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Characteristics and generalities on the Galilean satellites of Jupiter 

The Galilean satellites of Jupiter are remarkable bodies in many ways. These are the oldest known bodies not directly visible to the naked eye, they have similar sizes than Mercury and represent, rotating around Jupiter, a miniature solar system where all the problems of celestial mechanics are gathered: perturbation due to the flattening of Jupiter, by the Sun, Saturn and by mutual interactions. Their fast motions are easily observable and generate spectacular phenomena of eclipses. The Galilean satellites are very different from the other satellites of Jupiter which are only some rocky bodies similar to asteroids. Space probes have recently shown a wide geological variety (active volcanoes, ice crust, presence of water, ...). All this makes that these objects were extensively studied by astronomers and one of the main targets of the conquest of space. Let's see how our knowledge of these bodies have evolved, how we observe them and what we know today.

The four Galilean satellites have been named Io (J1), Europa (J2), Ganymede (J3) and Callisto (J4). 

The table below summarizes some physical data on the satellites. Their orbits almost belong to the same plane which is the one of Jupiter's equator and are almost circular. In order to calculate their positions with high precision, it is important to determine the deviations of these satellites relative to the plane of Jupiter's equator and compared to perfect circular orbits. These are the multiple gravitational perturbations acting on them that cause these differences. 

 Io - J1 
 Europa - J2    Ganymede - J3    Callisto - J4  
periode of rotation 1.8 days 3.6 days 7.2 days 16.7 days
eccentricity of the orbit 0.004 0.009 0.001 to 0.002 0.007
inclination on the equateur of Jupiter 0°.04 0°.47 0°.19 0°.25
diameter 3630 km 3138 km 5262 km 4800 km
magnitude 5.0 5.3 4.6 5.6
geometric albedo (V band) 0.61 0.64 0.42 0.20
elongation maximum 2' 18" 3' 40" 5' 51" 10' 18"

See the pages on celestial mechanics for the definition of the parameters of the ellipse. We notice that they are very bright and could in fact be observed with the naked eye if Jupiter had not a powerful light halo. The maximum elongation shows that Callisto does not move more than 10 minutes degree from Jupiter (one third of the diameter of the Moon), which is not sufficient for it to be separarted enough from the planet and to be visible with the naked eye. 

The figure at left shows the comparative sizes of the satellites of Jupiter compared with the Moon, Mercury, Mars and Pluto. We see that they are indeed true small planets. The biggest asteroid has a diameter which reaches a quarter of the smallest Galilean satellite.

The discovery

The telescope was existing for several years when Galileo had the idea of ​​directing it towards the sky. For that he had to improve existing models that but this use opened a new field for the exploration of the sky. It was a revolution, as today space exploration whenn seing the discoveries which followed.
Listen to Galileo talking about his first observations

" On January 7, 1610, at one in the morning, then I was exploring the sky with my telescope, Jupiter presented itself to me, and because I had built a powerful instrument, I could see three little stars next to him. Although I considered as stars, I was very surprised because they seemed exactly aligned on a line parallel to the ecliptic and because they were much nicer than the other stars of the same magnitude. Their positions were as follows: 

that is to say there were two at east and one to the west. The further at east and the one on the west side seemed slightly brighter than the third. I did not pay attention to their distance from Jupiter since as I already said, I thought it was fixed stars. When, on January 8, I do not know why, I did the same observation, I saw a completely different configuration: three stars were now all on the west side of Jupiter, and they were closer to each other than the night before, at equal distances from each other, as in the following figure: 

being to such a phenomenon, and unable to conceive that stars can change their relative positions, I wondered how Jupiter could be found east of the stars that day, while it was in the west of two of them the day before. Its motion was direct, in contradiction with astronomical calculations and was it by its own motion he had moved among the stars? 

I waited until the next night with impatience, but I was disappointed because the sky was cloudy all sides.


Le 10 janvier cependant, ces étoiles apparurent dans la configuration suivante par rapport à Jupiter : 


There were only two stars, and both in east of Jupiter, and the third was, I suppose, hidden by the planet. They were, as before, aligned with the planet, and exactly in the zodiac. Seing that, understanding that these changes of configuration were not due to Jupiter and confident that these stars were the same as the previous days, my doubts were transformed in astonishment. I realized that this change of relative positions were not due to Jupiter but to the stars themselves. For this reason, I decided to continue observations with great care.


