Multimedia Mania/abacus-multimedia-mania.iso/dp/0116/01167.txt

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$Unique_ID{bob01167}
$Pretitle{}
$Title{Pioneer
Chapter 6: Part 3 - Results At The New Frontier}
$Subtitle{}
$Author{Fimmel, Richard O.;Allen, James Van;Burgess, Eric}
$Affiliation{Ames Research Center;University Of Iowa;Science Writer}
$Subject{jupiter
atmosphere
planet
clouds
pioneer
red
earth
temperature
heat
spot}
$Date{1980}
$Log{}
Title:       Pioneer
Book:        Pioneer: First To Jupiter, Saturn, And Beyond
Author:      Fimmel, Richard O.;Allen, James Van;Burgess, Eric
Affiliation: Ames Research Center;University Of Iowa;Science Writer
Date:        1980

Chapter 6: Part 3 - Results At The New Frontier

The Planet Jupiter

     The Pioneer spacecraft permitted close looks at Jupiter as well as the
environment surrounding it.  These close looks were made possible by the
spin-scan imaging technique, infrared and ultraviolet experiments, and the
radio occultation and celestial mechanics experiments.  As a result,
astronomers were able to refine theories about the internal composition and
the meteorology and atmosphere of Jupiter.  Although the spin-scan images are
discussed in detail in chapters 8 and 9, it is appropriate here to summarize
the current theories about Jupiter, which were strengthened by or were evolved
from the Pioneer results.  Jupiter appears to be almost entirely fluid, with
possibly only a very small core of silicates and metals.  Jupiter's center may
have a temperature of 30,000 C (54,000 F), a result of heat from continued
gravitational contraction and a reserve of residual primordial heat.  Since
the temperature of the Jovian cloudtops is about - 148 C (- 34 F), there is a
wide range of temperatures within the planet so that millions of cubic miles
of the atmosphere could be at room temperature.

     Atop the bulk of the planet is a turbulent region of atmosphere, possibly
970 km (600 miles) thick.  The top regions of this atmosphere produce clouds
that are the visible surface of Jupiter as seen from Earth.  A transparent
atmosphere extends above the visible clouds and ultimately leads to a
multilayered ionosphere of highly rarefied, electrically charged gas.

     Jupiter exhibits convective circulation patterns, but the rapid rotation
of the planet and outward flow of internal energy makes the weather patterns
very different from those of Earth.

     From changes to the paths of the Pioneers determined from their radio
signals, the density distributions within Jupiter imply that the planet must
be largely liquid, with no concentrations of mass and no detectable crust or
solid surface.  But Jupiter could still possess a small, liquid, rocky core of
a few Earth masses consisting of iron and silicates.  The composition of
Jupiter is not precisely like that of the Sun since there is a fivefold
enhancement of heavy materials on Jupiter, probably in the form of silicates
and ices of ammonia, methane, and water.  Scientists cannot yet define how
these heavier materials are distributed throughout the planet.

     Jupiter is probably 85 to 90% hydrogen; despite the high internal
pressures, the hydrogen is most likely liquid because of Jupiter's high
internal temperatures.  However, the pressure within Jupiter at about 24,000
km (15,000 miles) below the visible cloudtops is sufficient to convert liquid
hydrogen into a metallic form that more readily conducts heat and electricity.

     Temperatures and pressures are enormously high in the interior of
Jupiter.  At 970 km (600 miles) below the cloudtops, the temperature is
probably about 2,000 C (3,600 F).  At 2,900 km (1,800 miles), the temperature
is believed to be 6,000 C (11,000 F).  At 24,000 km (15,000 miles), the
temperature may reach 11,000 C (20,000 F), and the pressure may be 3 million
Earth atmospheres.

