Visions of the Cosmos

Planetary Science

The Solar System

The Moons of Jupiter

The Galileo Mission

            On 7 January 1609, the Italian astronomer Galileo focussed his telescope on the heavens and made a stunning discovery - he found that Jupiter had four large moons. It was a turning point in the science of astronomy.

Galileo named the moons after the members of the Medici family - Cosimo, Francesco, Carlo and Lorenzo.  These titles are now long forgotten and in obedience to convention the moons were later named after four of the mythological lovers of the great god Jupiter - Io, Europa, Ganymede and Callisto.

In October 1989, a spaceship was launched by NASA to investigate the Jupiter System. As a tribute to the genius of the great scientist the project was named 'The Galileo Mission'.  The spacecraft consisted of two parts

1) the orbiter which was designed to orbit the giant world and make a large number of close fly-bys round the four large moons

2) a probe which was designed to droe into the atmosphere and take measurements of the atmosphere of the gas giant planet during its descent.

After an inauspicious beginning, the mission proved a spectacular success.

The disaster on 28 January 1986 when the Challenger spacecraft exploded killing all the seven astronauts on board led to many modifications being made in the Galileo spacecraft and caused a delay of three years in the take-off date. The Galileo mission was previously designed for a direct flight of about 2-1/2 years to Jupiter. Changes in the launch system after the Challenger accident precluded this direct flight. Trajectory engineers designed a new interplanetary flight path using gravity assists, once with Venus and twice with Earth, to build up the speed to reach Jupiter, taking a total of just over 6 years.

Galileo was launched in October 1989 and on 7 December 1995 entered orbit around Jupiter. The spacecraft's mission is to conduct detailed studies of the giant planet, its largest moons and the Jovian magnetic environment. The Jet Propulsion Laboratory, Pasadena, CA manages the mission for NASA's Office of Space Science, Washington, DC.

Because the spacecraft used a much less powerful booster rocket than was originally intended it had to take a circuitous route through the inner Solar System to pick up sufficient speed to reach the vicinity of Jupiter and its moons.  It took a route nicknamed the VEEGA Trajectory, which stands for Venus-Earth-Earth-Gravity-Assist.   The space-vessel was first aimed towards Venus.  By sweeping round the planet, it gained gravitational energy and therefore speed.  This is called THE SLINGSHOT EFFECT in which the gravitational energy of a planet is converted into kinetic energy, which is then used to accelerate the spacecraft.   The 'Galileo' returned to the vicinity of Earth where it picked up more gravitational energy.  After a trip round the asteroid Gaspra it returned again to the Earth's orbit and acquired a third accelerating force to speed it on its way. Finally it headed towards its destination passing the asteroid Gaspra once again and another asteroid Ida before finally arriving in a Jupiter orbit on 7 December 1995. 

         The Space Shuttle Atlantis carried the Galileo spacecraft into Earth orbit on October 18 1989. A two-stage solid fuel rocket accelerated the spacecraft out of Earth orbit toward the planet Venus.  

After its voyage round Venus the spacecraft picked up more energy by performing a slingshot round the Earth on 8 December 1990. 

Nine months after its first pass round the Earth the spacecraft, the spacecraft passed about 1,600 kilometres from Gaspra at a relative speed of about 29,000 kilometres per hour; several pictures of Gaspra were taken as well as measurements to indicate composition and physical properties.

Thirteen months after the Gaspra encounter, the spacecraft completed its 2-year elliptical orbit around the Sun and arrived back at Earth. It needs a much larger elliptical orbit (with a 6-year period) to reach as far as Jupiter, and the second flyby of Earth increased the orbit up to that size.

On its way to Jupiter the spacecraft made an encounter with the asteroid Ida.  Ida is 56 kilometres long. Like Gaspra, it is believed to represent the majority of main-belt asteroids in composition, though there are believed to be differences between the two. One of the surprising discoveries was that Ida has a small moon, christened Dactyl. This small moon is 1.5 km across and from measurements of its orbit around Ida the mass of the asteroid could be determined. The result was surprisingly low and indicated that Ida must contain many holes in its . 

In the final stages of the voyage to Jupiter thye orbiter and the probe separated.  Early in December 1995 the  probe approached Jupiter.. A few hours later, the probe entered the upper atmosphere, about 6 degrees north of Jupiter's equator, at more than 160,000 kilometres per hour, and was slowed down by aerodynamic braking for about 2 minutes before deploying its parachute and dropping its heat shields. It then floated down about 200 kilometres through the clouds, passing from a pressure of 1/10 that on Earth's surface to about 25 Earth atmospheres in 75 minutes. The probe batteries were not expected to last beyond this point, and the radio-communications link was terminated.

The orbiter went into orbit round Jupiter and obtained numerous images of the giant planet itself  and the moons Amalthea, Io, Europa, Ganymede and Callisto. Shortly after arrival in 1995 it observed the impact of the comet Shoemaker-Levy 9 with the Jovian atmosphere.

