Produced by Stephen Hutcheson and the Online Distributed
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[Illustration: A special color reconstruction of the eruption of the
volcano Loki on the Jovian satellite Io. The picture was taken by
Voyager 1 from a range of about half a million kilometers.
[P-21334C]]
NASA SP-439
[Illustration: Voyage To Jupiter]
Voyage To Jupiter
David Morrison
and Jane Samz
[Illustration: NASA]
Scientific and Technical Information Branch 1980
National Aeronautics and Space Administration
Washington, DC
For sale by the Superintendent of Documents
U.S. Government Printing Office, Washington, D.C. 20402
Library of Congress Catalog Card Number 80-600126
FOREWORD
Few missions of planetary exploration have provided such rewards of
insight and surprise as the Voyager flybys of Jupiter. Those who were
fortunate enough to be with the science teams during those weeks will
long remember the experience; it was like being in the crow’s nest of a
ship during landfall and passage through an archipelago of strange
islands. We had known that Jupiter would be remarkable, for man had been
studying it for centuries, but we were far from prepared for the torrent
of new information that the Voyagers poured back to Earth.
Some of the spirit of excitement and connection is captured in this
volume. Its senior author was a member of the Imaging Team. It is not
common that a person can both “do science” at the leading edge and also
present so vivid an inside picture of a remarkable moment in the history
of space exploration.
April 30, 1980
Thomas A. Mutch
Associate Administrator
Office of Space Science
INTRODUCTION
The two Voyager encounters with Jupiter were periods unparalleled in
degree and diversity of discovery. We had, of course, expected a number
of discoveries because we had never before been able to study in detail
the atmospheric motions on a planet that is a giant spinning sphere of
hydrogen and helium, nor had we ever observed planet-sized objects such
as the Jovian satellites Ganymede and Callisto, which are half
water-ice. We had never been so close to a Moon-sized satellite such as
Io, which was known to be dispersing sodium throughout its Jovian
neighborhood and was thought to be generating a one-million-ampere
electrical current that in some way results in billions of watts of
radio emission from Jupiter.
The closer Voyager came to Jupiter the more apparent it became that the
scientific richness of the Jovian system was going to greatly exceed
even our most optimistic expectations. The growing realization among
Voyager scientists of the wealth of discovery is apparent in their
comments, discussions, and reports as recounted by the authors in their
descriptions of the two encounters.
Although many of the discoveries occurred in the few weeks around each
encounter, they were, of course, the result of more than those few weeks
of effort. In fact, planning started a decade earlier, and the Voyager
team of engineers and scientists had been designing, building, and
planning for the encounters for seven years. The Pioneer spacecraft made
the first reconnaissance of Jupiter in 1973-1974, providing key
scientific results on which Voyager could build, and discoveries from
continuing ground-based observations suggested specific Voyager studies.
Voyager is itself just the second phase of exploration of the Jovian
system. It will be followed by the Galileo program, which will directly
probe Jupiter’s atmosphere and provide long-term observations of the
Jovian system from an orbiting spacecraft. In the meantime, the Voyager
spacecraft will continue their journey to Saturn, and possibly Uranus
and Neptune, planets even more remote from Earth and about which we know
even less than we knew of Jupiter before 1979.
As is clearly illustrated in this recounting of the voyage to Jupiter,
scientific endeavors are human endeavors; just as Galileo could not have
foreseen the advancement in our knowledge initiated by his discoveries
of the four Jovian moons in 1610, neither can we fully comprehend the
scientific heritage that our exploration of space is providing future
generations.
April 1980
Edward C. Stone
Voyager Project Scientist
ACKNOWLEDGEMENT
The authors are grateful to the many members of the Voyager Project team
who not only made this historic mission of exploration possible but also
took time from their busy schedules to offer us assistance, information,
and encouragement in the preparation of this book. Among many too
numerous to name individually, we particularly thank E. C. Stone, A. L.
Lane, C. H. Stembridge, R. A. Mills, E. Montoya, M. A. Mitz, and B. A.
Smith, L. Soderblom, and their colleagues on the Voyager Imaging Team.
We are grateful to F. E. Bristow, D. L. Bane, L. J. Pieri and especially
J. J. Van der Woude for their assistance in obtaining optimum versions
of the photographs printed here. C. B. Pilcher and I. de Pater kindly
made available their groundbased pictures of the Jovian magnetosphere.
