Voyagar
Enc punte rs
J up Her

 

 

   

      
 
  

  

‘ tmosphere was made from ten color
‘ images taken by Voyager 1 during a single
ten-hourrotation of the planet.

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s A ~ ' «n pole. This videlpwe v .
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black area at the pole results from
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Voyager
Encpunte rs
I up 1ter

July 1979

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Foreword

 

 

In late summer of 1977. the United States launched two unmanned Voyager spacecraft on
an extensive reconnaissance of the outer planets, a decade-long odyssey that could take them
to 3 planets and as many as 18 planetary satellites. The first encounter was with the giant
Jovian planetary system, 645 million kilometers (400 million miles) away. Passing by Jupiter
and its complex satellite system in 1979, the Voyager spacecraft have collected and returned to
Earth an enormous amount of data and information that may prove to be a keystone in under-
standing our solar system.

This publication provides an early look at the Jovian planetary system and contains a
selected sample from the more than 30,000 images collected during this phase ofthe Voyager
mission. While Voyager achieved an impressive record of accomplishments, full realization of
the scientific value ofthis program must await the remaining Voyager encounters with Saturn
and perhaps Uranus. and a detailed analysis ofthe data from all the spacecraft investigations.

ROBERT/A.FROSCH,AdanBUWHW
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Introduction

In March 1979 Voyager 1 swept past Jupiter. photo-
graphing both the giant planet and tive of its moons. Four
months later. a companion spacecraft. Voyager 2. made a
similar encounter. Now. with Jupiter receding behind them.
both spacecraft are headed toward the outer reaches of our
solar system. In November 1980. Voyager 1 will fly past Sat-
urn. Voyager 2. traveling at slower speeds. will reach the
same way station in August 1981. Beyond there. the itin-
erary is less certain. In January 1986. eight years after its
departure from Earth. Voyager 2 may sail within range of
Uranus. taking closeup pictures ofthat distant planet for the
first time. Long after they have exhausted their fuel supplies
and their radios have fallen silent. both spacecraft will con-
tinue their traverse through space and beyond our solar sys-
tem. on an endlessjourney.

Preliminary results ofthe Voyager encounters with Jupiter
are presented in this booklet. As you examine the pictures.
you will be participating in a revolutionaryjourney ofexplo-
ration. Living in a society where many accomplishments
and products are billed as "extraordinary." “stupendous.“
“once in a lifetime.” or “unique." we sometimes lose our per-
spective. Conditioned to hyperbole. we fail to recognize those
advances that are truly exceptional. We need a historian‘s
vantage point to identify the events that can literally change
the course of civilization. So it is that every student of his-
tory recognizes the importance of the Renaissance. an
extraordinary time when man looked outward. reaching
beyond the traditions of the past to study his place in the

natural world. The results were apparent in art. architecture.
and literature. in new philosophic and governmental sys—
tems. and in the staggering scientific revolution exemplified
by Galileo‘s first examination of the heavens with a tele-
scope. and in his stubborn support ofthe heretical assertion
that the Earth was not the center of the solar system.

Historians writing a hundred or two hundred years from
now may well look on the latter part of the twentieth century
as another turning point in civilization. For the first time. we
explored beyond Earth wfirst the Moon. then the neighbor-
ing planets. and finally the outermost planets. the very fringe
of our solar system.

How will the historian evaluate this period of explora—
tion‘? First. perhaps. he will describe the Apollo program as
a visionary example ofgreat cooperative ventures that can
be accomplished by many individuals. private companies.
and government institutions. He will describe the subse—
quent space ventures that weave a fabric of cooperation and
goodwill between nations.

He will point out the technological advances incor-
porated in unmanned spacecraft. sophisticated robots able
to control their own activities and solve their own problems.
He will mention the revolution in microelectronics ithe art
of fabricating complex electrical control circuits so small the
eye cannot perceive them. a revolution accelerated by the
requirement to conserve weight and generate performance
in interplanetary spacecraft. He will point to the introduc-

 

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m/ma' (UNI/HM ri/lt'r .i’U [NONI/H (\pmm‘c on lunar surmt t',

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\A l‘ 0/ \lilrv

 

(ialileo orbiter rim/probe mission to Jupiter in [985 will expand upon the
“Auger inrest/gallons o/ the Jovian si'stein.

tion of new products, particularly in areas of communica-
tion. medical treatment. and energy conversion.