The Voyage of the Galileo Space Probe.

 Diagram Credit NASA/JPL

The Moons of Jupiter

It was already known from the Pioneer and Voyager Missions that the Jupiter system was a collection of five planets amazing in their diversity.  There was Jupiter itself, almost halfway to being a star, and then its four remarkable moons, each one a world in its own right and each one so different to all the others.   Table 10.1 lists the four large moons of the Gas Giant with their main features.  Details of the Earth's Moon are given for comparison.

 The Moons of Jupiter.


Mean Distance from Jupiter in kilometres

Diameter in  kilometres

Mass Earth's Moon =1

Density in grams per cc

Surface Gravity

























Earth's Moon

Mean Distance from Earth = 384,392


Earth = approx: 81 times the Moon's Mass



Earth = 1.000


Table 10.2 shows the surface areas of the moons of Jupiter with the Earth and Moon for comparison.

Details of Surface Areas of the Planets



Equatorial Diameter   kms

Equatorial Radius kms

Surface Area square kilometres





sea  360,300,000      land  149,800,000












Ice cap 30,800,000










From the table it can be seen that the total surface area of the four small planets is 232,300,000 square kilometres which is considerable greater than the land surface area of the Earth.


The Geology of Io

Io is a planet with a mean equatorial diameter of 3,642 kilometres which is slightly larger than the Earth’s Moon.  It has a mass of just over 1.2 times that of the Moon.   It is one of the strangest planets in the Solar System.   It lies deep within the huge gravitational field of Jupiter.  The tidal effects are so enormous that it is distorted into an egg shape.  It is also affected at regular intervals by the gravitational pulls of Europa and Ganymede, which help to perturb its orbit into an elliptical shape. The most outstanding characteristic of the planet is its vulcanism.  The planet abounds in volcanic caldera and vents, from which lava appears to have flowed in sinuous streams.   During the Galileo Mission, a minimum of 15 massive volcanoes were  photographed.  Massive plumes of sulphur dioxide gas, sulphur and sulphur dioxide 'snow' are ejected from the vents of the volcanoes up to altitudes of 300 kilometres or more.  The cause of the intense vulcanism is the massive tidal effects produced by Jupiter, Europa and Ganymede. The energy necessary for the rapid and violent volcanic activity is largely derived from the tidal forces of Jupiter, Europa and Ganymede .  According to some calculations it is equivalent to tides capable of producing 100 metre waves on the Earth.

The density of Io indicates that it is a typical rocky body like the Earth's Moon.  All the evidence points to the existence of a thick core of at least 1,800 kilometres in diameter composed of iron with iron sulphide surrounded by a mantle that is believed to consist mainly of iron and magnesium silicates, which is overlain by a semi-molten asthenosphere of silicate magma.   A silicate crust covers the magma.  

Landscape - Geomorphology of Io

Because of the intense volcanic activity, the silicate crust seems to be almost completely covered by sulphur in its many allotropic forms and by a number of its compounds, chief among them being sulphur dioxide.  In the main, the surface shows little evidence of exposed silicate materials although there are a few high rugged mountains that appear to be silicate structures, emerging through the almost ubiquitous sulphur layer. 

Io appears to be totally devoid of impact craters.  Craters a few kilometres in diameter should be formed at a rate of about one every 10,000 years.  Clearly there must be a mechanism which is erasing all evidence of impacts very rapidly - either the surface must be buried by volcanic debris or the crust must be subducted on a time scale of 10,000 years or less.

The surface of Io is dominated by volcanic landforms.   The overall topography is flat but occasionally prominent mountains thrust up through the plains. The peaks are obviously not part of a range but resemble isolated mountains known as 'inselbergs' to geologists.  They are thought to be volcanic in origin.  Sulphur is not a strong material and the presence of steep scarps 2 kilometres high suggests the presence of an underlying stronger material, which is believed to be silicate rock.  The total topographical relief is about 9 kilometres.  

Reflection spectroscopy confirms that ices are absent on the surface of Io.   The overall albedo of the surface of the planet has the exceptionally high value of 0.6.  This is the kind of albedo given by the cloud layer of Venus, very fresh snow surfaces or sheets of glazed white paper.  

The leading hemisphere of Io as it orbits Jupiter is covered with a bright deposit, which has a strong infrared absorption band at a wavelength of 4.08 mm.  It seems clear that the bright white areas are covered with a frost of SO2.   On performing a scan from 3.5mm to 4.2mm in the infrared, the resulting reflectance spectrum shows a clear correlation with a laboratory spectrum of solid sulphur dioxide. 

The colours of the other (trailing) hemisphere of the planet are the typical yellows, oranges, reds and browns associated with the colours of the many allotropes of sulphur. 

Remarkable photographs of the surface of Io were taken by the Voyager Spacecraft and during the Galileo Mission.   .

