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$Unique_ID{bob01168}
$Pretitle{}
$Title{Pioneer
Chapter 6: Part 4 - 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{saturn
ring
pioneer
rings
particles
saturn's
field
jupiter
magnetosphere
km
see
pictures
see
figures
see
tables
}
$Date{1980}
$Log{See Saturn w/Rings*0116801.scf
See Ring Table*0116801.tab
}
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 4 - Results At The New Frontier

Encounter with Saturn

     The Pioneer 11 spacecraft, after traveling more than 3.2 billion
kilometers (2 billion miles) on a journey of nearly 6.5 yr, spent 10 days
gathering a wealth of new information about Saturn and its ring system.
Pioneer hurtled through the plane of the rings twice, on inbound and outbound
passages, at a distance of about 38,000 km (23,600 miles) from the outside
edge of the visible ring system.  The outbound crossing was slightly nearer
the planet than the inbound crossing.  The trajectory carried encounter,
Pioneer made important discoveries about the planet's magnetosphere, magnetic
field, and trapped radiation, as well as Saturn's satellites, rings, and the
planet itself.

Saturn's Magnetosphere

     The most fundamental discovery by Pioneer was that Saturn has a magnetic
field and a magnetosphere.  Although scientists thought it likely that Saturn
has a strong magnetic field - because of the planet's similarity to Jupiter,
which has a strong field - there had previously been no conclusive evidence of
a magnetic field.

     As the Pioneer spacecraft approached Saturn, a bow shock, where the solar
wind is deflected by the planet's magnetic field, was detected on August 31,
1979, at a distance of 1,446,000 km (898,540 miles).

     Pioneer approached Saturn almost along the noon meridian and encountered
the bow shock three times at 24.1, 23.1, and 20.0 Saturn radii; it then
crossed the magnetopause at 17.3 planetary radii.  Saturn was shown to have a
detached bow shock and magnetopause similar to those of Earth and Jupiter.
High resolution data from some of the instruments showed that the shock was
very turbulent; its precise position was difficult to determine.  Across the
shock, the solar wind speed was observed to change from 470 km/sec (292
miles/sec) to less than 140 km/sec (87 miles/sec).  Its temperature increased
from 30,000 K to nearly 470,000 K.  As Pioneer passed inward through the
magnetic field of Saturn, another important discovery was made Saturn's
magnetic field was unique in that its orientation corresponded almost exactly
with the axis of rotation of the planet, the magnetic axis being tilted less
than 1 degree.  This contrasts markedly with the magnetic fields of Earth,
Jupiter, and Mercury, which are tilted 10 to 20 degrees with respect to their
axes of rotation.  The surprising lack of any appreciable tilt to Saturn's
magnetic axis causes difficulties in explaining the field by some dynamo
theories of the generation of planetary fields.  It also prevents the accurate
measurement of the period of rotation of Saturn's interior by use of magnetic
field observations.

     The magnetic moment of Saturn is 540 times stronger than Earth's, and 35
times weaker than Jupiter's.  The strength of the field at the equatorial
cloudtops of Saturn is about 0.20 gauss - Earth's field at the surface is 0.31
gauss.  Polar fields of Saturn are 0.63 gauss (north) and 0.48 gauss (south).
The center of Saturn's field is displaced northward some 2400 km (1490 miles)
along the axis of the planet.  Like the field of Jupiter, the polarity of
Saturn's field is opposite to that of Earth.

     Because there is only a relatively small quadrupole component in Saturn's
magnetic field, scientists believe this means that the field must originate
far below the visible surface and that Saturn must have a core of metallic
hydrogen smaller than Jupiter's metallic hydrogen core.

     Like Earth and Jupiter, Saturn has a detached, strong bow shock and a
magnetopause.  The magnetosphere of Saturn is very responsive to changes in
the solar wind, but on a smaller scale than that of Jupiter.  The observed
dimensions of the magnetosphere of Saturn were, however, perhaps half the
average because of the markedly enhanced solar wind pressure when Pioneer
encountered the planet.  The blunt sunward side of the magnetosphere, where
the bow shock is located, moves in and out as the pressure of the solar wind
varies.  The outbound leg of the trajectory, along the dawn meridian, carried
Pioneer across the magnetopause five times between 30 and 40 Saturn radii, and
nine times across the bow shock between 49 and 102 Saturn radii.