Many colleagues have read and provided helpful comments on parts of the
manuscript, among them G. A. Briggs, S. A. Collins, S. Cruikshank, J.
Doughty, A. L. Guin, A. L. Lane, R. A. Mills, E. Montoya, E. C. Stone,
J. L. Ward, and especially C. R. Chapman.
CONTENTS
Foreword v
Introduction vii
Acknowledgment ix
Chapter 1. The Jovian System 1
Chapter 2. Pioneers to Jupiter 11
Chapter 3. The Voyager Mission 23
Chapter 4. Science and Scientists 33
Chapter 5. The Voyage to Jupiter—Getting There 47
Chapter 6. The First Encounter 63
Chapter 7. The Second Encounter: More Surprises From the “Land” of
the Giant 93
Chapter 8. Jupiter—King of the Planets 117
Chapter 9. Four New Worlds 139
Chapter 10. Return to Jupiter 169
Appendix A. Pictorial Maps of the Galilean Satellites 177
Appendix B. Voyager Science Teams 195
Appendix C. Voyager Management Team 197
Additional Reading 199
[Illustration: Jupiter is the largest planet in the solar system—a
gaseous world as large as 1300 Earths, marked by alternating bands
of colored clouds and a dazzling complexity of storm systems. The
Voyager mission gave us our first close look at this spectacular
planet. [P-21085]]
CHAPTER 1
THE JOVIAN SYSTEM
Introduction
In the Sun’s necklace of planets, one gem far outshines the rest:
Jupiter. Larger than all the other planets and satellites combined,
Jupiter is a true giant. If intelligent beings exist on planets circling
nearby stars, it is probable that Jupiter is the only member of our
planetary system they can detect. They can see the Sun wobble in its
motion with a twelve-year period as Jupiter circles it, pulling first
one way, then the other with the powerful tug of its gravity. If
astronomers on some distant worlds put telescopes in orbit above their
atmospheres, they might even be able to detect the sunlight reflected
from Jupiter. But all the other planets—including tiny inconspicuous
Earth—would be hopelessly lost in the glare of our star, the Sun.
Jupiter is outstanding among planets not only for its size, but also for
its system of orbiting bodies. With fifteen known satellites, and
probably several more too small to have been detected, it forms a sort
of miniature solar system. If we could understand how the Jovian system
formed and evolved, we could unlock vital clues to the beginning and
ultimate fate of the entire solar system.
Ancient peoples all over the world recognized Jupiter as one of the
brightest wandering lights in their skies. Only Venus is brighter, but
Venus, always a morning or evening star, never rules over the dark
midnight skies as Jupiter often does. In Greek and Roman mythology the
planet was identified with the most powerful of the gods and lord of the
heavens—the Greek Zeus; the Roman Jupiter.
As befits the king of the heavens, the planet Jupiter moves at a slow
and stately pace. Twelve years are required for Jupiter to complete one
orbit around the Sun. For about six months of each year, Jupiter shines
down on us from the night sky, more brightly and steadily than any star.
During the late 1970s it was a winter object, but in 1980 it will
dominate the spring skies, becoming a summer “star” about 1982.
Early Discoveries
Even seen through a small telescope or pair of binoculars, Jupiter looks
like a real world, displaying a faintly banded disk quite unlike the
tiny, brilliant image of a star. It also reveals the brightest members
of its satellite family as starlike points spread out along a straight
line extended east-west through the planet. There are four of these
planet-sized moons; with their orbits seen edge-on from Earth, they seem
to move constantly back and forth, changing their configuration hourly.
In January 1610, in his first attempt to apply the newly invented
telescope to astronomy, Galileo discovered the four large satellites of
Jupiter. He correctly interpreted their motion as being that of objects
circling Jupiter—establishing the first clear proof of celestial motion
around a center other than the Earth. The discovery of these satellites
played an important role in supporting the Copernican revolution that
formed the basis for modern astronomy.
