JUPITER—KING OF THE PLANETS
A Star That Failed
More massive than all the other planets combined, Jupiter dominates the
planetary system. The giant revealed by Voyager is a gas planet of great
complexity; its atmosphere is in constant motion, driven by heat
escaping from a glowing interior as well as by sunlight absorbed from
above. Energetic atomic particles stream around it, caught in a magnetic
field that reaches out nearly 10 million kilometers into the surrounding
space, embracing the seven inner satellites. From its deep interior
through its seething clouds out to its pulsating magnetosphere, Jupiter
is a place where forces of incredible energy contend.
At its birth, Jupiter shone like a star. The energy released by
infalling material from the solar nebula heated its interior, and the
larger it grew the hotter it became. Theorists calculate that when the
nebular material was finally exhausted, Jupiter had a diameter more than
ten times its present one, a central temperature of about 50 000 K, and
a luminosity about one percent as great as that of the Sun today.
At this early stage, Jupiter rivaled the Sun. Had it been perhaps 70
times more massive than it was, it would have continued to contract and
increase in temperature, until self-sustaining nuclear reactions could
ignite in its interior. If this had happened, the Sun would have been a
double star, and the Earth and the other planets might not have formed.
However, Jupiter did not make it as a star; after a brief flash of
glory, it began to cool.
At first Jupiter continued to collapse. Within the first ten million
years of its life, the planet was reduced to nearly its present size,
with only a few percent additional shrinkage during the past 4.5 billion
years. The luminosity also dropped as internal heat was carried to the
surface by convection and radiated away to space. After a million years
Jupiter emitted only one-hundred thousandth as much radiation as the
Sun, and today its luminosity is only one-ten billionth of the Sun’s.
Jupiter’s internal energy, although small by stellar standards, has
important effects on the planet. About 10¹⁷ watts of power, comparable
to that received by Jupiter from the Sun, reach the surface from the
still-luminous interior. The central temperature is still thought to be
about 30 000 K, sufficient to maintain the interior in a molten state.
Scientists generally agree that Jupiter is an entirely fluid planet,
with no solid core whatever.
Composition and Atmospheric Structure
Because of its great mass, Jupiter has been undiscriminating in its
composition. All gases and solids available in the early solar nebula
were attracted and held by its powerful gravity. Thus it is expected
that Jupiter has the same basic composition as the Sun, with both bodies
preserving a sample of the original cosmic material from which the solar
system formed.
[Illustration: Jupiter is a gas giant, composed of the same elements
as the Sun and stars—primarily hydrogen and helium. Its internal
structure is dominated by the properties of hydrogen, its most
abundant constituent and by the high temperatures in the deep
interior that remain from its luminous youth. Most of the interior
is liquid: metallic hydrogen at great depths and high pressures, and
normal hydrogen nearer the surface. In the upper few thousand
kilometers, the hydrogen is a gas. The primary known or suspected
cloud layers are, from the top down, thin hydrocarbon “smog”;
ammonia; ammonium hydrosulfide; water-ice, and liquid water.
[260-828]]
Cloud top-aerosols
Ammonia crystals
Ammonium hydrosulfide clouds
Ice crystal clouds
Water droplets
Trace compounds
Fluid molecular hydrogen
Transition Zone
Fluid metallic hydrogen
Possible core
The primary constituents of Jupiter have long been suspected to be
hydrogen and helium, the two simplest and lightest atoms. However, it
has proved impossible to derive accurate measurements of the abundance
of these two elements from astronomical observations. On the basis of a
rather simple infrared measurement, Pioneer investigators found He/H₂ =
0.14 ± 0.08. On Voyager, IRIS was able to obtain much improved infrared
spectra, yielding an initial value of He/H₂ = 0.11 ± 0.3. Voyager
scientists expect that further analysis will reduce the uncertainty to
about ± 0.01. The ratio of 0.11 is in excellent agreement with the solar
value of about 0.12, supporting the idea that Jupiter and the Sun have
similar elemental compositions.
Astronomers have known for a long time that, in addition to hydrogen and
helium, the compounds methane (CH₄) and ammonia (NH₃) are present in the
visible atmosphere of Jupiter. In the 1970s, additional spectra in the
infrared resulted in the discovery of water (H₂O), ethane (C₂H₆),
germane (GeH₄), acetylene (C₂H₂), phosphine (PH₃), carbon monoxide (CO),
hydrogen cyanide (HCN), and carbon dioxide (CO₂). All these are trace
constituents, with two of them, ethane and acetylene, apparently formed
at high altitudes by the action of sunlight on methane.