     The magnetosphere has four distinct regions.  An outer magnetosphere had,
at Pioneer encounter, a corotating plasma in which the flux of charged
particles varied considerably with time.  From 15 to 6 Saturn radii, the
observed direction of 1-MeV proton streaming is consistent with full
corotation.  On the outbound leg of its trajectory, Pioneer found that there
may be a magnetotail or a magnetodisk generated in the outer magnetosphere.
The magnetic field in this outer magnetosphere was compressed near noon and
extended equatorially near dawn, probably because of the presence of a current
sheet in or near the magnetic equator and possibly associated with formation
of a magnetotail.  There was, however, no evidence for a magnetodisk in the
planet's dayside magnetosphere.  But near dawn in Saturn's magnetosphere, the
field lines became equatorial instead of north-south, thereby showing the
presence of a current sheet.

     Inside about 6 Saturn radii, the inner boundary of the first region, the
numbers of low-energy protons dropped markedly.  This effect is believed to
result from the sweeping effect of the satellites Dione, Tethys, and Enceladus
and by a thin ring of dust (E ring).  This second region has been called the
"slot" because of its reduced level of radiation.

     Starting at 4 Saturn radii, the third region, the inner magnetosphere,
has a complex spectrum of very energetic charged particles.  Protons with
energies greater than 80 MeV were measured to a maximum flux of 2.5 x 10^4/
cm^2/sec, and electrons with energies greater than 0.56 MeV, to a flux of 9 x
10^6/cm^2/sec.

     There was a distinct region associated with the satellite Mimas where
particles were depleted.

     Analysis of particle fluxes, spectra, and distributions shows that:  (a)
The low-energy (1-MeV) protons in the outer magnetosphere come either from the
solar wind or from solar energetic particle beams . (b) The electrons from 40
keV to several MeV throughout the magnetosphere come from the solar wind.  (c)
The very high-energy (80-MeV) protons in the inner magnetosphere come from the
decay of neutrons produced in the atmosphere and rings of the planet by cosmic
ray bombardment.

     In the fourth region of the magnetosphere, internal to the outer edge of
ring A, the rings have swept up nearly all trapped radiation to create an
environment almost free of radiation.

     Thus, Pioneer discovered that Saturn, like Jupiter and Earth, has belts
of trapped energetic particles, mainly protons and electrons.  The trapping
boundary for energetic particles was observed to be accurately coincident with
the magnetopause on both inbound and outbound legs of the trajectory.  The
particles are present around Saturn in quantities comparable to but greater
than those in Earth's radiation belts, but they extend over a much greater
region because Saturn's magnetosphere is so much larger than that of Earth.
Saturn's radiation belts are some 10 times larger in linear dimensions than
Earth's radiation belts.  Even so, the total radiation dosage of electrons of
energy greater than 0.56 MeV experienced by Pioneer during its flyby of Saturn
was 7 x 10^10 electrons/cm^2, which is about the same as that encountered
during only 5 min of the spacecraft's passage through the most intense region
of Jupiter's inner radiation belt.

     The radiation environment of Saturn is made much less intense by the
presence of the rings, which have a marked effect on the radiation belts of
the planet.  Particles in radiation belts oscillate up and down across the
equatorial plane of the planet, first toward one pole and then, as though
reflected by a mirror, back toward the other.  As particles gradually diffuse
inward toward the planet, they are absorbed by the rings.  Any high-energy
electrons and protons that collide with ring particles are wiped out
completely.  From these observations the outer edge of the A ring is found to
be at a radial distance of 2.292 +/- 0.002 Saturn radii.  Also, the general
magnetic field of the planet reduces the level of high-energy cosmic
radiation, thereby making the region inward of the outermost edge of the rings
perhaps the most radiation-free space within the Solar System.  However, a
weak flux of high-energy electrons was discovered under the rings.  It had an
intensity 4 to 5 times the interplanetary flux of such electrons during quiet
periods of solar activity.

     As Pioneer approached Saturn, the spacecraft's instruments measured a
maximum intensity of very energetic protons at 2.67 Saturn radii.  Anomalies
in the rate of increase, and some decreases in the number of particles inward
toward the planet, provided crucial information on the origin of these
particles as summarized above.

     Generally, the trapped radiation in the inner magnetosphere was spaced
symmetrically around the planet, thereby showing that Saturn's magnetosphere
is much more stable than Jupiter's.  The effects of satellites in sweeping
energetic particles from the radiation belts was more clearly defined at
Saturn than at Jupiter because of the regular nature of Saturn's
magnetosphere.