A few decades later the satellites of Jupiter were used to make the
first measurement of the speed of light. Observers following their
motions had learned that the satellite clock seemed to run slow when
Jupiter was far from Earth and to speed up when the two planets were
closer together. In 1675 the Danish astronomer Ole Roemer explained that
this change was due to the finite velocity of light: The satellites only
seemed to run slow at large distances because the light coming from them
took longer to reach Earth. Knowing the dimensions of the orbits of
Earth and Jupiter and the amount of the delay (about fifteen minutes),
Roemer was able to calculate one of the most fundamental constants of
the physical universe—the speed of light (about 300 000 kilometers per
second).
[Illustration: Galileo’s notes summarizing his first observations of
the Jovian satellites Io, Europa, Ganymede, and Callisto in January
1610 were made on a piece of scratch paper containing the draft of a
letter presenting a telescope to the Doge in Venice. These
observations were the result of Galileo’s first attempt to apply the
telescope to astronomical research.]
The four great moons of Jupiter are called the Galilean satellites after
their discoverer. Their individual names—Io, Europa, Ganymede, and
Callisto—were proposed by Simon Marius, a contemporary and rival of
Galileo. (Marius claimed to have discovered the satellites a few weeks
before Galileo did, but modern scholars tend to discredit his claim.)
Io, Europa, Ganymede, and Callisto are names of lovers of the god
Jupiter in Greco-Roman mythology. Since Jupiter was not at all shy about
taking lovers, there are enough such names for the other eleven Jovian
satellites, as well as for those yet to be discovered.
In the century following Galileo’s death, improvements in telescopes
made it possible to measure the size of Jupiter and to note that it
bulged at the equator. The equatorial diameter is known today to be 142
800 kilometers, while from pole to pole Jupiter measures only 133 500
kilometers. For comparison, the diameter of the Earth is 12 900
kilometers, only about one-tenth as great, and the flattening of Earth
is also much smaller (less than one percent). By measuring the orbits of
the satellites and applying the laws of planetary motion discovered by
Johannes Kepler and Isaac Newton, astronomers were also able to
determine the total mass of Jupiter—about 2 × 10²⁴ tons, or 318 times
the mass of the Earth.
Once the size and mass are known, it is possible to calculate another
fundamental property of a planet—its density. The density, which is the
mass divided by the volume, provides important clues to the composition
and interior structure of a planetary body. The density of Earth, a body
composed primarily of rocky and metallic materials, is 5.6 times the
density of water. The mass of Jupiter is 318 times that of Earth; its
volume is 1317 times that of Earth. Thus Jupiter’s density is
substantially lower than Earth’s, amounting to 1.34 times the density of
water. From this low density, it was evident long ago that Jupiter was
not just a big brother of Earth and the other rocky planets in the inner
solar system. Rather, Jupiter is the prototype of the giant, gas-rich
planets Jupiter, Saturn, Uranus, and Neptune. These giant planets must,
from their low density, have a composition fundamentally different from
that of Mercury, Venus, Earth, Moon, and Mars.
Jupiter Through the Telescope
Jupiter is a beautiful sight seen with the naked eye on a clear night,
but only through a telescope does it begin to reveal its magnificence.
The most prominent features are alternating light and dark bands,
running parallel to the equator and subtly shaded in tones of blue,
yellow, brown, and orange. However, these bands are not the planet’s
only conspicuous markings. In 1664 the English astronomer Robert Hooke
first reported seeing a large oval spot on Jupiter, and additional spots
were noted as telescopes improved. As the planet rotates on its axis,
such spots are carried across the disk and can be used to measure
Jupiter’s speed of rotation. The giant planet spins so fast that a
Jovian day is less than half as long as a day on Earth, averaging just
under ten hours.
During the nineteenth century, observers using increasingly
sophisticated telescopes were able to see more complex detail in the
band structure, with wisps, streaks, and festoons that varied in
intensity and color from year to year. Furthermore, observations
revealed the remarkable fact that not all parts of the planet rotate
with the same period; near the equator the apparent length of a Jovian
day is several minutes shorter than the average day at higher latitudes.
It is thus apparent that Jupiter’s surface is not solid, and astronomers
came to realize that they were looking at a turbulent kaleidoscope of
shifting clouds.
Although the face of Jupiter is always changing, some spots and other
cloud features survive for years at a time, much longer than do the
largest storms on Earth. The record for longevity goes to the Great Red
Spot. This gigantic red oval, larger than the planet Earth, was first
seen more than three centuries ago. From decade to decade it has changed
in size and color, and for nearly fifty years in the late eighteenth
century no sightings were reported, but since about 1840 the Great Red
Spot has been the most prominent feature on the disk of Jupiter.
