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Atlas of the Galilean Satellites

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  • 795 colour illus.
  • Page extent: 406 pages
  • Size: 276 x 240 mm
  • Weight: 1.66 kg

Library of Congress

  • Dewey number: 523.9/850223
  • Dewey version: 22
  • LC Classification: QB404 .S43 2010
  • LC Subject headings:
    • Galilean satellites--Pictorial works
    • Galilean satellites--Maps
    • Jupiter (Planet)--Satellites--Pictorial works
    • Jupiter (Planet)--Satellites--Maps

Library of Congress Record

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 (ISBN-13: 9780521868358)

  • Also available in Adobe eBook
  • Published September 2010

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$201.00 (R)
Atlas of the Galilean Satellites
Cambridge University Press
9780521868358 - Atlas of the Galilean Satellites - By Paul Schenk

1    Introduction

1.1    The revolutionary importance of the Galilean satellites

Watershed moments, upon which the fates of nations, continents, or peoples hinge, are rare in human history. The Battle of Salamis in 480 BC, the Battle of Zama in 202 BC (my sympathies are with Carthage), the defeat of the Moors by Charles Martel in AD 732, the Sack of Constantinople in 1204, the death of Ogedei Khan as his armies approached Wien (Vienna) in 1241, the coming of the Black Plague in the fourteenth century, and the Czar's and Kaiser's decision to mobilize in August 1914 come to mind.

In this year 2009, we approach the 400th anniversary of another of these watershed events: the discovery of Jupiter's four large Galilean satellites, Io, Europa, Ganymede and Callisto, in January 1610 by Italian scientist Galileo Galilei. Galileo, sometimes falsely credited with invention of the telescope, perfected the basic instrument and was the first to point one at the heavens in earnest. Importantly for Galileo, he was quick to understand the revolutionary import of what he saw. Every object he observed, starting with the Moon, followed by Venus and Jupiter, revealed fundamental truths hidden to the naked eye that profoundly altered our perception of how the Universe worked, and in turn our worldview of ourselves and our place as a species in the Universe. Other revolutions were to follow in astronomy, chief among them Edwin Hubble's discovery that spiral nebulae are in fact millions of island galaxies like our own in a vast Universe, but Galileo's revolution permanently broke our myopic anthropocentric view of our place in the Universe, although it would take a few centuries for this new view to finally permeate the collective mass consciousness (and in some minds it never has).

The four moons Galileo saw are binocular objects, and would be visible to the naked eye outside the glare of Jupiter itself. Although there are claims (for Chinese astronomer Gan De in 362 BC, for example) that those with exceptional sight can actually detect the brighter moons with

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Figure 1.1 A page from Galileo's notes of his discovery of the Galilean satellites January 7, 1610. Jupiter is the large star and the four moons are the small shifting stars to either side. These are among the most valuable scientific documents in history. Credit: NASA.
the eye, their existence was inconceivable as December 1609 rolled to a close. Looking at Jupiter, Galileo saw three new “stars” in a line very close to the planet (Figure 1.1). After several days of observation, it was clear that there were in fact four new objects, and that they were all in orbit around Jupiter, not the Earth. Galileo's observations, and those of the Jovian moons in particular, thus gave a critical boost to the emerging Copernican Sun-centered worldview.

For more than a thousand years, it had been generally assumed that everything revolved around the Earth, which a casual observation of the heavens would imply. Copernicus helped relaunch the ancient Greek theory (by Aristarchus) of the Sun-centered (or heliocentric) Solar System in the early 1500s, but by the early 1600s the theory had received a decidedly ambivalent response. There were a few believers to be sure, but most had never heard of it, or remained unimpressed or uninterested. What Galileo saw on the Moon, Venus and Jupiter demonstrated that the celestial bodies are not immutable and that they do not all revolve around the Earth. (We now know that nothing is the center of anything, but the major point had been achieved.) Although it would be decades before the debate was won, mainly against yet another of the many reactionary responses from the conservative wing of the church to independent human thought, the first great astronomical revolution was now inevitable (and required only Kepler's “invention” of the elliptical orbit to be complete).

1.2    Post-discovery

A few years after Galileo's announcement in the Sidereus Nuncius (Stellar Message), German astronomer Simon Marius claimed to have discovered the four moons at about the same time. Today, Galileo is given credit, but it is Marius who is credited with the names by which we know these four moons, all named after Jove's indiscretionary loves in Greek mythology. These names did not enter common use till the twentieth century. Today, Marius and Galileo are both honored with names of large provinces on Ganymede.