A total of approximately 100 000 infrared spectra, many of small regions
on the disk, were obtained by IRIS. These spectra generally show
hydrogen, helium, methane, ammonia, phosphine, ethane, and acetylene. In
addition, excellent spectra were obtained in “hot spots,” regions in
which breaks in the upper clouds permit radiation from deeper layers to
escape. (The hot spots generally correspond to dark brown regions on
photographs of the planet.) IRIS measured temperatures in the hot spots
up to -13° C but no higher; apparently this temperature corresponds to
the top of a deeper cloud deck. Spectral features indicative of the
presence of water vapor and germane were clearly seen in the hot spots.
Further analysis of the IRIS spectra will be required to derive the
abundances of the gases detected. However, even the preliminary data
showed how variable Jupiter can be, especially in its upper atmosphere.
The two hydrocarbons, ethane and acetylene, vary in relative abundance
with latitude; there is less acetylene near the poles. In addition to
this planetwide trend, smaller variations were seen from place to place
and between the observations in March and July. All the variations will
eventually provide information on the processes of formation,
transportation, and destruction of hydrocarbons in the upper atmosphere.
ELEMENTS DETECTED IN THE JOVIAN MAGNETOSPHERE
Element Atomic Instruments
Number
Hydrogen (H) 1 UVS, Plasma, LECP, CRS
Helium (He) 2 Plasma, LECP
Carbon (C) 6 LECP
Nitrogen (N) 7 CRS
Oxygen (O) 8 UVS, Plasma, LECP, CRS
Neon (Ne) 10 CRS
Sodium (Na) 11 LECP, CRS
Magnesium (Mg) 12 CRS
Silicon (Si) 14 CRS
Sulfur (S) 16 UVS, Plasma, LECP, CRS
Iron (Fe) 26 CRS
Voyager did not make any direct measurements of the chemical composition
of the clouds, but theorists generally agree that the uppermost clouds
are ammonia cirrus, and that layers of ammonium hydrosulfide (NH₄SH) and
water exist at deeper levels. All these clouds are formed in the
troposphere, the layer of the atmosphere in which convection takes
place. The top of the ammonia cloud deck is thought to have a pressure
of about 1 atmosphere and a temperature of about -113° C.
Ammonia cirrus is white, yet Jupiter’s clouds display a spectacular
range of colors. Voyager did not determine the nature of the coloring
agents; they may be minor constituents—trace impurities in a sea of
white clouds. Perhaps organic polymers, formed from atmospheric
chemicals such as methane and ammonia that have reacted with lightning,
are responsible for the oranges and yellows. The color of the Red Spot
could be caused by red phosphorus (P₄). According to this theory,
phosphine (PH₃) from deep in Jupiter’s atmosphere is brought to high
altitudes by the upwelling of the Great Red Spot. Ultraviolet light,
penetrating the upper reaches of the Red Spot, splits the phosphine
molecules, and, through a series of chemical reactions, converts the
phosphine into pure phosphorus. However, this theory fails to explain
the existence of the smaller red spots on Jupiter; these spots are not
at such high altitudes as the Great Red Spot (which is the highest and
coldest of Jupiter’s visible clouds), so it is unlikely that ultraviolet
light could react with any phosphine in these areas to produce red
phosphorus.
[Illustration: Although the Voyager spacecraft never flew over the
poles of Jupiter, it is possible to reconstruct from several images
the View that would be seen from directly above or below the planet.
Note the absence of a strong banded structure near both poles. The
regular spacing of cloud features is obvious. In the Southern
hemisphere, the three white ovals are 90 degrees apart in longitude,
but a fourth oval at the other quadrant is missing. The irregular
black areas at each pole are places for which no Voyager data exist.
The resolution of the original pictures from which these polar
projections were made was about 600 kilometers.]
[Illustration: North pole. [P-21638C]]
[Illustration: South pole. [P-21639C]]
Various forms of elemental sulfur might be responsible for the riot of
color we see on Jupiter. Sulfur forms polymers (S₃, S₄, S₅, S₈,) that
are yellow, red, and brown, but no sulfur in any form has been detected
on Jupiter. “We never promised you we were going to identify the colors
on Jupiter with this mission,” one of the atmospheric scientists
remarked, “but we will have a probe that is going into the atmosphere in
the mid-1980s—Galileo.” Perhaps the mystery of the Jovian clouds will
have to wait till then.