Turning his attention to the environment. the historian
will almost surely suggest that the first widespread realiza-
tion of the fragile natural balances on Earth came at a time
when we were first able to see our Earth in its entirety. The
impact ofa picture of Earth from deep space. a luminously
blue globe surrounded by darkness. has probably been more
persuasive than lengthy treatises describing the complex ways
in which our system of rocks. plants. animals. water. and air
is interrelated. ()n a more practical level. the historian will
point to the new understanding of our terrestrial environ—
ment. The composition and structure of other planetary
atmospheres“ on Venus. Mars. and Jupiter— provide
important clues to what may happen in our own atmo-
sphere. especially if we disrupt the chemical composition.
Study of the primitive crusts of the Moon. Mars. and Mer-
cury permits us to reconstruct the first billion years of Earth
history. a time when chemical elements were being concen-
trated in activity ultimately leading to the formation of
important ore deposits. Unmanned spacecraft missions to
the Sun increase our understanding ofthat most fu ndamen-
tal ofall energy sources. paving the way for the efficient con-
version ofsolar energy into many practical applications. and
releasing us from dependence on ever-decreasing reserves
of fossil fuels. Spacecraft circling the Earth study the upper
atmospheric processes that play major roles in controlling
our weather. These same spacecraft look down on Earth.

 

.1 solar electric propulsion spacecraft would eject an instrumented probe
toward Hit/let‘s comet in [980 and continue on to renden‘ous u‘itli
another comet. Tempe/ 3.

aiding us with increasingly accurate forecasts ofweather and
crop productivity.

Looking beyond matters of technology and the envi-
ronment. the historian may cite the latter part of the twen-
tieth century as a time ofexplosive exploration. comparable
to the 15th and 16th century exploration of the Earth‘s oceans
and the distant lands that bounded them. In a sense. explo-
rationiiwhether it is physical or intellectualmprovides its
own rewards. The United States has always been a nation
that moves forward. pushing back the frontiers ofthe West.
pushing back the frontiers of social and economic develop-
ment. and now pushing back the frontiers of space. It is
arguable that this spirit of exploration is indispensable to a
vigorous society. and that any society that ceases to explore.
to inquire. and to strive is only a few years from decline.

And so the historian may recall the early days oflunar
exploration. the Apollo project. the landing of unmanned
Viking spacecraft on Mars. and the encounters of Voyager
spacecraft with Jupiter and Saturn as the first steps in a sus—
tained program of space exploration Va program that is
profoundly changing man‘s perspective of himself. of the
Earth. and ofthe larger cosmos beyond.

THOMAS A. MUTCH. Associate Administrator
for Space Science
National Aeronautics and Space Administration

Images of Jupiter and Its Satellites

 

1 x24 ,/ 79 40 million km (25 million ml)

46 3 mllhon km (28 7 million ml)

5 ,r’ 9 / 79

 

 

Jupiter’s atmosphere is undergoing constant
Change. presenting an ever—shifting face to
observers. The Great Red Spot has
undergone three major periods of activity
in the last 15 years. These images ofJupiter.
taken by Voyager 1 (top) and Voyager 2
(bottom) almost four months apart. show
that Cloud movement in the Jovian
atmosphere is not uniform because wind
speeds vary at different latitudes. For
example. the white ovals which appear
below the Great Red Spot dramatically
shifted between January and May. the time
interval between these two pictures. The
bright “tongue” extending upward from the
Great Red Spot interacted with a thin.
bright cloud above it that had traveled twice
around Jupiter in four months. Eddy pat-
terns to the left ofthe Great Red Spot.
which have been observed since 1975.
appear to be breaking up.

<1Jupiter is the largest planet in our solar system. with a diameter ll times that of Earth.
Jupiter rotates very quickly, making one full rotation injust under ten hours.
Composed primarily of hydrogen and helium. Jupiter’s colorfully banded atmosphere
displays complex patterns highlighted by the Great Red Spot. a large. circulating
atmospheric disturbance. Three ofJupiter’s l3 known satellites are also visible in this
Voyager 1 photograph.The innermost large satellite. lo. can be seen in front ofJupiter
and is distinguished by its bright. orange surface. To the right ofJupiter is Europa. also
very bright but with fainter surface markings. (‘allisto is barely visible beneath Jupiter.
These satellites orbit Jupiter in the equatorial plane and appear in their present

position because Voyager is above the plane.

