Pioneer Saturn has already greatly expanded our knowledge of Saturn, its
rings and moons. We now know that Saturn, in many ways, represents an
intermediate case between Jupiter, the largest planet in the solar
system, and Earth. The composition of Saturn’s interior is essentially
the same as Jupiter’s, differing only in the size and extent of the
various internal layers. Measurements for Saturn are consistent with a
central core of molten heavy elements (probably mostly iron) which is
the approximate size of the entire Earth, but about three times more
massive. Surrounding the central core is an outer core of highly
compressed hot, liquefied volatiles such as methane, ammonia, and water.
This outer core is equivalent to approximately nine Earth masses. These
core regions, however, represent a very small fraction of the planet,
which is composed primarily of the very lightest gases, hydrogen and
helium, and is almost 100 times the mass of the Earth. Because of the
high pressure in Saturn’s interior, the hydrogen is transformed to its
liquid metallic state. Above this metallic hydrogen shell are liquid
molecular hydrogen and Saturn’s gaseous atmosphere and clouds, which
make up the rest of the planet.
Electrical currents set up within the metallic hydrogen shell produce
Saturn’s magnetic field, which was measured by Pioneer. In spite of
Saturn’s large size, the magnetic field at the cloud tops is only
slightly weaker than the field at the Earth’s surface. Saturn is unique
in that its magnetic axis is nearly aligned with its rotation axis,
unlike Earth and Jupiter.
Saturn’s magnetic field is also much more regular in shape than the
fields of the other planets. At large distances from Saturn, the
magnetic field is deformed by the inward pressure of the solar wind.
Near the noon meridian (close to the inbound Pioneer trajectory), the
solar wind causes a compression of the field; in the dawn meridian
(close to the outbound trajectory), the field is swept back and
presumably forms a long magnetic tail. In both cases, Pioneer Saturn
crossed the outer boundary of the magnetic field several times as the
field moved in and out, responding to changing solar wind pressure.
Pioneer also observed inward and outward boundaries.
The magnetic envelope surrounding Saturn is intermediate in size and
energetic particle population between those of the Earth and Jupiter,
the only two other planets known to be strongly magnetized. The three
other planets investigated thus far (Venus, Mars, and Mercury) and
Earth’s moon have little or no magnetism. Virtually all our knowledge of
Saturn’s magnetic environment has been obtained by Pioneer. The
spacecraft found rings of particulate material and several small moons
near the rings, which strongly affect Saturn’s trapped radiation. These
features provide important diagnostic capabilities. A unique finding is
the nearly total absence of radiation belt particles at distances closer
to the planet than the outer edge of the visible rings.
The inner region of the thick magnetic envelope of Saturn, called the
magnetosphere, contains trapped high-energy electrons and protons, with
some evidence for heavier nuclei. The overall form of the magnetosphere
is simple and compact, more similar to that of Earth than of Jupiter.
The unique measuring capabilities of the Pioneer Saturn radiation
detectors led directly to the discovery of a diffuse new ring of
particulate matter in the region from about 10 to 15 planetary radii (1
Saturn radius = 60,000 km) from Saturn. This ring has been tentatively
designated as the G-ring. The G-ring clearly causes particle absorption
near the equatorial region. Moreover, Pioneer discovered another region
inside 7 or 8 planetary radii in which the radiation belt particles were
subjected to a strong loss or absorption, presumably caused by the
presence of an extensive cloud of plasma corotating with the planet.
Inside about 10 planetary radii, the trapped radiation shows a high
degree of axial symmetry around Saturn and is consistent with the
centered dipole magnetic field observed by Pioneer. Saturn’s rings
annihilate all trapped radiation at the outer edge of the A-ring,
leaving a shielded region close to the planet in which the radiation
intensity is the lowest so far encountered in this mission. This
shielding prevents the further buildup of electron intensities at lower
altitudes, which otherwise would have been present and would have made
Saturn a strong radio source observable from Earth.
Pioneer found that several of Saturn’s moons absorb trapped particles
from the radiation belts, producing prominent dips in the intensity. The
effectiveness of absorption at the moons Tethys and Enceladus is
particularly astonishing, and supports the idea that radiation belt
particles are drifting inward slowly across the moons’ orbits.
A precipitous decrease in particle intensity, lasting only for about 12
seconds, was observed over a wide range of energies for both protons and
electrons at a distance near 2.53 Saturn radii, 23 minutes after Pioneer
crossed the Saturn ring plane inbound. At about the same time, an
anomaly was also observed in the magnetic field measurements. These
phenomena have been tentatively interpreted as indicating the presence
of a nearby massive body absorbing the trapped radiation and perturbing
Saturn’s magnetic field. The estimated radius of this object lies in the
range of 100 to 300 km, based on the effectiveness with which it
absorbed the high-energy radiation. The total radiation dose received at
Saturn was equivalent to only 2 minutes in the Jovian radiation belts
because Saturn’s radiation belts were so much weaker.