     Jupiter also contains 10 to 15% helium which might, theoretically, be
soluble in liquid hydrogen.  It is speculated that, if conditions are not just
right, the helium might be insoluble within the hydrogen and form a shell
around the central core of Jupiter on top of which the liquid metallic
hydrogen would float.  There is no adequate theory yet on the miscibility of
metallic hydrogen and helium within a planet such as Jupiter.  There might be
precipitation of helium in the molecular hydrogen, which would be important to
layering and to convective processes within the planet.  In turn, these could
affect the magnetic field.  Additionally, there is the question whether rocks
might dissolve in a hydrogen-helium mixture at high temperature.  This could
prevent the formation of a discrete rocky core or could have dissolved such a
core that had already formed earlier in the history of the planet.

     The seething internal activity in the metallic hydrogen of Jupiter is
thought to be evidenced by the complex magnetic field of the planet.  Hydrogen
moving up from the center of Jupiter, like water coming to a boil in a
saucepan, would produce eddy currents that give rise to the magnetic field
through rotation of the planet.

     Somewhere around 970 km (600 miles) below the cloudtops, where the
pressure is low enough for liquid hydrogen to become a gas, the atmosphere of
Jupiter begins.  It is unlikely, however, that there is a sharp transition
surface similar to the surface of an ocean.  Rather, there is probably a
gradual change through a mixture of gas and liquid.  But the top 970 km (600
miles) of the planet, where hydrogen no longer exists in liquid form, is
defined as the Jovian atmosphere.

     Jupiter's atmosphere accounts for about 1% of the mass of the planet.  It
is predominantly hydrogen (about 85 to 90%) with 10 to 15% helium and less
than 1% of other gases.  These elements are found in the same proportions on
the Sun.  Although helium was believed present in Jupiter, the gas was not
positively identified until the Pioneer 10 flyby.

     Jupiter's atmosphere also contains small amounts of ammonia and methane,
and traces of deuterium, acetylene, ethane, and phosphine.  In recent years,
water vapor has been detected in small quantities as have carbon monoxide and
hydrogen cyanide.  Several trace gases have been discovered, and more are
being discovered, through the use of telescopes mounted on high-altitude
aircraft that surmount some of the masking absorptions of Earth's atmosphere.
In the atmosphere extending 32 km (20 miles) or more above and below the
visible cloudtops, solar heat and internal heat from the planet affect
circulation and modify the weather patterns.  Jupiter's clouds form in the
atmosphere by condensation, as on Earth.  But Jupiter's clouds appear to
contain ammonia and ammonia compounds as well as water.  The topmost clouds
are thought to be of ammonia crystals with water clouds confined to the lower
levels.

     An inversion layer 35 km (22 miles) above the visible clouds is thought
to be caused by a layer of aerosols and hydrocarbons such as ethane and
acetylene.  In this layer, sunlight is absorbed and adds heat to the cooling
atmosphere.  Methane, too, would absorb sunlight and contribute to this
inversion layer.

     Pioneer 10's occultation experiment at first produced results for the
temperature structure of the Jovian atmosphere that were in conflict with
ground-based observations and with the data from the infrared radiometer.
Moreover, the data from Pioneer 11 were consistent with those from Pioneer 10.
They were finally matched with the ground-based observations by taking into
account the great oblateness, or spin flattening, of Jupiter and its effects
on the path of the radio waves through the Jovian atmosphere.  For three
measurements - entry and exit of Pioneer 10 and exit of Pioneer 11 - the
occultation data were quite consistent.  They showed a temperature inversion
between the 10- and 100-mbar levels with temperatures between -133 and -113 C
(-207 and -171 F) at the 10-mbar level, and between -183 and -163 C (-297 and
-261 F) at 100 mbar.  At the 0.001-mbar level, the temperature of the Jovian
atmosphere, determined from an occultation of the star Beta Scorpii as
observed from Earth, was about -103 C (-153 F).  At the cloudtops, however,
the temperature as measured by Pioneer was about -148 C (-234 F).

     The Pioneer observations also showed that the poles and equatorial
regions of Jupiter have effectively the same temperature; the temperature is
also the same on the northern and southern hemispheres and on the day and
night sides.  Also, because the axis of Jupiter is inclined only a few degrees
to the plane of its orbit, the planet does not have seasons like those on
Earth.