The date ofeach photograph and the distance ofthe spacecraft from the planet

or satellite are included with each picture.

 

'l‘hc (Qr ‘ztt Red Spot on Jupiter is .’l ti'einendtitis ntntosphei‘ie SIOHTL t\\ tee the si/e tit
lizii‘th. thtit has heen tihsened thi‘ eentttt‘ies. the (hen Red Spot nitttles L‘Ollltlt‘t'eh‘n‘h“ ise
\Vilh one i‘thihititin e\ei') si\ d;1_\s, \Vind ettt'i‘ents on the tap titm east to \xest. tinti
etii‘i‘ents mt the htittmn titm west to enst lhis \tnngei‘ I piettii‘e shtMs the eoniple\ tin“

and ttit‘httlent patterns that result t‘niin the (hen Red Spot‘s lltlL‘I'JCIit‘Its \\11it these
flows, the large white mnl is it siniilni; hilt sinnliei‘. stoi’ni eentei' th.it hits C\l\1Cki

{or nhotit 40 M‘tlt‘s:

A comparison of the Voyager 2 photograph above with the preceding Voyager 1
photograph shows several distinct changes in the Jovian atmosphere around the Great
Red Spot. The white oval beneath the Great Red Spot in the first picture has moved
farther around Jupiter, and a different white oval has appeared under the Great

Red Spot in the Voyager 2 picture taken four months later. The disturbed cloud
regions around the Great Red Spot have noticeably changed. and the white zone west
of the Great Red Spot has narrowed.

 

7/3/79

6 million km

(3 72 million mi)

 

High-speed wind currents in the mid-
latitudes of Jupiter are shown in this high-
resolution Voyager 1 photograph. The pale
orange line running diagonally to the upper
right is the high-speed north temperate
current with a wind speed of about 120
meters per second (260 miles per hour), over
twice as fast as severe hurricane winds on
Earth. Toward the top of the picture, a
weaker jet of approximately 30 meters per
second (65 miles per hour) is characterized
by wave patterns and cloud features that
rotate in a clockwise manner.

The large brown-colored oval appearing in
this Voyager 1 picture was selected as one of
the targets to be photographed near closest
approach to Jupiter because it is probably
an opening in the upper cloud deck that
exposes deeper, warmer cloud levels. Brown
ovals (which can also be seen in the
photographs above and on the next page)
are common features in Jupiter’s northern
latitudes and have an average lifetime of one
to two years.

Jupiter’s Equatorial Zone is the broad. orange band that traverses the center of this
Voyager 2 picture. This zone is characterized by the wispy clouds along its northern edge.
The brown oval was observed by Voyager 1 four months earlier. illustrating the stability of
this type of feature in the Jovian atmosphere. In contrast. the turbulent region in the
lower right of the picture. which lies just to the left of the Great Red Spot. shows features
that are relatively short lived. With the exception of the cooler Great Red Spot. as colors
range from white to orange to brown. we are generally looking at deeper and warmer
layers in the Jovian atmosphere.

 

4 million km (2.5 million mi)

3/2/79

4 million km (25 million mi)

3/2/79

10 3 million km (6.4 million mi)

6/28/79

 

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2:3ng

 

 

The infrared image ()l‘Jupiter on the left “‘le taken trom lurth tind \lto\\\ heiit
radiating from deep holes in Jupiter‘s eloudx. Bright areux in the huge .ire higher

temperature regions than the dark :treux Lttttl eorrexpond to purtx ot‘the tunioxphere thiit
are relatively tree ol‘ohseuring clouds, 'l‘he (ireut Red Spot uppeitrx on the let‘t limh or
edge ol‘the planet. as Al durk urea eneireled h) it bright ring. indieiiting th.it the Spot ix
cooler than the surrounding region. 'l’he intiured image \\;1\ reeorded l‘.‘ the Itttt—ineh
llillC telescope on Mount l’ulonitir in (‘itlitornur l'he \'o_\.1ger l pieture on the right \Mix
also taken the same dd}; iihout one hour utter the intr.1red nudge.

