FOUR NEW WORLDS
Jupiter’s Satellite System
In a sense, the Voyager Mission revealed a new planetary system.
Astronomers had long been fascinated by the large Galilean satellites of
Jupiter, but they had only looked from afar, watching the dancing points
of light in their telescopes, and, occasionally, as the atmosphere
steadied, seeing these points resolve themselves into tiny disks before
dissolving again in the turbulence of the terrestrial atmosphere. Much
had been learned from telescopic studies, but not until the Voyager
flights had we truly seen the Galilean satellites. The historic hours as
Voyager 1 cruised past each satellite on March 5 and 6, 1979,
fundamentally altered our perspective. Four new worlds were revealed, as
diverse and fascinating as the more familiar terrestrial planets.
Although not yet household words, the names Io, Europa, Ganymede and
Callisto have now been added to Mercury, Venus, Moon, and Mars in the
lexicon of important “Earth-sized” bodies in the solar system.
Jupiter has fifteen known satellites, counting the two new satellites
discovered by Voyager. These moons vary greatly in size, composition,
and orbit. The four outermost satellites, Sinope, Pasiphae, Carme, and
Ananke, circle the planet in retrograde orbits of high inclination;
their distances from Jupiter vary between 20 and 24 million kilometers
(290 R_J to 333 R_J). These small bodies, none more than 50 kilometers
in diameter, require nearly two years for each orbit of Jupiter. It is
possible that they are captured asteroids, but so little is known about
them that astronomers cannot tell if their surface properties resemble
those of asteroids, or if these four satellites are even similar to each
other.
The next group of Jovian satellites consists of four small
difficult-to-observe objects. These are Lysithea, Elara, Himalia, and
Leda, the latter discovered by Charles Kowal of Hale Observatories in
1974. They have similar orbits, varying in distance from Jupiter between
11 and 12 million kilometers (about 160 R_J). Like the outer group,
these satellites have orbits of high inclination; unlike the outer
group, they move in the proper, prograde direction around Jupiter. The
largest, Himalia (170 kilometers in diameter) and Elara (80 kilometers
diameter), are known to be very dark, rocky objects, and it seems
probable that the others are similar. It is unlikely that the census of
the outer groups of irregular satellites is complete, and new satellites
less than 10 kilometers in diameter will probably be discovered.
The Jovian system is dominated, of course, by the large Galilean
satellites, which vary in size from just smaller than the Moon (Europa)
to nearly as large as Mars (Ganymede). These satellites are in regular,
nearly circular orbits in the same plane as the equator of Jupiter, and
all four lie within the inner magnetosphere of Jupiter, where they
interact strongly with energetic charged particles and plasma. Most of
this chapter will be devoted to a discussion of these fascinating
worlds.
We now know of three additional small satellites inside the orbit of Io,
orbiting close to Jupiter. The first, Amalthea, was discovered in 1892;
it orbits Jupiter in just twelve hours at a distance of 181 000
kilometers (2.55 R_J). A smaller object, Adrastea (officially 1979J1 for
the first new satellite of Jupiter discovered in 1979), is much closer,
at 134 000 kilometers (1.76 R_J). As described in Chapter 7, it skirts
the outer edge of the ring, circling Jupiter in just over seven hours.
The inner satellite moves faster than Jupiter’s rotation; seen from the
planet, it would rise in the west and set in the east. Both Amalthea and
Adrastea are buried deep within the inner magnetosphere where they are
continually bombarded by energetic electrons, protons, and ions.
Depletion of the Jovian radiation belt particles was observed at the
orbits of both satellites by Pioneer 11, which went much closer to
Jupiter than the Voyagers, testifying to the intensity of the
interaction between these objects and their surroundings.
[Illustration: Callisto was revealed by the Voyager cameras to be a
heavily cratered and hence geologically inactive world. This mosaic
of Voyager 1 images, obtained on March 6 from a distance of about
400 000 kilometers, shows surface detail as small as 10 kilometers
across. The prominent old impact feature Valhalla has a central
bright spot about 600 kilometers across, probably representing the
original impact basin. The concentric bright rings extend outward
about 1500 kilometers from the impact center. [260-450]]
SATELLITES OF JUPITER
Distance From Jupiter
Name 10³ Jupiter Period Year of
kilometers Radii (days) Discovery
Adrastea J14 134 1.76 0.30 1979
Amalthea J5 181 2.55 0.49 1892
1979J2 J15 222 3.11 0.67 1980
Io J1 422 5.95 1.77 1610
Europa J2 671 9.47 3.55 1610
Ganymede J3 1070 15.10 7.15 1610
Callisto J4 1880 26.60 16.70 1610
Leda J13 11 110 156 240 1974
Himalia J6 11 470 161 251 1904
Lysithea J10 11 710 164 260 1938
Elara J7 11 740 165 260 1904
Ananke J12 20 700 291 617 1951
Carme J11 22 350 314 692 1938
Pasiphae J8 23 300 327 735 1908
Sinope J9 23 700 333 758 1914
[Illustration: The state of the interiors of the Galilean satellites
can be judged from their sizes and densities. These cross-sectional
views represent the best guess following the Voyager flybys as to
the composition and structure of the objects. Io, with a density
equal to that of the Moon and a long history of volcanic activity,
is a dry, rocky object. Europa is less dense, and it probably has a
global ocean of ice as much as 100 kilometers thick over a rocky
interior. Ganymede and Callisto both have densities near 2 grams per
cubic centimeter, suggesting a composition about half water and half
rock. There is probably a rocky core surrounded by an icy mantle.]
Io
Active volcanoes
Sulfur and frozen SO₂
Molten silicate interior
Europa
Global fracture patterns
Ice crust
Rocky interior
Ganymede
Fresh craters expose ice
Young grooved terrain with intricate fracture
Old, dark cratered areas
Ice crust
Water or ice mantle
Callisto
Large basins reduced by ice flow
Fresh craters expose ice
Ice/rock crust
Water or ice mantle
Moon
Mercury
Long after the flybys of Jupiter, continued analysis of Voyager images
revealed another new satellite, Jupiter’s fifteenth. Initially
designated 1979J2, the unexpected new satellite orbits the planet at
3.17 R_J, between Io and Amalthea. Stephen Synnott of the JPL Optical
Navigation Team discovered the satellite on pictures taken during the
Voyager 1 events on March 5, 1979, while searching for additional images
of satellite 1979J1. It is about 75 kilometers in diameter, but nothing
else is known about its physical properties.
