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4
Giant Planets.
Keys to Solar System Formation
The gist planet Tory is Me story of the solar system. Each md the over small objects are leftovers from We
feed of gimt planet formation. As they formed' Me gimt planets (Figure 4.~) may have migrated inward or
outward' ejecting some objects from the solar system' pushing some into their parent stars md swallowing others.
Smaller thm stars' which have their own nuclear furnaces' the gist pawed rennin ~~S percent of the planetary
mass of the solar system. Their hydrogen-helium Ionospheres are similar to those of cooled-down mini-Suns' but
their roek-iee cores may resemble those of terrestrial planets.
The differences in composition md internal structure among Me gist pined reveal differences in how Hey
formed. The <~as gimpy' Jupiter md Saturn are mostly hydrogen md helium. These planets must have Kowtowed
~ portion of the solar nebula intact. The dice gimpy' Uranus md Neptune are made primarily of heavier stuff'
probably the next mod abundant element in He Sun oxygen' carbon' nitrogen' md sulfur. The e ore of each
gist planet is likely the ``seed', around which it secreted nebular gas.
Gist plme~ are laboratories in which to ~st our theories about geophysics, plasma physics' meteorology'
md even oceanography in ~ larger eon~xt. Jupiterts bo~omIess atmosphere' with its 300-year-old storms md
SOO-km~ winds, piques our inhered because it is so different from Earthts atmosphere. The gist planets'
enormous magnetic fields md infuse radiation belts test our theories of terres~i~ md solar electromagnetic
phenomena. The rings are pu=les' each ring system different from He others' reflecting different origins md
environments. So far' the main lesson of applying theories developed for Earn to the gist pawed is humility.
Gist pawed are also our link to He cosmos. Mmy have been found around other stars. We know something
about Heir orbits md masses' md we will soon know the radius, temperature' albedo' md partial composition for
several of these objects. To interpret these dark we mustunderst~d the Pets in He solar system. The two
dam sets are eomplemen~ry. Exhasolar gist plmets ~11 us how unusual we are How mmy over stars have
pawed md possibly plme~ry systems like our own: The solar systems gist planets provide ealibr~ion
standards. We em study Rem in situ' md we em ealeula~ what Hey would look like from the distance of ~ nearby
star. Together these two lines of research Caress He questions' Where did we come from: Where are we going:
Are we alone:
FIGURE 4.1 (facing page) A montage of the sour Ammo four gist paws. show to ~1~. they are pop ta bantam)
Jupi~r~ Satum, Ursula and Neptunc. Course of NASA/JPL.
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~4
HEW FR0~ IN =E 50~R HIM
UNIFYING THEMES FOR STUDIES OF THE GLINT PLANETS
Giant ply may ~ studied ~ whole object whose formation affected everything else in ~e solar system'
as me~orologica1 l~or~ories' as ringed worlds surrounded by puls~ing magnetospheres, md ~ standards for
calibrating observations of ply around other stars. The ideas discussed in this section are encompassed by ~e
following Area Demos:
Origin arid evolution'
Interiors arid atmospheres' md
L
Rings arid plasmas.
ORIGIN AND EVOLUTION
Is=c Newton (~642-1727) used ~e motions of ~e Galilear~ sa~lli~s to determine Jupi~r~s mass. William
Herschel (1738-~822) was aware thy Jupi~r~s density was ar~omalously low. In ~e 20~ century it became clear
thy only ~e lightest elements, hydrogen arid helium, could account for ~e low density. The inferred H/He ratio
was similar to thy of ~e Sun. From spectroscopy of H2 arid CH4 came ~e inference ~~ the C/H ratio ~ Jupiter
is similar to thy of the Sun. These studies gave rise to the solar composition model of girt plar~ets: take ~ piece
of the Sun' cool it down to planetary temperatures, md you have ~ gist planet like Jupiter or Saturn.
Modified Solar C~mp~ition Model
The solar composition model does not work for Uranus md Neptune' which are twice as dense as Saturn even
though they are smaller md therefore suffer less self-compression. Their densities are eonsis~nt with ~ mixture of
water, methane, md other ices ~ high temperatures md pressures. Since oxygen md carbon are the third md
fours most abundant element in the Sun after hydrogen md helium, this led to the modified solar composition
model. It starts with ~ mixture of element similar to that of He Sun, but Hen He hydrogen' helium' md other
noble gases are blown away. Stars like He Sun go Trough ~ chive BLAT Tauri,,) phase when Hey are young. The
powerful seller winds during the T Tauri phase are capable of blowing the gases out of the system. The mixture
that remains has solar composition except for the missing gaseous component. If the nebula is too hot He ices too
are lost, md only the rocks md metals remain. This modified solar composition model is supported by meteoric
composition, in which the elements that form solids ~ plme~ry temperatures are present in solar proportions.
Timing is critical. Gist plme~ have to form before the solar wind sweeps the gases out of He solar system.
That might explain He difference between He fee giants, Uranus md Neptune' md the gas giants' Jupiter md
Saturn. Gist planets form faster ~ the orbits of Jupiter md Saturn' where the density of the solar nebula is large
md collisions are more frequent. Perhaps Uranus md Neptune were just sorting to accumulate gases when the T
Tauri solar wind blew the gases out of the solar system.
The time ~~ it takes to produce ~ Jupiter-sized object depends on how it forms, md here some uncertainly
exits. The slow way is to first peered ~ roek-iee eve of approximately 10 Earn masses. Such ~ eve could form
by precipitation of less volatile materials as the solar nebula cools. The solid particles settle to the equatorial plane
of the eireumsolar disk md Hen coalesce by collisions. The dense solid objects are able to astray gas once Hey
reach He critical size of about 10 Earth masses' but the rate is limited by how fast He growing object em radian
id energy. The fast way to form ~ Jupiter-sized object is by hydrodynamic instability. Somewhere in He solar
nebula the density reaches ~ critical value, md the mixture collapses from its own gravitational self-at~aetion.
Stars form this way when the density in gist molecular clouds reaches ~ critical value. A Jupiter-sized object
formed by He second process (without ~ core) would be similar to ~ brown dwarf ~ subcellar object' insuffi-
eiently massive to sustain thermonuclear reactions in its core.
The way to choose between these hypotheses is to determine if all the gist planets have cores. Three out of
four do. Jupiter is He underpin one. ~ Measuring the size md mass of Jupiter ~ s eve is therefore ~ major objective.