Photograph of  Globe of Io taken from the Galileo Space Craft.

The above photograph was taken was taken on 7 September 1996 when the Galileo spacecraft was 487,000kilometres (302,000 miles) from Io.  The image was centred on the side of Io that always faces away from Jupiter.    The colour composition was taken using near infrared, green and violet filters with the Galileo solid-state imaging camera.  The photograph is colour enhanced to emphasize the variations in colour and brightness.  The black and bright red corresponds to the most recent deposits. Credit NASA/JPL

In December 1999, the Galileo spacecraft detected a dynamic eruption at Tvashtar Catena, a chain of volcanic calderas located near Io's north pole. The image below reveals a change in the location of hot lava over a period of a few months from late 1999 to early 2000.



In the second image, the orange and white areas on the left side are places where hot lava has recently erupted. The two small white spots are places where molten rock is exposed at the surface near the edges of the lava flows. The long, yellow and orange stream is more than 60 km long and is a cooling lava flow. The white color in the picture indicates the hottest material in the lava flow, while orange reflects the cooler temperatures. The dark deposits in the vicinity of the active flows were not seen in the image taken in the November, 1999 flyby

Known as Io's "Old Faithful", the Prometheus volcano has been active during every observation of it since it was first seen by Voyager 1 in 1979. The Prometheus plume is 80 km (50 miles) tall, and although its size and shape have remained constant over the years, its plume location has migrated about 85 km (53 miles) to the west. Its volcanic field is similar to those of Hawaiian volcanoes, but it is much larger and more active. The bright, ring-shaped deposit around the volcano forms when sulfur dioxide, ejected during the plume eruption, condenses into snow and falls back to the surface. Scientists have been especially interested in determining whether the Prometheus plume is erupting from a vent at the west end of the dark lava flow, or if it is being produced by advancing lava as it flows over a surface rich in sulfur dioxide. New images have helped to resolve this question.

Since the first photographs were taken during the Voyager missions and particularly during the Galileo mission dramatic changes have been observed in the surface of Io.   The whole planet is obviously in a constant state of violent activity

 Io has been laughingly compared to a pizza. (Images Courtesy NASA/JPL/University of Arizona)


The Cassini Spacecraft Visits the Jupiter System

In late December 2000 and early January 2001, the Cassini spacecraft passed through the Jupiter system on its way to Saturn.    Advantage was taken of this event to undertake joint studies of Io by both the Cassini and the Galileo spacecraft.   On the first and second of January 2001 Cassini photographed Io in both the visible and ultraviolet regions of the spectrum from a distance of about ten million kilometres (six million miles).  The ultraviolet was particularly good at investigating active volcanic plumes.  Two days earlier (30 and 31 December 2000) Galileo took high-resolution pictures from a tenth of the distance of Cassini.  The spacecraft photographed two gigantic active erupting plumes.  The pictures were released by NASA on 28 March 2001 taken by the two spacecraft.  The massive plumes from the Pele and Tvashtar regions can be clearly seen.The illustrations clearly show the sites of the two plumes and the huge red rings of material surrounding the volcanoesAcknowledgements NASA/JPL/University of Arizona


                             Io in natural colours

The photograph corresponds more to what the planet would look like to the unaided human eye  Credit NASA/JPL/University of Arizona

During the time that Galileo had been observing Io it had studied fifteen large active plumes in the mid latitudes of the planet.  In particular Mt Pele had been highly active over at least four years.  The plume had been measured and found to be about 390 kilometres (242miles) high.  At the time of the joint observations by the two spacecraft it was rivalled by a new massive plume in the Tvashtar volcanic area between 355-415 kilometres (221-259 miles) in altitude. The new plume originates from a volcanic feature named Tvashtar Catena near Io's north pole. Scientists were astounded to discover so large a plume so near the pole, because all active plumes previously detected on Io had been over equatorial regions and no others had approached Pele's in size.   Another opportunity was taken in May 2001 to observe Io from Galileo. 

The Compounds of Sulphur

When finally a space probe lands on Io, the planet will provide a wonderful practical laboratory for the study of the many allotropic forms of sulphur and its compounds with oxygen and other elements. 

The chemical element sulphur and a number of its compounds play an important role in the chemistry of Io.  Sulphur dioxide has already been mentioned as a major component of the surface frost that covers large parts of the planet.   It is possible that Io has a tenuous atmosphere mainly consisting of SO2 and S2 molecules since these substances are ejected in the plumes of active volcanoes

Voyager 1, which passed Io to within 420,000 kilometres of the planet, recorded eight eruptions and the ejection velocities were certainly of the order of 1 kilometre per second.  Some were more violent than Etna, Vesuvius or even Krakatoa.   The explosive plumes were from 70-300 kilometres high, with material rising to around 100 kilometres before spreading out into an umbrella-shaped cloud and slowly falling back to the ground.