[Illustration: This ground-based photograph of Jupiter showing the
Great Red Spot in the southern hemisphere was taken with the
Catalina Observatory’s 61-inch telescope in December 1966.]
It was not until the twentieth century that the composition of the
atmosphere of Jupiter could be measured. In 1905 spectra of the planet
revealed the presence of gases that absorb strongly at red and infrared
wavelengths; thirty years later these were identified as ammonia and
methane. These two poisonous gases are the simplest chemical compounds
of hydrogen combined with nitrogen and carbon, respectively. In the
atmosphere of Earth they are not stable, because oxygen, which is highly
active chemically, destroys them. The existence of methane and ammonia
on Jupiter demonstrated that free oxygen could not be present and that
the atmosphere was dominated by hydrogen—a reducing, rather than
oxidizing, condition. Subsequently, hydrogen was identified
spectroscopically. Although much more abundant than methane or ammonia,
hydrogen is harder to detect.
In the 1940s and 1950s the German-American astronomer Rupert Wildt used
all the available data to derive a picture of Jupiter that is still
generally accepted. He noted that both the low total density and the
observed presence of hydrogen-rich compounds in the atmosphere were
consistent with a bulk composition similar to that of the Sun and stars.
This “cosmic composition” is dominated by the two simplest elements,
hydrogen and helium, which together make up nearly 99 percent of all the
material in the universe. Wildt hypothesized that the giant planets,
because of their large size, had succeeded in retaining this primordial
composition, whereas the hydrogen and helium had escaped from the
smaller inner planets. He also used his knowledge of the properties of
hydrogen and helium to calculate what the interior structure of Jupiter
might be like, concluding that the planet is mostly liquid or gas. Wildt
suggested that there probably was a core of solid material deep in the
interior, but that much of Jupiter is fluid—extremely viscous and
compressed deep below the visible atmosphere, but still not solid. The
atmosphere seen from above is just the thin, topmost layer of an ocean
of gases thousands of kilometers thick.
Recent Earth-Based Studies of Jupiter
In the past, a great deal of planetary research was basically
descriptive, consisting of visual observations and photography.
Beginning in the 1960s, a new generation of planetary scientists began
to apply the techniques of modern astrophysics and geophysics to the
study of the solar system. Inspired in part by the developing space
programs of the United States and the Soviet Union, scientists began to
ask more quantitative questions: What are the surfaces and atmospheres
made of? What are the temperatures and wind speeds? Exactly what
quantities of different elements and isotopes are present? And how can
these new data be used to infer the origin and evolution of the planets?
[Illustration: The major features of Jupiter are shown in schematic
form. The planet is a banded disk of turbulent clouds; all its
stripes are parallel to the bulging equator. Large dusky gray
regions surround each pole. Darker gray or brown stripes called
belts intermingle with lighter, yellow-white stripes called zones.
Many of the belts and zones are permanent features that have been
named. One feature of particular note is the Great Red Spot, an
enigmatic oval larger than the planet Earth, which was first seen
more than three centuries ago. During the years the spot has changed
in size and color, and it escaped detection entirely for nearly
fifty years in the 1700s. However, since the mid-nineteenth century
the Great Red Spot has been the most prominent feature on the face
of Jupiter. [2935]]
N
North polar region
North north north temperate belt
North north temperate zone
North north temperate belt
North temperate zone
North temperate belt
North tropical zone
North equatorial belt
Equatorial zone equatorial band
South equatorial belt
N. component
S. component
South tropical zone
Great Red Spot
South temperate belt
South temperate zone
South south temperate belt
South south temperate zone
South polar region
S
Wildt had already suggested the basic gases in the atmosphere of
Jupiter: primarily hydrogen and helium, with much smaller quantities of
ammonia and methane. Undetected but possibly also present were nitrogen,
neon, argon, and water vapor. The abundance of helium was particularly a
problem; although it was presumably the second-ranking gas after
hydrogen, it has no spectral features in visible light and its presence
remained only a hypothesis, unconfirmed by observation.
Although the presence of a gas can usually be inferred from
spectroscopy, solids or liquids cannot normally be detected in this way.