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PLAIN
~5
Volatile Ahundn~c~
The temperature of ~e solar nebula ~ various distances from the Sun is critical in determining which
compounds were solid arid therefore likely to ~ incorporated into each diary Claret md which were not. The
Galileo prom found thy carbon, nitrogen, sulfur' argon' krypton' arid xenon are Writhed by similar amounts two
to four times solar Bunt. This result was unexposed, because ~e different elements are not equally Jollily.
~e theory is ~~ all the vol~iles condensed together ~ temperatures below 30 K' ~ or beyond ~e orbit of
Neptune' arid then migrated in to Jupi~r~s position.2 Another theory is thy the vol~iles were happed in ~e form
of claptrap hydrous in ~e feeding zone of Jupiter while the nebula was cooling down.3 The first exhilaration says
thy oxygen should also ~ enriched by ~ factor of two to four.
~ L
The second explosion requires ~ larger
enrichment for oxygen t~ ~ least e~ght-~~mes solar abun~iar~~ because ~e claptrap hydrae is molly wear
ice arid holds only ~ limited fraction of over molecules.
Unfortunately, the Galileo probe did not go deep enough to measure ~e perry abundar~e of wear. The
prom entered one of ~e dry downdrafts which apparently extend down well below cloud base ~ ~ to 7 bars
1~t to the 24-bar dupe ~ which the prom signal was lost. The over condensables, ammonia (NH:) arid
hydrogen sulfide (H2S), were depleted ~ cloud base but approached con~ar~t values ~ ~e d~pest levels. H2O
was still increasing win depth ~ ~e deepest 1~1~. Measuring the wear abundar~e in Jupi~r~s atmosphere is
thus ~ major objective.
Cooling History
Models of the interiors predict that gimt plmets cool slowly. They should still be radiating subst~tia1
amount of interns energy' md indeed' all but Uranus have measurable amount of hem emerging from their
interiors. Either Uranus cooled faster than the other gist planets did md id interior is now cold, or it is cooling
more slowly, in which ease the interior is hot but the hem ergot get out. For instance' ~ layered structure win He
high-density material near the center would inhibit convection. Uranus is the only gimt planet that spins on id
side. Whether this unique feature has Aching to do with the low hem flux is not known. The 980 obliquily is
evidence ~~ the final stage of plmet formation was ~ chaotic process involving collisions of Earth-sized objects
capable of Blaring He angular momentum of bodies He sin of Urmus. A gentle rain of small plmetesimals
would not do it.
Generally' He interns hem radiated by He planets today is eomp~ible with maculations of their cooling
histories. The uncertainty tenors on possible internal gradients in composition' He extent of convection zones'
the equation of sates md He possible gravi~tiona1 separation of hydrogen md helium as ~ additions source of
interns energy. ~~rna1 structure is revealed in the gravity field. The equation of sate is studied in He laboratory.
And the separation of hydrogen md helium leaves id mark on the He/H ratio in He Exosphere today. The
separation em occur only in Jupiter md Saturn' whose internal pressures are so high (A to 3 Hearst ~~ hydrogen
becomes ~ liquid meal. It is thought that ~ helium-rich phase will preeipi~te out of the hydro~en-helium metallic
1 ~ 1 ~ ~ ~ ~
mixture when me temperature cirops below ~ Ordeal value. Vellum crops setne toward me tenor ot me planed
leaving the layers Gove depleted in helium. Jupiter, because of its greater mass' cools more slowly md is just
entering this sager according to He ealeul~ions. Saturn' which has less mass' he cooled down far enough that id
atmosphere should be significantly depleted in helium.
The Galileo probe measured the Ionospheric Hem ratio for Jupiter. The value was higher than ~~ obtained
from Voyager remo~-sensing observations but Breed win the best estimates of solar composition. ~ other
words' preeipi~tion of helium has not yet produced significant depletion on Jupiter. This raises questions Bout
the in~rpre~tion of Voyager remo~-sensing observations for both Jupiter md Saturn. For the lat~r, remote
sensing is all we have, md it seems to imply that significant depletion has occurred. The Cassini Infrared
Spectrograph will resolve helium lines md provide additional dam; however, ~ probe into Saturn~s atmosphere
would settle Be issue.
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HEW FR0~ IN =E 50~R HIM
Extrmolar Bent Planets
An impressive fraction (~S percent) of stars surveyed to dam show evidence of ply. This number arid ~e
mass rar~ge will increase as more sensitive diction methods come online arid planets with longer-period orbits
weigh in. The discovery of gimt plays in highly eccentric tight orbits (radii c! AU) around other stars is
revolution—~ because it shows thy some perry systems are very different from our own. There is clearly art
observational bias to the results, because massive objects close to their stars are easier to detect by current
methods. But the results imply either ~~ gimt plums cart form in the high-~mperature environment close to
their parent stars, or thy they form farther out arid migrate in.4 Either way, the implications are profound. Diary
ply cart migrate' provided Hey interact with comparable masses ~ different orbital radii. Object in different
orbits repel each over: to conserve angular momentum when energy is dissipated, the inner object moves inward
arid the outer object shift outward. A gimiplar~th~ moves inward may have expelled other diary plumed' which
are now war~dering Trough in~rs~llar space. It will eider expel or devour arty ~rrestria1 ply thy are in
id path.
How cm the study of Jupiter, Saturn, Urar~us' arid Neptune contribute to the study of diary plar~ets around
over sparse The solar system provides ground-~uth. The extrasolar giant plar~e~ have clouds in their ~mospheres.5
Clouds lead to preeipit~ion arid release of 1~t hey. The gimt plmets close to their parent stars have large day-
night temperature gradients. The temperature gradients lead to winds, which affect the temperature field. Clouds'
preeipi~tion' temperature gradients, md winds are meteorological phenomena. We know something about these
things from studying Earth md other planets. The observations of exhasolar plme~ mass, radius, temperature'
md composition will be difficult to interpret unless we draw on our knowledge of gist plme~ in the solar
system. Even ~~ knowledge is incomplete, so further exploration is vital.
Important Questions for Origins and Evolution
Importmt questions about the origin md evolution of gimt plme~ em be divided into Dose speeif~eally
relying to the solar systems give md, more generally, Lose relating to extrasolar planets md brown dwarfs.
Importmt questions for the solar systems gist planets include the following:
How did He gist planets form:
Does Jupiter have ~ roek-iee cored
What are the elemental compositions of the gist plme~9
What are the internal structures md dummies of He gimt plmets:
What are the orbital evolutionary pays of the gimt plmets:
For extrasolar gist plme~ md brown dwarfs' He impor~t questions are these:
Around what types of stars are gist plmets found:
Are multiple Lint plme~ common in seller systems:
. ~ .