On January 11, I saw the following configuration : 


Only two stars were to the east of Jupiter, the central one being three times farther from Jupiter than the other star. The one the most easterly was twice brighter than the one at the center while the previous night they appeared to me to have the same magnitude. I admitted that there was in the sky, without a doubt, stars turning around Jupiter in the same way that Mercury and Venus revolve around the Sun ..."


It was only on January 13 that Galileo could observe the four satellites simultaneously. He then made this drawing : 

This discovery of Galilee would change our understanding of the solar system: we had an example of motion that was not around the Earth. And the Earth was no longer an exception since Jupiter had moons following it in its motion around the Sun. The Earth could become a planet like the others revolving around the Sun. Note, however, that it was only an observational fact, not a demonstration since the foundations of dynamics and gravitation necessary for that were still unknown. 

Galilee named these stars the "stars of the Medici" or Medicean stars (Medicea sidera) in honor of the prince of Medici (the name is still in use in Italy). It gave the following names: Principharus, Victripharus, Cosmipharus and Ferdinandipharus. The names Io, Europa, Ganymede and Callisto were given by Simon Marius in 1614 in his "Mundus Jovialis". Simon Marius claimed to have observed the satellites before Galileo, in November 1609. It is possible, but he did not understand what he was observing. The term "satellite" comes from the Latin "satelles, satellitis" meaning guard or escort, and was given by Ozanam. 

Following this discovery, how Galileo identified the different four satellites and how he determined their periods of rotation around Jupiter. The problem is less simple than it appears because it was impossible to observe all time: the day would interrupt the observations (there were then 10 hours night), when this was not the clouds ... According to the writings of Galileo, he would have identified the first satellite away most of Jupiter and calculated its positions in order to be able to identify the next one walking away most of the planet, and so on. Theoretically, this method works (it is possible to try with satellite ephemeris) but practically , the measures were not enough precise for that: Galileo was making measurements with a precision of the order of a minute degree. It is assumed that Galileo used the fact that the four satellites have not the same brightness, which greatly facilitated their identification which was made at the beginning of the year 1611. 


The eclipses 

On January 12, 1610, while observing Jupiter's satellites, Galileo witnessed an eclipse by Jupiter but did not understand its meaning before 1612. Galileo adopted a circular motion for the satellites of Jupiter in March 1610. To predict the positions, he had to build tables of the motion and for this it was necessary to be able to make observations of precise positions. Unfortunately, telescopes at the time did not permit it.

The first tables were made by Galileo in 1612 and Marius in 1614. It is only in the Hodierna tables appearing in 1656 that latitudes of satellites above a reference plane were given. Hodierna also makes predictions of eclipses. In 1643 Fontana observed a shadow of a satellite crossing the surface of Jupiter. The interest of the eclipses by Jupiter was rapidly understood. The observation required only to note the time of the phenomenon and it gave an accurate position of the satellite as entered or emerged from the shadow of Jupiter. Once predictions of eclipses performed, the observation of an eclipse allowed to obtain a common time scale to all terrestrial observers and therefore to determine the longitude of the place where they were observing. This method was used by many geographers mapping uncharted territories but did not serve at sea since observation from the deck of a ship was too difficult. Lalande also wrote in 1792 in his "Astronomy": "The Galilean satellites are continuously used by astronomers to determine the differences in longitude between the different countries of the world". 


The eclipses by Jupiter (see the specific page dedicated to them) are still observed nowadays. However, they have been supplanted by direct photographic observations of positions from the late nineteenth century to determine the positions of the satellites. The observation of an eclipse is indeed rendered inaccurate due to the thick atmosphere of Jupiter that refracts light rays at the edge of the shadow of Jupiter. But the observation of the light absorption by the atmosphere of the planet Jupiter during an eclipse was used to analyze this atmosphere, as the observation of the drop of light at the beginning of an eclipse in a large number of wavelengths eclipse (figure above).