Together, the 15 satellites circling giant Jupiter form a mini-solar
system. Perhaps the outer, irregular satellites were captured or
resulted from the catastrophic collisions of one or more larger
satellites with passing asteroids. The inner seven satellites constitute
a coherent system, almost certainly formed together with Jupiter and
sharing a common 4.5-billion-year history. They are fascinating as
individual worlds, and also as brothers and sisters, and the study of
their interrelationships undoubtedly will provide insights into the
general problems of planetary formation and evolution.
SIZES AND DENSITIES OF THE GALILEAN SATELLITES
Name Diameter Density (grams per
(kilometers) cubic centimeter)
Io 3640 3.5
Europa 3130 3.0
Ganymede 5270 1.9
Callisto 4840 1.8
Callisto
Callisto is the least active geologically of the Galilean satellites.
Basically a dead world, it bears the scars of innumerable meteoric
impacts, with virtually no sign of major internal activity. Callisto is
a world of craters, and to understand it we must explore the role that
cratering plays in molding planetary surfaces.
The space between the planets is filled with debris, ranging from the
larger asteroids, hundreds of kilometers in size, down to microscopic
grains of dust. Inevitably, each planet collides with some of these
fragments. The smaller particles do little damage; in the case of a
planet with an atmosphere, like Earth, they burn up as meteors before
reaching the surface, whereas on an airless planet, they erode the
surface by sandblasting the exposed rock. The larger impacts are another
matter, and the craters they produce can be the dominant features on the
surface of a planet.
[Illustration: Voyagers 1 and 2 photographed most of the surface of
Callisto at resolutions of a few kilometers or better. Shown here is
a preliminary shaded relief map. Additional measurements will
improve the accuracy of the coordinate system. [260-672]]
[Illustration: Callisto is a world of craters, as is well shown in
this Voyager 2 photomosaic taken from a distance of 400 000
kilometers. Craters about 100 kilometers in diameter cover the
surface uniformly. Many have bright rims, perhaps composed of
exposed water-ice. There are very few craters larger than 150
kilometers in diameter, however, indicating that the scars of very
large impacts do not survive on the surface of Callisto.
[P-21746B/W]]
[Illustration: The concentric rings surrounding Valhalla are perhaps
the most distinctive geological feature on Callisto. This Voyager 1
close-up shows a segment of the ridged terrain. The presence of
superposed impact craters shows that the rings formed early in
Callisto’s history; however, the density of craters is less here
than on other parts of the satellite, where the surface is older.
[P-22194]]
We who live on Earth tend not to realize the importance of cratering,
for the simple reason that our planet has very few craters, and these
are frequently of volcanic rather than meteoric origin. Why are we so
favored? Is there an invisible shield to protect us from the cosmic
shooting gallery? Clearly not; the Earth has experienced just as many
cratering impacts as has the Moon or other planets. The difference is
not that craters are formed less often, but that the great geological
activity of Earth—erosion, volcanism, mountain building, continental
drift, etc.—erases craters as fast as they are formed. On the average, a
10-kilometer-wide crater is formed on Earth about once every million
years, but all those older than a few million years have been eroded
away, filled in, or crushed beyond recognition by crustal motion.
If a planet lacks great internal geologic forces, large craters can
survive almost indefinitely. Such is the case for the Moon. Most of the
volcanism and other activity on the Moon ceased 3½ billion years ago, as
the dating of lunar samples obtained by the Apollo astronauts showed.
Since that time, the lunar surface has been passively accumulating
impact scars. The longer any particular surface area has been exposed,
the more densely packed are the craters. Thus crater density is the
first thing a planetary geologist looks for in photos of a new world.
Craters are the touchstone of this field, revealing the degree of
internal activity and allowing the determination of the relative ages of
different surface units.
On Callisto the density of craters is very high. In some places they are
packed as closely as one can imagine, particularly for craters several
tens of kilometers in diameter. Although no one knows the exact rate of
formation of impact craters on the Jovian satellites, geologists on the
Voyager Imaging Team estimate that it would require several billion
years to accumulate the number of craters found on Callisto. They
therefore conclude that Callisto has been geologically inactive almost
since the time of its formation.
Although superficially similar to the heavily cratered surfaces of the
Moon and Mercury, Callisto is far from identical to these rocky worlds.
One of the most obvious differences is a lack of craters larger than
about 150 kilometers on Callisto, together with a tendency for large
craters to have much shallower depths. Apparently the ice-rock
composition of Callisto alters the ability of the crust to retain large
craters. Geologists speculate that the ice flows over many millions of
years, filling in crater floors and gradually obliterating the largest
craters. There is also a conspicuous absence of mountains on Callisto,
again suggestive of a weak, icy crust.
[Illustration: The IRIS instrument measured the temperature of spots
on the surface of Callisto as each Voyager sped past. The
measurements shown here were all made at equatorial latitudes
(between -10° and 25°). Shown are very low predawn temperatures
(-190° C) followed by an increase to a noon-time maximum of about
-120° C, and then a drop again as the Sun sets. [260-735]]
The most prominent features in the Voyager pictures are the ghost
remnants of what must have been immense impact basins. The largest of
these, the “bullseye” of the Voyager 1 images, has been named Valhalla,
for the home of the Norse gods. These ghost basins have lost nearly all
their vertical relief. What remains is a central, light-colored zone
(probably the location of the original crater), surrounded by numerous
concentric rings of subdued, bright ridges. Such features had never been
seen before on any planet, and they appear to be the characteristic
geologic feature of an ice-rock planet.
Little is known about the composition of Callisto’s surface, the
material from which sunlight is reflected. It appears to be primarily
dark rock or soil, but it lacks diagnostic spectral features, except for
one infrared band due to water molecules bound in the soil. The many
lighter spots and arcs that outline craters in the high-resolution
pictures may be regions in which the ice is showing through, but these
cover only a very small fraction of the exposed surface. (It should be
noted that, although Callisto is the darkest of the Galilean satellites,
the term “dark” is relative, for even Callisto is brighter than Earth’s
moon.)
