The Moons of Jupiter

Jupiter has 63 confirmed natural satellites, but only the four largest discovered in 1610 by Italian Galileo Galilei [1] have sufficient mass to have attained a spherical shape; named after the lovers of the Greek god Zeus they are known collectively as the Galilean moons:

Io with a diameter of 3,642 kilometers is slightly larger than Earth’s moon and the third largest of Jupiter’s moons.

Voyager 1 and 2 on their 1979 flybys of Io expected to find a surface littered with impact craters, instead it found a relatively young surface covered in volcanoes many of which were active and spewing sulfur or sulfur dioxide hundreds of kilometers high. Along with the volcanoes, Io’s surface is a hodgepodge mosaic of calderas, lava flows and mountain ranges. Temperatures average -140 Celsius with hot spots of 1,700 Celsius or more. Heat produced by intense geologic activity is a result of Io’s gravitational interaction with Jupiter and other Galilean moons. Io has a tenuous atmosphere mostly composed of sulfur dioxide and unlike the other Galilean moons has little or no water.

Other missions have also added much to our knowledge of this amazing moon. The Galileo spacecraft sent back stunning images, temperature maps of the satellites surface and information leading to an amazing discovery, the moon has a gigantic molten iron core 1,800 kilometers wide, almost half its diameter.

The first few days of 2001 witnessed a unique event, the Cassini spacecraft temporarily joined Galileo in orbit around Jupiter before continuing on to Saturn. The space vehicle armed with a remote sensing payload is capable of infrared mapping and ultraviolet imaging. The Imaging Science Subsystem (ISS) is capable of returning photographic images of extraordinary beauty and clarity.

Ganymede with a diameter of 5,262 kilometers is the largest of Jupiter’s moons, it is also the largest moon in the Solar System.

Data gathered by the Galileo spacecraft indicates that Ganymede has a small molten iron core covered by a rocky silicate mantel then a possible layer of liquid salt water covered in turn by a crust of water ice. The surface temperature is -156 degrees Celsius and the atmosphere though tenuous is 100 percent oxygen.

Surface terrain is a mixture of ancient highly cratered dark regions and somewhat younger lighter regions grooved and ridged. Craters observed in both the light and dark areas are relatively flat probably due to the fluidity of the ice layer.

Galileo’s near infrared mapping spectrometer (NIMS) has detected materials on the surface which appear to be frozen magnesium brine indicative of a subsurface ocean. The spacecraft has also detected the presence of complex organic molecules a prerequisite for life.

With liquid water, heat generated by geothermal activity and organic compounds this moon is definitely a candidate for life elsewhere in the solar system, though due to the 170 kilometer thickness of its ice crust not the primary one.

Callisto is the outermost of the Galilean moons and with a diameter of 4,820 kilometers is almost the size of Mercury.

The mean surface temperature of Jupiter’s second largest satellite is -170 degrees Celsius. Its atmosphere is 100 percent carbon dioxide and extremely tenuous.

The surface itself is an ancient heavily cratered layer of dirty ice about 150 km thick. Data sent back by the Galileo spacecraft suggest that below this mantel is a salty ocean approximately 10 km deep (Callisto has a weak fluctuating magnetic field suggestive of a highly conductive subsurface liquid.) beneath which is a fairly uniform rock and ice interior. Callisto is approximately 60 percent rock and 40 percent ice.

The likelihood of life on this distant sphere is low but possible.

Europa is the fourth largest of Jupiter’s moons with a diameter of 3,122 kilometers.

The mean surface temperature is -160 degrees centigrade at the equator, colder at the poles. The atmosphere is very thin and 100 percent oxygen.

Europa is a body constantly stressed by gravitational interaction with Jupiter and the other Galilean moons (tidal flexing) a pushing and pulling that generates heat and allows for the existence of liquid water. Data recieved from the Galileo spacecraft seems to indicate the possiblity of a subsurface saltwater ocean that planetary scientists speculate could be as much as 100 kilometers deep, existing beneath a layer of ice that appears in some places to be only a few kilometers thick, is relatively young (geologicaly speaking) and smooth, covered with low-lying ridged plains, crisscrossing dark streaks (called lineae), icy flows and myriad fractures.

Both the Voyager and Galileo probes have sent back images that show areas eerily similar to Earth’s Arctic Ocean, indeed some images seem to show large pieces of splintered and broken ice that actually appear to be floating on liquid water.