Apart from Sulphur dioxide (SO2), sulphur forms a number of ‘exotic’ oxides such as S2O.  Thus S2O could also be present in any tenous atmosphere.  However, this would be rapidly removed since when S2O gas is condensed onto a cold surface it undergoes irreversible polymerisation to a number of highly coloured polysulphur oxides with the general formula (S7O2)n.  The colours of these oxides range from yellow, orange and red.

It has been suggested that, apart from the oxides, a number of other sulphur compounds play significant roles in the chemistry of Io.  Among them are the three iron sulphide minerals Iron pyrites (FeS), Troilite  (FeS2) and  pyrrhotite (Fe I-x S) and a number of metal sulphides such as sodium and potassium.   Magnesite (MgSO4) and anhydrite (CaSO4) are also likely to be present.

Water and hydrogen sulphide seem to be conspicuous by their absence although recently it has been claimed that H2S has been detected.  Neither of these substances would be expected to be present since free hydrogen would be released and the hydrogen would quickly escape from the planet where the temperatures are locally very hot due to volcanic activity and the escape velocity is low.

Because of the enormous importance of sulphur chemistry on the surface of Io, an account of some of the major chemical properties of sulphur and its most important compounds is given on another page of the web-site hyperlinked to this page.

 The Plasma Torus.

            Io lies within the plasma torus, which contains ions entrained in Jupiter's magnetosphere.  It is a ring of ions mainly oxygen and sulphur surrounding Jupiter in the vicinity of Io.    It also contains sodium and possibly potassium ions.  Photographs of the Jovian system in the sodium D-line show a remarkable large comet-like feature surrounding Io and orbiting tail first about Jupiter. This indicates the presence of sodium vapour in the cloud. 

 Sodium atoms have the most intense emission lines of any common atom.  A gas containing equal amounts of sodium and potassium would appear spectroscopically like pure sodium.  Potassium may also be present in the cloud.   It has been suggested sodium and potassium sulphides are present on the surface of Io and are ejected in the volcanic plumes.

The Ionosphere

In the late 1990s Galileo detected a high altitude ionosphere composed of ionizerd oxygen, sulphur and sulphur dioxide.  Sensors on the space craft detected a dense region of ionised oxygen gases at an altitude of 900 kilometres (555miles) but it seems as if the ionosphere may grow and sink in altitude and in intensity depending upon the variations in the volcanic activity

Magnetic field

There is a ‘hole’ in Jupiter’s magnetic field near Io and this has led to speculation that Io may well have a magnetic field of its own.



Europa is the smallest of the Galilean moons.  It shows no volcanoes, little relief and only a few impact craters. It has a slightly lower density than Io and the Earth's Moon.  Unlike these other small planets it is an icy satellite and contains considerable quantities of water.  It seems to be totally covered in an ice-cap and its fascination lies in the fact that there may be an ocean of liquid water beneath the ice and that in that ocean some form of life may have evolved.    It has a much higher density than Ganymede and Callisto, which suggests that it possesses a fairly large core.  The age and thickness of the ice-cap are unknown but it has been suggested that the thickness of the ice is about 50-70 kilometres.  Although it is by no means certain, the current thinking is that the ice-cap overlays a one hundred kilometre deep ocean of liquid water or slushy ice, which probably contains dissolved salts and ammonia. The ammonia will be in the form of ammonium ions since the ocean may contain sulphuric acid..  Beneath the ocean there is believed to be a core of silicate rock material, overlaying an iron or iron sulphide core.

The surface appearance of Europa is of a smooth globe with very little relief and relatively few impact craters.  The lack of a large number of impact craters indicates than the surface is relatively young - no more than a few hundred million years. The impact craters that are found on the planet indicate a thin, brittle surface which overlays a viscous sub-medium.   The surface is covered by long intersecting dark lines.   Some of this network is reminiscent of terrestrial Arctic Ice. The overall colour of the planet is a pale yellow, which is probably due to a scattering of sulphur from Io.  Europa is almost as bright as Io in the visible and up to 1.2mm in the infrared after which it gets quite dark and shows strong absorption bands for water ice near 1.6mm and 2.0mm.

Image of Europa from the cameras of the Galileo Spacecraft taken on 7 September 1996


Copy of PHOTO P-48040 caption issued by JPL Laboratory on 12 November 1996

This image shows two views of the trailing hemisphere of Jupiter's ice-covered satellite, Europa. The left image shows the approximate natural colour appearance of Europa. The image on the right is a false-color composite version combining violet, green and infrared images to enhance colour differences in the predominantly water-ice crust of Europa. Dark brown areas represent rocky material derived from the interior, implanted by impact, or from a combination of interior and exterior sources. Bright plains in the polar areas (top and bottom) are shown in tones of blue to distinguish possibly coarse-grained ice (dark blue) from fine-grained ice (light blue). Long, dark lines are fractures in the crust, some of which are more than 3,000 kilometres (1,850 miles) long. The bright feature containing a central dark spot in the lower third of the image is a young impact crater some 50 kilometres (31 miles) in diameter. This crater has been provisionally named 'Pwyll' for the Celtic god of the underworld.