The daytime surface temperature of Callisto, observed both from the
ground and by Voyager, is about -118° C. The Voyager infrared
interferometer spectrometer also determined the minimum temperature,
reached just before dawn, of -193° C. No atmosphere is expected at these
cold temperatures, and none was seen.
Analysis of Voyager images provided an improved diameter for Callisto of
4840 kilometers, yielding an average density of 1.8 grams per cubic
centimeter. As noted previously, it is this low density that leads to
the conclusion that ice or water is an important component of the
interior of Callisto. The ice has never been detected directly, but the
peculiar nature of the craters seen by Voyager adds strong
circumstantial support to this conclusion.
Callisto, with its heavy cratering, is the most familiar-looking of the
Galilean satellites; if all of them had turned out to be as geologically
dead as Callisto, planetary geologists would certainly have been
disappointed. However, each satellite, progressing in toward Jupiter,
presents increasing evidence of internal activity.
Ganymede
The largest of the Galilean satellites (5270 kilometers in diameter),
Ganymede was expected to be similar to Callisto in many ways. Both have
low densities (for Ganymede, 1.9 grams per cubic centimeter), indicating
a bulk composition of about half rocky materials and half water. In
addition, their diameters differ by only eight percent, and both are far
enough from Jupiter to escape the severe pounding Io receives from
magnetospheric charged particles. Thus it was with great interest that
Voyager scientists looked at the differences that emerged between these
two satellites.
[Illustration: Shaded relief map of Ganymede. [260-673]]
The surface of Ganymede as revealed by the Voyager cameras is one of
great diversity, indicating differing periods of geologic activity. At
one extreme there are numerous dark areas that resemble the surface of
Callisto in both albedo (reflectivity) and crater density. The largest
of these, Regio Galileo, stretches from the equator to latitude +45° and
is 4000 kilometers across, nearly as large as the continental United
States. This ancient terrain even preserves the remnants of a
Callisto-type impact basin in the form of a system of parallel, curving,
subdued ridges about 10 kilometers wide, 100 meters high, and spaced
about 50 kilometers apart. The central part of this ghost basin is
missing, however; it was presumably destroyed by subsequent geologic
activity.
Other regions of the surface of Ganymede are clearly the product of
intense internal geologic activity. Generally, these regions are of
higher albedo and consist of many straight parallel lines of mountains
and valleys. The Voyager geologists call these the grooved terrain
because of their appearance from a great distance. Typically, these
mountain ridges are 10-15 kilometers across and about 1000 meters high,
similar in scale to some sections of the Appalachian Mountains in the
Eastern United States. No higher relief exists, presumably for the same
reason it is absent on Callisto. In many places the grooved terrain
forms between areas of the older, darker surface, giving the appearance
of mountains extruded between separating plates of ancient crust. In
other areas the relationships are much more complex, with curved systems
of grooves and ridges overlying each other, displaying intricate
crosscutting relationships. Apparently Ganymede has experienced a series
of mountain-building events.
The grooved terrains show a substantial range in age, as indicated by
the crater densities. The oldest have nearly the same density as the
ancient, dark plains, suggesting that formation of the grooved terrain
began early, perhaps 4 billion years ago. The youngest grooved terrain
has only about one-tenth as many craters, but this is still as many as
are seen in the 3.5-billion-year-old lunar plains. The Voyager
geologists believe that even in these areas geologic activity ceased
billions of years ago.
[Illustration: The hemisphere of Ganymede that faces away from the
Sun displays a great variety of terrain. In this Voyager 2 mosaic,
photographed at a range of 300 000 kilometers, the ancient dark area
of Regio Galileo lies at the upper left. Below it, the lighter
grooved terrain forms bands of varying width, separating older
surface units. On the right edge, a prominent crater ray system is
probably caused by water-ice splashed out in a relatively recent
impact. [260-671]]
[Illustration: The sinuous nature of some of the narrower Ganymede
groove systems can be seen in this oblique view, obtained on March 5
by Voyager 1. The area shown is about the size of California, with
features visible as small as 5 kilometers across. The ridges appear
to be the result of deformation of the crust of Ganymede. [P-21235]]
Other types of surfaces are seen on Ganymede. Some regions are lightly
cratered and smooth, with no indication of mountain building. In one
place, there is a rough mountainous area that looks more like the
jumbled lunar mountains than the long ridges and valleys of the rest of
Ganymede. Many of the larger craters are distinguished by brilliant
white halos and rays that suggest that impacts may have splashed large
quantities of water or ice over the surface.
Many of the geologic features seen on Ganymede appear to have been
caused by breaking, faulting, or spreading of the crust. In a few cases,
there even seem to be indications of transverse, or sideways, motion
along faults. This evidence is extremely exciting to geologists, since
similar crustal motion on Earth is associated with the drift of
continental plates, drawn by convection currents deep in the mantle.
Such activity has never been seen before on another planet.
Astronomers on Earth had known since 1971 that about half the surface of
Ganymede was covered with exposed water ice and about half with darker
rock. An examination of the albedo variations in the Voyager pictures
suggests that the ice is exposed near large craters and, to a lesser
extent, in the grooved terrain, but no direct measurements were made by
Voyager of the composition of different parts of the surface.
The presence of ice on the surface suggested to many astronomers that
Ganymede might have a very tenuous atmosphere of water vapor or oxygen,
which might be released by the breakdown of water vapor by sunlight.
During the Voyager 1 flyby, a sensitive test for an atmosphere was made
by the ultraviolet instrument from observations of the star Kappa
Centauri as it was occulted by Ganymede. No dimming of the starlight was
seen, yielding an upper limit for the surface pressure of the gases
oxygen, water vapor, or carbon dioxide of 10⁻¹¹ bar, or one
hundred-billionth the atmospheric pressure at Earth.
The differences between the geologic histories of Ganymede and Callisto
are surprisingly large. No one knows the reason. Perhaps only a small
increase in internal temperature is necessary to initiate geologic
activity in an icy planet, and for some reason Ganymede crossed this
threshold for a part of its history, whereas Callisto did not.
[Illustration: The complex patterns of the grooved terrain on
Ganymede are apparent in high-resolution images. This picture, taken
by Voyager 1 on March 5, has a resolution of about 3 kilometers and
shows a region about the size of the state of Pennsylvania. The
mountain ridges are spaced about 10 to 15 kilometers apart and rise
about 1000 meters, similar to many of the mountains of Pennsylvania.