Thus the composition of Jupiter’s clouds could not be determined
directly. However, the presence of ammonia gas provided an important
clue. At the temperatures expected in the upper atmosphere of the
planet, ammonia gas must freeze to form tiny crystals of ammonia ice,
just as water vapor in the Earth’s atmosphere freezes to form cirrus
clouds. Most investigators agreed that the high clouds covering much of
Jupiter must be ammonia cirrus. But ammonia crystals are white, so the
presence of this material provides no explanation for the many colors
seen on Jupiter. Additional materials must be present—perhaps colored
organic compounds, produced in small amounts by the action of sunlight
on the atmosphere.
Because Jupiter is five times farther from the Sun than is the Earth, a
given area on Jupiter receives only about four percent as much solar
heating as does a comparable area on Earth. Thus Jupiter is colder than
Earth; even though it may be warm deep below its blanket of clouds,
Jupiter presents a frigid face.
[Illustration: These blue filter photographs of Jupiter were taken
at Mauna Kea Observatory, Hawaii. They show changes on Jupiter’s
surface between 1973 and 1978, with the dates of the observations.]
[Illustration: July 25, 1973]
[Illustration: October 5, 1974]
[Illustration: October 2, 1975]
[Illustration: November 20, 1976.]
[Illustration: January 28, 1978.]
[Illustration: December 19, 1978.]
The development of a new science, infrared astronomy, in the 1960s made
it possible to measure these low temperatures directly. In the case of a
cloudy planet like Jupiter, the infrared emission evident at various
wavelengths originates at different depths in the atmosphere. It is a
general property of any mixed, convecting atmosphere that the
temperature varies with depth; the rate of variation depends only on the
composition of the atmosphere, the gravity of the planet, and the
presence or absence of condensible materials to form clouds. On Jupiter
it is about 1.9° C warmer for each kilometer of descent through the
atmosphere. Thus, although the ammonia clouds are very cold, a little
above -173° C, if one goes deep enough one can reach temperatures that
are quite comfortable. With a variation of 1.9° C per kilometer,
terrestrial “room temperature” would be reached about 100 kilometers
below the clouds.
To measure the total energy radiated by a planet, it is necessary to
utilize infrared radiation at wavelengths more than one hundred times
longer than the wavelengths of visible light. Even when detectors were
developed that could measure such radiation, it was impossible to
observe celestial sources such as Jupiter because of the opacity of the
terrestrial atmosphere. Even a tiny amount of water vapor in our own
atmosphere can block our view of long-wave infrared. To make the
required measurements, it is necessary to carry a telescope to very high
altitudes, above all but a fraction of a percent of the offending water
vapor.
In the late 1960s a Lear-Jet airplane was equipped with a telescope and
made available by NASA to astronomers to carry out long-wave infrared
observations from above 99 percent of the terrestrial water vapor. In
1969 Frank Low of the University of Arizona and his colleagues used this
system to make a remarkable discovery: Jupiter was radiating more heat
than it received from the Sun! Repeated observations demonstrated that
between two and three times as much energy emanated from the planet as
was absorbed. Thus Jupiter must have an internal heat source; in effect,
it shines by its own power as well as by reflected sunlight. Theoretical
studies suggest that the heat is primordial, the remnant of an
incandescent proto-Jupiter that formed four and one-half billion years
ago.
[Illustration: Images of Jupiter in visible light (below) and
five-micrometer infrared light show the planet’s characteristic
belts and zones. The infrared image reveals areas that emit large
amounts of thermal energy. The source of the energy is thought to be
breaks in the Jovian cloud cover, which allow investigators a
glimpse of the deep regions of the atmosphere. One of the mysteries
of Jupiter concerns its heat balance: The planet appears to radiate
more heat than it receives from the Sun. [P-20957]]
[Illustration: Jupiter in visible light.]
At the same time that the internal heat source on Jupiter was being
revealed with long-wave airborne infrared telescopes, a new discovery
was being made from short-wave infrared observations. The clouds of
Jupiter are too cold to emit any detectable thermal radiation at a
wavelength of 5 micrometers (about ten times the wavelength of green
light). Nevertheless, images of Jupiter at 5 micrometers revealed a few
small spots where large amounts of thermal energy were being emitted.
The sources of the energy appeared to be holes or breaks in the clouds,
where it was possible to see deeper into hotter regions. The discovery
of these hot spots opened the possibility of probing deep regions of the
Jovian atmosphere that had previously been beyond the reach of direct
investigation.