     A speculative possibility is the effect of Jupiter on Saturn's
magnetosphere.  About every 20 yr, Jupiter may shield Saturn from the solar
wind when Saturn becomes immersed transiently in an extended magnetotail of
Jupiter.  Then Saturn's magnetosphere might expand dramatically.  Such a
condition may be observed when Voyager 2 encounters Saturn in 1981.

Saturn's Satellites

     Just after Pioneer passed through the plane of the rings on its inward
passage, the number of energetic particles decreased abruptly - to about 2% of
the prevailing value for 10 sec.  The experimenters attributed the absorption
of particles to a previously unknown satellite-sized body.  The satellite,
designated 1979 S2, had to be at least 170 km (106 miles) in diameter to
account for the effects observed.  It is the first satellite to be discovered
from an analsis of energetic charged particles, apart from the Pioneer 11
evidence for a ring or inner satellite of Jupiter noted earlier.  Its orbit
was within that of Mimas, about 14,500 km (9,010 miles) from the outer edge of
the visible rings at 2.53 Saturn radii from the center of the planet.

     On the previous day, the imaging photopolarimeter discovered a new
satellite.  It showed up on two of the computer-generated pictures of Saturn
as a small dot of light, consisting of three pixels only, near the outer edge
of the rings.  The small satellite was designated 1979) - 1979 S1 and 1979 S2
could be the same object detected independently by two quite same object
detected independently by two quite different techniques.  Moreover, the
satellite may be one of those detected by ground-based observations during the
edge-on presentation of the ring system in 1966.  Pioneer also found that
there was absorption of high-energy electrons and protons by the satellites
Enceladus and Tethys.  As a result of this absorption, low-energy charged
particles are sputtered from the satellites' surfaces and create an oxygen-ion
rich plasma torus at the orbit of each satellite.  Strong ultraviolet
radiation from these oxygen ions was detected by Pioneer's ultraviolet
photometer.

     There was no evidence of absorption of energetic particles by the
suspected satellite Janus at or near a distance of 2.66 Saturn radii, although
there were clear absorptions that might be associated with previously unknown
satellites at 2.34 and 2.82 Saturn radii.

     The Pioneer observations indicate that the F ring and the G ring contain
small satellites and that there may be small satellites associated with the A
and B rings.  Formation of Saturn's rings by tidal breakup of a single
satellite now seems unlikely.  It is postulated that although near Jupiter the
satellites Io and Europa formed, the lesser mass density of the solar nebula
at the distance of Saturn's orbit resulted in the formation of a system of
rings and small satellites.

     The previously known large satellites did perturb Pioneer's path through
the system.  From these effects, the masses of Iapetus, Rhea, and Titan were
determined more accurately; their mean densities were calculated as 1.8, 1.0,
and 1.32 gm/cm^3, respectively.  These satellites were confirmed as being
low-density icy worlds.  The particle absorption signature of Mimas appeared
unexpected because it suggests that Mimas has a diameter less than 180 km (112
miles) compared with the generally accepted value of about 360 km (220 miles).
If the smaller diameter is correct, the mean density of the satellite would be
5 gm/cm^3 - a surprisingly high value.

     The photopolarimeter obtained polarization measurements of Titan's
atmosphere over a wide range of phase angles.  Light from Titan was found to
be strongly polarized, and the data revealed the types of aerosols present in
the Titan atmosphere.  The data appear to be consistent with a haze of methane
particles extending high into the atmosphere.  Infrared radiation at 45 mu_m
from the clouds revealed a cloudtop temperature of only -198 C (-324 F), about
as expected for a body in equilibrium with solar radiation and one that
generates no internal heat.  There may be warmer regions below that are
obscured by aerosols, but nevertheless it appears that Titan does not have a
significant internal heat source.  A greenhouse effect, which would trap solar
radiation, was not ruled out; however, the results from Pioneer do not support
the possibility that life or precursors of life exist on Titan.  The images of
Titan obtained by the Pioneer spacecraft do not show any revealing detail.

     Data sets at red and blue wavelengths recorded by the imaging
photopolarimeter provided information to determine the radius of Titan more
precisely.  These radii are 2845 +/- 25 km and 2880 +/- 22 km for red and blue
wavelengths, respectively.  The difference may result from there being a thin
haze of submicrometer particles above the nominal haze layer.