The transections of different mountain systems indicate that they
formed at different times. A degraded crater near the left center of
the picture is crossed by ridges, indicating that it predated the
period of crustal deformation and mountain building. [P-21279]]
[Illustration: Ray systems of exposed water-ice are visible in this
high-resolution mosaic of Ganymede, obtained by Voyager 2 on July 9
at a range of about 100 000 kilometers. The rough mountainous
terrain at lower right is the outer portion of a large fresh impact
basin that postdates most of the other terrain. At the bottom,
portions of grooved terrain transect other portions, indicating an
age sequence. The dark patches of heavily cratered terrain (right
center) are probably ancient icy material formed prior to the
grooved terrain. The large rayed crater at upper center is about 150
kilometers in diameter. [P-21770B/W]]
[Illustration: The many variants of smooth and grooved terrain on
Ganymede suggest a complex geologic history for this satellite. Four
high-resolution views by Voyager 2 are grouped together. [260-678A]]
[Illustration: A band of low mountain ridges has apparently been cut
and offset by a fault.]
[Illustration: Multiple sets of mountain ridges transect at nearly
right angles. Some impact craters were formed before, and some
after, the grooved terrain.]
[Illustration: Many short parallel ridges butt into each other,
making a crazy-quilt pattern.]
[Illustration: In the center of this frame is an unusual smooth
area, perhaps the result of flooding of the surface by material that
filled in the grooves.]
Europa
As one proceeds inward through the Galilean satellites, these worlds
become less and less familiar to the planetary geologist. This was an
unexpected effect. Callisto and Ganymede were expected to have unusual
properties as a result of their large percentage of ice. The densities
of Europa and Io are more normal for the smaller, terrestrial-type
planets, and before Voyager many scientists expected these two
satellites might look much like the Moon, which they resemble in size.
Europa, with a diameter of 3130 kilometers, is about 15 percent smaller
than the Moon. Its density is 3.0 grams per cubic centimeter, indicating
a basically rocky composition. However, cosmic mixtures of rocky and
metallic materials are often a bit denser than this, leaving room for a
substantial component of ice or water. Calculations indicate that if all
the ice were at the surface, it might form a crust up to 100 kilometers
thick.
[Illustration: Shaded relief map of Europa. [260-659]]
Telescopic observations of Europa demonstrated long before Voyager that
this satellite is almost completely covered with ice. It is a white,
highly reflecting body, looking, from a great distance, like a giant
snowball. In the early Voyager pictures, Europa always showed a bland,
white disk, in striking contrast to the spottiness of Ganymede or the
brilliant colors of Io.
Voyager 1 never got closer to Europa than 734 000 kilometers, and at
that distance it remained a nearly featureless planet, with no obvious
impact craters or other familiar geologic structures. What did show in
the Voyager 1 pictures, however, were numerous thin, straight dark lines
crisscrossing the surface, some extending up to 3000 kilometers in
length. To the members of the Imaging Team, these features were
“strongly suggestive of global-scale tectonic processes, induced either
externally (as by tidal despinning) or internally (as by convection).”
It was with the greatest interest that the Voyager 2 images, taken from
about four times closer, were anticipated.
The spectacular pictures obtained of the satellite in July were perhaps
more confusing than clarifying. Europa is entirely covered with dark
streaks that vary in width from several kilometers to approximately 70
kilometers and in length from several hundred to several thousand
kilometers. Most streaks are straight, but others are curved or
irregular. The streaks lie on otherwise smooth, bright terrain,
featureless except for numerous random dark spots, most less than 10
kilometers in diameter.
Voyager 2 photos showed, in addition to the smooth terrain with its dark
streaks, regions of darker, mottled terrain. This mottled terrain
appears rough on a small scale, and it may contain small craters just on
the limit of resolution (about 4 kilometers). Only three definite impact
craters have been identified, each about 20 kilometers across. This
small number of craters suggests either that the surface is relatively
young or that craters are not preserved for long in the icy crust.
Although the dark streaks give Europa a cracked appearance, the streaks
themselves are not obviously cracks. They are not depressed below their
surroundings; in fact, they have no topographic structure whatever.
Europa is extraordinarily smooth, and the dark streaks look rather like
marks made with a felt-tipped pen on a white billiard ball. The streaks
are not even very dark; the contrast with adjacent smooth terrain is
only about 10 percent.
One of the most remarkable geologic phenomena discovered by Voyager is
the light streaks that appear on Europa. These are smaller than the dark
streaks, only about 10 kilometers in width, but much more uniform. Seen
at low Sun angle, they also show vertical relief of less than a few
hundred meters. These light ridges are seen best at low Sun and tend to
be invisible at higher illumination angles.
The most amazing thing about the light ridges is their form. Instead of
being straight, they form scallops or cusps, with smooth curves that
repeat regularly on a scale of 100 to a few hundred kilometers. In some
of the low-Sun-angle pictures, the surface of Europa seems to be covered
with a beautiful network of these regular curving lines. The impression
is so bizarre that one tends not to believe the reality of what is seen.
Nothing remotely like it has ever been seen on any other planet.
At present the geology of Europa remains beyond our understanding.
Presumably there is a thick ice crust, perhaps floating on a liquid
water ocean. Presumably there is sufficient heat coming from the
interior to have produced cracking or motion in the ice crust, and the
light and dark streaks preserve a pattern in some way related to this
internal activity. However, the actual mechanisms for producing the
observed features so lightly traced on this smooth white world remain
for scientists to decipher.
[Illustration: Europa looks like a cracked egg in this computer
mosaic of the best Voyager 2 images. In this presentation, the
variation of surface brightness due to the angle of the Sun has been
removed by computer processing, so that surface features can be seen
equally well at all places. The many broad dark streaks show up
well, but this presentation does not bring out the much fainter and
more enigmatic light streaks. These pictures were taken from a
distance of about 250 000 kilometers and show features as small as 5
kilometers across. [260-686]]
Io
The most spectacular of the Galilean satellites is Io. Even in
low-resolution images, its brilliant colors of red, orange, yellow, and
white set it apart from any other planet. The dramatic scale of its
volcanic activity confirms that Io is in a class by itself as the most
geologically active planetary body in the solar system.
The diameter of Io is 3640 kilometers, and its density is 3.5 grams per
cubic centimeter. Both values are nearly identical to those of our Moon.