Europa is about 3,160 kilometres (1,950 miles) in diameter, or about the size of Earth's moon. This image was taken on September 7, 1996, at a range of 677,000 kilometres (417,900 miles) by the solid state imaging television camera onboard the Galileo spacecraft during its second orbit around Jupiter. The image was processed by Deutsche Forschungsanstalt fur Luft- und Raumfahrt e.V., Berlin, Germany.

Information issued 0n 12 November 1996 by

PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011


During the early years, just after the planets had formed from the Solar Nebula, Europa would have been hot.  Jupiter was still contracting and emitting far more energy on its own account as it does at the present time.  The radioactivity from the rocks of Europa itself would have been ten times higher than it is now. There were a number of radioisotopes with half-lives of medium length which would have been highly active; for example iodine 129 and plutonium 244.   They were present in considerable amounts in the early Solar System and their decay products ('daughter nuclides') are found in chondrite meteorites.   Even those radionuclides still found on Earth would have been present in greater amounts, in particular Uranium 235.  With a relatively low half-life of 720 million years there would have been 84 times as much of this radioactive element present in the early Solar System than there is at the present time.    It is therefore probable that a liquid ocean would have completely covered the small planet.  Meteorites containing organic molecules such as amino acids would have plunged through the water to rest on the ocean floor.   As the radiation from Jupiter and the radioactivity from the silicate rock core of Europa itself decreased, the top layer of the ocean would have frozen.  Since the time of the great bombardments, which lasted between 4,500 and 3,900 million years ago, the evidence for any impact craters would have been erased long ago.   The few craters that now exist are of relatively recent origin.

Many scientists have believed that Europa has an ocean of liquid water, perhaps up to 100 km (62 mi.) deep.    Galileo has flown near Europa frequently since the spacecraft began orbiting Jupiter and its moons in December 1995.  Pictures from those flybys show patterns that support the theory.   In some, rafts of ice appear to have shifted position by floating on fluid below.  In others, fluid appears to have risen to the surface and frozen. 

On 16 September 1999 a team of scientists from the University of Arizona pointed out that an unusual set of cracks on the icy surface of Europa may be the most convincing evidence yet that it harbors a liquid water ocean under the ice.   They had a characteristic appearance described by the Arizona team as Flexi.  They are cycloidal cracks that appear as a series of arcs, joined together at each end to form a long, wavy crack across the surface. The cracks, unique to Europa, were first noticed in Voyager images in 1979 but defied explanation.   They have since been studied in more detail recently by the scientists of the Galileo team.., The varying distance of Europa from Jupiter and from Ganymede and Io cause tides in the hypothetical subsurface ocean to rise and fall.   These tides can rise and fall as much as 30 meters (110 feet), compared to the 1-2 meters (3.3-6.6 feet) for most terrestrial tides. "This causes Europa's ice shell to flex," said Gregory Hoppa the team leader.  “When the tidal stress exceeds the tensile strength of the ice, a crack forms.   Each arc in the flexi is 75 to 200 km long, and forms over the course of  3.5 days, the orbital period of the planet”.

The Arizona scientists believe this is the most convincing evidence yet for the under-ice ocean theory since no other mechanism can explain the formation of the flexi.


Illustration 9.8. Flexi-formations on the Surface of Europa

Cycloidal double ridges viewed in the northern hemisphere of Europa (60° N, 80° W).

Many thanks are due to Gregory Hoppa for his permission to reproduce this photograph in theweb-site

Very strong evidence for the ocean theory was announced in the journal ’Science’ in the 25 August 2000. It comes from magnetic readings by NASA's Galileo spacecraft.  The report explains that magnetic evidence for an ocean is possible because Europa orbits within the magnetic field of Jupiter. That field induces electric currents to flow through a conductive layer near Europa's surface, and the current creates a secondary magnetic field at Europa.  Margaret Kivelson and her team working at the University of California, Los Angeles pointed out that the changes in the readings on the Galileo magnetometer as the spacecraft flew close to Europa supported the conclusion that there was a conducting layer beneath the surface of the ice.   "We have good reason to believe the surface layers of Europa are made up of water that is either frozen or liquid," she said, pointing out that earlier gravity measurements show a low density, such as water's, for the moon's outer portions. "But ice is not a good conductor, and therefore we infer that the conductor may be a liquid ocean.  There is a very strong case that the source of the magnetic signature is a conducting layer near the surface.”  Galileo's project scientist, Dr. Torrence Johnson of NASA's Jet Propulsion Laboratory at Pasadena is cautious however and pointed out that the results could be explained by a past ocean that is now frozen. "This magnetometer data is the only indication we have that there's an ocean there now, rather than in the geological past," Johnson said.
Johnson thinks the case for liquid water on Europa is still not totally conclusive. 
The Atmosphere of Europa