Ephemerides and celestial mechanics 

As we have seen, the importance of the eclipses of Galilean satellites encouraged works for making predictions of these events and the construction of tables of the motion of these bodies. After Galileo, Marius and Hodierna, Cassini published in 1668 his "tables of the motion and calculation of eclipses". Based on a large number of observations of eclipses, these tables were much more accurate than previous ones but were further improved in 1693, after Roemer has showed that the speed of light was finite thanks to the observations of the eclipses of Io. Indeed, the distance between Earth and Jupiter varies during the year from 600 to 900 million kilometers so that the light will take from 30 to 50 minutes to come from Jupiter. Eclipses occurring regularly around Jupiter, the offset of 20 minutes was quickly noticed by observers: Roemer deduced that the light had a finite velocity and calculated it.
In 1749, Bradley publishes tables and notes the inequality of 437 days in the time of the eclipses of the three first satellites. Maraldi reported at that time the mutual action between the satellites and begins to suspect the eccentricities of the orbits and the nature of inequality. Wargentin will publish improved tables in 1757. At this time, the satellite motion is described by empirical equations, purely kinematic and Lalande could say in the "Connaissance des temps" in 1763 that "the inclinations and nodes of the orbits experience variations that are still not known".
But in the eighteenth century, from Newton to Laplace, the principles of dynamics and universal gravitation will be theorized. Everything will change for the modeling of the motions: it will be possible to write equations representing dynamical models. For the Galilean satellites, the problem will be difficult and is not yet fully solved even today: all the difficulties of the celestial mechanics can be found here. First, many forces acting on the satellites: the Sun, far but massive, the flattening of Jupiter, the presence of Saturn and also the mutual interactions between satellites. These interactions will result in a resonance that will force the motion of the satellites. The first three satellites do not move independently of each other, but have their longitudes L1, L2, L3 connected by the relation : L1 - 3 L2 + 2 L3 = 180°. This remarkable relationship will imply that some configurations between the satellites are not possible, for example the first three satellites can not be aligned on the same side of Jupiter. The following figure shows where should be the third satellite J3 when J1 and J2 are aligned with Jupiter (a), J2 and J3 (b) and finally J1 and J3 (c). 

The satellites have obviously tend to escape this constraint but they can not get away from more than one degree: the resonance takes them back to their imposed configuration.
From the dynamical equations, tables (i.e. ephemerides) would progress quickly: the first theories are due to Bailly and Lagrange in 1766, then comes the one of Laplace, the most complete in 1788. In 1791 Delambre built tables from Laplace's theory and observations of over 6000 eclipses. 

The XIXth century

The nineteenth century was the golden age of celestial mechanics and astrometric observations. From the point of view of the theory Damoiseau improved the Laplace's work and publishes ephemerides and predictions of eclipses with a better accuracy. Further improvement came from Souillart in 1880. Then comes the monumental work that realizes Sampson, a complete analytical theory of motion of Galilean satellites that was used to build the ephemerides from the late nineteenth century, but which was not published before 1921 due to the complexity of the task. It should be noted that the building of an analytical theory, that is to say, the construction of a solution of a system of differential equations describing the motion of the satellites, solution which is proved to do not exist and that we will ever only approximate and not exactly, was the only method at that time. Nowadays, computers allow us constructing purely numerical solutions easier to obtain but which have the disadvantage of being difficult to extrapolate in the future. Despite this, the Sampson's theory was in use until the end of the XXth century.
Along with theoretical work, observations progress: eclipses were dated more precisely. The satellites moving at about 10km/second, an error of 30 seconds on the time of an eclipse corresponds to an error of 300 km in the satellite orbit. From the late nineteenth century, the observation of eclipses became photometric, that is to say the decreasing or increasing brightness during an eclipse is no more evaluated according to specific criteria of each observer but by comparison with measured well calibrated photometric references. The increasing size and therefore the power of the instruments also allowed to make direct measurements of position when no eclipses were observable. The observation was to measure the apparent angle between two satellites at a given time and the position angle of the straight line connecting the satellites. A micrometer was used for measuring small angles on the sky. This type of observation was introduced by Bouguer in 1748 from Fraunhofer but the best instrument built especially for this was running near 1896 at the Cape Observatory in South Africa, instrument named heliometer because it was also used to measure the diameter of the Sun (the heliometer has a lens cut in two pieces that slid along their diameters, so we obtain two images that were put in coincidence and by reading the measurement on the graduated circles we get the observed positions). Daguerreotypes invented in 1837, did not serve to observe the satellites of Jupiter (only the Sun and the Moon). Contrarily, the plates first using collodion, and second using gelatino-bromure allowed to photography fainter stars. Compared with micrometric observations, photography was used to preserve the image and make measurements quietly after observation, although its accuracy was not better if using a telescope of equal power. For photographic plates of quality, the instruments built for visual observation could not match: lenses had to be achromatized (the eye is not sensitive to the same wavelength than photographic plates) and the driving of the telescope had to be improved to allow long exposures and compensate perfectly the diurnal motion of the Earth. Also, the astronomical photography progresses when the Henry brothers proposed their refractor named "équatorial photographique de la Carte du Ciel" having a 33cm aperture and a focal length of 3m 43. Photographic observations of the Galilean satellites could begin in the years 1880-1890 in parallel with improved observations of eclipses and continued during the 1920-1930. Curiously photographic observations were not used at that time to improve the ephemerides : the accuracy was limited due to the short focal length of the refractors of the "Carte du ciel" measuring rather large fields while the Galilean satellites remained clustered around Jupiter, too bright. Instruments with a longer focal length to enlarge the field of the Galilean satellites would be better but the low sensitivity of photographic plates at that time would not permit such observations. The observations made by heliometer, although more accurate than the observations of eclipses, were no more used to improve the ephemerides . See the pages on the instrumentation of astrometric observation and also the one on the telescopes for more details on the used technics.