Were it not for Io’s proximity to Jupiter, it would probably be a dead,
rocky world much like Earth’s satellite.
Careful examination of all the Voyager images of Io, some of which have
resolutions as good as 1 kilometer, has failed to reveal a single impact
crater. Yet the flux of crater-producing impacts at Io must be even
greater than for the other Galilean satellites, because of the focusing
effect of Jupiter’s gravity. The absence of craters alone would indicate
that Io has an extremely young and dynamic planetary surface, even
without the observation of active volcanoes. Calculations indicate that
craters on Io must be filled in or otherwise obliterated at a rate
corresponding to the deposition of at least 100 meters per million
years, and quite probably a factor of ten greater, or 1 meter every
thousand years.
[Illustration: One of the most remarkable of all the Voyager
discoveries was the arcuate white ridges on Europa. Visible only at
very low Sun angle, these curved bright streaks are 5 to 10
kilometers wide and rise at most a few hundred meters above the
surface. Their graceful scalloped pattern is unique to this planet
and has defied explanation. Also visible in this view, taken by
Voyager 2 on July 9 at a range of 225 000 kilometers, are dark
bands, more diffuse than the light ridges, typically 20 to 40
kilometers wide and hundreds to thousands of kilometers long.
[P-21766]]
[Illustration: Shaded relief map of Io as it appeared in early March
1979. [260-634BC]]
[Illustration: The great erupting volcanoes on Io produce
distinctive surface markings. These are three views of Prometheus
(P₃). The bright ring on the surface rims the areas of fallout from
the plume, and it is probably an area in which sulfur dioxide frost
is being deposited on the surface. [260-451]]
[Illustration: Prometheus.]
[Illustration: Prometheus.]
[Illustration: Prometheus.]
[Illustration: These views are of Loki (P₂) as seen by Voyager 1.
[260-451]]
[Illustration: The asymmetric structure of the plume can be seen.]
[Illustration: Loki.]
[Illustration: An ultraviolet image has been used to produce a
false-color composite; the large ultraviolet halo above the
visible-light plume may be due to scattering from sulfur dioxide gas
rather than solid particles.]
[Illustration: Perhaps the most spectacular of all the Voyager
photos of Io is this mosaic obtained by Voyager 1 on March 5 at a
range of 400 000 kilometers. A great variety of color and albedo is
seen on the surface, now thought to be the result of surface
deposits of various forms of sulfur and sulfur dioxide. The two
great volcanoes Pele and Loki (upper left) are prominent. [260-464]]
In place of impact craters, the surface of Io has a great many volcanic
centers, which generally take the form of black spots a few tens of
kilometers across. In a few cases, high-resolution pictures show the
characteristic shapes associated with volcanic calderas on Earth and
Mars, and, if the other volcanic centers are similar, about 5 percent of
the entire surface of Io is occupied by calderas. These are extremely
black, reflecting less than 5 percent of the sunlight; often they are
surrounded by irregular, diffuse halos nearly as black as the central
spot. The calderas seem more like the Valles caldera in New Mexico,
which is associated with vents that produced large quantities of ash,
than with those of Hawaiian-type shield volcanic mountains.
There is evidence in many of the Voyager photos of extensive surface
flows on Io. These originate in dark volcanic centers and either spread
to fan shapes, typically 100 kilometers across, or else snake out in
long, twisting tentacles. Some of the flows are lighter than the
background and some are darker. Most are red or orange in color, often
outlined by fringes of contrasting albedo.
The equatorial regions of Io are quite flat, with no vertical relief
greater than about 1 kilometer high; indeed, many of the volcanic
centers do not appear to correspond to mountains or domes at all. There
are, however, a number of long, curvilinear cliffs or scarps and narrow,
straight-walled valleys a few hundred meters deep. These appear to be
places in which the crust has broken under tension, somewhat similar to
terrestrial faults and the valleys called graben. A few rugged mountains
of uncertain origin are visible in low Sun elevation pictures.
Near the poles of Io the terrain is more irregular. There are few
volcanic centers, but more mountains, some with heights of several
kilometers. In addition, there are regions that appear to be made of
stacked layers of material. These so-called layered terrains are
revealed when erosion cuts into them, exposing the layers along the
cliff or scarp. The largest such plateau or mesa has an area of about
100 000 square kilometers. The scarps sometimes intersect each other,
suggesting a complex history of deposition, faulting, and erosion.
Voyager geologists believe that these scarps may be areas in which the
release of liquid sulfur or sulfur dioxide has undercut cliffs,
analogous to internal sapping by groundwater at similar scarps on Earth.
Perhaps the most distinctive surface features on Io are the circular or
oval albedo markings that surround the great volcanoes. The first of
these to be seen was the 300-kilometer-wide white donut of Prometheus,
on the equator at longitude 150°. Much more spectacular is the hoofprint
of Pele, about 700 by 1000 kilometers. These symmetric rings mark the
locations of the kinds of eruptions that generate large fountains or
plumes, and may be produced by condensible sulfur or sulfur dioxide
raining down from the volcanic fountain. At least one new ring appeared
during the four months between the Voyager encounters, centered at
longitude 330°, latitude +20°, but by the time Voyager 2 photographed
this area, no plume remained active.
During the Voyager 1 flyby, temperature scans of the surface of Io were
made with the infrared interferometer spectrometer (IRIS). A number of
localized warm regions were found, the most dramatic being just south of
the volcano Loki. Here the images showed a strange, U-shaped black
feature about 200 kilometers across. The IRIS team interpreted its data
to indicate a temperature for the black feature of 17° C (or room
temperature), in contrast to the surrounding surface at -146° C. Perhaps
the dark feature was some sort of lava lake, either of molten rock or
molten sulfur. The melting point of sulfur is 112° C. If there were a
scum of solidifying sulfur on top of the “lake,” this interpretation
might well be the correct one.
The brilliant reds and yellows of the surface of Io immediately suggest
the presence of sulfur. When heated to different temperatures and
suddenly cooled, sulfur can assume many colors, ranging from black
through various shades of red to its normal light-yellow appearance.
Even before Voyager, laboratory studies had shown that sulfur matches
the overall properties of the spectrum of Io, including the low albedo
in the ultraviolet and the high reflectivity throughout the infrared.
