Preliminary science results of the Voyager 1 encounter of the planet Saturn are reported. On August 22, 1980, the spacecraft was 109 million km (68 million mi) from Saturn. Closest approach to Saturn took place on November 12, at 3:46 p.m. (PDT), when the spacecraft passed 126,000 km (78,000 mi) from the cloud tops. Measurements of the atmosphere, wind speed, radiation, six surrounding rings, and the planet's old and newly found satellites were recorded. The encounter ended December 15, 1980. The spacecraft took more than 17,500 photographs of Saturn and its satellites

Bane, D.

English

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National Aeronautics and
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Charles Redmond
Headquarters, Washington, D.C.
(Phone: 202/755-3680)
Don Bane
NASA Jet Propulsion Laboratory, Pasadena, Calif.
(Phone: 213/354-5011)
For
IMMEDIATE
RELEASE NO: 81-44
PRELIMINARY SCIENCE RESULTS OF VOYAGER 1 SATURN ENCOUNTER
| Voyager 1's encounter with Saturn began Aug. 22, 1980, when
the spacecraft was 109 million kilometers (68 million miles) from
Saturn. Closest approach to Saturn took place at 3:46 p.m. (PDT)
Nov. 12, when the spacecraft passed 126,000 km (78,000 mi.) from
the cloud tops. The encounter ended Dec. 15, 1980. The space-
craft took more than 17,500 photographs of Saturn and its
satellites.
March 30, 1981
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Scientific results of the encounter are as follows:
Saturn >-jfl6<" ^ ^ v^b^Ol^
From the spacecraft, as from Earth, Saturn's atmosphere
appears grossly similar to Jupiter's, with alternating dark belts
and light zones, circulating storm regions and other discrete
dark and light cloud markings. Unlike Jupiter, however, Saturn's
markings are strongly muted by a thick haze layer above the
visible cloud tops. (Jupiter has a similar haze layer, but it is
not as optically thick as Saturn's.) Because Saturn is "colder,
the cloud layers are deeper in the atmosphere than at Jupiter and
appear blander.
Wind speeds in the atmosphere of Saturn are substantially
higher than on Jupiter, and do not appear closely tied to the
belt-zone boundaries. Highest winds (about 1,770 km or 1,100 mi.
an hour) blow eastward at the equator and are four times stronger
than on Jupiter. The velocity decreases smoothly to near zero at
about 40 degrees latitude north and south.
The bulk of Saturn's atmosphere is composed of hydrogen.
Helium accounts for only about 11 percent of the mass of the at-
mosphere above the clouds, compared to an abundance of about 19
percent at Jupiter. The difference is consistent with gravita-
tional separation of helium and hydrogen in Saturn's interior and
could generate the excess energy radiated by Saturn over that
received from the Sun.
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Voyager 1 found aurora-like emissions near the illuminated
limbs of the planet, and auroras in ultraviolet light were found
in a ring near 80 degr'eVs'''-south*" iaftrtu3eOS:ci-entists faced a dif-
ficult and unexpected problem in attempts to detect lightning on
the dark side of Saturn, since the rings reflected too much light
onto the night side. Although lightning has not been detected in
images of Saturn's dark face, radio emissions typical of electric
discharges have been observed. Those discharges originated in the
rings.
Radio emissions, primarily from the north polar region and
near 90 degrees longitude, indicated that the body of Saturn and
its magnetosphere rotate with a period of 10 hours, 39 minutes
and 26 seconds.
The Rings
Voyager 1 found that the classically observed A-, B- and C-
rings consist of hundreds of small ringlets, a few of which are
elliptical in shape. Even the classical gaps were seen to con-
tain ringlets; the Cassini Division appears to contain at least
five, each of which shows finer detail. The F-ring, discovered
by Pioneer 11 in 1979, is composed of three separate ringlets
that appear to be intertwined. The inner and outer limits of the
F-ring are controlled by two shepherd satellites, 1980S26 on the
outside and 1980S27 on the inside. The intertwining or "braiding"
phenomenon is unexplained — although it may be related to elec-
trostatic charging of the dust-size particles comprising the
ring.
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Near the outer edge of the A-ring orbits the newly discovered
satellite 1980S28. All three of those satellites (1980S26, 1980S27
and 1980S28) were discovered by Voyager 1. .
Voyager 1 also photographed the D- and E-rings, during pas-
sage through Saturn's shadow, and confirmed a new ring near 2.8
Saturn radii from the center of the planet. The existence of
that ring had been postulated on the basis of Pioneer 11 fields
and particles data.