            Even from the immense distance of about 750 million kilometres, the surface of Europa still receives ultraviolet radiation from the Sun.  There is some evidence that slow photolysis of the water molecules of the surface ice is providing the small world with a very tenuous atmosphere of oxygen.  Hydrogen peroxide molecules have also been detected.  Because of the low escape velocity, this will not be a 'permanent atmosphere'.  However, the oxygen and peroxide will be constantly replaced to give a 'steady state atmosphere' whose concentration will depend on the rate of photolysis and the rate of escape of oxygen and hydrogen peroxide to space.  It is also possible that sulphuric acid is present due to oxidation of sulphur.

The Ocean Beneath the Ice.

The temperature at the surface is around -110º Celsius.  Although this is a very low temperature by our standards, it should be noted that this is far above the temperature of interstellar space.   'The warmth' is mostly due to radiation received from the Sun from a distance of 740-816 million kilometres.

Although the tidal effect of Jupiter is only about 10% of the effect felt by Io, it is still very considerable and must be a major factor in heating the small planet.

Another major heat source is still radioactivity.  The ratio of rock to ice in the case of Europa is far greater than for the other two icy satellites Ganymede and Callisto.   Therefore, there will be a higher heat production per unit mass due to the presence of radioactive isotopes in the silicate mantle of the planet  

Depression of the Freezing Point by Substances in Solution

The presence of dissolved substances will depress the freezing point of water.   Salts such as sodium chloride have a significant effect.  There is some evidence that deposits of salts have been found on the surface of the ice.  These may have made their way through cracks and fissures in the ice.

An even greater factor in depressing the freezing point is the possible presence of ammonia (or perhaps ammonium sulphate in solution as 2NH4+ and SO4ions).     Just as the addition of ethylene glycol (anti-freeze) to the water in a car radiator prevents freezing in winter, ammonia depresses the freezing point dramatically.  

In the early years of this twenty-first century special missions will be launched to investigate Europa and find out if there really is an under-ice ocean.  Initially it is planned to send a space craft specially to orbit Europa.

Europa has gained the rank of one of the highest priority targets for an outer Solar System exploration mission. Life on Earth has been discovered at great ocean depths in thermal vents, beyond the penetration of sunlight, thriving on upwelling chemical nutrients from the interior of the planet. Many scientists believe the pictures and other investigations by the Galileo spacecraft reveal a relatively young surface of ice, possibly quite thin in places. Internal heating on Europa due to Jupiter's tidal pull could melt the underside of the icepack, forming an ocean of liquid water underneath the surface.

NASA is making plans to send a mission to orbit Europa and measure the thickness of the surface ice and detect an underlying liquid ocean if it exists. Using an instrument called a radar sounder to bounce radio waves through the ice, the Europa Orbiter spacecraft would be able to detect an ice-water interface, perhaps as little as 1 km below the surface. Other instruments would reveal details of the surface and interior processes. This mission would be a precursor to lander missions that would make detailed studies of the surface characteristics, such as composition, seismology, and physical state. Unlike the Galileo and Cassini missions it is proposed to send the Europa Orbiter directly to the Jupiter System.

Europa Orbiter Mission Trajectory

Europa Orbiter - 2003 Launch - Example Jupiter Direct Trajectory

Acknowledgements NASA/JPL/University of Arizona


To get to the Jupiter system it is necessary for the spacecraft to undergo a change in direction termed the Broken Plane Manoeuvre -- Imagine a flat plane representing Jupiter's path about the Sun. Now imagine another plane representing the Earth's orbit about the Sun. Jupiter's plane is tilted slightly with respect to Earth's. A spacecraft travelling from Earth's plane to Jupiter's has to "climb a hill" to reach the plane of the gas-giant planet. A spacecraft can accomplish this at any time after its launch from Earth but the best place to start this climb is at the intersection of the two planes. This is the point labelled "Broken Plane Maneuver" in the figure above. It will take an amount of energy equal to changing the velocity of the spacecraft by 227 meters per second (508 miles per hour).

The mechanics of space flight are quite complicated.   Even after reaching the Jupiter system the spacecraft will have to undergo a series of manoeuvres involving Jupiter and the other three moons before it can finally be inserted into the Europa orbit which will last a period of one to two years.

The Europa orbiter was scheduled for launch in November 2003 and was due for arrival in the Jupiter system in the summer of 2006.  However the mission has been postponed and at the moment is scheduled to start in 2006 for arrival around 2009.   When it finally arrives in the system it will loop round in a series of about a dozen gravity assists to shrink the orbit until it finally undergoes orbital insertion around Europa about 2010.

It will be designed to withstand seven times as much radiation as Galileo since at the distance of  670,000 kilometres (Europa’s orbit) from Jupiter it will be subjected for a long time within the fierce radiation belts of the giant planet.