     Because the Sun's radiation is more concentrated per unit area in the
equatorial regions than in the polar regions, the equator would be expected to
be warmer than the poles, as on Earth and other planets; however, the
temperatures do not differ.  Two theories were proposed to account for the
even distribution of temperature as measured by infrared radiation from
Jupiter:  The first holds that the circulation within the atmosphere should be
very efficient in redistributing the solar heat; the second suggests that the
heat flux from inside Jupiter is sufficiently greater at the poles to balance
the lesser solar input there.  Since no equator-to-pole atmospheric flow
pattern is seen on Jupiter, the second theory seems more likely to fit
conditions on the planet.  It is believed that convection is so effective over
the entire planet that it eliminates any temperature differences due to the
solar input variations with latitude.  Thus, at the poles, where the cloud
temperatures would be expected to fall, convection brings heat from the
interior and keeps the temperature constant.  At the equator, where the clouds
are warmed more by the Sun, convection is reduced accordingly.  Thus, the
planet acts as though controlled by a natural thermostat.

     It has been speculated that the spots on Jupiter, including the Great Red
Spot, are probably large, hurricane-type features consisting of groups of
persistent air masses that rise like gigantic thunderstorms.  For reasons
mentioned above, it is no longer believed that the Great Red Spot is a column
of gas anchored to some feature on a hypothetical surface of Jupiter.  The
core of Jupiter is now believed to be much too small to produce effects that
would extend to the visible surface of the clouds.  The Pioneer spacecraft
revealed no noticeable density differences to suggest that the Great Red Spot
extends toward the core.

     Fundamental questions remain unanswered:  What causes the Great Red Spot?
Why has it lasted so long?  Speculative theories are constantly being advanced
as, for example, that the Great Red Spot is the Jovian equivalent of a
hurricane but the validity of these theories remains in doubt.  Equations that
describe the atmospheric flow on a rapidly rotating planet with an internal
heat source can be solved by powerful computers.  Several scientists have
developed mathematical models to explain the Great Red Spot.  Whether these
new hydrodynamic solutions do, in fact, apply to the real Great Red Spot must
await careful comparison of the predictions of the spot's behavior and
characteristics.  Time-lapse motion pictures obtained with the Voyager
spacecraft later threw more light on the complex motions and their probable
causes.

     One of the most significant images from Pioneer 10 showed a similar red
spot, though much smaller, in the northern hemisphere at the same latitude as
the Great Red Spot.  Its shape and structure confirmed that these red spots
are meteorological features in the atmosphere.  The Great Red Spot appeared to
rotate counterclockwise as seen from above, a motion clearly defined in the
Voyager pictures.  It is thus anticyclonic and behaves as an ascending mass of
gas flowing out at the level of its top which pokes several miles above the
surrounding clouds.

     By looking at sunlight reflected off a cloud, it is not possible to tell,
even on Earth, what is under the cloud.  But the nature of this reflected
light reveals much about the size, distribution, and refractive index of the
droplets comprising the cloud.  There was no haze over the Great Red Spot as
observed by the Pioneer near the limb.  At the terminator, the Great Red Spot
showed a bluish tint where the sunlight was scattered into space.  Scientists
speculated that the red color of the spot may result from phosphine being
carried to great heights where it is broken down by solar ultraviolet to
produce red phosphorus.

     The views of the north polar regions of Jupiter were unique in that such
views are not possible from Earth.  Pioneer's pictures showed that north of
the North Temperate Belt, the dark belts and light zones characteristic of
regions closer to the equator became successively less organized.  The band
structure changed into oval and circular patterns within 30 of the pole.  The
details were greater in the red images of the polar regions thereby suggesting
that the atmosphere is thicker over the polar clouds than over the temperate
and equatorial regions of the planet.

     Photopolarimetry was also used to estimate the optical depth of the
atmosphere above the cloudtops.  It appeared to be three times greater at
latitudes above 60 degrees than in the equatorial zone.  But the effects may
have been caused by a thin, high cloud layer or an unknown absorber in the
upper atmosphere.