When it comes to the likelihood of extraterrestrial life within our solar system Europa is the jewel in the crown. The extent and proximity to the surface of a (admittedly still hypothetical) subsurface ocean have space scientists drooling in anticipation of future missions. These might include: crashing an object into the ice in order to create a plume for analysis by an accompanying spacecraft, a Lander that would collect frozen surface detritus for examination, or a “melt probe” designed to melt its way downward through the icy covering releasing a robotic underwater vehicle upon encountering the under-ice ocean, this "hydrobot" designed to function independent of human interaction would then send back information including images to an anxiously awaiting Earth.

The 59 minor moons orbiting Jupiter are much smaller without the mass necessary to assume a spherical shape:

Metis is irregularly shaped and has a diameter of 40 km.
Adrastea is irregularly shaped and has a diameter of 20 km.
Amalthea is reddish in color, irregularly shaped and has a diameter of 189 km.
Thebe is irregularly shaped and has a diameter of 100 km.
Themisto has a diameter of 16 kilometers and little other data.
Leda has a diameter of 10 kilometers and little other data.
Himalia has a diameter of 170 kilometers and little other data.
Lysithia has a diameter of 24 kilometers and little other data.
Elara has a diameter of 80 kilometers and little other data.
Carpo has a diameter of 3 kilometers and little other data.
Euporie belongs to the Ananke group and has a diameter of 2 km.
Thelxinoe belongs to the Ananke group and has a diameter of 2 km.
Euanthe belongs to the Ananke group and has a diameter of 3 km.
Helike belongs to the Ananke group and has a diameter of 4 km.
Orthosie belongs to the Ananke group and has a diameter of 2 km.
Iocaste belongs to the Ananke group and has a diameter of 5 km.
Praxidike belongs to the Ananke group and has a diameter of 7 km.
Harpalyke belongs to the Ananke group and has a diameter of 4 km.
Mneme belongs to the Ananke group and has a diameter of 2 km.
Hermippe belongs to the Ananke group and has a diameter of 4 km.
Thyone belongs to the Ananke group and has a diameter of 4 km.
Ananke is an asteroid fragment and has a diameter of 28 kilometers. It is light red in color and gives its name to a group of satellites with similar characteristics to itself.
Aitne belongs to the Carme group and has a diameter of 3 km.
Kale belongs to the Carme group and has a diameter of 2 km.
Taygete belongs to the Carme group and has a diameter of 5 km.
Chaldene belongs to the Carme group and has a diameter of 4 km.
Erinome belongs to the Carme group and has a diameter of 3 km.
Aoede belongs to the Pasiphae group and has a diameter of 4 km.
Kallichore belongs to the Carme group and has a diameter of 2 km.
Kalyke belongs to the Carme group and has a diameter of 5 km.
Carme gives its name to a group of satellites, which at some point may have been a single unit until shattered by impact with another body. It has a diameter of 46 km.
Callirrhoe belongs to the Pasiphae group and has a diameter of 9 km.
Eurydome belongs to the Pasiphae group and has a diameter of 3 km.
Pasithee belongs to the Carme group and has a diameter of 2 km.
Cyllene belongs to the Pasiphae group and has a diameter of 2 km.
Eukelade belongs to the Carme group and has a diameter of 4 km.
Pasiphae with a diameter of 60 kilometers is the largest fragment in the Pasiphae group, remnants of a collision with another body.
Hegemone belongs to the Pasiphae group and has a diameter of 3 km.
Arche belongs to the Carme group and has a diameter of 3 km.
Isonoe belongs to the Carme group and has a diameter of 4 km.
Sinope has a diameter of 38 km and though possibly a member of the Pasiphae group is more likely to be an independent object.
Sponde belongs to the Pasiphae group and has a diameter of 2 km.
Autonoe belongs to the Pasiphae group and has a diameter of 4 km.
Megaclite belongs to the Pasiphae group and has a diameter of 5 km.
Kore belongs to the Pasiphae group and has a diameter of 2 km.
Herse belongs to the Carme group and has a diameter of 2 km.
The other thirteen moons S/2000 J 11, S/2003 J 12, S/2003 J 3, S/2003 J 18, S/2003 J 16, S/2003 J 19, S/2003 J 15, S/2003 J 10, S/2003 J 23, S/2003 J 4, S/2003 J 9, S/2003 J 5, and S/2003 J 2, which were discovered in 2000 and 2003 are as yet unnamed. They range from 2 km to 4 km in size.

On September 21, 2003, after exploring the Jovian system for eight years and in order to avoid the contamination of possible life on Europa, the Galileo spacecraft plunged into the dense atmosphere of Jupiter in a last unselfish act. At 48.26 kilometers per second (nearly 108,000 mph) it was a fiery end to an extraordinary mission.

[1] The German astronomer Simon Marius also laid claim to the discovery but failed to adequately publish his observations and although it's probable that both he and Galileo discovered the moons independently credit was given to Galileo (Marius was, however, allowed to name them).

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