In what ways do gist plme~ differ from brown dwarfs:
What are He properties of extrasolar gist plmets (radii' effective temperatures, compositions' clouds'
moons' winds, magnetic fields' hey flows)9
~ How em we use the gist plme~ in the solar system to calibrate speehoseopie observations (optical'
infrared, radio) of extr~olar gist plmets:
Future Dictions
As identified by the Gim(Plmets Panel, He most importmt directions for research on the origin md evolution
of gist plme~ for He next decade are as follows:
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PLAIN
~7
~ Bomb Jup`~s `~`or wad I ~~ my memurem~ from polo I.
As win Earth' one cart probe the interior win tools from geophysics ~~ utilize wismic, gravity' arid magnetic
observations. Two kinds of oscillations are relevar~: acoustic modes excited by convection or other interior
dynamics, md tidal modes excited by the sullies. The tidal bulges show up in ~e plus gravity field' which
affects the spacecraft orbit. A spacecraft in ~ low-periapse polar orbit around Jupiter could detect the sa~lli~-
induced tides arid also improve the determination of the axisymmetric terms in the gravi~ field. Both measure-
men~ contain information about the core.6 The magnetic field structure provides information about convection in
the deep interior' arid may also contain ~ signature of ~ solid core, in urology win Earths magnetic field' which
contains ~e signature of the solid irmer core.)
Memun~g I Cup atmosp~c compos`oon why mult~p~ entry probe cwd m`~row~e rumor
sensing. Probes thy operate down to ~e lOO-bar pressure 1~1 ~ ~ variety of latitudes are needed. Remote
sensing ~ wavelengths grower Bars 10 cm cart detect wear ~ depths down to hundreds of bars. The combination
of Probes arid remote sensing is n—deaf to provide both ground-truth arid 0106:31 context. The water :~bundar~e
· . - · .T T. - T
~ ~ O O
bears on how the diary ply got Weir volatile elements arid whiner significar~t migration of plar~simals
occurred in the early solar system. It is importers to measure ~e volatile abundar~es for all the gimt ply
beginning with Jupiter.
Acqu`~`ng ~d `~mrpret~ng Ear~-bmed obsessions of boor system ~d extrmo~r gore paws. The
effects of clouds' winds' arid chemist on the spectra of solar system girt planets need to be determined' taking
into account the orientation of the planet with respect to the Sun Id the observer. This information will enable us
to expand He scope of comparative plmetology to include extrasolar gist planets Id brown dwarfs.
INTEllIOllEi AND ATMOSPHERES
The gist planets do not have surfaces in He usual sense, but Hey have what amounts to the same Ding from
the point of view of ~ external observer: ~ barrier to both remote sensing Id direct probing With currently
foreseeable technology' this occurs near He 100-bar pressure level. Below this level' properties must be inferred'
just as properties of Earths interior are inferred from near-surface measurement. The methods of inference are
the same for my planet; the interiors of He gist plme~ are relatively urn own only because near-surface day are
relatively sparse compared with day for the ~rrestria1 plme~. The distinction between interior Id Ionosphere is
largely ~ operational one ~ the gimt plme~, Id Be two domains are proudly more intimately coupled in He
gist planets than in Be Arrest plmets precisely because He gists lack ~ conventional surface.~~i
Interior Stru~um
Present models of gist planet interiors are contained by observed properties, including planetary mass'
radius' shape, rotation period, hey flow, gravitations moments' magnetic moments' Id elemental composition.
The first five of these observable properties are known to sufficient accuracy; the It Free are not. Laboratory
measurement Id ~eoretiea1 modeling of the properties of hydrogen, helium, Id Bake element ~ very high
pressures also provide critical eons~aints for interior models. Previous spacecraft measurements have provided
mmy of He observable parameters needed for the development of meaningful interior models. However, He
uneer~inties in boy observational constraints Id high-pressure material behavior are such that He interior
density Id temperature structure, the variation of composition Id phase ~~ with depth' Id the size of ~ dense
central roek-iee eve (or even its existence' in He ease of Jupiter) ergot be ascertained with eonf~denee.
Our present view of He interiors of Jupiter Id Saturn divides each planet into three distinct regions: ~ dense
central roek-iee e ore with ~ mass of up to 10 Each masses ~ Jupiter Id ~ to 17 Earth masses ~ Saturn, ~ fluid
metallic hydrogen region ~ pressures greater than about ~ Mbars' Id ~ outer shell of molecular hydrogen. The
question of the presence or absence of ~ dense eve ~ Jupiter is ~ key missing link in our underfunding of Jupiter is
interior structure Id hence its formation history. Other key urn owns include the nature of the phase transition
between metallic Id molecular hydrogen' Id He presence or absence of <`radiative zones,, where the deep
atmosphere is not fully convective.
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Is
HEW FR0~ IN =E 50~R HIM
Uranus md Neptune are distinct from Jupiter arid Saturn in the the former contains ~ much larger fraction of
elements heavier chart hydrogen md helium. Their interior structures are more underpin. Three-layer models
have men developed for these Clarets ~~ include ~ small central ``rock,, core' art extensive ``ice,' region compris-
ing most of ~e ply arid ~ methar~-rich hydrogen-helium gas envelope. (In this context "ice,' metros ~ mixture
of volatile elements whom original form was wa~r, merry ammonia arid other ice-forming molecules but
whom present form is fluid rather chart solid arid is probably not composed of input molecules.) The small interns
hem flux observed ~ Urar~us may imply ~~ parts of the interior are neither convective nor homogeneous.
Clout and Composition
Though we still do not know what truce chemicals give the clouds of the g~ girts their familiar colors' we
have reamed ~ Brew deal Tout ~e bulk composition arid structure of Weir visible atmospheres from Earth-b~ed
arid spac~raft-based remote sensing.~4 Different wags probe different levels of ~e ahnosphere, arid this
fact has ~~n exploited to plwe constrains on the pressure levels' bulk compositions, arid over properties of ~e
various cloud arid he layers. The clouds of Jupiter md Saturn are thought to comprise Area distinct layers'
composed of ammonia ~ the top' ammonium hydrosulfide in the middle' arid ~ wa~r-solution cloud ~ the bosom.
Analogous cloud decks may also exit ~ ~e ice giar~ts, Urar~us arid Neptune, with ~e addition of ~ methar~e cloud
~ high altitudes arid perhaps ~ hydrogen-sulf~de cloud rather Bars ~ ammonia cloud just below. The Galileo
end probe ~ Jupiter, while confirming mmy of Me result of earlier remote-sensing observations' found little or
no evidence of Me expend water md ammonia clouds in the I- to S-bar pressure r~ge'i~ probably as the result
of entering ~ Pompous Ionospheric hot spot. The resolution of this underwing is critical not only to our
understanding of Jupiter~s origin md evolution, ~ described in the previous section, but also to jovimme~orology.