Contemporary with the Voyager flybys, additional telescopic observations
and laboratory studies by Fraser Fanale at JPL and Dale Cruikshank at
the University of Hawaii identified another component on Io, sulfur
dioxide. Sulfur dioxide is an acrid gas released from terrestrial
volcanoes, where it combines with water in the Earth’s atmosphere to
produce sulfuric acid. At the temperature of the surface of Io, sulfur
dioxide is a white solid. Researchers guessed that the extensive bright
white areas in the Voyager pictures of Io might be covered with sulfur
dioxide frost or snow. The presence of this material on Io was confirmed
when the infrared IRIS instrument obtained a spectrum of sulfur dioxide
gas over the erupting volcano Loki during the Voyager 1 encounter.
[Illustration: Close-ups of Io reveal a wide variety of volcanic
phenomena. This Voyager 1 view of an equatorial region near
longitude 300° shows several large surface flows that originate in
volcanic craters or calderas. At the right edge is a light flow
about 250 kilometers long. Another dark, lobate flow with bright
edges is just left of center, with an exceedingly dark caldera to
its left. [260-468A]]
The discovery of the ongoing eruptions on Io, made shortly after the
Voyager 1 flyby, did much to clarify the confused evidence pouring in
concerning the apparent youth of Io’s surface. Here, under the very eyes
of Voyager, eruptions were taking place on a scale that dwarfed anything
ever seen before. The discovery picture alone, taken from a distance of
4 million kilometers, showed two eruptions (Pele and Loki), each of
which was much larger than the most violent volcanic eruption ever
recorded on Earth.
Voyager 1 found eight giant eruptions, with fountains or plumes rising
to heights of between 70 and 280 kilometers. To reach these altitudes,
the material must have been ejected from the vents at speeds of between
300 and 1000 meters per second, several times greater than the highest
ejection velocities from terrestrial volcanoes. Although widely spaced
in longitude, these volcanoes were concentrated toward the equator;
seven of the eight were at latitudes between +30° and -30°, and the
eighth at -44°.
When Voyager 2 arrived four months later, it was able to reobserve seven
of the eight volcanoes. (To be identified reliably, the volcanic plumes
must be silhouetted against dark space at the edge of the disk.) Six of
these were still erupting; one, Pele, the largest plume seen by Voyager
1, had ceased activity. The plume associated with Loki had also changed
markedly, increasing in height from 100 to 210 kilometers in visible
light. (All the plumes appear larger when viewed in the ultraviolet.)
Loki had developed a more complex structure; in March it appeared to
originate near the south end of a 250-kilometer-long dark feature, but
in July there was a double plume, with activity at both ends of the dark
feature.
[Illustration: Differences in surface elevation can clearly be seen
in a few of the Io close-ups from Voyager 1. This remarkable picture
is of the center of the great volcano Pele, at latitude 15°S and
longitude 224°. A low mountain with flow features can be seen. In
the background, there are several large irregular depressions with
flat floors that appear to be the result of collapse. The diffuse
dark features in the center are probably the ejecta plumes being
erupted from the Pele vent. [P-21220B/W]]
[Illustration: At the highest resolution obtained by the Voyager
cameras, Io revealed some landscapes that looked familiar to
terrestrial geologists. This picture, taken by Voyager 1 at a range
of only 31 000 kilometers, shows a region about the size of the
state of Maryland at a resolution of 300 meters. Clearly seen is a
volcano not too different from some of those on the Earth or Mars.
At the center is an irregular composite crater or caldera about 50
kilometers in diameter with dark flows radiating from its rim. The
style of volcanism illustrated here is quite different from the
explosive plumes or fountains with their associated rings of bright
material deposited on the surface. This volcano is located at about
longitude 330°, latitude 70°S. [260-502]]
[Illustration: In addition to its giant volcanic plumes or
fountains, Io possesses other indications of current volcanic
activity. One of these takes the form of intermittent blue-white
patches that may be caused by gas venting from the interior. In this
pair of photographs, the same region of the surface is shown about
six hours apart. On the right, there is an arcuate bright gas cloud;
on the left the same region is black. It is believed that the
venting gas is sulfur dioxide, and that the condensation of this gas
produces fine particles of “snow” that look blue. [260-508]]
[Illustration: Some of the most dramatic changes in the surface of
Io between March and July took place in the vicinity of the volcano
Loki, at longitude 310° and latitude 15°N. On the left is a Voyager
1 view; on the right, one from Voyager 2. The “lava lake” associated
with Loki has become less distinct, apparently as a result of
deposits that fell on the northern part of the dark U-shaped
feature. Perhaps the surface had also cooled between these photos.
In the upper left center, a new dark volcanic caldera with bright
spots near it and a large, faint bright ring had appeared by July,
although it was not active at the time Voyager 2 flew by.
[260-687AC]]
No new eruptions were seen by Voyager 2; between them, the two
spacecraft effectively surveyed the whole surface of Io for plumes down
to 40 kilometers height. Interestingly, the smallest plume seen was 70
kilometers high; there appeared to be a real absence of smaller
eruptions.
From the number and size of the observed eruptions, it is possible to
calculate the resurfacing rate for Io due to these plumes. The result is
that each plume is erupting about 10 000 tons of material per second, or
more than 100 billion tons per year. This quantity corresponds to about
10 meters of deposition over the whole surface in a million years. When
additional note is taken of surface flows, the deposition rate could
easily be ten times higher, or 100 meters per million years, in
agreement with the rate estimated from the absence of impact craters.
Energy for the Io Volcanoes
Clearly, something extraordinary is happening to Io to generate the
observed level of volcanism. The primary heat source for the interiors
of the terrestrial planets is the decay of the long-lived radioactive
elements thorium and uranium. But Io would have to be supplied with a
hundred times its quota of these elements to explain the observed
activity.
A way out of this difficulty was provided by a theoretical investigation
carried out by Stanton Peale of the University of California at Santa
Barbara and Pat Cassen and Ray Reynolds at the NASA Ames Research
Laboratory. Working in the months before the first Voyager flyby, they
calculated that the tidal effects of Jupiter on Io could generate
large-scale heating of the satellite. Io is about the same distance from
Jupiter as the Moon from Earth, but the much greater mass of Jupiter
raises enormous tides in its satellite. These tides distort its shape,
but no other effect would be present if Io remained at a constant
distance from Jupiter. What Peale, Cassen, and Reynolds realized was
that the distance of Io from Jupiter varies as the result of small
gravitational perturbations from the other Galilean satellites.
