The Legacy of Galileo - Keck Institute for Space Studies - Caltech

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Challenging the Paradigm: The Legacy of Galileo paintings, but rather a spotted and scarred orb modeled after his friend’s drawings made with aid of the telescope 3 The regular motions of Jupiter’s moons around the giant planet were quickly recognized as providing an invaluable celestial clock. The speed of light (which Galileo tried unsuccessfully to measure) was determined from Ole Roemer’s measurements of satellite eclipse timings in 1676. Use of Galilean satellite eclipses became a standard way of determining longitudes by the eighteenth century. A planisphere from the collection of a French nobleman dating from 1745-1749 illustrates the impact of these astronomical advances on general culture of the era. Now part of the decorative arts collection of the Getty Museum in Malibu, California 4 , this work of art includes a dial for predicting the “Eclipses du Ier Satellite de Jupiter”⎯that is, an “Io dial” with 42.5 divisions around it’s circumference (Io orbital period in hours)(Wilson et al., 1996). Galileo’s “Medicean stars” also helped advance the understanding of celestial mechanics, with Laplace studying their orbital resonances, now known to be a major source of energy for heating some of the satellites. It was not until the early twentieth century, however, that we began to get an idea of these moons as worlds, in Galileo’s sense. By the time of the launch of the Voyager spacecraft in 1977, telescopic observations of ever-increasing sophistication and data from the Pioneer 10 and 11 spacecrafts provided the information illustrated in Figure 2, which comes from a briefing package used to explain Voyager’s scientific goals during launch preparations. At that time, the satellites individual characters were beginning to emerge. We knew, for instance, that Io was red (although not why) and was surrounded by a tenuous cloud of atomic sodium atoms escaping into Jupiter’s magnetosphere. By contrast, spectra showed that Europa, Ganymede and Callisto had water ice on their surfaces. We had reasonably accurate densities, showing that Io and Europa were mostly rocky in composition while Ganymede and Callisto were mixtures of rock and ice. Theoretical models for the satellites’ thermal evolution suggested the possibility of global liquid water oceans beneath the icy surfaces of at least the larger satellites. The state of knowledge prior to the Voyager mission is reviewed in Planetary Satellite. 5 3 Ostrow, S. F., 1996. Cigoli’s Immacolata and Galileo’s Moon: Astronomy and the Virgin in the Early Seicento. Art Bulletin. 78, 219-235 4 Wilson, G., et al., 1996. European Clocks in the J. Paul Getty Museum. The J. Paul Getty Museum, Los Angeles 5 In: J. A. Burns, (Ed.), Planetary Satellites. University of Arizona Press, Tucson, 1977 21
Challenging the Paradigm: The Legacy of Galileo Figure 2: Data from the Pioneer 10 and 11 spacecrafts. Even given these high expectations, the results of the Voyager flybys in 1979 surpassed our most optimistic projections, and completely revolutionized our view of Galileo’s moons, completing their transformation from points of light seen through a telescope to complex worlds, each with its individual properties and geologic history. Figure 3 shows a composite of the four Galilean satellites as seen by Voyager. Voyager’s data revealed that Io is an active, volcanic world, heated by tidal forces and emitting sulfurous gases that explain its yellowreddish hue. Callisto’s dark, ancient cratered surface was seen in stark contrast to Ganymede’s more geologically youthful surface, with older regions cut by smooth, resurfaced lanes. And Europa appearance defied easy explanation, with its bright, smooth icy planes almost free of large impact scars but crisscrossed by ridges and fracture patterns of global extent. Europa was immediately added to the list of moons suspected of harboring liquid water oceans. Voyager results for the satellites are reviewed in Satellites of Jupiter 6 and Satellites. 7 6 In: D. Morrision, (Ed.), Satellites of Jupiter. University of Arizona Press, Tucson, 1982 7 In: J. A. Burns, M. S. Matthews, (Eds.), Satellites. University of Arizona Press, Tucson, 1986 22
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