The gist planet Ionospheres are so cold that volatile species such as whorl hydrogen sulfide' md ammonia
condense. hiethme is sufficiently volatile to be present as ~ gas throughout the upper Ionospheres of the gist
plme~, though it too em partially condense ~ Uranus md Neptune. Methane molecules are broken apart ~ high
altitudes by ultraviolet solar photons md by preeipi~ting magnetospherie charged particles' md Me fragment em
ream to form more complex hydrocarbon molecules, producing the array of organic molecules ~~ have been
observed in the upper atmospheres of the gist planets. A better understanding of this process may provide clues
to how heavy organic molecules, including biogenie molecules, originated on early Earth. At Jupiter md Saturn'
ultraviolet photons em penetrate to levels where ammonia phosphine, md perhaps sulfur-bearing gases are
present, giving rise to additional photoehemis~y. Studying these photoehemiea1 md ~ermoehemiea1 processes
the gist plme~ will guide He interpretation of speeds obtained from brown dwarfs md extrasolar gist plme~.
An extended layer of hue particles envelops the upper atmospheres of He gist plme~. The hues are
probably produced by aurora1 ehemisby in the polar regions md by photochemistry throughout the upper
atmospheres. Global high-~titude winds may carry polar hues to lower latitudes. The he is interesting not only
because of its influence on atmospheric optical properties, thermal structure' md global circulation, but also
because of He possibility of synthesis of unusual md complex organic molecules.
The impact of Comet Shoemaker-Levy ~ on Jupiter in 1994 dramatically illustrates He feet that new ma~ria1
is being introduced into gist planets atmospheres. The externally supplied oxygen from comets, interplanetary
dust, md sa~lliteiring debris is observed as H2O md CO2 in He upper atmospheres' md provides clues about He
exchm~e of material between different parts of He solar system.
Thermal Structure
Just above the clouds lies the tropopause, He coldest layer of He Exosphere. Below this level He ~mpera-
ture increases wig depth in ~ mower that is generally consistent with upward convective hem transport from ~
interns source. Above the tropopause, He temperature increases with height as the atmosphere is increasingly
exposed to solar radiation. However' He observed increase of temperature in He shato sphere is greater ~m
predicted on the basis of solar absorption alone, especially ~ Neptune, implying additional heating meehmisms.
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Representative terms from entire chapter:
gimt planets
PLAIN
In the upper steno sphere, molecular diffusion begins to affect atmospheric composition as ~e density of
species heavier Bars hydrogen falls off rapidly with height. Because them heavier molecules (primarily hydro-
carbons) are responsible for cooling ~e stratosphere by infrared radiation, the temperature rises rapidly win
altitude' reaching ~ plateau of 400 to 1000 K in ~e thermosphere. The thermospheric temperatures ~ ~1 diary
ply are higher by ~ factor oftwo to four chart would ~ expected on the basis of solar extreme-ultraviolet (EW)
he~ing.~7 Additional high-altitude hem sources are clearly operating. Possibilities include ionospheric Joule
he~ing, charged particle precipitation' arid dynamo action.
In the upper atmospheres of the diary plar~ts, ~e impact of EW solar photons arid m~n~ospheric charged
particles produces ionization, as it does on Earth. The Knin electrical conductivity of ~e ionosphere gives rise to
spectacular arid d~amic aurora1 displays thy reveal the elec~od~amic coupling of ~e atmosphere with ~e
magnetosphere arid with the embedded sa~lli~s. As ~ Earth' ionospheric structure is affected by upper-
atmospheric winds, magnetic-field structure' arid electric fields induced by motions of plasma. In contrast to
Earth' however' perry rotation plays ~ dominmt role in driving arid shaping the upper atmosphere arid
ionosphere. Spacecraft radio occultations have revealed dramatic Spain tared probably temporal) variations of
ionospheric structure ~ all giant plumed' md ~e relative roles of chemist md dummies in producing ~e
observed behavior are not well understood.
The upper ahnospheres of the diary plmets provide natural laboratories where we earl ~st md refine our
understanding of ionospheric structuring arid aurora1 processes ~~ occur under very different bounds conditions
Earth md elsewhere in the universe.
Winy
The cloud patterns are constrained by winds that blow parallel to lines of eonst~t latitude. Instead of one
eastward jet stream in each hemisphere, as ~ Earth' Jupiter has six or seven. The large-scale weaner patterns are
remarkably stable. The Grew lied Spot has been in existence since ~ lead 1664 md possibly much loner.
llemarkably' Me winds do not decrease as one moves outward in Be solar system Neptune~s winds are
3 times sponger than Jupiter ts, even though the power per unit area bow from sunlight md from internal head is
about 20 times less ~ Neptune ~m ~ Jupiter.20
Principal questions revolve around the depth of Be winds, Be role of interns hem versus solar hem in driving
them, md the meehmisms ~~ maintain them. For inshore, Be Gre~lled Spot md Be large jet streams regularly
devour smaller spots' but where the smaller spots get their energy is still ~ mystery.
Atmospheric dummies is intimately eor~E~ee~d with thermal structure md composition. The energy sources
for atmospheric dummies include internal head solar insolation' md' ~ the higher levels, aurora1 Joule hewing
md eharged-partiele precipitation. The internal energy source evidently dominoes atmospheric dummies ~ md
below Be cloud level, md the influence of rapid planets rotation is obvious in the preponderance of Tonal (east-
west) winds.
Condensation, evaporation' md hmsport of eloud-forming species also drive Be meteorology of Be gist
plme~ Trough Heir effect on pressure gradients md Be redistribution of energy, primarily in the form of latent
hem. The Galileo orbiter observations of water-rich convective storms ~soeia~d with lighting md eyelonie
shear zones have shed new light on Be role of moist convection in the maintenance of Tonal jets on Jupiter; ~us'
knowing the abundance of water is ~ major objective for Jovian me~orology.2i
The Tonal jets are visually prominent ~ Be gas gibes Jupiter md Spurn md less so ~ the fee gists Uranus
md Neptune.~~~5 At Jupiter' Tonal wind speeds (~ the cloud level) are greatest ~ the boundaries between Be
lighter~olored <~ones', of upwelling warmer atmosphere md Be darker~olored <
HEW FR0~ IN =E 50~R HIM
the wind pattern revealed by cloud patterns have lifetimes raging from monks to decades in most cases' to
centuries (~ 1~) in the case of Jupi~r~s Greg Red Spot. The longevity of Case structures is nof underwood.