Therefore the tidal distortions also vary, in effect squeezing and
unsqueezing Io each orbit. Such flexing pumps energy into the interior
of Io in the form of heat; theorists calculated that the heat supplied
could be as high as 10¹³ watts. They predicted, in a paper published
just three days before the Voyager flyby of Io, that “widespread and
recurrent surface volcanism might occur,” and that “consequences of a
largely molten interior may be evident in pictures of Io’s surface
returned by Voyager.”
[Illustration: Voyager 2 obtained beautiful views of the volcanic
eruptions during its ten-hour Io volcano watch on July 9. On the
edge of the crescent image are the volcanoes Amirani (P₅) and below
it Maui (P₆), each sending up fountains about 100 kilometers above
the surface. The blue color is probably the result of sunlight
scattered by tiny particles of sulfur dioxide snow condensing in the
erupting plume. [P-21780]]
VOLCANIC ERUPTIONS ON IO
Plume Name Location Height During Activity During
Number (latitude/longitude) Voyager 1 Voyager 2 Flyby
Flyby
(kilometers)
1 Pele -20°/255° 280 ceased
2 Loki +20°/300° 100 increased
3 Prometheus -5°/155° 70 increased
4 Volund +20°/175° 95 no data
5 Amirani +25°/120° 80 similar
6 Maui +20°/120° 80 similar
7 Marduk -25°/210° 120 similar
8 Masubi -40°/ 50° 70 similar
[Illustration: Between the two encounters, the volcanic eruption at
Loki (P₂) changed character. The single plume emanating from the
western end of an apparent dark fissure seen in March was joined by
a second fountain of similar size about 100 kilometers to the east.
The plume also increased in height, from about 120 kilometers in the
Voyager 1 image (left) to 175 kilometers in the Voyager 2 image
(right). [260-662A and B]]
[Illustration: The detailed structure near the volcano Loki is like
nothing seen elsewhere on Io. [260-642B]]
[Illustration: When this Voyager 1 picture was taken, the main
eruptive activity (P₂) came from the lower left of the dark linear
feature (perhaps a rift) in the center. Below it is the “lava lake,”
a U-shaped dark area about 200 kilometers across. In this specially
processed image, detail can be seen in the dark surface of this
feature, possibly due to “icebergs” of solid sulfur in a liquid
sulfur lake.]
[Illustration: (Bottom) The IRIS on Voyager 1 found this “lava lake”
to be the hottest region on Io, with a temperature about 150° C
higher than that of the surrounding area.]
[Illustration: One model for the structure of Io indicates that an
ocean of liquid sulfur with a solid sulfur crust covers most of the
satellite. Heat escapes from the interior in the form of lava, which
erupts beneath the sulfur ocean. Secondary eruptions in the sulfur
ocean heat liquid sulfur dioxide, which is mixed with solid sulfur
in the crust. The rapid expansion of sulfur dioxide gas then
produces the great eruptive plumes, which consist of a mixture of
solid sulfur, sulfur dioxide gas, and sulfur dioxide snow.]
Solid sulfur + solid SO₂
Solid sulfur + liquid SO₂
Molten sulfur
Solid silicate
The Voyager observations appear to confirm the theoretical calculations.
The tidal heat source has presumably been acting since Io was formed
more than 4 billion years ago. With a totally molten interior and
continuing large-scale volcanism, Io has had an opportunity to
thoroughly sort out its composition. In the process it would have lost
all the volatile gases such as water and carbon dioxide, explaining why
Io now has no appreciable atmosphere in spite of the outpouring of
material from the interior. In addition, most of the sulfur from the
interior could have risen to the surface, where it would be constantly
recycled through volcanic activity.
The presence of large amounts of sulfur on the surface may help explain
the extraordinary nature of the Io volcanoes. One model considered for
the satellite postulates that it is covered by a sea of liquid sulfur
several kilometers deep, with a crust of solid sulfur and, below the
surface, liquid sulfur dioxide. Calculations by Sue Kieffer of the U.S.
Geological Survey and others indicate that the expansion of the sulfur
dioxide in such a model can explain the observed eruption velocities of
up to a kilometer per second.
The volcanic plumes on Io appear to be made primarily of sulfur and
sulfur dioxide. Both are molten as they emerge from the vent, but they
quickly cool as the plume rises 100 or more kilometers into the near
vacuum of space. Unlike terrestrial volcanoes, there is almost no gas in
the plumes. It requires about half an hour for the fine particles of
solidified sulfur and sulfur dioxide snow to fall back to the surface,
where they form the colorful rings that mark the major eruptive sites.
Almost all the roughly 100 000 tons of material erupted each second by
the Io volcanoes snows back to the surface. But apparently a
part—perhaps 10 tons per second—escapes from Io and is captured by the
Jovian magnetosphere. Another part contributes to an ionosphere—a
tenuous atmosphere of electrons and ions—that surrounds Io. The
injection of several tons of particles each second into the
magnetosphere has dramatic consequences that can be seen even from
Earth.
[Illustration: Direct evidence of an atmosphere on Io was obtained
during the Voyager 1 flyby by the IRIS. In the region near the
volcano Loki and its associated “lava lake,” infrared spectra
clearly showed the signature of sulfur dioxide gas. It is not known
whether this gas was a temporary feature associated with the
eruption of Loki or if it might be present on Io more generally.
Other evidence, however, points to the sulfur dioxide atmosphere as
a transient feature. A small amount of the sulfur dioxide escapes
and is broken apart by sunlight to provide the oxygen and sulfur
ions observed in the Io torus.]
Sulfur dioxide gas cloud
Plume
-235° F
Hot surface areas (45° F)
The Io Torus
Surrounding Jupiter at the distance of Io is a donut-shaped volume, or
torus, of plasma that originates at the satellite. At first, the atoms
escaping from Io expand outward as a gas, but soon they are stripped of
electrons and become electrically charged. Some of these gases, such as
sulfur dioxide, apparently originate in the large volcanic eruptions;
other, such as the sodium cloud being studied with Earth-based
telescopes, result from sputtering of surface materials by energetic
particles in the magnetosphere. After they are ionized by the loss of
one or more electrons, the atoms are caught by the spinning magnetic
field of Jupiter and become a part of what is called a co-rotating
plasma, spinning at 74 kilometers per second with the same ten-hour
period as Jupiter itself.