The Jovian Magnetosphere
The discovery of planetary magnetospheres began in 1959 when the first
U.S. Explorer satellite detected the radiation belts around the Earth.
Named for James Van Allen of the University of Iowa, whose
geiger-counter instrument aboard Explorer 1 first measured them, these
belts are regions in which charged atomic particles—primarily electrons
and protons—are trapped by the magnetic field of the Earth. They are one
manifestation of the terrestrial magnetosphere—a large, dynamic region
around the Earth in which the magnetic field of our planet interacts
with streams of charged particles emanating from the Sun.
At almost the same time that the terrestrial magnetosphere was being
discovered by artificial satellites, astronomers were finding evidence
to suggest similar phenomena around Jupiter. Radio astronomy is a branch
of science that measures radiation from celestial bodies at radio
frequencies, which correspond to wavelengths much longer than those of
visible or infrared light. All planets emit weak thermal radio
radiation, but in the late 1950s investigators found that Jupiter was a
much stronger long-wave radio source than would be expected for a planet
with its temperature. This radiation bore the signature of higher-energy
processes. Physicists had seen similar emissions produced in synchrotron
electron accelerators, huge machines in which electrons are whirled
around at nearly the speed of light so that they can be used for
experiments in nuclear physics. The Russian theorist I. S. Shklovsky
identified the Jovian radio radiation as also resulting from the
synchrotron process, due to spiraling electrons trapped in the planet’s
magnetic field. From the intensity and spectrum of the observed
synchrotron radiation, it was clear that both the magnetic field of the
planet and the energy of charged particles in its Van Allen belts were
much greater than was the case for Earth.
Using radio telescopes of high sensitivity, astronomers determined the
approximate strength and orientation of the magnetic field of Jupiter.
Although they were able to measure synchrotron radiation only from the
innermost parts of the Jovian magnetosphere, they could infer that the
total volume occupied by the magnetosphere was enormous. If our eyes
were sensitive to magnetospheric emissions, Jupiter would look more than
twice the diameter of the full moon in the sky.
All four Galilean satellites orbit within the magnetosphere of Jupiter;
in contrast, our Moon lies well outside the terrestrial magnetosphere.
Striking evidence of the interaction of the satellites and the
magnetosphere was provided when it was found that the innermost large
satellite—Io—actually affects the bursts of radio static produced by
Jupiter. Only when Io is at certain places in its orbit are these strong
bursts detected. Theorists suggested that electric currents flowing
between the satellite and the planet might be responsible for this
effect.
The Jovian Satellites
For nearly three centuries after their discovery in 1610, the only known
moons of Jupiter were the four large Galilean satellites. In 1892 E. E.
Barnard, an American astronomer, found a much smaller fifth satellite
orbiting very close to the planet, and between 1904 and 1974 eight
additional satellites were found far outside the orbits of the Galilean
satellites. The outer satellites are quite faint and presumably no more
than a few tens of kilometers in diameter, and all have orbits that are
much less regular than those of the five inner satellites. Four of them
revolve in a retrograde direction, opposite to that of the inner
satellites and Jupiter itself.
In 1975 the International Astronomical Union assumed the responsibility
for assigning names to the non-Galilean satellites of Jupiter. Following
tradition, they named the inner satellite Amalthea for the she-goat that
suckled the young god Jupiter. The outer eight were named for lovers of
Jupiter: Leda, Himalia, Lysithea, Elara, Ananke, Carme, Pasiphae, and
Sinope. For the non-Galilean satellites, the “e” ending is reserved for
satellites with retrograde orbits; those with normal orbits have names
that end in “a.”
Because they are so large, the Galilean satellites have attracted the
most attention from astronomers. More than fifty years ago large
telescopes were used to estimate their sizes, and a careful series of
measurements of their light variation showed that all four always keep
the same face pointed toward Jupiter, just as our Moon always turns the
same face toward Earth. Also, the subtle gravitational perturbations
they exert on each other were used to determine the approximate mass of
each.
[Illustration: The pattern of the Galilean satellites changes from
hour to hour, as seen from Earth. Viewed edge-on, the nearly
circular orbits produce an apparent back and forth motion with
respect to Jupiter. These images recreate the kinds of observations
first made by Galileo in 1610.]