The Galilean satellites then lay dormant in human thought for several centuries. True, they were useful for terrestrial longitude determination and for estimating the speed of light (based on eclipse timings). In the seventeenth century, Laplace explained the curious mathematical timing, or resonance, between the three inner moons in which their orbital periods

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Figure 1.2 The Solar System was a relatively simple, sedate place as the Space Age dawned. Only one paragraph is granted the four Galilean satellites in this volume that once occupied my bookshelf, space enough to assert that Ganymede has canal-like lines, an obliquely prophetic statement as it turns out.(All About the Planets, P. Lauber, Random House, 1960.)
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Figure 1.3 Pioneer 10 and 11 images of the Galilean satellites in 1973 and 1974 (from top to bottom: Io, Europa, Ganymede, and Callisto). These images have effective resolution of 200 to 400 kilometers. The right side versions have been heavily processed to enhance what little detail is present. Nowadays, the Hubble Space Telescope routinely acquires images with 10-fold improvement in resolution. Credit: NASA/Ames Research Center.
are related by simple integers (this Laplace Resonance is named for him). The profound consequences of this orbital dance were not understood for another 200 years, however.

With the advent of “modern” telescopic instruments and techniques, the Jovian moons began to emerge as real planetary bodies. Still, by the dawn of the Space Age in the late 1950s, little was known about these worlds (Figure 1.2). Pioneers 10 and 11 were the first visitors to Jupiter a few years before (I listened to the hourly radio news summaries for word of Pioneer 10's successful launch). Although the imaging systems were “primitive,” they did show a few fuzzy global features that can now be identified on our maps (Figure 1.3). Earth-bound observers saw dark “polar caps” on Io, bright caps on Ganymede and a dark equatorial band (or patches) on Europa. These features proved real, but most other apparent markings did not.

Spectroscopic observations found water ice on all the moons except Io, which also looked oddly yellowish. Instead, sodium clouds were found in Io's orbit. These scant facts lead to perhaps the best-known pre-Voyager speculation, which suggested that Io was covered by the salty deposits of a dried-up ocean. There was also the curious coordinated timing between Jupiter's radio emissions and Io's rotation period. By the mid 1970s, it was apparent something odd was going on in the Jupiter system.

1.3    Voyager and Galileo: Global mapping begins

The Galilean satellites have launched another revolution in our own time, the importance of which is not yet fully manifest. This revolution began in 1979. Prior to spring that year, it was commonly assumed that the satellites orbiting the four giant outer planets were essentially relicts of planetary formation, perhaps even cold dead worlds. Voyagers 1 and 2 were the first to explore the Jovian system with what we would call modern scientific instruments, including high-definition television cameras. What they revealed fundamentally altered our perception of the Outer Solar System. All four moons proved to be unique planetary bodies, as these pages document. The monopoly of Mars on our imagination was broken.

Voyager acquired high-resolution images of all four satellites, but the politics of celestial dynamics, competing mission requirements, and a date with Saturn demanded that the Voyagers give Europa less attention than the other Galilean satellites. It required the focused and detailed

observations of another Galileo, in this case a robotic explorer launched from the human home world, to reveal the fundamental nature of this ice-covered moon, demonstrating that Europa most likely possesses an ocean of liquid water beneath its surface. This marks Europa as one of several objects in the Outer Solar System possessing liquid water, hydrocarbons (perhaps), and internal heat sources. Each is a required element of any potential habitat for life, as we understand such things. What really lies or grows (?) inside Europa is not yet known, but Europa leads an impressive group of active icy worlds, including Triton, Titan, Enceladus, and perhaps even Ganymede. Where this fundamental shift in thinking will take us in the next decades no one can say, at least until we return to Europa.

A total of eight spacecraft have visited the Jupiter system since 1972, the most recent in 2007. Of those carrying dedicated cameras, only three have passed within the confines of the Galilean satellites, and only one has lingered for more than a day (Appendix 4). These three spacecraft, the two Voyagers and Galileo, have changed our perceptions of these moons, yet no truly global mapping data sets exist for the Galilean satellites. The global mosaics presented here are cobbled together from hundreds of images taken by the Voyager and Galileo spacecraft during their flybys of these satellites, beginning in 1979 and resuming in 1996.