Plans are being made to send a lander equipped with an instrument to bore a hole through the ice and send down a tiny submarine probe.  Once it arrives in the ocean the instruments carried in the submarine will measure many parameters such as temperature, hydrostatic pressure, pH, the presence and concentration of dissolved substances and the speeds of ocean currents.  It will be equipped with a television camera and the means to radio the results back to Earth via the lander and an orbiting satellite.  Above all it will search for life.   With the discovery of hydrothermal vents on our own planet, there is a possibility that similar forms of life may have evolved in the waters of Europa.   Some scientists believe that the seeds of life's precursors came in the form of amino acids and other biochemical substances carried by meteorites. Perhaps in the distant days, when the Solar System was young, meteorites plunged into the warm ocean of Europa before the surface froze. May be they carried with them pre-biotic chemicals which helped to initiate life processes at the vents of underwater volcanos.

If the Europa Orbiter does indeed find an ocean and ways to reach it through the ice, then a follow-on mission to explore those alien seas with a series of hydrobots would be the logical next step in planetary exploration.

Boston National Space Society Vice President Larry Klaes has  proposing a project to design such a mission to Europa.   He has proposed the name Europa Ocean Explorer Mission.    Larry envisions a main spacecraft, or "mothership", which would either flyby or orbit the moon and drop a number of small probes onto the planet. Some of these landers would explore the surface, while others would find their way beneath the ice into the Europan waters and reveal their long-held mysteries.

Larry is inviting interested individuals from diverse backgrounds to form a committee to flesh out this proposed space mission. You do not have to be a member of the NSS, just enthusiastic and willing to share your ideas. The goal as it presently stands is to eventually produce a detailed mission layout, working models of the spacecraft, and a presentation to NASA. You may contact Larry for further details at:


Ganymede is the largest moon in the Solar System.  It has an equatorial diameter of 5,262 kilometres (3,280 miles) which gives it a larger volume than the planet Mercury.  However it is only about half the mass of Mercury because its density is much lower.  It is far less dense than Europa (Ganymede 1.93 Europa 3.03) and is thought to contain about 50% water to 50% silicate rock.  The crust appears to be composed of ice mixed with some silicate rock.  There is some possibility that the outer crust may cover a convecting layer of water or soft ice. 

            Both Voyager Missions obtained detailed views of the surface and maps of the planet have been drawn down to a resolution of 5 kilometres.   The illustration shows a picture of Ganymede taken by the Voyager Spacecraft.  Credit NASA/JPL

There are two main types of geological unit.  There is the dark, heavily cratered terrain and the brighter younger regions, which consist of long parallel grooves.  It is thought that the grooved terrain developed gradually after the surface froze. Ganymede appears to have passed through a period when tectonic ice-plates (analogous to the Earth's tectonic plates of silicate rock) formed.  This may have been as a result of the slowing down of the axial rotation by the tidal forces from Jupiter, which produced the present synchronous orbit.   (Ganymede and the three other Galilean moons all keep the same face turned to Jupiter just as our Moon always keeps the same face turned towards the Earth).

            Half of the surface of Ganymede is dark in colour and very heavily cratered dating back to a very early period in Solar System history.  The other half is dominated by younger much less heavily cratered terrain that consists of rifts filled with parallel bands of ice.  The bands appear to have been produced by upwelling from the mantle in a manner similar to the production of new oceanic crust on Earth at mid-ocean spreading centres.

Galileo has detected a strong magnetic field, with a strength of about 1/40th of the Earth's field, around Ganymede despite the fact that Ganymede has a mass of only 0.025 that of the Earth.  It has been attributed to an iron core between 400 and 1200 kilometres in diameter overlaid by a rocky silicate mantle, topped by a layer of ice 800 kilometres deep.  It has been suggested, by Graeme Sarson of the University of Exeter, that the strong field of Jupiter has helped to initiate the circulation of the core, which gives Ganymede such a strong magnetic field for a relatively small planet.  There is also some evidence for aurora at the poles.  Illustration 9.11 shows a picture of the globe of Ganymede.

It was reported on 17 December 2000 that there was evidence of an enormous salt water ocean possibly about 120 kilometres beneath the ice.  This is supported by magnetic evidence and by the presence of surface salty minerals detected by infrared spectrometry readings.


Callisto is the most heavily cratered planet in the Solar System.  Virtually the whole surface is covered with craters nearly shoulder to shoulder. 

It is unique among the cratered planets in having no plains where craters have been obliterated by more recent events. Most of the cratering is believed to have occurred during the period of major bombardments between 4,500 and 3,900 million years ago.  In some places large ring structures exist which were caused by very large impacts.   The illustration shows the globe of  Callista taken from the Galileo spacecraft


The Illustration on the right shows a picture of the Valhalla region of Callisto taken from the Galileo spacecraft.

