.. art of the process by which Jupiter itself formed.” (Fimmel) “Accretion is the accumulation of dust and gas into larger bodies.” (Astronomy) Ganymede is the largest satellite in the solar system; with a diameter of 3,268 mi (5,262 km), it is larger than the planet Mercury. In 1979 Io was observed by Voyager I and II and was found to have several active volcanoes actually in eruption during the spacecraft flyby. Io is the innermost of the Galilean satellites. Io’s size and density is very similar to our own Moon, and it the most dense of the Galilean satellites.
“When the Galileo spacecraft flew by Io in December 1995; it discovered that Io has an iron inner core. A high-altitude ionosphere was also revealed by the Galileo flyby. In 1979, the Voyager spacecraft flew through the Jovian system, and one of the most exciting discoveries made by the Voyager spacecraft was the presence of active erupting volcanoes on Io.” (Helicon). It was discovered that Io was the most volcanically active planet in the solar system, even more active than the Earth. “The volcanism on Io is due to the internal heat generated by the tidal tug-of-war between Jupiter, Europa and Ganymede.”(Helicon). The largest volcano on Io is named Pele.
Pele was the first volcano discovered on Io and it was actively erupting with a plume an astonishing 300 km high at the time of its discovery. The Voyager spacecraft observed eleven active volcanoes during their flyby. “Hundreds of volcanic calderas have also been observed.” (Fimmel). There are no impact craters on Io. Therefore, the surface of Io is believed to be younger than a millions years old, and is continually being resurfaced by volcanic activity.
Also, the surface is very colorful, covered with red, yellow, white and orange black markings. The surface composition on Io consists largely of sulfur with deposits of frozen sulfur dioxide. The surface on Io is mostly flat plains rising no more than 1km. Mountain ranges up to 9 km high have also been observed. “A torus of sodium gas along with sulfur ions is spread out over Io’s orbit.
This torus is so large that it has been observed from Earth.” (Io;Helicon) Europa is a strange looking moon of Jupiter with a large number of intersecting features. It is unlike Callisto and Ganymede with their heavily cratered crusts. “Europa has almost a complete absence of craters as well as almost no vertical relief.” (Europa;Helicon). As one scientist put it, the features “might have been painted on with a felt marker” (Seeds). There is a possibility that Europa may be internally active due to tidal heating at a level one-tenth or less that of Io. Models of Europa’s interior show that beneath a thin 5 km (3 miles) crust of water ice, Europa may have oceans as deep as 50 km (30 miles) or more. “The visible markings on Europa could be a result of global expansion where the crust could have fractured, filled with water and froze.” (Europa;Helicon).
Ganymede is the largest moon of the planet Jupiter, and the largest moon in the Solar System, 5,260 km/3,270 mi in diameter which is larger than the planet Mercury. It orbits Jupiter every 7.2 days at a distance of 1.1 million km/700,000 mi. ” Its surface is a mixture of cratered and grooved terrain. Molecular oxygen was identified on Ganymede’s surface in 1994″ (Ganymede;Helicon). “The space probe Galileo detected a magnetic field around Ganymede in 1996; this suggests it may have a molten core.” (Hamilton).
Galileo photographed Ganymede at a distance of 7,448 km/4,628 mi. The resulting images were 17 times clearer than those taken by Voyager 2 in 1979, and show the surface to be extensively cratered and ridged, probably as a result of forces similar to those that create mountains on Earth. “Galileo also detected molecules containing both carbon and nitrogen on the surface March 1997. Their presence may indicate that Ganymede harbored life at some time” (Hamilton). Callisto is the eighth of Jupiter’s known satellites and the second largest.
It is the outermost of the Galilean moons and was discovered by Galileo and Marius in 1610. Unlike Ganymede, Callisto seems to have little internal structure; However, there are signs from recent Galileo data that the interior materials have settled partially, with the percentage of rock increasing toward the center. “Callisto is about 40% ice and 60% rock/iron ” (Callisto;Helicon). Callisto’s surface is covered entirely with craters. The surface is very old, like the highlands of the Moon and Mars.