Voyagers' discoveries at Jupiter are spread throughout this Atlas, but the story began in 1966 as a simple concept to use Jupiter to accelerate a spacecraft towards the other outer planets. Although the concept of gravity assist was known, Ph.D. student Gary Flandro discovered the opportunity that became the germ of the Voyager project. Voyager started life as the Grand Tour, a fleet of four spacecraft to visit all five outer planets, including Pluto, during a grand alignment of planets that occurs only every 173 years or so. The budget was not awarded to fit this profile, so in 1972 four spacecraft became two, and five planets became two (plus two: Uranus and Neptune were optioned for Voyager 2 only if Voyager 1 succeeded at Jupiter and Saturn). Pluto was not physically within reach of either Voyager and only now is a spacecraft on its way to that remote orb.

The two Voyagers were targeted to observe opposite hemispheres of each satellite, but effective resolution seldom exceeded 1 km, and significant mapping gaps remained, especially on Io and Europa. Even before Voyager arrival, a follow-on mission, the Jupiter Orbiter Probe, was designed in the mid 1970s for a 1982 mission to capitalize on and complete the Voyager

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Figure 1.4 The Galileo engineering test model, on display at the Jet Propulsion Laboratory, Pasadena, CA. The wire mesh antenna at top is shown in its real-life jammed configuration.
discoveries. Renamed Galileo, it would remain in Jupiter orbit for at least 2 years of extended studies of the planet and its moons.

It was Galileo's mission during its repeatedly delayed grand orbital tour of Jupiter (later extended by four more years) to pass within a few hundred kilometers of Europa, Ganymede, and Callisto with a battery of remote sensing instruments. (Io was targeted for a close pass during the first orbit in 1995 but due to the extreme radiation environment, additional passes were awarded only after the primary mission had succeeded. A tape recorder anomaly caused the cancellation of these first high-resolution Io observations.) Among other investigations, Galileo was expected to essentially replace the partial Voyager maps with nearly global mapping at resolutions of a few hundred meters and acquire very high resolution images of high-priority targets at 10 to 100 meter resolutions. Information on interior structure and magnetic fields were acquired but compositional mapping was severely restricted. Galileo was never able to achieve more than a tiny fraction of its global mapping mission.

The principal devil in this is the High-Gain Antenna, or HGA (Figure 1.4). The HGA onboard Galileo was designed to furl like an umbrella inside the Space Shuttle and be opened in space to provide the primary data link to Earth at 140 000 bits per second. The additional delays incurred due to the Challenger accident weeks before the scheduled launch in 1986 had unforeseen consequences. After three more years on the ground and two years in space (furled to protect the gold-plated mesh from the Sun), the antenna refused to open properly for reasons that today remain obscure. The secondary antenna on Galileo could only transmit at roughly 10 to 20 bits per second, no better than during the first Mars mission back in 1965, when it took more than a month to transmit 22 small images back to Earth from Mars. After years of frustrated effort, the antenna remained unusable and Galileo would return only a tiny fraction of its intended data.

Once it was realized that the antenna would never work, JPL engineers did a superb job in teasing as much information from the probe as possible. Onboard and ground-based upgrades increased data transmission to ∼150 bits per second by the time Galileo arrived in late 1995, an improvement but still crippling (compare this to your current cable or wireless capacity). The onboard tape recorder was also very limited, with a total capacity of only 115 megabytes, less than a CD-ROM. Using onboard data compression similar to JPEG, together with upgrades

to receiving antennae, a valuable data set was obtained, including amazing high-resolution images of each satellite. Still, the imaging instruments had to share this downlink capacity with ten other instruments. The intense radiation environment at Jupiter also inflicted a toll on the spacecraft, requiring further engineering efforts to keep the machine operational. Towards the end of the mission, Galileo succeeded in obtaining its programmed objectives about as often as it failed. Despite the great success of all these efforts, the loss of potential data was staggering.

The science teams responsible for guiding Galileo's tour of Jupiter faced a cruel choice: how best to use the sparse resources provided by the tiny backup antenna and recorder to achieve some of the original mission goals. Typical imaging results for any given Jupiter orbit during the original two-year prime mission (not including NIMS data) were only 150 to 180 images for Jupiter, its rings, and satellites, and quite a few of those were only partly returned. The allocation ratio for imaging increased slightly during the extended missions, which focused heavily on the new Europa and Io discoveries. With the exception of very limited success at Europa and Io, however, global mapping was sacrificed in favor of (reduced) high-resolution imaging (see Appendix 3). In fact, the global maps of Ganymede and Callisto are still heavily dominated by Voyager images, and the hemisphere of Io observed by Voyager 1 was never seen well by Galileo at all. As a result, the best resolution that can be sustained at global scales on any of these satellites is about 1 km, the resolution of all global and quadrangle maps in this Atlas.

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