12

The largest aurora ever observed. nearly 29.000 kilometers ( 18.000 miles) long.
appears in this Voyager 1 photograph. taken on the dark side ot‘Jupiter six hours after

closest encounter. The auroral lights are brighter than any northern lights seen on
Earth. Jupiter’s north pole is approximately midway along the auroral arc. This timed
exposure ofthe aurora also shows what appear to be lightning storms several thousand
kilometers below the aurora. The strength ofthe lightning bolts is comparable to that
ofsuperbolts seen near cloud tops above Earth. Lightning had been suspected to exist
on Jupiter. but at lower levels in the atmosphere.

3,“.‘35 79 515 000 km (320,000 ml)

 

.fi

"s

 

it

The first evidence ofa ring around Jupiter is
seen in this photograph taken by Voyager 1.
This photograph was part ofa sequence
planned to search for such rings around
Jupiter. The multiple image ofthe extreme];
thin. faint ring appears as a broad light
band crossing the center ofthe picture This
multiple image and the elongated. wavy
motion of‘the background stars are due to
the ll~minute. l2—sec0nd exposure and the
very slow natural oscillation of the
spacecraft. The ring. which is in Jupiter's
equatorial plane. is imisible from Earth
because ofits thinness and transparenc} and
because ofJupiter‘s brightness. The black
dots in the picture are calibration points in
the camera.

Because ofV'oyager l‘s (liscoi'er) ota ring around Jupiter. \twager 2 was programmed to
take additional pictures ol‘the ring We three \twagcr 3 images on the new page show
Jupiter's ring in progressiwh higher resolution The pictures were taken when Jupiter
was eclipsed b_\ the Sun. and the ring appears unusuall_\ bright because ot‘ the torward
scattering ol‘sunlight lw small ring particles. In the top tour—picture mosaic. the arms ot‘
the ring cuning toward the spacecraft (on the near side ot‘ the planet) are cut otl‘ b_\ the
planet‘s shadow. Scientists estimate that the distance trom the .lo\ ian cloud tops to the
outer edge ot‘the ring is 55.000 kilometers (35.000 miles). In the center picture. which is
composed ot‘six images. there is c\ idence ot‘structure within the ring. btit the spacecraft
motion during these long e\posures obscured the highest resolution detail. llowe\ er. there
is speculation that the ring width. estimated at 0000 kilometers (4000 miles). contains
more than one ring. l‘he bottom photograph is an enlargement ot‘ the isolated let‘t trame
in the lirst picture and re\‘eals a densit} gradient olWet‘} small particles e\tendtng inward
li‘om the ring. The thickness ol‘the ring has been estimated at less than one kilometer
(0.6 mile) although the ring appears about 30 kilometers t 1*) miles) thick in the image.
due to camera motion and tinite resolution. Composition ot'the low —albedo tdark)
particles is not know it. btit particle si/c probably ranges from microscopic to at most a
lcw meters in diameter. ll‘collected together to torm a single bod_\. the total mass ot‘ the
.loVian rings would lorm an object with a diameter less than twice that ot‘tin} .»\malthea.

      

7 7O 79 ‘ASmHMon km (900,000 ml)

7 TO 79 ’55 mHHofl km {961000 rm)

   

 

7 10 79 145 mllhon km (900 000 mm

15

 

Jupiter and two ”fits planet-sized satellites. 10.11 [MI and {wow .u thL are \ MW:
in this. Voyager 1 picture. Jupiter‘s {our lurgcxt NHCHHL‘N lo. I uropu. {hm} mcdc Jud
(ullislo wcrc discmrrcd in 101013} (‘1;1lilcofiulllc1. The No uulcr‘ (Mlllk‘JH \ucHucx
arc(};111_\'Incdc;1ml (l’lllislu. not slnmn in [his picture. \11 Iluu‘ \ucllum prolmbl}
formed ulmut Ibur hilliun _\c;ui\ ugu but their .xurtlu‘ux mm m ugc le‘zucmhuhl}. [0.11M
liurupu haw _\nungcr_ more ucmc surllu‘m than (him mcdc .md (ullmu. l ikc our
Mom]. lhc .xutclliu‘s keep the sumo 111cc Immrd Jupiter. lu 1111s picture. Ihc sub oflhc

sulcllilcs [hut ulmus [11cc ;1\\;1\ Hum Ihc pluucl .u’c \ ixlhlc.