Temperature maps of Jupiter were obtained by IRIS in radiation arising
at different levels above the clouds. Maps show temperatures at
pressures of 0.8 atmosphere near the clouds, and 0.2 atmosphere near the
top of the troposphere. In addition to the low temperatures over the
bright zones and the higher temperatures over dark belts, there is a
great deal of smaller scale structure. It is interesting that a cold
area corresponding to the Great Red Spot is clearly visible even near
the top of the troposphere, indicating that this feature disturbs the
atmosphere to very high altitudes.
The structure of the atmosphere of Jupiter above the troposphere was
investigated through the radio occultation experiment as well as by
IRIS. The level in which the minimum temperature of about -173° C occurs
has a pressure of 0.1 atmosphere. Above this point lies the
stratosphere, in which temperatures increase with altitude as a result
of sunlight absorbed by the gas or by aerosol particles resembling smog.
At 70 kilometers above the ammonia clouds, the temperature is about
-113° C. Above this level, the temperature stays approximately constant,
although at extreme altitudes the temperature again rises in the
ionosphere.
[Illustration: If one could “unwrap” Jupiter like a map, views such
as these would be obtained. The comparison between the pictures
shows the relative motions of features in Jupiter’s atmosphere. It
can be seen, for example, that the Great Red Spot moved westward and
the white ovals eastward during the time between the acquisition of
these pictures. Regular plume patterns are equidistant around the
northern edge of the equator, while a train of small spots moved
eastward at approximately latitude 80° S. In addition to these
relative motions, significant changes are evident in the
recirculating flow east of the Great Red Spot, in the disturbed
region west of the Great Red Spot, and as seen in the brightening of
material spreading into the equatorial region from the more
southerly latitudes. [P-21771C]]
[Illustration: The planet as it appeared about March 1.]
[Illustration: As it was in early July.]
Weather on Jupiter
The Voyager pictures reveal a planet of complex atmospheric motions.
Spots chase after each other, meet, whirl around, mingle, and then split
up again; filamentary structures curl into spirals that open outward;
feathery cloud systems reach out toward neighboring regions; cumulus
clouds that look like ostrich plumes may brighten suddenly as they float
toward the east; spots stream around the Red Spot or get caught up in
its vortical motion—all in an incredible interplay of color, texture,
and eastward and westward flows. Such changes can be noticed in the
space of only a few Jovian days.
[Illustration: Differing characteristics of Jupiter’s meteorology
are apparent in high-resolution images, such as this one taken by
Voyager 1 on March 2 at a range of 4 million kilometers. The
well-defined pale orange line running from southwest to northeast
(north is at the top) marks the high-speed north temperate current
with wind speeds of about 120 meters per second. Toward the top of
the picture, a weaker jet of approximately 30 meters per second is
characterized by wave patterns and cloud features which have been
observed to rotate in a clockwise manner at these latitudes of about
35°N. These clouds have been observed to have lifetimes of one to
two years. [P-21193C]]
On a broader time scale, greater changes on the face of Jupiter can be
seen. Features drift around the planet; even the large white ovals and
the Great Red Spot slide along in their respective latitudes. Belts or
zones intrude upon each other, resulting in one of the banded structures
splitting up or seeming to squeeze together and eventually disappear.
Small structures form, then die. The largest spots may slowly shrink in
size, and the Red Spot itself changes its size and color.
The Jupiter of Pioneers 10 and 11 was quite unlike the planet seen by
Voyager 1. At the time of the Pioneer exploration, the Great Red Spot,
embedded in a huge white zone, was more uniformly colored, and pale
brown bands circled the northern hemisphere. In the intervening years,
the south temperate latitudes have changed completely, developing the
complex turbulent clouds seen around the Red Spot by Voyager 1. Yet,
even between the two Voyagers, Jupiter appeared to be undergoing a
dynamic “facelift.” At a quick glance, Voyager 2 photographs showed the
visage that had been familiar since early in 1979, but a closer look
showed that it is not quite the same. The white band below the Great Red
Spot, fairly broad during the first flyby, had become a thin white
ribbon where it rims the southern edge of the Spot. The turbulence to
the west of the Red Spot had stretched out and become “blander” than it
was before. Small rotating clouds seemed to be forming out of the waves
in this region. The cloud structure that had been east of the Red Spot
during the Voyager 1 flyby spread out, covering the northern boundary
and preventing small clouds from circling the huge red oval. The Red
Spot itself also changed. Its northern boundary seemed—at least
visually—to be more set off from the clouds that surround it, and the
feature appeared to be more uniform in color, perhaps reverting back to
the personality it had in Pioneer days.
[Illustration: Jupiter’s cloud patterns changed significantly in the
few months between the two Voyager flybys. Most of the changes are
the result of differential rotation, in which the prevailing winds
at different latitudes shift long-lived features with respect to
those north or south. Thus, for example, the three large white ovals
shifted nearly 90 degrees in longitude, relative to the Great Red
Spot, between March and July. [P-21599]]
[Illustration: Cloud patterns from Voyager 2.]