     The linear polarization of the integrated disk of Titan in red and blue
light at phase angles between 15 and 97 provided information about the sizes
of particles in the atmosphere of this large satellite.  A polarization of 54%
measured in blue light at 90 phase angle implies that the particles near the
top of Titan's atmosphere must have radii smaller than about 0.09 mu_m if they
have a refractive index of 2.0.  A smaller polarization in red light (41%)
implies that the optical thickness of the layers of small aerosols is about
0.6 above an effective depolarizing surface.  The shape of the polarization/
phase curve in blue light suggests increasing particle size with increasing
depth in the atmosphere.

     Pioneer's ultraviolet instrument discovered a cloud of hydrogen atoms
around Titan, extending at least 300,000 km (186,400 miles) along the orbit
and about 180,000 km (112,000 miles) thick.  This discovery suggests that the
methane in Titan's atmosphere is being broken down into hydrogen and carbon by
solar radiation.  Hydrogen atoms would possess sufficient energy to escape
into space - hence the observed hydrogen cloud.  Since the heavier carbon
atoms do not travel fast enough to escape, they would be expected to remain in
the atmosphere and possibly to produce aerosol clouds, the particles of which
ultimately fall to Titan's surface.

The Rings of Saturn

     The Pioneer missions provided valuable information on the magnificent
ring system of Saturn.  Discovered by Galileo in 1610, the true nature of this
ring system - swarms of small orbiting bodies - was not understood until the
speculations of Huygens in 1659.  This ring system has divisions and gaps
where the orbiting particles are fewer in number than in the visible rings.
The most prominent of these is Cassini's division.  Before the Pioneer
encounter, the division was believed to be about 6400 km (4000 miles) wide,
but Pioneer refined the dimension to 4200 km (2600 miles).  The division
separates the two main bright rings, A and B.

     Some of the most spectacular pictures from any of the space missions were
taken by Pioneer when it obtained images of Saturn's ring system illuminated
from behind.  These pictures provided valuable new information that could
never be obtained from Earth.  Those rings, which normally appear bright when
viewed from Earth, appear dark in the Pioneer pictures, and the dark gaps in
the rings as seen from Earth appear as bright rings in the Pioneer picture.
The gaps appear bright because they are not entirely free of material and the
particles within the gaps scatter the sunlight.  However, the particles within
the bright rings are sufficient to intercept most of the sunlight and permit
only a small amount of it to pass through.  These new viewpoints based on
light transmitted through the rings allow scientists to assess much more
accurately the thickness of the ring material.

[See Saturn w/Rings: A general view of Saturn and its magnificent ring system
during the approach of Pioneer. The rings are illuminated from behind.]

     Several surprises came from Pioneer's observations of the ring system.
No visual trace was found of an outermost E ring (sometimes refered to as the
D1 ring), first referred to by Fournier when the rings were viewed edge-on in
1907.  He wrote of a faint, transparent, and luminous ring outside the
principal rings of Saturn.  Other astronomers claimed they had seen the E ring
when the rings were again edged in 1952 through 1954; it was photographed by
W. A. Feibelman and G. P. Kuiper in the 1966 period.  Some radar data appeared
to support the presence of the E ring but in the imaging data from Pioneer
there was no trace of the ring.  Nonetheless, the energetic particle
measurements suggest confirmatory evidence for a thin tenuous E ring.  Also,
the dust detector provided evidence for the E ring and suggested a thickness
of 1800 km with an optical depth (opacity) greater than 10^-8.

     The A ring, the bright outer ring seen from Earth, was found by Pioneer
to transmit in part appreciable amounts of sunlight; it appeared bright in the
Pioneer pictures.  Considerable structural details could be seen in the ring;
the outer 25% was substantially darker than the rest of the ring, thereby
showing that it contains more material.  The inner parts showed several
regions that have a low particle density, but the innermost edge contains much
material and is almost as opaque as the B ring.  This B ring, the brightest
ring seen from Earth, is almost completely opaque to sunlight striking it on
the side away from the observer.