The Io torus was easily detected on Voyager by the ultraviolet
spectrometer, even from a distance of 150 million kilometers. The
strongest ultraviolet radiation comes from twice-ionized sulfur (atoms
that have lost two electrons) (S III), emitting a wavelength of 69
nanometers or about one-eighth the wavelength of visible light. The
spectrometer also detected glows from atoms of triply ionized sulfur (S
IV) and twice-ionized oxygen (O III).
[Illustration: At the time of the Voyager 1 encounter, the most
abundant heavy ions in the Jovian magnetosphere were sulfur and
oxygen. Multiply ionized sulfur and oxygen both emit strongly in the
ultraviolet, where they could be observed by the ultraviolet
spectrometer. This spectrum of the Io torus registers the tremendous
amount of ultraviolet energy (about a million million watts) being
radiated. To emit so strongly, the temperatures in the torus must be
near 100 000 K.]
[Illustration: Direct measurement of the heavy ions associated with
the Io torus were made by the Voyager 1 LECP instrument. Here the
amounts of various elements are shown for two cases: the Jovian
inner magnetosphere (solid line) and a typical solar event. Both the
Jovian and the solar particles have been scaled to show similar
amounts of oxygen, but the solar particles are also rich in carbon
and iron, whereas Jupiter has a great deal of sulfur. Evidence such
as this demonstrates that the sulfur does not come from the Sun;
rather, the sulfur and most of the oxygen appear to be the product
of the Io sulfur dioxide volcanic eruptions.]
Scans across the torus showed that it had a thickness of 1.0 R_J and was
centered at a distance of 5.9 R_J from Jupiter. The torus is centered on
the magnetic, rather than the rotational, equator of the planet. To
produce the intense glow observed, the electron temperatures in the
torus must be 100 000 K, with an electron density of about 1000 per
cubic centimeter. The brightness in the ultraviolet corresponds to a
radiated power of more than a million million (10¹²) watts. This
enormous amount of energy must be continuously supplied by the
magnetosphere.
The ultraviolet emissions from the Io torus seen by Voyager were
dramatically different from those seen in 1973 and 1974 with the simpler
ultraviolet instrument on board Pioneers 10 and 11. These changes
correspond to more than a factor of 10 in brightness. As noted by the
Voyager Team, “Because of the remarkable differences we conclude that
the Jupiter-Io environment has changed significantly since December
1973. The observed differences are so spectacularly large that this
conclusion does not depend on a detailed comparison of the two
instruments, their calibrations, or the observing geometry.” The reason
for this change, or the degree to which it reflects a large-scale
variation in the volcanic activity of Io, is one of the major questions
arising from the Voyager mission.
The Io torus can also be observed from the ground. In 1976, spectra
showed the glow of singly ionized sulfur (S II), and in 1979 singly
ionized oxygen (O II) was detected. The observations of S II and O II
are particularly interesting because they provide a measure of the
density and temperature of the plasma. In April 1979, between the two
Voyager flybys, Carl Pilcher of the University of Hawaii succeeded in
obtaining a telescopic image of the torus in the light of S II. He
measured nearly the same ring diameter (5.3-5.7 R_J) as had Voyager;
interestingly, both agree that the sulfur torus is centered slightly
inside the orbit of Io (6.0 R_J).
[Illustration: The emission of light from sulfur ions in the Io
torus is so strong it can be measured from the Earth. In these
pictures, University of Hawaii astronomer Carl Pilcher photographed
the torus in the light of ionized sulfur on the night of April 9,
1979. As the planet rotates, the torus is seen first partly opened,
then edge-on, and again opened in the opposite direction. The dark
band on the right of each image is due to light from Jupiter
scattered in the telescope, as shown in the bottom picture, which
contains the scattered light only.]
Direct measurements of the torus were made from Voyager 1 as the
spacecraft passed twice through this region, once inbound and once
outbound. The low energy charged particle instrument and the cosmic ray
instrument both determined that the composition of the ions in the Io
torus was primarily sulfur and oxygen. Ionized sodium was also observed.
For several years, ground-based telescope observations had revealed a
cloud of neutral sodium around Io; the Voyager instruments picked up
these atoms after they had each lost an electron and become trapped in
the magnetosphere. These instruments also derived the electron and ion
density (about 1000 per cubic centimeter) and confirmed that the ions
were co-rotating with the inner magnetosphere.
Another Io-associated phenomenon searched for by Voyager was the Io flux
tube. As a conductor moving through the Jovian magnetic field, Io
generates an electric current, estimated to have a strength of about 10
million (10⁷) amperes and a power of the order of a million million
(10¹²) watts. The region of space through which this current flows from
the satellite to Jupiter is called the flux tube.
Voyager 1 was targeted to fly through the Io flux tube. This was an
important decision, since this option precluded the possibility of
obtaining occultations by either Io or Ganymede. The event was to take
place on March 5, just after closest approach to Io. The effects of the
flux tube were clearly observed by the magnetometer, the LECP
instrument, and other particle and field instruments; however,
subsequent analysis indicated that the spacecraft had not penetrated the
region of maximum current flow; it probably missed the center of the
flux tube between 5000 and 10 000 kilometers.
The flux tube is not the only connection between Io and Jupiter. Radio
emissions from the atmosphere are triggered by Io’s orbital position,
and the aurorae that illuminate Jupiter’s polar regions are the result
of charged particles falling into the planet from the Io torus. Other
charged particles can occasionally escape outward and be detected as far
away as Earth.
Io is unquestionably a remarkable world. The only planetary body known
to be geologically more active than the Earth, it provides many extreme
examples to test the theories of geoscientists. Its intimate
interconnections with the Jovian magnetosphere and the planet itself
provide a unifying theme to the complex processes taking place in the
inner parts of the Jovian system.
[Illustration: The Galileo Probe will make a fiery entry into the
Jovian atmosphere, carrying a payload of scientific instruments for
the first direct sampling of the atmosphere of a giant planet. Shown
here is the moment, at a pressure level of about 0.1 bar, when the
parachute is deployed and the still-glowing heatshield drops free
from the Probe.]