The most obvious features in the atmosphere of Jupiter, after the banded
belts and zones, are the Great Red Spot and the three white ovals. These
have often been described as “storms” in Jupiter’s atmosphere. The ovals
are about the size of the Moon, and the Red Spot is larger than the
Earth. Voyager has revealed that in many respects the white ovals, which
formed in 1939, resemble their ancient red relative. All four spots are
southern hemispheric anticyclonic features that exhibit counterclockwise
motion; hence they are meteorologically similar. Other smaller bright
elliptical and circular spots also exhibit anticyclonic motion, rotating
clockwise in the northern hemisphere and counterclockwise in the
southern hemisphere. In general, these features are circled by
filamentary rings that are darker than the spots they surround. Hints of
interior spiral structure can be seen in some of these spots. All the
elliptical features in the southern hemisphere lie to the south of the
strong westward-blowing jet streams. The spots tend to become rounder
the closer they are to the poles.
Along the northern edge of the equator are a number of cloud plumes,
which appear to be regularly spaced all around the planet. Some of the
plumes have been observed to brighten rapidly, which may be an
indication of convective activity; indeed, some of the plume structures
seem to resemble the convective storms that form in the Earth’s tropics.
The plumes travel eastward at speeds ranging from about 100 to 150
meters per second, but they do not move as a unit.
[Illustration: The Great Red Spot of Jupiter is a magnificent sight,
whether viewed in normal or exaggerated color. These pictures were
taken by Voyager 1 at a range of about 1 million kilometers; the
area shown is about 25 000 kilometers, with features visible on the
originals that are as small as 30 kilometers across. The Red Spot is
partly obscured on the north by a thin layer of overlying ammonia
cirrus cloud. South of the Red Spot is one of the three white ovals,
which are also anticyclonic vortices in the atmosphere.]
[Illustration: This frame is in natural color. [P-21430C]]
[Illustration: The red and blue have been greatly exaggerated in
this frame to bring out fine detail in the cloud structure.
[P-21431C]]
The most visible cloud interactions take place in the region of the
Great Red Spot. Material within the Red Spot rotates about once every
six days. Infrared measurements show that the Red Spot is a region of
atmospheric upwelling, which extends to very high altitudes; however,
the divergent flow suggested by this upwelling seems to be very
small—one bright feature was observed to circle the Red Spot for sixty
days without appreciably changing its distance from the spot’s center.
During the Voyager 1 flyby, spots were seen to move toward the Red Spot
from the east, flow along its northern border, then either flow on to
the west past the Red Spot or into the outer regions of its vortex. A
spot caught on the outer edge of the Red Spot flow might break in two as
it reached the eastern edge of the spot, with one piece remaining in the
vortex and the other moving off to the east. Alternatively, a spot
floating toward the Red Spot from the east might be pushed northward to
join the eastward current flowing north of the giant red oval.
By the time of the second Voyager encounter, a ribbon of white clouds
curled around the northern border of the Red Spot, blocking the motion
of small spots that might otherwise have been caught up in the vortex.
Spots approaching the Red Spot from the east just turned around and
headed back in the direction from which they had come.
[Illustration: Voyager 2 captured the Red Spot region four months
after Voyager 1, when some changes had taken place in the cloud
circulation pattern around it. This is a mosaic of Voyager 2 frames,
taken on July 6. The white oval to the south is not the same one
that was present in a similar location during the Voyager 1 flyby,
because of differential rotation at the two latitudes. [260-606]]
In the northern hemisphere, small brown anticyclonic features speed
around the planet, often colliding with one another. On collision, the
spots may combine and roll around together for a while. Ultimately, part
of the mass of the combined spots is ejected as a streamer, and the
remaining material continues on its eastward path.
Order out of Chaos
Despite all the turbulence in Jupiter’s atmosphere—this ever-changing
chaotic mixture of cyclonic and anticyclonic flows, of ovals and
filaments, of reds, browns, and whites—a pattern may be emerging: There
is an underlying order to the seemingly random mixing of patterns we see
in the Jovian atmosphere.
First, the changing weather patterns are in some sense cyclic. The fact
that Jupiter may be reverting to the appearance it presented at the time
of Pioneers 10 and 11 is not surprising. From Earth-based studies,
astronomers have found that the face of Jupiter often goes through a
major change every few years. The transition is very rapid, but the
planet maintains its new “look” for some time—until the next major
transition. At the time of the Pioneer flybys, “The Red Spot was
prominent, but it was surrounded by intense cloud. There was no visible
structure at all in the south tropical zone—it was totally bland. You
couldn’t see the turbulent area to the west,” explained Garry Hunt.