At the Galileo probe envy aims ~e Tonal winds increased win depth' lending support to the hypothesis thy
the Tonal jets extend deep into Jupi~r~s ahnosphere.26 Furler measurements of deep atmospheric winds arid
interior structure are needed to dear mine how the observed ahnospheric winds relay to motions' including
possible nonuniform rotation' in the deep atmospheres arid interiors of ~e gimt plus.
ELey Question
Importers questions about ~e interiors arid atmospheres of Diary ply include ~e following.
In~`ors
Atmospheres
What is the nature of convection in Tiara Claret interiors
How does the composition vary with depth
What is the nature of phase ~ar~sitions within the Diary plar~e~9
How is energy ~ar~spor~d through the deep Ionospheres Do radiative layers exists
How md where are plar~e~ry magnetic fields generated:
What energy source maintains the Tonal winds' md how do they v~ with depth:
What role does wear md moist convection play:
How md why does atmospheric temperature vary win depth latitude' md longitude:
What physical md chemical processes eonhol He atmospheric composition md the formation of clouds
md he layers:
How does the aurora affect the global composition, temperature' md hue formation:
What produces the intrigue vertical structure of gimt planet ionospheres:
At what rate does ex~rna1 material enter gist planet atmospheres, md where does this material come
from:
What em organic chemist in gimt planet atmospheres tell us about the atmosphere of early Earth md He
origin of life:
Future Di~tiom
The most importmt directions for research on the interiors md atmospheres of gist plme~ for He next
decade are identified as follows:
~ ~so~g Jfi~-~ str~re of t~ gore ~ ~d maniac f ~4 to ei~m t~ ~rsmnor s~re ~d t~
mec~s~sms of energy trouper; ma~-fie~ grenerat~o~ ~d convection wean Jug. The acquisition of
high-order gravi~tiona1 md magnetic moments, combined with sa~lli~ tides md possibly observations of acoustic
oscillations within He atmosphere' will enable us to <`image'' He deep atmosphere md interior of Jupiter. Deep
winds' if they are strong enough' will show up in He gravity field, because their centrifugal forces cause ~
rearrangement of masses in the deep interiors These observations will provide critical constraints for models of
interior structure, energy transport fluid motions' md magnetie-f~eld generation ~~ have far-reaching planets
md astrophysical applications. Improved observational constrains will qualitatively enhance our understanding
of planet formation md evolution md our abilily to understand similarities md differences among our own gist
plme~, exhasolar gist plme~, md brown dwarfs.
~ Memun~g c°~-gm ~~es (02 O. NH3, CH4, ~d HSt temperature we'd veW~, ~d
cloM opium down to t~ 100-~r pressure ~~! at Jo. The Galileo probe provided critical information on
PLANETS
jo]
elemental abundm~s in Jupi~r~s ahnosphere, but ~e limited dep~ arid unusual location of its end prevented ~
definitive measurement of the deep tropospheric wear Thunder. The wear abundar~e is especially critical' not
only because it distinguishes among different plmet-form~ion scenarios, but also because condensation of wear
arid the resulting release of leant hem drive atmospheric dummies. The Galileo measurement of ammonia
abundar~e is also uncertain' owing to experimental problems rel~d to the behavior of Anionic within the mass
spec~ome~r. Ammonia is ~ critical cloud-forming molecule in the atmospheres of Jupiter arid Saturn, arid remote
sensing of its abundar~e has yielded contradictory result. Multiple in situ proms arid microwave sounders cart
resolve Base issues. Multiple probes cart also provide clues to mmy outstanding questions Bout Ionospheric
temperature profiles' tropospheric dynamics, arid cloud structure.
~ Ace u' r`ng Ear~-~ m~cop`c obse~~ons of atmospher`c compos`~`on, stru~ clomp, I
auroras, ~d ~o~c of. Earth-~ased (or orbital) observations Thigh spectral arid sp~ia1 resolution cart
reveal the ~ree-dimensiona1 distributions of composition' temperature' md winds. These three variables are
intimately relend. Composition affects the absorption of solar radiation arid ~e readmission of infrared radiation'
thus regulating ~e thermal structure. ~ ^^ ~ -
compos~~on croci thermal structure ~~t onion arid release of leant
heat thereby affecting ~e wind pattern. The winds in turn affect composition arid Wormy structure by transporting
malaria arid hem. To understar~d how these in~rcormected procesms operate' we need simultaneous observations
of temperature, composition' arid winds. This is difficult even ~ Earth let alone ~ the girt plmets. Previous
telescope arid spaceport observations have put several pieces of this complex puzzle in place, butthree-dimensiona1
information is still limited. Brewer access to large ground-based md space-based telescopes md advances in
Instrument tee~o~ogy art me next decade should greatly improve He situation. Observations of global acoustic
oscillations' although difficult to obtain' are of particular interest because they shed light on interior structure.
r. ~ ~ ~ . ~ ,] ,. ~ , ]. ~ ,' ~ ~ . ~ ,, ~ ,] , 7. .
· . . . 1 1 . · .1
rer~orm`ng '~orc~tory ~a t~eore~a`~ st~s ok lee ve~`or oh matter =~r lee extreme conmt~ons
present `n goat peat 'manors ad atmospheres. The ~ermod~amie, kinetic, radiative' md speeba1 properties
of the relevant gases md ices, md Be high-pressure equation of ~~ of Be relevant hydrogen-helium mixtures are
critical to Be interpretation of dam on bow solar system md extrasolar gift plme~. Observations need to be
combined win theoretical models in order to undersold Be underlying physical md chemical processes. L~o-
ratory dam are critical for analyzing observational dam md plying future observations.
ICING AND PLASMAS
Disks are ubiquitous in Be universe' md the solar systems gi~tplmets provide numerous examples amenable
to in situ study. These range from visible rings composed of macroscopic objects dominated by gravity' to fully
ionized plasma disks dominated by electromagnetic forces' win intermediate eases (~`dusly plasmas,') where bow
forces are competitive. The diversity of present-day disk structures ~ the gist planets offers glimpses into Be
various sages of solar system formation md other astrophysical processes.