     The Pioneer observations of Jupiter added considerably to our basic
knowledge of the atmospheric dynamics of cloudy planets by providing
information on very deep atmospheres in rapid rotation without any solid
surface interactions with the atmosphere.  They also provided information
about atmospheres driven mainly by heat from below rather than from the Sun.

     Pioneer results seemed to confirm earlier theoretical deductions that the
Great Red Spot and the light-colored zones are regions of well developed
clouds, swirling anticyclones, and rising air masses.  The darker belts, by
contrast, are cyclonic, sinking masses of air leading to depressed clouds.
The belts and zones of Jupiter reflect sunlight in very different ways.  It is
speculated that the belts may appear dark because of dark aerosols suspended
in the gaseous atmosphere there.  On Jupiter, the familiar cyclones and
anticyclones of Earth are stretched into linear or hook-shaped features on
this rapidly rotating planet, with extremely turbulent areas separating
adjacent bands of different velocities, areas in which there are many examples
of classical von Karman vortices.

     Whereas a storm system such as a hurricane on Earth may last for several
days or weeks, storm systems on Jupiter last much longer.  The Great Red Spot
has been observed for nearly three centuries, although at least twice it has
virtually disappeared.  On Earth there are strong interactions between
atmospheric systems and the land masses over which the systems travel.  These
masses tend to break up an atmospheric system passing over them.  In addition,
Earth systems are powered by solar heat concentrated in the tropics during
daylight.  Thus, they tend to break up when they move away from the tropics
and into the night hemisphere of Earth.  However, Jupiter's storms are powered
mainly by internal heat flow that is more evenly distributed planet wide and
over the day and night hemispheres.  It is not known why Jovian weather
systems can last so long, although it is clear that the huge mass of a
swirling body of gas has immense rotational inertia and consequently has a
long lifetime.

     Some of the bright zones on Jupiter may be analogous to tropical
convergences on Earth, which show up plainly on satellite photographs as bands
of thunderstorms, a few degrees north and south of the equator.  On Earth they
are caused by the trade winds, blowing toward the equator, and moist air
rising in the tropics.  The consequent thunderstorms spread their tops into
cirrus clouds which then flow back toward the poles.  Similarly, on Jupiter,
rising air masses may produce great anvil-shaped masses of cumulus clouds,
which appear as bright bands in the North Tropical and South Tropical Zones.

     A problem still not resolved is why, when ammonia and water are both
colorless when condensed, Jupiter displays bands of colored clouds and red
spots.  Certain ammonia compounds, if sufficiently exposed to ultraviolet
radiation, produce colors like those on Jupiter.  Sufficient solar radiation
does penetrate to the cloud levels.  Perhaps carbon compounds or traces of
sulfur and phosphorus - all believed to be present in primordial material -
supply some of the color.  Only traces would be needed to react in sunlight
and produce the colors seen on Jupiter.  It could very well be that, because
the gas of the Great Red Spot rises so high, it is subjected to irradiation by
solar ultraviolet which triggers a different set of photochemical reactions
that deepen the color.

     However, since solar ultraviolet radiation penetrates to lower cloud
levels, that is, to the belts, the Great Red Spot may result from a different
type of chemical reaction, from low temperature, or from longer exposure to
the radiation because its gases are less mixed than those of the belts.

     The presence of free radicals could also explain the colors on Jupiter.
At very low temperatures, such as those experienced in the higher cloud
layers, chemical compounds can exist with some of their normal complement of
atoms missing and still be relatively stable - these are called free radicals
and they are generally highly colored.

     Limb darkening on Jupiter shows that the clouds of the planet consist of
a thin upper layer, which is semitransparent to red light, above a more dense
lower layer.  The particles of Jupiter's upper clouds are much smaller than
particles in Earth's clouds.

     A precise modeling of the Jovian cloud layers was still continuing at the
time of the Voyager encounters in 1979.  Generally, two cloud layers appeared
to be present on the planet:  a thick, low deck with a gaseous atmosphere
above and a thin, high layer topped by aerosols.  The Jovian cloud particles
were not spherical (unlike the sulfuric acid droplets of the Venusian
atmosphere). Instead, the Jovian particles seemed irregular and probably
larger than the wavelength of light.  Clouds seemed to be lower at the poles.
But, alternatively, the upper cloud layers might have been diffuse with many
aerosols suspended in the upper atmosphere.