“And, I believe that the buildup of cloud that we’re seeing to the east
of the Red Spot is the beginning of the transition that will produce the
Pioneer look.”
There is even more order underlying Jupiter’s changing atmosphere. This
order is revealed in part in the alternating belts and zones. It is
believed that the cloud-covered zones are regions of rising air, and the
belts are regions of descending air. The internal energy of Jupiter
provides the power to maintain this pattern of slow vertical
circulation. In addition, there are horizontal or zonal flows that are
much more regular than the changing cloud patterns.
[Illustration: The three large white ovals are the longest-lived
features in Jupiter’s atmosphere, after the Great Red Spot. Like the
Red Spot, they are anticyclonic, or high-pressure, regions. During
Voyager 2 encounter, one of the ovals was just south of the Red
Spot. This picture shows the other two ovals as they looked in early
July. The clouds show very similar internal structures. To the east
of each of them, recirculating currents are clearly seen.
[P-21754C]]
[Illustration: In this frame, a similar structure is seen to the
west of the cloud.]
[Illustration: A sequence of pictures of Jupiter, taken once per
rotation (about ten hours), can be used to construct a time-lapse
movie of the circulation of the Jovian atmosphere. These frames are
from the Voyager 1 “Blue Movie” of the Great Red Spot region. Every
odd Jovian rotation is shown, so the 24 frames correspond to 48
Jupiter days, or about 20 Earth days. White spots can be seen
entering the Red Spot from the upper right and being carried around
by its six-day rotation until they are ejected toward the lower
right. Above the Red Spot, the flow is toward the right; below it,
toward the left. The rotation of the Spot is counterclockwise, or
anticyclonic. [260-449]]
[Illustration: At different latitudes, the strong prevailing zonal
winds produce different apparent rotation rates. Plotted here are
the horizontal velocities measured from a pair of Voyager images
taken one rotation (about ten hours) apart. Also shown for
comparison are older ground-based measurements obtained from careful
timings of the apparent rotation rate at different latitudes. The
excellent agreement of the two plots indicates the stability of the
zonal winds. Also, the wind pattern shows much greater symmetry
between northern and southern hemispheres than do the more
superficial cloud patterns.]
“At first, Voyager seemed to do nothing but emphasize the chaos, not the
order in Jupiter’s atmosphere,” Andy Ingersoll stated. “There are
turbulent regions in which individual little spots seem to change every
Jovian rotation. And the whole texture in certain turbulent regions is
unrecognizable in one earth day.” With the Voyager spacecraft, more
detail could be seen than ever before; in addition, changes could be
observed on a small timescale, as they happened. “It became much more of
a mystery how large-scale order could exist in the face of all this
small-scale chaos. But I think we are beginning to see the order
underneath. What we are looking at when we observe Jupiter are minute
cloud particles representing only a small fraction of the mass of the
atmosphere.” The large-scale order the scientists had found was a
regular alternation of eastward and westward jets. “If we take all the
measurements from Earth-based observations over the last 75 years, we
find that every current that has ever been seen from the Earth over 75
years is visible in one ten-hour rotation. They’re all there—they were
just invisible.” The ever-changing appearance was dancing above a
regular, almost stationary pattern of alternating flows, which may come
from deep within Jupiter’s atmosphere. Why this alternating pattern
persists remains a mystery. Even if the underlying pattern can be
thought of as a sort of Jovian climate, it still does not explain the
mechanics of “the minor sideshow”—the changing weather patterns.
Analysis of the Voyager pictures is sure to keep planetary
meteorologists busy for many years to come.
Lights in the Night Sky
Toward the end of the first encounter period, Voyager 1 flew behind
Jupiter, and the spacecraft’s wide-angle camera scanned the northern
hemisphere on the nightside of the planet, searching for aurorae and
lightning bolts. The most impressive darkside feature found was a
tremendous aurora in the north polar region. But this was not the first
time Jovian aurorae had been detected. Very-high-energy auroral
emissions resulting from ultraviolet glows of atomic and molecular
hydrogen had been detected prior to encounter on the bright side of
Jupiter by the ultraviolet spectrometer. The ultraviolet observations
indicate that atmospheric temperatures in the auroral regions are at
least 1000 K. In both the visible and the ultraviolet spectra, the
aurorae are confined to the polar regions and result from charged
magnetospheric particles striking the upper atmosphere. The ultraviolet
aurorae are created when high-energy particles from the Io plasma torus
spiral in toward Jupiter on magnetic field lines.