In addition to images of Saturn, the brightness, color, and polarization
of the reflected light were also measured by the imaging
photopolarimeter on Pioneer Saturn. These measurements are used to study
the cloud layers of Saturn and Titan and to model the vertical structure
of the atmospheres of these two bodies. In the scans that made the
images, the banded structures of Saturn and of the rings were obtained
in fine detail. These are essential in studying the atmosphere, rings,
and moons. A new Saturnian ring, which has been tentatively designated
the F-ring, was discovered in the images. It is narrower than 500 km in
width, but is important because it forms an outside barrier to the
bright A- and B-rings. The gap between the F- and A-rings has been
designated the Pioneer Division by the Pioneer team. A small moon, which
either was previously unknown or had been previously discovered from
Earth but lost again, was found in the Pioneer Saturn images. After its
initial discovery, this new moon continued on its 17-hour orbit around
the planet and passed near Pioneer as the spacecraft entered the ring
system. It is quite conceivable that this moon is the same one that
perturbed the radiation belt particles and produced the anomaly in the
magnetic field measurements.
Infrared observations obtained during the Saturn flyby revealed the
temperatures in the atmospheres of Saturn and the rings and in the
atmosphere of Titan. It was found that Saturn has a temperature of about
100 K (about 280°F below zero) and, according to these observations, has
an internal heat source of enough strength that the planet emits
approximately 2.5 times as much energy as it absorbs from the sun. The
equatorial yellowish band observable in many of the images was found to
be several degrees colder than the planet at other latitudes and is
probably a zone of high clouds resembling similar zones on Jupiter. As
expected, the rings were extremely cold, 65 to 75 K (about 330°F below
zero), at the time of encounter. The temperature differences between the
illuminated and unilluminated sides of the rings, and the rate of
cooling as the ring particles go into Saturn’s shadow, suggest that the
ring particles are at least several centimeters in diameter and the
rings themselves are many particle diameters thick. The very minor
perturbation to Pioneer’s trajectory, as it passed under the visible
rings, indicates that the rings probably consist of ices.
As Pioneer passed through Saturn’s ring system, very sensitive meteoroid
detectors observed the impact of five particles on the spacecraft,
particles that were about 10 micrometers (0.0005 inch) in diameter. Two
impacts occurred while the spacecraft was above the rings and three
while the spacecraft was below the rings. No impacts were detected going
through the ring plane, but the Pioneer instrument cannot detect
individual impacts that occur less than 77 minutes apart. This
characteristic would have prevented detection of ring particles because
of the impacts detected just before both ring plane crossings. It is
uncertain whether the micrometeoroids detected by Pioneer Saturn were
stray ring particles deflected out of Saturn’s ring plane or whether
they were particles from interplanetary space drawn inward toward Saturn
by its strong gravitational field.
Close to the point of closest approach to Saturn, the spacecraft’s radio
transmissions were affected by Saturn’s ionosphere. The manner in which
the radio signals were absorbed indicates that Saturn has an extensive
ionosphere composed of ionized atomic hydrogen with a temperature of
about 1250 K in its upper regions. This high temperature requires an
extensive energy source other than the sun. This phenomenon was also
observed at Jupiter.
Pioneer measured ultraviolet glow throughout the Saturnian system. This
ultraviolet glow is due to the scattering of the light from the sun by
atomic hydrogen. The observations of ultraviolet emission from an
extensive cloud of hydrogen gas surrounding Saturn’s visible rings are
especially interesting. The rings themselves are presumably the source
of this hydrogen. On the planet’s disk, the ultraviolet observations
show significant latitude variations, suggesting the possibility of
aurora near Saturn’s polar regions. A similar extensive cloud of
hydrogen was also seen partially surrounding Titan’s orbit.
These very preliminary findings by Pioneer Saturn represent only a small
fraction of what will ultimately be learned about Saturn and its
environment as the spacecraft data are analyzed in greater detail over
the weeks and months ahead.
[Illustration: Schematic of the solar wind interaction with Saturn’s
magnetosphere. The solar wind arrives from the direction of the sun,
is deflected at Saturn’s bow shock, and flows around Saturn in the
magnetosheath (orange region), as indicated by the arrows. The sizes
of the magnetosheath and radiation belts change in response to the
external solar-wind pressure, becoming smaller when the external
pressure is larger and vice versa.]
[Illustration: Diagram of Saturn’s inner trapped radiation belts.
The energetic particle fluxes generally become more intense closer
to Saturn. Decreases in particle flux at the locations of Saturn’s
moons are due to the sweeping up of the energetic particles caused
by particles striking the moons and being absorbed by them. Also,
there are decreases in particle flux at the outer edge of the rings,
where the energetic particles are also absorbed, so that a region
free of trapped radiation is created from the outer edge of the
rings to Saturn.]
[Illustration: Infrared radiometer image of Saturn and its rings.
Brightness in the image is related to temperature, with the
brightest areas at about 100 K (about -280°F). The left version
shows contrast in the colder regions (the rings). The right version
shows contrast in the warmer regions (the planet). The image
contains many separate scans from top to bottom. Each scan is
displaced to the right from the one before by the motion of the
spacecraft. The spacecraft was below the ring plane during most of
the 3-hour observation period and was much closer to the planet at
the end of the period (right of image) than at the start (left of
image). Thus, the images are quite distorted. The small-scale
pattern is instrument noise. In the left image, the warm infrared
radiation from the planet is seen through Cassini’s Division between
the A- and B-rings. The brightness level in the right image implies
that Saturn emits heat at a rate that is 2.5 times the rate that it
absorbs energy from the sun.]