l6

1.25 mlllion km (780,000 mi)

3/4/79

695,000 km (430,000 mi)

3/4/79

425,000 km (264,000 mt)

3/5/79

 

 

 

 

 

 

 

Amalthea, Jupiter‘s innermost satellite.

was discovered in 1892. It is so small and
close to Jupiter that it is extremely difficult
to observe from Earth. Amalthea’s surface
is dark and red. quite unlike any ol‘the
Galilean satellites. The three Voyager 1
pictures on the left and the one Voyager 2
picture above (seen against the disk of
Jupiter) reveal a small. elongated object.
about 265 kilometers ( I65 miles) long and
150 kih)nneters(9()ntues)in dianieter
Amalthea keeps its long axis pointed toward
Jupiter as it orbits around the planet every
12 hours. Amalthea was observed end-on in
the beagerZ pkflure.uluch haslfieen
computer-processed to enhance the image.

(350 000 mil

560.000 km

9 /9

I

 

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hrighlncsx. l'his \knuucx' l Il»l11‘~pu'll|1\‘111«>_\m‘\l1u\\\ Ink cumplm polmmuon u!‘ rui—

nl’nnguhl;1c1\.;md“hilcI‘cgimeAIMIhchwnmm Iopugmphly IL‘JIIII‘CV \nlmmg
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lmmghl In Ihc surllu‘v In \nlc.1nlx'.1cll\11_\.

(80,500 mi)

129.600 km

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66.000 km

The bright area at the upper right in

this Voyager 1 picture of lo appears to be a
caldera (collapsed volcano) that is venting
clouds of gases. The clouds may condense
to form extremely fine particles that scatter
light and appear blue. Because the infrared
spectrometer discovered sulfur dioxide on
Io. scientists believe this gas may be the
main component ofthe clouds. Sulfur
dioxide clouds would rapidly freeze and
snow back to the surface. It is also possible
that dark areas in the floors of the calderas
are pools of encrusted liquid sulfur.

Evidence of erosion in 10’s southern polar
region is visible in this Voyager 1 high-
resolution image. The picture has been com—
puter-enhanced to bring out surface detail
while suppressing bright markings. A
depressed segment of the crust, bounded by
faults. is seen near the terminator in the
upper right portion of the image. At the
lower center are complicated scarps (slopes)
and portions of isolated elevated terrain that
geologists interpret as “islands” left behind
as the scarps eroded. Scientists speculate
that sulfur dioxide (as a subsurface liquid)
may be a determinant in the creation of
these features.

 

 

 

 

 

 

 
 

 
 
 

1.2 million km (750.000 mi)

7/8/79

Europa’s surface is probably a thin ice crust
overlying water or softer ice (slush) about

100 kilometers (60 miles) thick that covers a
silicate interior. The tectonic processes on
Europa’s surface create patterns that are
drastically different from the fault systems seen on
Ganymede’s surface, where pieces of the

crust have moved relative to each other. On
Europa, the crust evidently fractures. but the
pieces remain roughly in their original position.
This Voyager 2 picture is composed of

three images.

 

7/9/79 240000 km (150.000 ml)

<1Eur0pa, approximately the same size and density as our Moon. is the brightest
Galilean satellite. The surface displays a complex array ofstreaks. indicating that the crust
has been fractured. In contrast to its icy neighbors Ganymede and Callisto. Europa has
very few impact craters. The relative absence of features and low topography
indicate that the crust is young and probably warm a few kilometers below the
surface. The warmth is probably due to a combination of radioactive and tidal
heating. The tidal heating within Europa is estimated to be ten percent that ofthe
stronger tidal heating elTect within Io. The regions that appear blue in this Voyager 2
image are actually white.

 

2 6 million km (1.6 million mi)

d/4/l9

7/7/79 1.2 million km (750,000 ml)

 

The dark, cratered, circular feature in this Voyager 2 photograph is about 3200
kilometers (2000 miles) in diameter and is on the side of Ganymede opposite to that
shown in the previous picture. This region is apparently the largest piece of ancient.
heavily cratered crust left on Ganymede. The light branching bands are ridged and
grooved terrain which are younger than the more heavily cratered dark regions.
Despite the dramatic surface appearance. Ganymede is relatively devoid of topo-
graphic relief due to the consequences of glacier-like “creep” in the icy crust.