Several meteor trails were also evident in the darkside pictures of
Jupiter’s atmosphere. Traveling at roughly 60 kilometers per second as
they entered, these fireballs brightened quickly and seemed to survive
for about 1000 kilometers before they died.
Clusters of lightning bolts—indicative of electrical storms—were also
discovered on Jupiter’s nightside. This particular phenomenon does not
seem to depend on latitude. The Voyager 1 photograph that captured the
huge Jovian aurora also caught the electrical discharges of 19
superbolts of lightning, and Voyager 2 photographs located eight
additional flashes. Radio emission (whistlers) from lightning discharges
were also detected by the Voyager radio astronomy receivers and the
plasma wave instrument.
[Illustration: The night side of Jupiter is not dull. A large aurora
(northern light) arcs across the northern horizon, while farther
south about twenty large bolts of lightning illuminate electrical
storms in the clouds. Similar pictures also revealed fireballs, or
large meteors, burning up in the atmosphere of Jupiter.
[P-21283B/W]]
Magnetic Field
Deep in the interior of Jupiter, the pressures are so great that
hydrogen becomes an electrical conductor, like a metal. Currents driven
by the rapid rotation of the planet are thought to flow in this metallic
core. The result is a magnetic field that penetrates the space around
Jupiter.
Direct measurements of the Jovian magnetic field were first made by the
Pioneers, and Voyager results generally confirm the initial findings.
The strength of the Jovian field is about 4000 times greater than that
of the Earth. The dipolar axis is not at the center of Jupiter, but
offset by about 10 000 kilometers and tipped by 11 degrees from the axis
of rotation. Each time the planet spins, the field wobbles up and down,
carrying with it the trapped plasma of the radiation belts. The Voyager
particles and fields instruments concentrated not on the planetary
magnetic field but on the processes taking place in the magnetosphere.
[Illustration: A probe of the Jovian atmosphere is obtained each
time a spacecraft passes behind the planet as seen from the Earth.
Passage through the ionosphere and atmosphere alters the phase of
the radio telemetry signal, and subsequent computer analysis allows
members of the Radio Science Team to reconstruct the profile of the
atmosphere. Shown here is the atmospheric temperature as a function
of pressure as derived from the Voyager 1 X-band occultation data,
corresponding to a point at latitude 12°S, longitude 63°. The two
curves represent extreme interpretations of the same data; the best
fit lies somewhere between. Accuracy is high at greater depths but
poor at levels above a pressure of about 0.03 bar. Clearly shown is
the temperature minimum near 0.1 bar and the steady increase of
temperature with depth as the radio beam probed toward the cloud
level near 1.0 bar.]
[Illustration: At a wavelength near 5 micrometers, the primary gases
in the atmosphere of Jupiter are particularly transparent, and the
infrared radiation from the planet comes from relatively great
depths. At these depths, it is possible to see evidence of gases
such as water vapor that condense at higher altitudes where the
temperatures are lower. This IRIS spectrum in the 5-micrometer
spectral region shows features identified with water (H₂O), germane
(GeH₄), and deuterated methane (CH₃D), as well as the more easily
detected ammonia (NH₃).]
Wavelength (µm)
H₂O
CH₃D
GeH₄
NH₃
Voyager 1
Voyager 2
Wave number (cm⁻¹)
Voyager 1 brightness temperature (K)
Voyager 2 brightness temperature (K)
[Illustration: The structure of the Jovian atmosphere can be derived
from infrared spectra as well as from the radio occultation data.
This profile of temperature as a function of pressure covers the
same range of altitudes as does the preceding figure and is in good
agreement. Both profiles locate a temperature minimum of about 110 K
near a pressure of 0.1 bar (1000 mb = 1 bar = 1 atm). The IRIS data
also show variation of structure with position, including a cooler
minimum temperature (about 100 K) over the Great Red Spot.]
[Illustration: The structure of the atmosphere can be inferred from
IRIS spectra at many locations over the disk of Jupiter. Scientists
are beginning to assemble this vast amount of information into maps
that show the temperatures at a given pressure. The temperature
contours are labeled in degrees Kelvin. The banded structure, with
higher temperatures near the dark equatorial belt, is most clearly
evident at the lower altitude. Surprisingly, the cool region
associated with the Great Red Spot (latitude 23°S) is more apparent
at high altitude.]
[Illustration: The observed temperature at a depth near the cloud
tops (0.8 bar).]
[Illustration: The observed temperature at an altitude about 30
kilometers higher (0.15 bar).]