Callisto, the outermost Galilean satellite, is larger than the planet
Mercury. It also has the lowest reflectivity, or albedo, of the four,
suggesting that its surface may be composed of some rather dark,
colorless rock. Callisto takes just over two weeks to orbit once around
Jupiter.
Ganymede, which requires only seven days for one orbit, is the largest
satellite in the Jovian system, being only slightly smaller than the
planet Mars. Its albedo is much higher than that of Callisto, or of the
rocky planets such as Mercury, Mars, or the Moon. In 1971 astronomers
first measured the infrared spectrum of reflected sunlight from Ganymede
and found the characteristic absorptions of water ice, indicating that
this satellite is partially covered with highly reflective snow or ice.
Europa, which is slightly smaller than the Moon, circles Jupiter in half
the time required by Ganymede. Its surface reflects about sixty percent
of the incident sunlight, and the infrared spectrum shows prominent
absorptions due to water ice; Europa appears to be almost entirely
covered with ice. However, its color in the visible and ultraviolet part
of the spectrum is not that of ice, so some other material must also be
present.
Io, innermost of the Galilean satellites, is the same size as our Moon.
It orbits the planet in 42 hours, half the period of Europa. Like
Europa, it has a very high reflectivity, but, unlike Europa, it has no
spectral absorptions indicative of water ice. Before Voyager,
identification of the surface material on Io presented a major problem
to planetary astronomers.
When the sizes and masses of these satellites were measured, astronomers
could calculate their densities. The inner two, Io and Europa, both have
densities about three times that of water—nearly the same as the density
of the Moon, or of rocks in the crust of the Earth. Callisto and
Ganymede have densities only half as large, far too low to be consistent
with a rocky composition. The most plausible alternative to rock is a
composition that includes ice as a major component. Calculations showed
that if these satellites were composed of rock and ice, approximately
equal quantities of each were required to account for the measured
density. Thus the two outer Galilean satellites were thought to
represent a new kind of solar system object, as large as one of the
terrestrial planets, but composed in large part of ice.
In 1973 the attention of astronomers was dramatically drawn to Io when
Robert Brown of Harvard University detected the faint yellow glow of
sodium from the region of space surrounding it. It seemed that this
satellite had an atmosphere, composed of the metal sodium! Continued
observations showed, however, that this was not an atmosphere in the
usual sense of the word. The gas atoms were not bound gravitationally to
Io, but continuously escaped from it to form a gigantic cloud enveloping
the orbit of the satellite. Fraser Fanale and Dennis Matson of the
Caltech Jet Propulsion Laboratory suggested that bombardment of Io by
high-energy particles from the Jovian Van Allen belts was knocking off
atoms of sodium by a process called sputtering, releasing these atoms
and allowing them to expand outward to form the observed sodium cloud.
No one anticipated then that powerful volcanic eruptions on Io might
also be contributing to this remarkable gas cloud.
[Illustration: The Galilean satellites in orbit around Jupiter,
along with the outer satellites, constitute a miniature solar
system. Here they are shown relative to the size of Mercury and that
of the Moon. The portrayal of their internal and external
composition is based on theoretical models that preceded the Voyager
flybys. [PC-17054AC]]
[Illustration: This image of Io’s extended sodium cloud was taken
February 19, 1977, at the Jet Propulsion Laboratory’s Table Mountain
Observatory. A picture of Jupiter, drawings of the orbital geometry,
and Io’s disk (the small circle on the left) are included for
perspective. The sodium cloud image has been processed for removal
of sky background, instrumental effects, and the like. This
photograph demonstrates that the cloud is highly elongated and that
more sodium precedes Io in its orbit than trails it. [P-20047]]
This picture of the satellites was developed just as the first space
probe reached the Jovian system. In the next chapter we describe the
Pioneer program by which scientists reached out across nearly a million
kilometers of space to explore Jupiter, its magnetosphere, and its
system of satellites.
[Illustration: Pioneer 10 was launched on March 2, 1972, at 8:49
p.m. from Cape Canaveral, Florida. A powerful Atlas-Centaur rocket
served as the launch vehicle, which propelled the space probe to its
goal nearly a billion kilometers away. The beauty of the night
launch was enhanced by the rumbling thunder and flashing lightning
of a nearby storm.]