Measurements show that the E- and F-rings have large popu-
lations of particles smaller than about 2/10,000th of an inch in
diameter. Radio measurements of the C-ring indicate that it has
an effective particle size of about 2 m (6 ft.), and, at the same
time, suggest a wide distribution of particle sizes. Effective
particle sizes for the A-ring and the Cassini Division are 10 and
8 m (33 and 26 ft.), respectively.
On its inbound leg, Voyager 1 discovered a series of trans-
ient, spoke-like features that radiate outward across the B-ring;
they first appear as the ring emerges from darkness and seem to
dissipate within a few hours. In photos taken during the inbound
portion of the flight, the spoke-like features appeared darker
than the surrounding ring material.
After it passed Saturn, Voyager 1 took photographs of the
spoke-like features that showed they were brighter than surround-
ing material. The material in the spokes forward-scatters sun-
light effectively, implying extremely small particles.
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It is possible that the fine material is levitated above the
rings. The electric discharges in;:the Brings may be associated
with the spokes.
New Satellites
Photographs of the new satellites were used to measure their
size. The innermost satellite, temporarily designated 1980S28, is
about 30 km (19 mi.) in diameter. The next satellite, 1980S27,
which shepherds the inner edge of the F-ring, is about 220 km (137
mi.) in diameter. Next is 1980S26, outer shepherd of the F-ring,
about 200 km (124 mi.) in diameter. 1980S3 and 1980S1, which
share an orbit 91,120 km (56,620 mi.) from the clouds, are 90 by
40 km (56 by 25 mi.) and 100 by 90 km (62 by 56 mi.), respec-
tively. And 1980S6, the new satellite that occupies the same
orbit as Dione (about 60 degrees ahead) is about 160 km (100 mi.)
in diameter. Other satellites may be discovered as voyager scien-
tists sift through the mountains of data returned from the space-
craft's encounter — or from photos to be returned in the summer
of 1981 by Voyager 2.
Other Satellites
Voyager 1 observed all Saturn's known satellites except
Phoebe. Mimas, Enceladus, Tethys, Dione and Rhea are approx-
imately spherical in shape and appear to be composed mostly of
water ice.
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Tethys in particular seems to be almost pure ice, while
Dione has a density that indicates it is 40 percent rock. All
five are of sizes not previously explored by spacecraft. Their
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measured diameters, accurate"to about 20 km (12 mi.), are: Mimas,
390 km (240 mi.); Enceladus, 500 km (310 mi.); Tethys, 1,050 km
(650 mi.); Dione, 1,120 km (700 mi.); and Rhea, 1,530 km (950
mi.).
Mimas, Tethys, Dione and Rhea are all cratered; Enceladus
appears smooth. One very prominent crater on Mimas is so large it
covers about one-third the diameter of the satellite. Stretching
for 750 km (465 mi.) across the surface of Tethys is a valley
that is 60 km (37 mi.) wide. The valley appears to be a great
fracture in the crust of the satellite. Several sinuous valleys,
some of which appear to branch, are visible on Dione1s surface,
as are smooth plains, suggestive of internal processes and re-
newal of portions of the surface.
Both Dione and Rhea have bright, wispy streaks that stand
out against an already-bright surface. The streaks are probably
the result of relatively fresh ice that has migrated from the in-
terior fairly recently (on a geologic time scale).
Enceladus shows no evidence, at 12 km (7 mi.) resolution, of
any impact craters. Because the maximum intensity of the E-ring
occurs at the orbit of Enceladus, it has been speculated that the
satellite is the source of particles for that ring; it also may
be that tidal stresses cause a surface renewal similar to
Jupiter's satellite Europa.
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Enceladus was not a prime target for Voyager 1; Voyager 2
will fly closer and return higher-resolution pictures of that
satellite.
Hyperion's and lapetus1 masses are poorly known, so their
densities are uncertain. It is, however, likely that they too
are mostly water ice, although lapetus is peculiar because it has
one bright and one dark hemisphere. The dark side, which faces
forward as lapetus orbits Saturn, reflects only one-fifth as much
light as the bright, trailing side1* Hyperion, which also has a
somewhat darker surface, has a diameter of about 290 km (180
mi.), and lapetus has a diameter of about 1,440 km (895 mi.).