The Magnetosphere
Giant Jupiter has an enormous realm—from the size of its satellite
system to its tremendous aurorae and superbolts of lightning, to the
huge planet-sized cloud features that surround its atmosphere. The most
gargantuan Jovian feature is its magnetosphere, which envelopes the
satellites and constantly changes in size, pumping in and out at the
whim of the solar wind. The Pioneer and Voyager spacecraft provided four
cuts through this dynamic region, showing that its borders in the upwind
solar direction lie between 50 R_J and 100 R_J from Jupiter. Downwind,
away from the Sun, the magnetosphere extends much farther; some
scientists postulate that a magnetotail may reach as far as the orbit of
Saturn.
Charged particles in the magnetosphere are subject to powerful forces.
Tightly embedded in Jupiter, the magnetic field spins with a ten-hour
period as the planet rotates. The particles are caught in the spinning
field and accelerated to high speeds. The result is a co-rotating plasma
in the magnetic equator of Jupiter, extending outward to at least 20
R_J. Beyond this distance, the flow breaks up and the magnetosphere is
more unstable. Within the co-rotation region, the spinning plasma sets
up a powerful electric current girdling the planet.
Charged particles can be accelerated in the magnetosphere to high
energies, corresponding to speeds tens of thousands of kilometers per
second. Some of these particle streams escape from the inner parts of
the magnetosphere and can penetrate the magnetopause and be ejected from
the Jovian system. On Voyager 1, the low energy charged particle
instrument began detecting these streams of “hot” plasma on 22 January,
when Voyager 1 was still 600 R_J (almost 50 million kilometers) from the
planet. Voyager 2 first detected Jovian particles at an even greater
distance, 800 R_J. Hydrogen and helium ions (protons and alpha
particles) dominate the magnetosphere at great distances from Jupiter,
but increasing amounts of sulfur and oxygen appeared as the spacecraft
crossed the magnetopause. The heavier ions presumably originate from Io.
In the inner magnetosphere, the Galilean satellites have a powerful
influence on the populations of fast-moving particles. During the
Voyager 1 encounter, the primary effect was observed at Io, where the
satellite apparently sweeps up energetic electrons. In the million-volt
energy range, these particles are depleted near Io, with peaks observed
both inside and outside the satellite’s orbit. Voyager 2 passed close to
Ganymede, and here also major effects were seen, with the satellite
apparently absorbing electrons. In the wake created by the motion of the
co-rotating plasma past Ganymede, the particle populations showed large
and complex variations.
[Illustration: The magnetosphere of Jupiter can be “seen” from Earth
by its emissions at radio wavelengths. The recent development of
imaging radio telescopes in Great Britain, The Netherlands, and the
United States allows frequent mapping of the large-scale features of
the innermost magnetosphere, inside the orbit of Io. These six
images showing one rotation of Jupiter were obtained near the time
of the Voyager 1 flyby by Imke de Pater at the Westerbork Radio
Observatory near Leiden. The brightest emission is shown by dark red
or black. The size of the planet is indicated by a dotted white
circle. The tilt of the magnetosphere relative to the rotation axis
of the planet can be seen by the wobble of the magnetosphere as
Jupiter rotates.]
Just inside the Jovian magnetosphere is the “hot spot” of the solar
system: a 300-400 million degree plasma detected by Voyager 1 while it
was still about 5 million kilometers from Jupiter. T. P. Armstrong
commented, “Even the interior of the Sun is estimated to be less than 20
million degrees.” S. M. Krimigis added that the temperature of this
plasma is “the highest yet measured anywhere in the solar system.”
Fortunately for Voyager, this region of incredibly hot plasma is also
one of the solar system’s best vacuums. The spacecraft was in little
danger because the bow shock protects this region from the solar wind,
and most of the particles in Jupiter’s magnetosphere are held in much
closer to the planet.
[Illustration: Each Voyager passed through the boundaries of the
magnetosphere—the bow shock (BS) and the magnetopause (MP)—on both
the inbound and the outbound legs of its passage through the Jovian
system. In this diagram, the heavy solid line represents the
spacecraft trajectory, as seen looking down from the north. Also
shown are the positions of the bow shock in March and of the
magnetosphere in both March and July.]
[Illustration: Several regions of plasma (charged particles) make up
the Jovian magnetosphere. These sketches, based on Voyager data,
show the magnetosphere as viewed from above (a) and as seen from the
Jovian equatorial plane (b). Most of the plasma co-rotates with the
planet and is confined near the magnetic equator, where it forms a
broad plasma sheet about 100 R_J across.]
