including our own. By studying Jupiter’s satellites we hope to determine the effects of initial conditions, size, energy sources, meteorite bombardment, and tectonic processes on the way planets evolve.

Observations of the magnetosphere will help us to understand the complex interactions between magnetic forces and matter throughout the universe. We should always remember that even though we have many ideas about what we anticipate Galileo will accomplish, the most exciting results are often unexpected. 

Practical exercise

Activity description: If possible, show the Great Red Spot rotation movie and/or complete the Great Red Spot rotation speed exercise. Where is the storm rotating the fastest? The slowest? Does it all rotate in the same direction?

It takes 6 days for the Great Red Spot to make one rotation. Assuming that each part of the storm takes the same amount of time to rotate (which isn't quite true), the outer part of the storm must travel faster because it must travel farther in those 6 days.

Have some students stand in a line and have them rotate like a pinwheel (representing a "slice" or "spoke" of the storm). To keep them in a straight line, have them hold hands. Either way, students will quickly realize that those on the outside must take bigger steps (hence travelling a greater distance for each unit of time, which in the second case is one "step" command).

Who is moving the fastest? The slowest? Add another line of students like a spoke and have them continue rotating. What happens? What if there are 3 or 4 lines of students rotating? What if you represent the whole storm (like a crowded skating rink)?

Students will start bumping into each other, especially on the outside where they must move faster. This is representative of what happens in cyclonic storms like the Great Red Spot. Make the storm move faster or slower and notice the frequency with which students bump into each other.

This activity can lead to a number of spin-off discussions about atmospheric dynamics, rotation speed, and the difficulty of modelling complex, dynamic activity.