Titan
Because of its unique atmosphere, Titan may turn out to be
the most important and interesting body, from a terrestrial per-
spective, in the solar system.
For almost two decades, space scientists have searched for
clues to the primeval Earth. At Titan, they found an atmosphere
similar to the one that would have evolved on Earth, had Earth
formed at Titan's distance from the Sun.
Titan was thought from Earth-based observations to be the
largest satellite in the solar system. Voyager's close approach
and diametric occultation show it to have a diameter of 5,140 km
(3,194 mi.) — slightly smaller than Ganymede, Jupiter's largest
satellite. Both are larger than the planet Mercury.
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Titan's density thus appears to be about twice that of water
ice, requiring Titan to be composed of nearly equal amounts of
rock and ice, as is Ganymede.
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Titan's surface cannot be seen in photos from Voyager 1; it
is hidden by a dense, optically thick haze. Several distinct,
detached haze layers can be seen above the visibly opaque haze
layer. The layers merge into a darkened hood over the north pole
of Titan. The southern hemisphere is slightly brighter than the
northern, possibly the result of seasonal effects. When Voyager 1
flew past, the season on Titan was the equivalent of early April
on Earth, or early spring in the northern hemisphere and early
fall in the south.
The atmospheric pressure near Titan's surface is about 1.6
bars, 60 percent greater than Earth's. The atmosphere is composed
mainly of nitrogen, the major constituent of Earth's atmosphere.
The surface temperature appears to be about 93 degrees
Kelvin (-294 degrees F), close to the triple-point temperature of
methane. Methane, therefore, probably plays the same role on
Titan as water does on Earth — as rain, snow, ice and vapor.
Rivers of methane may cut through glaciers of methane under a
nitrogen sky. Clouds may drop liquid-methane rain on the surface.
Titan's methane, through continuing organic chemistry, is
converted to ethane, acetylene, ethylene and (when combined with
nitrogen) hydrogen cyanide.
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The last is an especially important molecule, since it is a
building block of amino acids. However, Titan's low temperature
undoubtedly inhibits mOr-erc6mp.le^ -r/6rganic^ hejnl?s]try.
Titan has no intrinsic magnetic field; therefore it has no
electrically conducting liquid core. Its interaction with
Saturn's magnetosphere does create an induced dipolar field in
Titan's wake, however. The big satellite also serves as a source
for neutral and charged particles in Saturn's magnetosphere.
The Magnetosphere
While Saturn's magnetosphere is only about one-third the
size of Jupiter's, it is still a huge stucture, extending nearly
a million miles from the planet toward the Sun before the flow of
charged particles in the solar wind overcomes the effects of
Saturn's magnetic field. As at Jupiter, charged particles in
Saturn's magnetic field are dragged along by the magnetic field
and circle the planet once in 10 hours, 39 minutes, 26 seconds.
At Titan's orbit, those particles pass the satellite at a
relative speed of 200 km (124 mi.) per second. The size of
Saturn's magnetic field fluctuates as the solar wind pressure
increases and decreases. As a result, Titan is generally inside
but sometimes outside the magnetosphere.
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Titan was inside Saturn's magnetosphere as Voyager 1 flew
past and observed the wake resulting;^from^fehe flow past Titan of
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the electrically chargecl i"bhs carried'around by Saturn's rotating
magnetic field. Radio emissions were also observed emanating
from the wake region.
Surrounding Titan and its orbit, and extending to Rhea's
orbit, is a cloud of uncharged hydrogen atoms forming an enormous
hydrogen torus. A disk of plasma, composed of hydrogen and pos-
sibly oxygen ions, extends from outside the orbit of Tethys al-
most to the orbit of Titan. The plasma is in nearly full corota-
tion with Saturn's magnet field.
Voyager 1 is now headed out of the solar system. Its scan-
platform instruments were turned off Dec. 19, 1980. Most fields
and particles instruments continue to operate, monitoring the
solar wind and its changes with distance and time.- Although the
exact location of the heliopause (the outer edge of the solar
wind) is unknown, it is possible Voyager 1 will reach it sometime
around 1990. Even before that time, however, Voyager may detect
low-energy cosmic rays penetrating into the outer reaches of the
solar system from nearby supernova remnants.
Voyager 2, meanwhile, will make its closest approach to
Saturn Aug. 25, 1981, and then continue on a trajectory toward
Uranus.
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(Index: 6, 23, 36, 44)
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Preliminary science results of Voyager 1 Saturn encounter

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