<1Ganymede, Jupiter’s largest satellite. is about one and one-halftimes the size ofour

Moon but only about half as dense and is composed of about 50 percent water or ice
and the rest rock. The bright surface of Ganymede is a complex montage ofancient,
relatively dark and cratered terrain, grooved terrain that resulted from a dramatic
history oftectonic movement in the icy crust. and bright young ray craters that expose
fresh ice. This photograph was taken by Voyager l.

 

 

 

 

 

Several different types of terrain common to (lanymede‘s surface are Visible in this
Voyager 2 picture. The boundary ofthe largest region of dark ancient terrain (also sho\\ n
in the previous photo) can be seen to the right. re\'ealing the light linear features that ma}
be the remains ofshock rings from an ancient impact. The broad light regions are the
typical grooved structures contained within the light regions on Ganymede. On the lower
left is another example of what might be evidence of large—scale lateral faulting in the

crust: the band appears to be offset by a linear feature perpendicular to it. These are the
first Clear examples of lateral faulting seen on any planet other than liarth.

 

7/8/79 313.000 km (194.500 mi)

 

This color reconstruction of part of Ganymede’s northern hemisphere. taken by Voyager
2, encompasses an area about 1300 kilometers (800 miles) across. It shows part of a dark,
densely cratered region that contains numerous craters, many with central peaks.

The large bright circular features have little relief and are probably the remnants of
old, large craters that have been annealed by the flow oficy material near the

surface. The gradually curving lines that press through the dark region suggest the
presence of a large impact basin to the southwest. which has been obliterated by the
subsequent formation of younger grooved terrain.

31

 

 

 

 

2.3 million km (14 million mi)

7/7/79

The prominent concentric ring structure
shown in this Voyager 1 four-picture
mosaic of Callisto is believed to be a large
impact basin, similar to Mare Orientale

on the Moon and Caloris Basin on Mer-
cury. The bright circular spot is about 600
kilometers (360 miles) across, and the outer
ring is about 2600 kilometers (l560

miles) across. This is the first recognized
basin in the Jovian system and supports the
assumption that Callisto’s surface is old.
The lack of high ridges. ring mountains. or
a large central depression suggests that the
impacting body caused melting. some flow.
and shock waves. and that the refreezing
occurred in time to preserve the concentric
shock rings.

 

3/679 350000 km (217.000 mi)

<1Callist0, only slightly smaller than Ganymede. has the lowest density of all the
Galilean satellites. implying that it has large amounts of water in its bulk composition
lts surface is darker than the other Galilean satellites. although it is still twice as bright
as our Moon. This Voyager 2 image shows (,‘allisto to have the most heavily cratered
and. therefore, the oldest surface of the Galilean satellites. probably dating back to the
period of heavy meteoritic bombardment ending about four billion years ago.

 

(3.7-meter diameter)

H IG H-FIELD
MAG N ETOM ETER

LOW-FIELD
MAG NETOME'I‘ER (2)

RADIOISOTOPE
TI I ERMOELEC'I‘RIC GENERATOR (3)
\

\

PLANETARY RADIO ASTRONOMY ANI) /
PLASMA WAVE ANTENNA (2)

 

HIGH-GAIN ANTENNA

   
 
 
  
    
 
 
 

I 'L'I‘RAVIOLET SPECTROME'I'ER
PLASMA IMAGING

COSMIC RAY
‘\

INFRARED
SPECI‘ROME'I‘ER
AND RADIO.“ ETER

 

PIIOTOI’OI .ARIM ETER

LOW-ENERGY
(‘IIARGED PARTICLE

OPTICAL CALIBRATION
’I‘ARGE'I‘

 

 

I in‘ugi’r spacecraft and .x't'l'cmi/ic [Irv/rimmrrs.

the planet at about the orbit of lo. Then. both spacecraft

disappeared behind .lupitcr. out of view of Earth and Sun.

for about two hours. During this time. measurements were
taken on the planet's dark side. Each spacecraft took over
l5.000 photographs of Jupiter and its satellites.