The modern period: the first physical observations 

After the extensive work of the nineteenth century and the many observations made at that time, the dynamical study of the Galilean satellites will stop in the 1920s since it is estimated not to go further (at that time the accuracy on positions is of about 800km) and astronomers started to look at their nature. The astrophysics studing the radiation emitted by the stars was then progressing rapidly. The increasing power will permit instruments to try to see details in the surface of the satellites. Unfortunately observation can only be visual and the satellites have an apparent diameter very small, this diameter being determined during occultations by the Moon. Measuring the amount of light reflected by the satellites show that they are particularly bright, especially Io and Europa. The measurement of the light by the photoelectric effect might provide variations with time. This have demonstrated the synchronous rotation of the satellites (they show always the same face to Jupiter). The determination of their density will call dynamical studies. By measuring the mutual interactions between satellites, we may determine their masses and hence, their density. Measurements were made in 1928 with an error of 20%, which is very honorable for the difficulty. In the 1950s techniques improve, measurements are more accurate. Photometry, polarimetry and spectrophotometry (see pages on these topics) were developed. We measure colors, we look at atmospheres. In the 1960s, it was suggested the existence of water on Europa and Ganymede, one noticed the high reflectance of Io in red and some authors even consider the existence of a volcanism generated by the powerful tidal effects of Jupiter on Io. 


The modern period: new dynamical studies

The arrival of space technology in the years 1960 changed the observations. Sending a probe near Jupiter and its moons is the best way to get observations of a quality that is not possible with a ground-based instrument. The preparation of space missions encouraged astronomers to look at the ephemerides for a better accuracy. In the 1970s, observational campaigns were initiated. Long focus refractors were used associated with very sensitive photographic plates. A neutral density filter was used to hide Jupiter, too dazzling, and much more precise astrometric positions were obtained than at the beginning of the century. We then saw that theories were far from having the accuracy announced by their authors at the beginning of the century. Ephemeris accuracy was only 3000km ! Researchers at the Jet Propulsion Laboratory (USA) and Bureau des Longitudes (in France) will resume the theory of Sampson published in 1921 for computer programming and tried to correct defects, especially due to the fact that all the calculations were made ​​by hand at that time. Observations were then systematically conducted every year to provide a database to allow the building of ephemerides using the theory of Sampson "renovated". All photographic and heliometric old good precision observations which had not been used previously were analyzed and used with the new ones.
Dynamical studies of the Galilean satellites are not yet completed: determining a possible acceleration (or deceleration) of the motion of Io, close to Jupiter is still a goal for astronomers. What are the tidal effects? What is the influence of the torus of dust in which Io orbits? The theory can estimate this effect quantitatively and only more accurate observations will allow us to detect and quantify this effect. 


The modern period: the renewal of terrestrial observations

In addition to the series of photographic observations which have been made in the 1970s, other types of observations were studied, practiced or planned for the future.
Difficult to predict before the arrival of computers, the mutual events of satellites provide valuable information when observed. Relative positions with an accuracy of a few milliseconds of degree where other types of observation do not give the tenth of a second of degree, obtaining data on the nature of the surfaces, detection of the volcanoes of Io were possible through the observation of these events. International campaigns of observation were organized for these phenomena requiring photometric receptors for astrometric purpose.
The appearance of CCD has achieved more easily observations, these receptors for both observation positions and phenomena.
Note finally the distance measurement Earth-satellite through radar shooting on the Galilean satellites. Indeed, it is impossible to perform radar shots on Jupiter that does not reflect waves while the satellites are available. These shots were made through the Arecibo radio telescope in Puerto Rico.