     The composition and sizes of the particles comprising the rings have for
many years been a matter of debate and speculation.  The celestial mechanics
experiment showed that the total mass of the rings is less than 3 millionths
that of Saturn itself.  Where the rings were illuminated by sunlight, Pioneer
measured a temperature of -208 C (-342 F).  Where the planet's shadow fell on
the rings, the temperature was -210 C (-346 F).  This small difference in
temperature indicates that the rings receive energy from the dark hemisphere
of Saturn.  The temperature of the unilluminated face of the rings was about
-218 C (-360 F).  This temperature indicates that considerable infrared
radiation is transmitted through the rings, but the rings are thick enough to
insulate the dark side from the warmer sunlit side.  The rings cannot be more
than 4 km (2.5 miles) thick.  The size of the ring particles appeared to be in
the centimeter range.  However, Pioneer observations of the unilluminated side
of the rings indicate a distribution of much smaller particles also -
approximately 100 mu_m in diameter.  These may result from collisions between
larger particles.

     The dark Cassini division between the A and B rings appeared quite bright
when seen from Pioneer because it contains particles, and these scatter light
through the division.  A less bright region near the middle of the division
indicated a gap there.

     The C ring or Crepe ring, discovered by Bond in 1850, is a very faint
dusky ring inside the B ring.  The C ring was clearly identified on the
Pioneer pictures as a bright ring since it also scatters light.  Particles in
this ring were apparently as diffuse as in Cassini's division.

     Pioneer confirmed the existence of another ring division that had been
suggested by several French astronomers.  Pioneer pictures of the shadows of
the rings on the clouds of Saturn clearly revealed a division between the B
and C rings.  The division, about 3600 km (2200 miles) wide, was called the
French or Dollfus division.

     Although no optical trace was found of the E ring (apparently it is too
faint to be detected by the imaging photopolarimeter), a narrow ring appeared
outside the A ring on the Pioneer pictures.  The Pioneer experimenters called
the new ring the F ring.  It is quite narrow, less than 800 km (500 miles)
wide, and it is separated from the outer edge of the A ring by a gap of about
3600 km (2240 miles) - named the Pioneer division by the spacecraft team.
Details of the measurements of the ring system from Pioneer data are given in
Table 6-2, "Ring Table."


[See Ring Table: Table 6-2. Dimensions of Saturn's Ring System from Pioneer
Data]

     Pioneer also discovered a substantial glow of atomic hydrogen around the
ring system, which was enhanced at the B ring.  The presence of hydrogen is
attributed to the dissociation of water molecules sputtered off the rings by
sunlight.

The Planet Saturn

     Measurements of the trajectory of Pioneer past Saturn allowed the shape
and gravity field of the planet to be determined more precisely than ever
before.  Because of the planet's rapid rotation, Saturn's polar diameter is
about 10% less than its equatorial diameter.  The polar flattening was
determined more precisely from the spin-scan imaging data.  By overlapping
graphical predictions of geometric distortions of spin-scan images with raster
scans of the raw data (both displayed on the same scale), the precise geometry
of each data-seeking sequence was established.  The predictions are quite
sensitive to the dimensions assumed for Saturn and its rings.  As a result,
scientists concluded that the ratio of polar-to-equatorial radius is 0.912 +/-
0.006 and that the Encke gap in the rings is 133,500 km (82,960 miles) from
Saturn's center.

     Analysis of the gravity field, coupled with a temperature profile based
on the measurements of heat emitted from the clouds in excess of that absorbed
from the Sun, allowed the experimenters to develop a new view of the interior
of the planet.  The planet's core of about 18 Earth masses appears to have two
distinct regions.  An inner core, about the size of Earth but with a mass
about 3 times that of Earth, is a mixture of iron-rich rocky materials.  An
outer core, of about 9 Earth masses, is thought to consist of ammonia,
methane, and water.  It probably extends from the center of the planet to
about 23% of the radius, that is, to about 13,800 km (8,575 miles).  Above the
core and extending to about 58% of Saturn's radius, there appears to be a
region of liquid metallic hydrogen, a form of hydrogen at high temperature and
under great pressure so that it readily conducts electricity.  The presence of
this material was also indicated by the characteristics of the planet's
magnetic field.  The nature of this field implied that the metallic hydrogen
dynamo region of Saturn must be substantially smaller than that of Jupiter:
0.5 Saturn radii compared with 0.75 Jupiter radii, respectively.

     The effective temperature of Saturn measured by the infrared experiment
was -177 C (-287 F), some 30 C less than that of Jupiter.  Saturn was found to
radiate 2.8 +/- 0.9 times more heat than it absorbs from the Sun.  Photometric
data lead, however, to a greater number.  Analysis of photometric observations
at large phase angles in red and blue light led to an important conclusion.
If Saturn scatters light similarly at other wavelengths, its bolometric
geometric albedo together with the effective temperature of -177 C (reported
by the infrared experimenters) imply that Saturn radiates three times as much
energy as it receives from the Sun.