At first glance it is less ~m obvious that studies of planets rings will tell us Aching about ashophysiea1
disks because of Be grew mismatch in Be relevant sp~ia1 md temporal scales on which they operate. Nevertheless'
grew similarities exist. Most of the underlying physics is seale-invari~t, md while it is true ~~ there are
import~tdifferenees between eireums~llar disks md ring~for example, Be presence of gas in early types of Be
former md the absence of gas in Be later their dummies are essentially the same. Moreover' the study of Be
dummies of ~ disk of particles in Be absence of gas provides one essential ingredient in understanding Be
dummies of ~ disk of panicles in Be presence of gas.
lying
Among the solar sys~m~s four known ring systems, more differences than similarities exist. Saturnts spee-
t~eular rings contain by far Be most mass of ~1 the rings. Urmus~s narrow dark sheds are interspersed win
small' dusty particles. Jupi~rts rings are exceptionally tenuous, md Neptune is rings contain possibly long-lived
are structures. The eomplexi~ md variety of structure in Be various ring systems, as revealed in the Voyager
images md occultations' came as ~ complete surprise.28
jo)
HEW FR0~ IN =E 50~R HIM
The theoretical life spars of arty of ~e observed ring systems is much shorter chart ~e age of ~e solar system.
Angular momentum transfer between rings arid nearby sa~lli~s causes ring particles to fall inward, as does gas
drag from ~e upper reaches of the plus ahnosphere. Duration of small charged grains with ma$netospheric
particles md fields cart produce inward or outward migration as well as erosion. Continual bombardment by
in~rplm~ary particles should darken ~e rings. Although ~e rings of Uranus are indeed quip dark' thou of
Saturn are as bright as fresh ice.
Where did rings come from in ~e first place: Are Hey leftover bits of myriad ~~ did nof get swept up into
the plmet or sa~lli~9 Are Hey ~e result of the catastrophic destruction of ~ sa~lli~9 Are they remainders of art
errors comb thy strayed too close to the plmet arid was torn apart by tidal forced Or are Hey continuously
replenished by myriad sputtered from nearby sa~lli~s by magnetospheric ionic Knowledge of ring dynamics
arid particle properties cm help to grower Best questions.
Recent studies have identified complex gravitational intrusions among the rings arid Heir retinues of
at~ndar~t sullies. There are It examples of Lindblad arid corotation resonances (two different Ems of
eccentricity resonar~ees first invoked in the context of galactic disks)' electromagnetic resonar~ees' spiral density
waves arid bending waves' narrow ringlets thy exhibit internal modes due to collective instabilities, sharp-edged
gaps maintained by tidal torques from embedded moonlets' arid tenuous dust belt created by meteoroid impact on
or collisions among parent bodies. These processes earl amount for some of the features observed within the ring
systems arid in so doing earl provide He begirming of art explosion for the survival of the rings beyond their
predicted life spans.
The composition, size, md shape of the particles remain major urn owns. Infrared spectroscopy md miero-
wave radiometry reveal that Saturn~s rings are mostly water ice. Different regions of the rings show noticeable
color derisions on all sexes. The compositions of the rings of Jupiter' Urmus, md Neptune are urn own but
apparently different from one mower. The Jovian ring system shares He reddish color of the nearby satellite
Amal~ea~ but the brightness of the particles is urn own because their seadering cross section is urn own. The
urmim rings are nearly colorless, md those of Neptune are so dark that nothing is known about their color.
The study of extrasolar protoplme~ry disks em be influenced by studies of planetary rings in He solar
system. The concept of He migration of bodies due to angular momentum exchange with surrounding ma~ria1
was first advanced in the ring eon~xt md is now ~ mainstay of planetary formation models. The fine structure of
plme~ry rings with embedded bodies has motivated md guided studies that have application to disks of much
larger male.
Plump
The gimt planets have redefined our concept of planetary magnetospheres. Unlike the ~rreshia1 planet
magnetospheres (Ear~ md Mercury), the gist planet magnetospheres are internally driven, powered by planetary
rotations energy that is exuded by plasma of internal origin.29 Jupi~rts magnetosphere (Figure 4.~} is the
largest Id the most rapidly rotting Id therefore the most powerful Id He most differentfrom Earths. Jupi~rts
rotations energy is extracted Id dissipated by plasma originating in Io, s volcanoes. The transport Id energiz~ion
of this Iogenie plasma gives rise to ~ astonishing variely of rem only observable emissions across He eleetro-
magnetie spectrum from radio to x-rays.~° Mod of these emissions have strong spin-period modulations that are
presumably analogous to Lose of ashophysiea1 pulsars.
Major urn owns include He meehmism~s) by which to injects magnetospherie plasma Id He way~s) in
which this plasma is energized Id transported outward to power the magnetosphere Id ultimately to generate
plme~ry wind. These processes leave distinctive footprint in He plenary aurora1 emissions that are resolvable
from Earth or Each orbit. However, in situ measurements ~ low altitudes Id high latitudes are needed to provide
the key for extracting information about magnetospherie processes from Ear~-based aurora1 images (Figures 4.3
Id 4.4}
Sutures magnetosphere is ~ smaller analog to Jupiter~s, but more eomplie~ed because the internal sources of
plasma are multiple Id widely dispersed. Saturn is unique in the solar system in having ~ magnetic dipole
moment that is almost exactly aligned with its spin axis. lleports exist of pulsar behavior ~ Saturn which, although
PLAIN
107
research. Jupiter, the prototypical gas gimt is the highest privily for ~ new mission. Neptune' ~e prototypical ice
givers is ~e next-highest privily.
First+ Determine the Maw and Size of Jupiter,~ Ire
~e theory of diary Claret formation says ~~ ~ rock-ice ``s~d'' of some 10 Each masses is necessary to
ath act the lither gases hydrogen arid helium. Another theory says ~~ Jupi~r-si~d objects cm form as stars
do' attracting gas, ice' md dust directly from ~e nebula. The lair process produces art object without ~ core. The
two scenarios have vastly different consequences for diary plmet arid solar system formation. The core mar~ifes~
itself bow in the rotations bulge, which is the response of the plum to id own rotation' arid in ~e tidal bulge'
which is ~e response of the plar~et to ~e gravi~tiona1 pull of ~e sa~lli~s. Bow bulges have signatures in ~e
plus gravi~ field' which cm be measure from ~ inclined orbit with periapse close to the ply. An
inn technique uses distortions of the magnetic field near ~e pole to infer ~e core ~ s radius' in Orology win
Earths magnetic field, which reveals ~e inner corers radius.~3 A low-T'riaose T'olar-orbitino soacecr~ eauiomd
with ~ radio har~spon~r arid vector magnetometer is required.