Volcanoes of Io discovered by the Voyager spacecraft were subject to observations from the ground. Both probes Voyager flew Io and detected volcanoes were not quite the same for the two probes: the volcanic activity was so strong that volcanoes changed their appearance in a very short time. Variable phenomena as quickly deserved to be observed regularly from Earth. 
The first method which was used was the occultation of Io by another satellite. These mutual phenomena occur only every six years and it is necessary to take profit of the opportunities. So, observations of the flux and the position of the volcanoes were made.
The second method is observing directly the surface of Io in infrared. Thanks to the technics of adaptative optics, images of its surface have been obtained as illustrated below.  One will find here an animation showing the volcanoes of Io during a revolution of the satellite around Jupiter with a passage in the shadow of the planet. This observation has been made on the Keck telescope with adaptive optics. 

Credit : F. Marchis/ESO
Above the aspect of Io on October 20, 1996  showing visible volcanoes.

Credit : F. Marchis/ESO
Io on Octob20, 1996, L' band
Above Io observed with the system ADONIS/COMIC in infrared : the volcano Loki, very active, is quite visible.

The modern period: what space probes told us on the physical nature of the satellites

Space probes that have approached the Jupiter system and observed the Galilean satellites made a larger harvest of data for the exploration of solar system. New worlds, very different from what we knew before were revealed. Let's see what we know today after the passage of several spacecrafts in their environment. 

Io :  
The proximity of Jupiter influences Io : even before receiving images from the space probes, it was suggested that tides from Jupiter could create an active volcanism on Io. The infrared observations revealed that the surface of Io was warmer than expected.

The photos from Voyager actually revealed nine highly active volcanoes, some ejecting matter up to 300 km altitude. In addition to the tidal forces from Jupiter, the gravitational perturbations of Europa and Ganymede on Io deform the surface of a hundred meters (for comparison, the Moon and the Sun have an effect of some tens of centimeters on Earth) and we understand the warming of the surface. The surface temperature is 130K (-143°C) and the one of the volcanoes or hot spots is 290K (17°C).

Credit : C.J. Hamilton/JPL, NASA
Image from Voyager taken on March 4, 1979 showing the volcano P3 Prometheus erupting as seen on the limb of Io.
Credit : C.J. Hamilton/JPL, NASA.
The mountain "Mons Haemus" near the south pole of Io. Its base measures 100 x 200 km and some of its peaks reach an altitude of 10 km.

Io is made of rocks containing little iron; its surface, covered with sulfur and sulfur dioxide presents, in addition to volcanoes, hot spots and outpourings of lava, a relief of mountains up to 10 km altitude. The interior of the satellite is probably composed of an iron-nickel core surrounded by a rocky mantle as suggested by the Galileo probe measurements.
Io interacts more violently with the magnetosphere of Jupiter who pulls the material (one ton per second) that will form a torus around Jupiter. 

Europa :  
Although different from Io, Europa, however, has also amazing features: it is a body without relief whose surface is made of a crust of a water ice the thickness of which being 5 miles under which there is an ocean 50 kilometers deep. Deeper, the mantle is rocky then comes an iron-nickel core.
Europa's surface shows no marking relief without any craters. Meteoritic impacts however ejected a darker material than the surface. The craters are probably filled quickly by eruptions of ice gushing fractures of the ice, created also by tidal forces. 

Ganymede :  
Ganymede is the largest of the Galilean satellites. Its lower density suggests a large rocky core surrounded by a mantle of ice and silicates. The surface crust is rocky but also contains water ice. The magnetic field measurements made by Galileo suggests an important ocean of salt water under the surface crust.
Ganymede's surface has a significant relief of valleys, mountains and craters which ejected ice from underground. The surface temperature varies from 90 to 160K (-113 to -183°C).

Callisto :  
Callisto is characterized by a very old surface (4 billion years!) having a significant number of craters (the highest density of all bodies of the solar system!). This surface is composed of rocks and of ice and does not present a significant relief. Craters impact are surrounded by concentric rings and ejected ice form clear areas on the surface.
The small density of Callisto and measures made by Galileo suggest that the crust is 200 km thick over an ocean depth of 10 km around an inner core which does not have a uniform conformation. 

The drawings above show the interior of the galilean satellites from measurements made by the space probe Galileo. Water is shown in blue. 

Credit : JPL/NASA

Credit : J.E. Arlot/IMCCE