From the moment of launch. the Voyager spacecraft

have been monitored by a worldwide tracking system of

nine giant antennas strategically located around the world
in California. Spain. and Australia to ensure constant radio
contact with the spacecraft as the Earth rotates. Radio
contact with Voyager‘s l and 2 has not been instantaneous.
however. When Voyager I flew past Jupiter. radio signals
between Earth and the spacecraft took 37 minutes; when
Voyager 2 arrived. the signals took 52 minutes because by
then the planet was farther from Earth.

The pictures in this book were taken by a shuttered

television—type camera. Each picture is composed of

640.000 dots. which were converted into binary numbers
before being radioed to Earth. \Vhen the signals reached
Earth. they were reeonverted by' computer into dots and
reassembled into the original image. Most of the color
pictures are composed of three images. each one taken
through a dill‘erent color filter: blue. orange. or green. The

images were combined and the original color was recon—
structed by computer. The computer eliminated many of
the imperfections that crept into the images. and enhanced
some of the images by emphasizing ditlerent colors.

Designed to provide a broad spectrum of scientitic
investigations at Jupiter. the science instruments investi—
gated atmospheres. satellites. and magnetospheres. The
scientific investigations for the Voyager mission and their
Jovian encounter objectives are shown in the table on
page 40.

After their closest approaches to Jupiter. both space—
craft tired their thrusters. retargeting for their new goal.
the Saturn system. Scientists will still be studying the wealth
of new information about Jupiter when Voyager 1 reaches
Saturn in November 1080. and Voyager 3 follows in August
IQSI. After Voyager l encounters Saturn. Voyager 3 may
be retargeted to fly past l'ranus in 1980. l'pon comple«
tion oftheir planetary missions. both spacecraft will search
for the outer limit of the solar wind. that boundary some-
where in our part ofthe Milky Way where the influence of
the Sun gives way to other stars of the gala\y‘. Voyagers l
and 3 will continue to study interstellar space until the
spacecraft signals can no longer be received.

Scientific Highlights

 

Some of the most important information gathered by
Voyagers l and 2 on the Jovian system is presented picto-
rially in this book and is supplemented here with brief
summaries of the major discoveries. observations, and
theories.

Jupiter

The atmosphere of Jupiter is colorful, with cloud
bands of alternating colors. A major characteristic of the
atmosphere is the appearance of regularly spaced fea-
tures. Around the northern edge of the equator, a train of
plumes is observed, which has bright centers representa—
tive of cumulus convection similar to that seen on Earth.
At both northern and southern latitudes, cloud spots are
observed spaced almost all the way around the planet,
suggestive of wave interactions. The cloud structures in
the northern and southern hemispheres are distinctly dif-
ferent. However, the velocities between the bright zones
and dark belts appear to be symmetric about the equator,
and stable over many decades. This suggests that such
long—lived and stable features may be controlled by the
atmosphere far beneath the visible clouds. The Great Red
Spot possesses the same meteorological properties of
internal structure and counterclockwise rotation as the
smaller white spots. The color of the Great Red Spot may
indicate that it extends deep into the Jovian atmosphere.
Cloud-top lightning bolts, similar to those on Earth, have
also been found in the Jovian atmosphere. At the polar
regions, auroras have been observed. A very thin ring of
material less than one kilometer (0.6 mile) in thickness
and about 6000 kilometers (4000 miles) in radial extent has
been observed circling the planet about 55.000 kilometers
(35,000 miles) above the cloud tops.

Amalthea

Amalthea is an elongated. irregularly shaped satellite
of reddish color. It is 265 kilometers (165 miles) long and
150 kilometers (90 miles) wide. Just like the large Galilean
satellites, Amalthea is in synchronous rotation, with its long
axis always oriented toward Jupiter. At least one significant
color variation has been detected on its surface.

Io

Eight active volcanoes have been detected on Io, with
some plumes extending up to 320 kilometers (200 miles)

above the surface. Over the four-month interval between the
Voyager 1 and 2 encounters, the active volcanism appears to
have continued. Seven of the volcanoes were photographed
by Voyager 2, and six were still erupting.

The relative smoothness of 10’s surface and its volcanic
activity suggest that it has the youngest surface of J upiter’s
moons. Its surface is composed of large amounts of sulfur
and sulfur dioxide frost, which account for the primarily yel-
low-orange surface color. The volcanoes seem to eject a su f-
ficient amount of sulfur dioxide to form a doughnut-shaped
ring (torus) of ionized sulfur and oxygen atoms around Jupiter
near Io’s orbit. The Jovian magnetic field lines that go through
the torus allow particles to precipitate into the polar regions
of Jupiter, resulting in intense ultraviolet and visible auroras.