L L L ~ ~ ~ 1 ~
Second+ M - ure Elemental Ahun~nc~ (H. He, 0' C' N' S)
The wear abundance (hence' the O/H ratio) in Jupi~r~s Exosphere is uncertain by ~ order of magnitude'
even though oxygen is expend to be the ~ird-most-abund~t clement after hydrogen Id helium. Wear plays
important role in gist planet formation. The O/H ratio tells us how gist plme~ got their vol~iles (H2O, CH4'
NH:' Id H2S) md, in particular, the extent to which He volatiles were carried in from beyond Neptune~s orbit to
the ironer solar system on icy plme~simals. Wear is also impor~t to He meteorology of gist planets, as it is on
Earth. The Galileo probe penetrated below the Jovian clouds, but the composition was still varying when He probe
reached id maximum depth ~ 24 bars. The feet ~~ the probe entered ~ unusually hot ~ region hindered He
in~rpre~tion. Beder coverage in latitude Id penetration to greater depth are needed. This need em be met win
the following two complementary approaches.
~ Mubip~ awry probes carrying ~~s so. A dedicated spacecraft should be able to carry three
probes that enter within 30 degrees of He equator Id reach depths of 100 bars. The carrier makes ~ polar pass
where it collects the dam from each probe, md then transmits to Earth. Having only Free probes is ~ limitation'
but it is sufficient to resolve the ambiguity left by He single Galileo probe. Ammonia is measured separately by
monitoring He adenu~ion of He probers radio signal (measuring ammonia by mass speehome~y is not accurate
because it coax He walls of the chamber).
~ M`~row~ Amy. This technique uses thermal emission from the planet ~ wavelengths between
10 Id 100 em to measure the water md ammonia abundance from 10 to hundreds of bars. To avoid interference
from Jupi~rts radiation belts' He measurement must be made when the spacecraft is less ~m sever al Housed
kilometers above the tops of He clouds. A polar-orbiting spacecraft is best because it gives latitude Id longitude
coverage while avoiding radiation-belt md ring-particle hoards. interpretation of He radiomen results is
facilitated by knowledge of He temperature profile' which em be measured by ~ entry probe. Multiprobes
provide ground-tru~ for interpreting the radiometer observations, which in turn provide global coterie ~~ is not
obtainable from ~ limiM number of probes.
Third+ Investigate Deep WincL and Interns nvection
Jupiter~s jet streams Id oval storms may get Heir lOO-year longevily from massive jet streams Id convection
cells in Jupi~rts interior. The degree of coupling between motions in the visible atmosphere Id the interior
depends on He thermal structures which itself is urn own. A probe em measure both thermal structure Id winds
the latter using the Doppler shift in the probers radio signs. A probe em also measure clouds' sunlight' Id
gaseous composition' but only to depths of 100 bars' ~ least for Jupiter. emotions ~ deeper levels may be inferred
job
HEW FR0~ IN =E 50~R HIM
from ~e planets gravity field. For inseam' if ~e observed jet streams extend down to kilobar pressures' ~e
gravity field will look noticeably ``rougher'' Bars if ~e interior is in solid body rotation.34
An orbiting spacecraft the skims close to the top of the atmosphere cart measure this fine structure of ~e
gravity field. It could also measure the fine structure of the magnetic field, which might ~11 us if the winds extend
to the dep~ where the fluid Gnomes art electrical conductor: The tiled dipole field appears to ~ time-dependent
in the reference frame of the moving fluid' arid the time-dependence produces elec~ica1 currents thy caum
observable charities in the fielder
Fourth+ lip the Structure of Magnetic Field
The goal is to understar~d how plenary dynamos operas. Previous spacecraft did nof sped enough time
clom to Jupiter or my of the other girt ply to measure the fine structure arid ~mpora1 variations of ~e
magnetic field. The ex~rn~ field cm be extrapolated down to ~e 1~1 where the fluid becomes ~ elechica1
conductor. At Each this 1~1 is the liquid iron core, arid there the spectrum of the magnetic field is flat ~e
different harmonic components of the field all have comparable amplitudes. This may be ~ fundamental property
of perry dynamos. The fields of ~e girt planets provide art opportunity to find out.
Fifth+ Explore Polar M~et~pheres
The solar wind, the satellites, ~e rings, md the planet cm all act both as sources md as sinks of charged
particles ~~ populate the magnetosphere. The polar regions, where magnetospherie particles interact win He
plme~ry ahnosphere to produce the aurora md related radio emissions, are particularly important. There'
magnetic field lines from He distant magnetosphere md from in~rplme~ry space reach the plmetts atmosphere.
Previous spacecraft missions to He gist plme~ have not explored He aurora1 zone because they were designed to
visit satellites in the equ~oria1 plane md to avoid radiation md ring particle hoards close to the planet. However'
~ polar orbiter win ~ near-equ~oria1 periapse just above He cloud tops will traverse the polar region ~ ~ disagree
of ~ to 3 planetary radii from the planets tenor' while avoiding He rings md mod of the radiation Bela
(Figure 4.~. Existing instruments em sample He composition' density, md veloeily distribution of the charged
particles md learn where they come from. Jupiter is interesting because it he the largest md most powerful
magnetosphere md because our knowledge of it is largely rescind to He equatorial plane. Neptune is interesting
because He tilt of the field exposes the polar cusp to the solar wind on every rotation. This I6-hour periodicily
allows one to see the sources md sinks in operation on very short time scales. A Neptune orbiter that reaches high
latitudes could take advantage of this opportunity.
Sixth+ Determine the Properties of Planetary flings
Composition' particle size, number density, eollisiona1 eff~eieney' md collective behavior are some of He
mod important properties of plme~ry rings. The Cassini orbiter he the potential to do ~ exquisite job win He
mod massive rings in He solar system. A Cassini extended mission would provide dam on deeada1 echoes'
including thermal effects when the rings are edge-on to the Sun, d~amiea1 effects when nearby satellites pass in
their orbits md secular eh~ges brought about by collisions with in~rplme~ry bodies. However, eele~ia1
meehmies prehend Cassini from hovering over the rings. Such hovering would allow one to follow individual
ring particles as they collide win each other' but technological development are needed to accomplish ~is.
Seventh+ Map Atmospheric Properties ~ Functions of Depth' Latitude' and Longitude
The Cassini mission will provide ~ weals of new information about the ~ree-dimensiona1 structure of
Sutures atmosphere. However' Earth-b~ed ~leseopie observations are ~ essential complement to in situ studies
~ Jupiter md Saturn md are the only source of such information for Uranus md Neptune in the next decade.
Three-dimensiona1 distributions of atmospheric composition' temperature, aerosols, winds' md aurora1 emissions
~~ PLANETS
jop
FIGURE 4.5 Minim rear Alum doubled as ~ radio ~lewopc to Hollow chew images showing the variations in Jupi~r~s
trapped radiation beds over ~ lO-hour period. The radio emission has ~ wavelength of 2.2 cm and original from cleckons
trapped in Jupiter5s inane made tic field. The ~1~s wobble with respect to the superposed optical images laud Jupi~r,
magnetic axis is inclined with respect to its rotation axis. Soured of NASA/JPL.