     It is suggested that only about half the planet's heat is generated by
leftover heat of formation and by a continuing compression of the plane's core
by the enormous weight of al layers of material above it.  Additional heat is
probably being evolved by a separation of the planet's two major
constituents, hydrogen and helium.  At Saturn's temperature, which is lower
than Jupiter's, helium does not remain mixed with hydrogen as it might be
within Jupiter.  The denser helium, gradually sinking to Saturn's core, is
generating heat.  Infrared and radio occultation data show that the outer
atmosphere of Saturn is about 90% hydrogen and 10% helium.

Saturn's Atmosphere and Cloud Systems

     The pictures of Saturn's clouds showed surprisingly little contrast.  The
infrared data suggest that these clouds are thicker than the Jovian clouds.
The images showed some scalloping on the edges of belts and zones, as on
Jupiter.  Such scalloping is caused by differences in velocities between
adjacent air masses.  There were subtle colors in the clouds away from the
poles, but at the poles there was a clearly blue-green color.  This was
believed to result from Rayleigh scattering of light in the atmosphere.
Saturn's cloudtops appeared lower at the poles than at the equator.  Hence the
planet's gaseous atmosphere could be seen above the clouds, ranging in color
from dark blue to slightly green.

     Saturn's pastel colon changed to brownish belts at about 55 latitude in
both hemispheres.  Near the terminator, the colors darkened, indicating that
light was being scattered by the molecules of the atmospheric gases.

     The polarization measurements of Saturn indicated an atmosphere of clear
gas to a pressure of 2 atm, then a region of small absorbing particles, then
an ammonia haze, with a topmost ammonia cloud deck.  The cloudtops appeared to
be at a pressure of about 750 mbar.  The colored clouds may be deeper in the
atmosphere of Saturn than they are in the Jovian atmosphere, thereby
accounting for the lack of strong coloring on Saturn.  Generally, Saturn
appeared to have more and narrower belts and zones than Jupiter.  There
appeared to be features like jet streams, one at about 70 north latitude and
another near the equator.  A plume in the northern hemisphere, similar to that
seen on Jupiter by Pioneer, was also discovered.  The jet stream is much
faster than that of Jupiter:  350 km/hr (217 mph) compared with 150 km/hr (93
mph).

     An ultraviolet glow, intensified at the polar regions, could be caused by
aurorae or it might be a limb-brightening effect.

     Infrared measurements of the globe of Saturn showed that temperature
dropped within 8 of the equator corresponding to the high clouds of the
equatorial yellow band.  Also, the temperature of the belts and zones differed
by about 2.5 C (4.5 F).  The temperature of the upper atmospheric layers was
about 5 C (9 F) warmer than expected, thereby confirming suggestions that
substantial heat moves from inside the planet outward, almost certainly by
convective circulation.

     Saturn's ionosphere, as measured by the occultation experiment near the
terminator, extends much higher than that of Jupiter, but the inferred
temperature is about the same, of the order of 1000 C (1830 F).  Two peaks of
electrons were identified.  The highest, at about 1800 km (1100 miles) above
the cloudtops, has about 1.1 x 10^4 electrons/cm^3.  The other, at 1200 km
(745 miles) has a peak electron density of 9 x 10^3/cm^3.

     The S-band radio signal also penetrated the neutral atmosphere of Saturn
to the level of about 150 mbar (19% of Earth's atmospheric pressure at sea
level).  The temperature structure derived from these data matches the
temperatures derived from the infrared radiometer measurements for a
composition of 90% hydrogen, 10% helium.  The minimum temperature was about
-185 C (-301 F) at a level of about 100 mbar.

     The mission of the two Pioneer spacecraft to explore the giant planets of
our Solar System was remarkably successful.  Many questions had been answered
and many discoveries made.  Nonetheless, these giant planets, with their
intriguing satellites, ring systems, and complex magnetospheres, posed many
new questions.  Some of these are being answered by the Voyager spacecraft,
but others will remain for future missions to place orbiters around these
giants and to dispatch probes deep into their atmospheres.  The Pioneer
trailblazers opened the outer Solar System to mankind, but a full exploration
of the many planetary worlds there will take decades of human effort.  This
exploration will undoubtedly reveal surprises equally as great as those of the
Pioneers.