The very hot plasma in the outer magnetosphere discovered by Voyager is
thought to play an important role in establishing the size of the Jovian
magnetosphere. Although the density is low, only about one charged
particle per hundred cubic centimeters, this plasma actually carries a
great deal of energy because of the high speed of the particles. It is
this plasma pressure, rather than the magnetic field pressure, that
appears to hold off the pressure of the solar wind. However, the balance
between hot plasma inside the magnetopause and the solar wind outside is
not very stable. The Voyager experimenters suggest that a small change
in solar wind pressure can cause the boundary to become suddenly
unstable. A large quantity of the hot plasma can then be lost, producing
the bursts seen at large distances and permitting a sudden collapse of
the outer magnetosphere. Continued injection of hot plasma from within
would then reinflate the magnetosphere, which would expand like a
balloon until another instability developed. Processes of this sort may
be the cause of the rapidly varying magnetospheric boundaries observed
by both Voyager spacecraft.
[Illustration: The rings of Jupiter are best seen when looking
nearly in the direction of the Sun, since the small particles that
comprise them are good forward scatterers of sunlight. This mosaic
is of Voyager 2 images (two wide angle and four narrow angle)
obtained from a perspective behind the planet and inside the shadow
of Jupiter. The spacecraft was 2 degrees below the equator of
Jupiter and 1.5 million kilometers from the rings. The shadow of the
planet can be seen to obscure the near segment of the ring near the
edge of the planet. The brightest region of the ring is about 1.8
R_J from the center of Jupiter. [260-678B]]
Rings of Jupiter
One of the spectacular discoveries of Voyager was the existence of a
ring system of small particles circling Jupiter. Saturn and Uranus were
known to have rings, but none had been seen before at Jupiter.
As revealed by the Voyager cameras, the rings extend outward from the
upper atmosphere to a distance of 53 000 kilometers above the cloud
tops, 1.8 R_J from the center of the planet. The main rings, however,
are much narrower, spanning from 47 000 to 53 000 kilometers above
Jupiter. There are two main rings, a 5000-kilometer-wide segment, and a
brighter, outer 800-kilometer segment. The thickness of the rings is
unknown, except that it is certainly less than 30 kilometers, and
probably under 1 kilometer.
[Illustration: Structure within the ring can be seen in the best
Voyager 2 images, taken about 27 hours after closest approach to
Jupiter. This enlarged portion of a wide-angle picture taken with a
clear filter shows a bright core about 800 kilometers across with a
dimmer region a few thousand kilometers across on the inside, and a
narrow dim region on the outside. [260-674]]
The rings of Jupiter are quite tenuous, which explains why they are
invisible from Earth [although they have been detected from Earth since
Voyager]. Seen face on, the brightest part of the ring blocks less than
one part in ten thousand of the light passing through, making it
essentially transparent. In fact, the ring does not even offer much
resistance to a spacecraft; Pioneer 11 traversed the ring in 1974 with
no obvious ill consequences. Apparently the individual particles that
make up the ring are widely dispersed. They can be seen only when the
rings are viewed nearly edge-on, or toward the Sun, where they show up
well in forward scattered light. It is this extra brilliance when
backlit that created the excellent photos taken by Voyager 2 from inside
the shadow of Jupiter.
The individual ring particles are probably dark, rocky fragments that
are very small—essentially dust grains. They move around Jupiter in
individual orbits, circling the planet in 5 - 7 hours. Scientists
postulate that such orbits are not stable and that the particles fall
slowly in toward Jupiter. Apparently the rings are constantly renewed
from some source, which may be the satellite Adrastea (J14), discovered
by Voyager 2. There has also been speculation that Adrastea may
influence the ring structure by sweeping particles out of the ring. At
present the rings of Jupiter remain mysterious. They are clearly very
different from the rings of Saturn and Uranus, and reaching an
understanding of their origins and dynamics presents many challenges to
planetary scientists.
[Illustration: The four Galilean satellites of Jupiter are
planet-like worlds, revealed by Voyager to be as diverse and
fascinating as the terrestrial planets Mercury, Venus, Earth, and
Mars. In this Voyager 1 composite, all four are shown in their
correct relative size, as they would appear from a distance of about
1 million kilometers. Relative color and reflectivity are also
approximately preserved, although it is not possible to show on a
single print the full range of brightness from the dark rocky
surface of Callisto to the brilliant white of Europa or orange of
Io. [260-499C]]
[Illustration: Io (longitude 140°).]
[Illustration: Europa (longitude 300°).]
[Illustration: Callisto (longitude 350°).]
[Illustration: Ganymede (longitude 320°).]