The Galilean Moons of Jupiter

Galileo first discovered Jupiter's four largest moons when he observed Jupiter with a telescope on January 7, 1610. These four largest moons are named Ganymede, Europa, Callisto, and Io. Of course, now we know these 'Galilean' moons are interesting little worlds in their own right. Perhaps 'little' is an inappropriate description, since Ganymede, Callisto, and Io are each larger in diameter than Earth's Moon.

Albedo markings were detected on all the Galilean moons with large telescopes and good seeing long before the spacecraft missions of the 1970s, although great detail was never detected and the observations were always suspect. The Voyager spacecraft missions detected a wealth of detail on the surfaces of these moons, along with other kinds of data. The Galileo spacecraft mission that followed provided even greater detail and a new cache of data that scientists will be studying for years. Yet, there are many interesting phenomena that amateurs can observe with modest telescopes. Since the Galilean moons orbit Jupiter in its equatorial plane, ground based telescopes can observe transits of the moons in front of the planet. And, we can observe the shadows of these moons on Jupiter's surface, appearing ink black against Jupiter's bright cloud tops. We can observe eclipses of the moons as they disappear and reappear in and out of Jupiter's shadow. We can also observe occultations of the moons by Jupiter, as they disappear behind the limb of the planet and reappear on the other side. About every six years, when the Jovian system is edge-on to Earth, we can see the moons occult and eclipse each other. Mutual occultations of the moons offer one of the only chances to

£ C x o detect color differences between the moons. Schedules for all of these events can be found at various scientific web sites and in schedules published monthly in Sky and Telescope and Astronomy magazines. And, with the great advances in CCD cameras and web cams, now even experienced amateurs are capturing images of Jupiter's moons that show amazingly detailed albedo markings. Yes, it can be very rewarding to observe Jupiter and its moons and to show them to a person for the first time. The movement of the moons in relation to Jupiter can be detected in a relatively short period.

The orbits of the Galilean moons are eccentric, or non-circular. The Galilean moons deform the others orbits, while Jupiter's tidal forces try to circularize them. Io is most affected by this tug of war since it is closest in to Jupiter. The effect of all of this is to cause tidal heating of Io and Europa [300].

Io is a fascinating world! We have already seen in Chap. 5 that Io is a major polluter of the Jovian environment. Mainly due to volcanic activity, Io creates clouds

5 .15 of neutral atoms, sheds dust, deposits sulfur on the surfaces of its neighboring

U moons, and supplies charged particles to the Io torus. Indeed, the Galileo space

© g craft detected significant variability in the concentrations of dust and charged

I— ifl particles from one spacecraft orbit to another, probably due to varying intensi ties of volcanic activity [301]. What a busy moon! As we shall see, Io displays some of the most intriguing volcanic related activity in the solar system [302]! Astronomers were greatly surprised when the first close look at Io revealed that its surface was devoid of craters (Fig. 6.1)! We now know that this is due to the great geologic activity on this moon and the constant resurfacing of its surface. Much new data was collected during the Galileo era and scientists have delved into it to further our understanding of Io. Galileo was launched in October 1989 and ended by plummeting into Jupiter in September 2003.

Io's diameter is 3,642.6 km [303] and it is so close to Jupiter that it completes an orbit around its parent in just 1.769 days [304]! It has a mean density of

Fig. 6.1. High Resolution Global View of Io by the Galileo Spacecraft. Io is the most volcanic body in the solar system. Its surface reveals rugged mountains, layer materials forming plateaus, and many i rregular depressions called volcanic calderas. Several of the dark, flow-like features correspond to hot spots, and may be active lava flows. There are no impact craters seen, as volcanism covers the surface with new deposits more rapidly than impacts from comets and asteroids can occur. (Credit: NASA/JPL/University of Arizona/PIRL)

Fig. 6.1. High Resolution Global View of Io by the Galileo Spacecraft. Io is the most volcanic body in the solar system. Its surface reveals rugged mountains, layer materials forming plateaus, and many i rregular depressions called volcanic calderas. Several of the dark, flow-like features correspond to hot spots, and may be active lava flows. There are no impact craters seen, as volcanism covers the surface with new deposits more rapidly than impacts from comets and asteroids can occur. (Credit: NASA/JPL/University of Arizona/PIRL)

3.5294 ± 0.0013 g cm-3 [305]. The tidal force of Jupiter acting on Io melts its interior, drives its volcanoes, covers its surface with sulfur, provides all the wonderful colors on its surface, and spits out a transient atmosphere of sulfur and sodium that makes its way into the Io torus cloud [306]. Yes, due to the tidal forces caused by its close proximity to Jupiter, Io is a tortured world. Io's surface is pockmarked with calderas, lava lakes, and plumes. Its volcanic activities are not evidenced so much by pyroclastic clouds, like volcanoes on Earth, but rather geysers similar to "Old faithful" in that they are concentrated and powerful. These 'geysers' are referred to officially as 'plumes' on Io, and they are not ordinary by any sense of the word (Fig. 6.2).

However, volcanic eruptions are not the only event on Io. During a 5-year period, the Galileo spacecraft recorded 82 surface changes on Io, from various processes

[307]. Io is so close to Jupiter that differential gravity has distorted the moon's shape. The surface of the hemisphere that faces Jupiter forms a 100-m tall bulge

[308]. Io's atmosphere is tenuous at best. Being composed of SO2, its atmosphere can reach 10-7 bar at midday or under volcanic plumes, but is completely frozen out at night. Surface temperature on Io can range from 120-130 K at midday to 60-90 K at midnight [309].

The plumes on Io are fascinating. During the Galileo spacecraft mission, two types of plumes were observed. These two types of plumes are categorized based upon their size and the colors of their deposits, or fallout. According to Geissler et al.,

Fig. 6.2. Active volcanic plumes on Io. The Galileo spacecraft capture two active plumes on Io. One plume is clearly seen on the bright limb, or edge, of the moon. This plume is 140 km (86 miles) high. Prometheus, the second plume is, is seen near the terminator. The shadow of the 75-km high plume can be seen extending to the right of the eruption vent. The vent is near the center of the bright and dark rings. (Credit: NASA/JPL/Caltech)

Fig. 6.2. Active volcanic plumes on Io. The Galileo spacecraft capture two active plumes on Io. One plume is clearly seen on the bright limb, or edge, of the moon. This plume is 140 km (86 miles) high. Prometheus, the second plume is, is seen near the terminator. The shadow of the 75-km high plume can be seen extending to the right of the eruption vent. The vent is near the center of the bright and dark rings. (Credit: NASA/JPL/Caltech)

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