Europa

Europa, the brightest of Jupiter’s Galilean satellites.
may have a surface ofthin ice crust overlying water or softer
ice, with large—scale fracture and ridge systems appearing in
the crust. Europa has a density about three times that of
water. suggesting it is a mixture of silicate rock and some
water. Very few impact craters are visible on the surface.
implying a continual resurfacing process. perhaps by the
production of fresh ice or snow along cracks and cold
glacier-like flows.

Ganymede

Ganymede, largest of Jupiter’s l3 satellites. has bright
“young” ray craters; light, linear stripes resembling the outer
rings of a very large, ancient impact basin; grooved terrain
with many faults; and regions of dark, heavily cratered ter-
rain. Among the Galilean satellites, Ganymede probably has
the greatest variety of geologic processes recorded on its su r-
face and may be the best example for studying the evolution
of Jupiter’s inner satellites. lmbedded within J upiter’s mag-
netosphere, Ganymede is subjected to the influences of the
corotating charged-particle plasma and an interaction may
exist with this plasma. No atmosphere has been detected.

Callisto

The icy, dirt-laden surface of Callisto appears to be
very ancient and heavily cratered. The large concentric rings
indicate the remains of several enormous impact basins, cre-
ated by huge meteors crashing into the surface, and since
erased by the flow ofthe crust. Callisto‘s density (less than
twice that of water) is very close to that of Ganymede. yet

39

there is little or no evidence of the crustal motion and inter—
nal activity that is visible on Ganymede.

The Magnetosphere

Perhaps the largest structure in the solar system is the
magnetosphere of Jupiter. This is the region of space which
is filled with J upiter’s magnetic field and is bounded by the
interaction of that magnetic field with the solar wind. which
is the Sun’s outward flow of charged particles. The plasma
of electrically charged particles that exists in the magneto—
sphere is flattened into a large disk more than 4.8 million
kilometers (3 million miles) in diameter. is coupled to the

magnetic field, and rotates around Jupiter. The Galilean sat-
ellites are located in the inner regions of the magnetosphere
where they are subjected to intense radiation bombardment.
It appears that lo is a source of the sulfur and oxygen ions
which fill the magnetosphere. Another magnetospheric
interaction is the electrical connection between 10 and J upi-
ter along the magnetic field lines that leave Jupiter and inter-
sect lo. This magnetic flux tube was examined by Voyager 1
and a flow of about five million amperes of current was
measured, which was considerably more than anticipated.
Voyager also discovered a new low-frequency radio emission
coming from Jupiter. which is possibly associated with the
lo torus.

Scientific investigations of the Voyager mission

 

Investigation

Typical Jovian encounter objectives

 

Imaging science

High—resolution reconnaissance over large phase angles: atmospheric

dynamics: geologic structure of satellites

Infrared radiation

Atmospheric composition. thermal structure and dynamics: satellite

surface composition and thermal properties

Photopolarimetry
Radio science

Ultraviolet spectroscopy

Magnetic fields

Plasma particles

Plasma waves

Planetary radio astronomy

Low-energy charged particles

Cosmic ray particles

Atmospheric aerosols: satellite surface texture and sodium cloud
Atmospheric and ionospheric structure. constituents. and dynamics

Upper atmospheric composition and structure: auroral processes:
t istribution of ions and neutral atoms in the Jovian system

Planetary magnetic field: magnetospheric structure: lo flux tube currents

Magnetospheric ion and electron distribution: solar wind interaction with
Jupiter: ions from satellites

Plasma electron densities: wave particle interactions: low-frequency wave
emissions

Polarization and spectra of radio frequency emissions: lo radio modulation
process: plasma densities

Distribution. composition. and flow ofenergetic ions and electrons:
satellite energetic particle interactions

Distribution. composition. and flow of high—energy trapped nuclei:
energetic electron spectra

 

 
 

featurgs to about 20 degrees south latitude. V
The black area at thefpule results from

 

 

National Aeronautics and
Space Administration

Jet Propulsion Laboratory
California institute of Technology
Pasadena. California

 

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JPL 40024 7/79