     The pictures of Jupiter revealed several surprises about the clouds.  The
detailed cloud structures in intermediate latitudes were unexpected.  The
billows and whirls near the edges of belts and zones confirmed that the
direction and speed of the winds change rapidly there.  Motions in latitude as
well as in longitude seem to be evidenced by trends and slants in the North
Tropical Zone, for example.  The plume in the Equatorial Zone was revealed in
remarkable detail, which provided structural information so important to
understanding these common cloud forms of that zone.

     Infrared observations of Jupiter have been made from the ground at
wavelengths of 5 mu_m where there is a window of transparency in the
atmospheres of both Earth and Jupiter.  Maps of Jupiter at this wavelength,
made at the Hale Observatories, reveal belts and zones very much the same as
shown in photographs of Jupiter taken by visible light.  But the dark visible
belts are light (hotter) in the infrared pictures, and the light visible zones
are dark (cooler).  The infrared radiation comes from deep within the
atmosphere and shows that the dark visible belts are lower, or thinner, hotter
clouds, while the bright visible featuress are higher, or thicker, cooler
clouds.  There is also a close correlation between infrared maps of the dark,
bluish-gray regions, which are interpreted as dark holes in the clouds.  These
show as regions of increased infrared radiation.  The 5-mu_m pictures also
correlate well with the Pioneer pictures of visible features; the prominent
plume and various cells and wave features are clearly the same.

     The Pioneer 10 and 11 spacecraft also made infrared maps of Jupiter, but
at 20 and 40 mu_m where, although there is less detail because of less
penetration and less temperature contrast, the planet emits more infrared
radiation than it does at 5 mu_m.  These maps also confirm the high and low
clouds and provide information on the general heat balance of the planet -
namely, that Jupiter emits more heat than it absorbs from the Sun.

     Despite the loss of some data about the northern hemisphere of Jupiter
because of radiation effects on the instrument, the infrared radiometer
onboard Pioneer 11 provided two infrared spinscan images of the planet.  A
complete image was centered at 41 south latitude and a partial image at 52
north latitude.  The ratio of total thermal energy to absorbed solar energy
was revised to 1.7 to 0.2; previous estimates were 2.5 to 0.5.  The fact that
both Pioneer 10 and Pioneer 11 data yield this result increases confidence in
the new value.  Thus Jupiter does not appear to be emitting as much internal
heat as was once thought.  The new value lends support to the idea that
Jupiter loses internal energy by cooling and contraction only, not by
separation of helium from hydrogen or other mechanisms.

     Jupiter's ionosphere rises 4000 km (2500 miles) above the visible
surface.  It is 10 times thicker and 5 times hotter than predicted.  Also, the
ionosphere has at least 5 sharply defined layers of different density, similar
to Earth's ionospheric layers that permit long-range radio communications
around the bulge of Earth by returning certain radio frequencies to the
ground.

     Confirmation that Jupiter has a warm, extended, hydrogen-rich atmosphere
has important implications for further exploration of this giant planet.

     Before the measurements by the Pioneer spacecraft, engineers generally
considered that a probe into Jupiter's atmosphere could not withstand the
intense heat at entry.  Now, the new determinations of the Jovian atmosphere
suggest that a probe can be built to survive entry into the Jovian atmosphere
and to directly measure its characteristics and constituents.  Thus, the path
was cleared for NASA's Project Galileo:  a Jupiter probe and orbiter.

     The Pioneers stimulated a great increase in theoretical and ground-based
planetary astronomy and confirmed or revealed enough information about Jupiter
to provide a basis for further missions.  Pioneers 10 and 11 also demonstrated
for the first time that such exploration is within the capabilities of
present-day space technology.  There is now the opportunity to sample directly
what may be the primordial material of our Solar System, moving back 4.5
billion years in time.