This close up of Callisto shows the heavily cratered surface and the prominent ring structure of Valhalla.  This enormous impact site contains at least 25 concentric rings or ring arcs.  It is one of the most unusual impact sites in the Solar System.  The very large number of rings may be due to impact occurring into a thin icy lithosphere which fractured very easily when the basin collapsed (compare to the Moon and Mercury where impact was into hard silicate non-icy rocks)  Image by Paul M. Schenk, Lunar and Planetary Institute, Houston, TX.

Acknowledgements NASA/JPL/University of Arizona

The shapes of the craters on Callisto look superficially very similar to those of the Moon and Mercury.   However they are much flatter than those found in the rocky terrestrial planets.   The larger craters also lack the central depressions flooded by volcanic material and the surrounding ring such as those found in the Imbrian Basin on the Moon and the Caloris Basin on Mercury.   Instead Callisto shows many high albedo features with or without concentric rings and radial structures.   During the 'daytime' in the equatorial regions the surface temperatures on Callisto reach -133 to -143º Celsius.  Although at these temperatures ice behaves more like rock, it will undergo glacial like flow over geological periods of time.   This will give a softer appearance than that found for silicate rocks and will result in a smoother topographical profile.

Margaret Kivelson, who is the principal investigator for the Galileo spacecraft's magnetometer experiment , has reported recent surprising discoveries.  Kilveson and her co-workers have found that Callisto has a magnetic field.   Galileo measurements of Europa and Callisto show that as Jupiter's intense magnetic field rotates past the moons it induces electrical currents in Callisto and Europa that in turn generate secondary magnetic fields.  One possible explanation offered is that each moon possesses a salty ocean beneath the solid icy crust to which the electrolytes dissolved in the water will give considerable electrical conductivity.   This is not surprising in the case of Europa but is unexpected in the case of Callisto, which is such a long way from Jupiter that tidal heating would be very low indeed.   One explanation offered is that the presence of the slow decay of radioactive substances in the silicate core of Callisto are sufficient to cause partial melting with the salt in the water acting as a natural anti-freeze.  Clip from Nature October 23, 1998: Until now most scientists thought Jupiter's moon Callisto was a dead and boring moon, an unchanging piece of rock and ice. Data reported in today's issue of Nature could change all that. It appears that Callisto, like another of Jupiter's moons Europa, may have an underground liquid ocean and at least some of the basic ingredients of life

 The strongest clues to life on Callisto and Europa may lie right here at home. In 1996, radio sounding and altimetry measurements revealed the the presence of an underground lake in Antarctica near the Russian Vostok Station. Lake Vostok is overlaid by about 3,710 meters (12,169 ft) of ice and may be 500,000 to 1 million years old. Since the discovery, drilling has gone slowly while procedures are worked out to keep it pristine. No one has seen or sampled the lake - the deepest ice sample is from 100 meters (328 feet) above the liquid surface - nor is anyone sure why it is liquid, hence the scientific curiosity. Scientists are hopeful that Lake Vostok can one day serve as a terrestrial laboratory to help us understand better the oceans on the distant moons of Jupiter

The strongest clues to life on Callisto and Europa may lie right here at home. In 1996, radio sounding and altimetry measurements revealed the the presence of an underground lake in Antarctica near the Russian Vostok Station. Lake Vostok is overlaid by about 3,710 meters (12,169 ft) of ice and may be 500,000 to 1 million years old. Since the discovery, drilling has gone slowly while procedures are worked out to keep it pristine. No one has seen or sampled the lake - the deepest ice sample is from 100 meters (328 feet) above the liquid surface - nor is anyone sure why it is liquid, hence the scientific curiosity. Scientists are hopeful that Lake Vostok can one day serve as a terrestrial laboratory to help us understand better the oceans on the distant moons of Jupiter.

Internal Structure of Callisto

Unlike the other moons of Jupiter it is believed that Callisto does not have an iron or iron sulphide core but that the whole of the interior is occupied by a rocky silicate core surrounded by rock and ice mixtures.  Like Europa and Ganymede it may have an ocean under the crust and a magnetic field.

 Other Moons of Jupiter

            Jupiter has four sets of satellites consisting of four moons in each case;-

The first four lie between 128,000 and 222,000 kilometres from Jupiter.

The second four lie between 422,000 and 1,883,000 kilometres from Jupiter.  They are - Io, Europa, Ganymede and Callisto.

 The next four lie between 11,110,000 and 11,740,000 kilometres from Jupiter

           The last four lie between 20,700,000 and 23,700,000 kilometres from Jupiter.

  It also has a host of very small minor satellites

The largest of the inner group is Amalthea, named after the mythological goat that looked after the god Jupiter when he was   a baby. This moon is noteworthy in that, like Europa, it is coloured by sulphur derived from Io.

Sulphur Chemistry with reference to Io  Io and Sulphur forms a separate page of this web on its own.

The Solar System