“Callisto has the oldest, most cratered surface of any body yet observed in the solar system; having undergone little change other than the occasional impact for 4 billion years” (Callisto;Helicon). “The largest craters are surrounded by a series of concentric rings that look like huge cracks but which have been smoothed out by eons of slow movement of the ice. The largest of these has been named Valhalla (right). 4000 km in diameter, Valhalla is a dramatic example of a multi-ring basin, the result of a massive impact ” (Callisto;Helicon). “In terms of the mass of Earth’s Moon, the masses of the Galilean satellites in order of distance from Jupiter were found to be: Io, 1.21; Europa, 0.65; Ganymede, 2.02; and Callisto, 1.46. The mass of Io was 23% greater than that estimated before the Pioneer odyssey.
The density of the satellites decreases with increasing distance from Jupiter and was refined as a result of Pioneer’s observations. Io’s density is 3.52; Europa’s, 3.28; Ganymede’s, 1.95; and Callisto’s, 1.63 gm/cm^3. The outer satellites, because of their low density, could consist largely of water and ice. All four satellites were found to have average daylight surface temperatures of about-140 C (-220 F) ” (Columbia). A second group is comprised of the four innermost satellites–Metis, Adrastea, Amalthea, and Thebe.
Discovered by E. E. Barnard in 1892, Amalthea has an oblong shape and is 168 mi (270 km) long. Metis and Adrastea orbit close to Jupiter’s thin ring system; material ejected from these moons helps maintain the ring. The final group consists of the eight remaining satellites, none larger than c.110 mi (180 km) in diameter. “Four of the outer eight satellites located from 14 million to 16 million mi from Jupiter (22 million-26 million km), have retrograde motion, i.e., motion opposite to that of the planet’s rotation. The other four have direct orbits.
It is speculated that all eight might be captured asteroids” (Seeds). When it is in the nighttime sky, Jupiter is often the brightest “star” in the sky (it is second only to Venus, which is seldom visible in a dark sky). The four Galilean moons are easily visible with binoculars; a few bands and the Great Red Spot can be seen with a small astronomical telescope. Jupiter is very gradually slowing down due to the tidal drag produced by the Galilean satellites. How will this effect it and its moons? We currently know that the same tidal forces that are slowing Jupiter down are changing the orbits of the moons, very slowly forcing them farther from Jupiter. As additional data is gathered and technology enables a new fronitier, only then will we know the fate of Jupiter. Until then we can merely speculate it’s final life as a Jovian planet.
Bibliography Bibliography The Columbia Encyclopedia, Fifth Edition. Copyright 1993, Columbia University Press. Licensed from Lernout & Hauspie Speech Products USA, Inc. Pioneer: First to Jupiter, Saturn, and Beyond: Chapter 6A Results At The New Frontier; Fimmel, Richard O.; Van Allen, James; Burgess, Eric; 09-01-1990 Ganymede; ( The Hutchinson Dictionary of Science ) ; 01-01-1998, Helicon Publishing Ltd. 1998.
Io ; ( The Hutchinson Dictionary of Science ) ; 01-01-1998, Helicon Publishing Ltd. 1998. Callisto; ( The Hutchinson Dictionary of Science ) ; 01-01-1998, Helicon Publishing Ltd. 1998. Europa; ( The Hutchinson Dictionary of Science ) ; 01-01-1998, Helicon Publishing Ltd. 1998. Seeds, Michael A., Foundations of Astronomy; copyright 1994, Wadsworth Inc.
Copyright 1997-1999 by Calvin J. Hamilton. Copyright 1998 The Association of Universities for Research in Astronomy, Inc. Author not available, Astronomy: Common Terms in Astronomy. , The New York Public Library Science Desk Reference, 01-01-1995.