Pro
HEW FR0~ IN =E 50~R HIM
are poorly known for the outer plows. This situation cart ~ improved dramatically in ~e next ~~e by utilizing
large ground-based telescopes win adaptive optics' Id Mentors, arid the expend wavelength coverage
available from space-~ased ~lemopes.
SPACE MISSIONS FOR GIANT PLANET EXPLORATION
Space Missions
The Cassini orbiter is scheduled to begin its exploration of ~e Satum system in 1~ 2004. The success of this
historic effort is ~ manor of the highest scientific priority arid it should also be ~ maker of the highest program-
matic importer.
The Diary Ply Pared has identified ~ single medium~lass new mission thy addresses most of ~e key
questions described above for ~ gas girt ~ Jupiter polar orbiter with Area atmospheric entry prows. This
mission requires only incremental ~chnologica1 development. It cart arid should ~ launched in this decade. For
the longer Arms ~e party he identified ~ single large-class mission ~~ addresses most of the key questions for art
ice diary ~ Neptune orbiter win multiple enLy probes. The Neptune mission' among others' requires new
Ethnology development ~~ should ~ initiated in this decade to enable consideration in the following decade.
Table 4.l summaries how them missions arid other activities address ~e key science questions discussed above.
Jumper Bode 076~r why proms
The Jupiter Polar Orbiter with Probes (JPOP) mission combines several smaller missions ~~ have recently
been proposed or studied by NASA teams. Combining Hem as one mission reduces trmsport~ion eons md
enhances He mienee return, because the measurement complement each over in important ways. The element
of the mission are as follows:
~ A polar orbiter (periapse cl.l 1~) spacecraft for atmospheric remote sensing' gravity analysis, panicles md
fields measurements md probe day relay; md
~ Three a~nospherie probes that em penetrate to He 100-bar pressure level md thy em sample ~ range of
latitudes within 30 degrees of He equator for a~nospherie sounding.
JPOP carries ~ microwave radiometer ~~ is used for remote sensing of atmospheric composition when it is
inside the radiation belts; thus, the periapse of the orbiter must be close to the planet. The polar inelin~ion Id low
periapse are also essential to avoid radiation Id ring hoards. The radiometer obtains estimates of He wear Id
ammonia Trundles to depths of hundreds of bars. The pole-to-pole coverage complement He mass spee~ome~rs
on He probes, which sample ~ range of latitudes within +30 degrees. The mass speebome~rs on He probes
provide ground-bush for the microwave radiometer. The probes measure composition' winds, temperatures'
clouds, Id sunlight as functions of pressure to 100 bars.
After dropping off the probes Id relaying Heir signals to Early JPOP spends ~ year or more in ~ highly
inclined orbit win periapse near the equ~oria1 plane cl.l 1~ from He planet center. It measures the magnetic
field' charged particles' Id plasma waves close to the plmet. lladio occultations probe He atmospheric d~amiea1
structure. The orbit itself is sensitive to the fine structure of He gravity field. Both the axisymmetrie part due to
Jupi~r~s interns mass distribution Id the non~xisymmetrie part due to sa~llite-indueed tides are measured. The
microwave radiometry away from periapse, provides He first ~ree-dimensiona1 map of Jupiterts radiation Bela.
Additions remote sensing (ultraviolet visible, infrared) would be desirable but is nof critical to He success of He
. .
mission.
~~ PLANETS
cms`~] No - ~~! M]~`on
The performers of ~e Gemini spacecraft arid instrument during the December 2000 Jupiter flyby bomb
well for ~e po~ntia1 success of ~e Cassini orbiter mission ~ Saturn. To realize this potential' hard decisions have
to ~ made concerning science priorities. Past economies arid added taxes have affected ~e run-out costs of this
program' threading ~e communizes Vilify to ingest arid interpret ~e dam. As the mission progresses arid ~e
capabilities of ~e instrument complement are known' ~e run-out budget should ~ enhanced to allow optimal
ar~alysis by Ham mem~rs arid the larger science community.
Chasm` Ext=~d M`~`on
After the nominal Cassini mission ends' coverage of mmy pare of ~e Saturn system' including Tithes
surface md the polar regions of the plar~et md its magnetosphere' will be incomplete. A Cassini ascended mission
should ~ formulated arid priced in order to obtain optimal science-to-cost ratio. The Cassini Satum science
complements ~~ from ~e proposed Jupiter Polar Orbiter win Proms mission. Critical choices should be made
to optimize the bomb yield from them missions.
Neptune 0~r wad Probes
The objectives of this longer-~rm mission span the plmet, rings' magnetosphere, md sa~lli~s, particularly
Triton. The spacecraft would carry remo~-sensing ins~umm~ as well as instruments to sample particles md
fields. Compared win the Jupiter Polar Orbiter win Probes' He Neptune mission would be more comprehensive'
as befits ~ planet about which less is known. Trade-offs would have to be made among the orbit payload' power'
telemetry, md other resources. Satellite objectives are described in other Shapers. Here the panel describes
objectives arising from the plmet the rings, md He magnetosphere.
As win Jupiter, knowing He volatile abundances has high priority. The cloud base for water may be deep
within He planet' so He atmospheric Trundle might reflect the saturation vapor pressure rather ~~ the bulk
water abundance of the interior. Other vol~iles such as CH4' NH:, md His may have cloud bases within He range
of probes md microwave remo~-sensing observations, so it would be possible to sample the well-mixed planetary
interior for these compounds. Both the gravi~ field md magnetic field are of grew interest. Voyager showed that
the magnetic field is <
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Other Mission Concepts
Two other promising concepts for longer-~rm missions are listed here' without arty raking. As is ~e
Neptune Orbiter with Probed they are dependent on the development of enabling technologies.
satum R`~g 065~r
On ~ Saturn Ring Observer mission' advanced propulsion would be used to hover clom above the ring plural
for longhorn study of collisions arid over microphysica1 processes.
Ur~ 0761~r why Probes
The science objectives arid payload of ~ Urar~us Orbiter win Probes mission would be similar to those of ~e
Neptune Orbiter with Proms mission.
ELey Enabling Technologies nnil Enrth-Bnsed Facilities for Ginnt Planet Exploration
Tec~olo,gy D - ~lopment
Two incremental ~chnologica1 developments are needed for atmospheric probes to the 100-bar pressure level
Jupiter during the present decade:
Lightweight hem shields' md
~ Lightweight (a few kilograms) mass spectrometers.
To enable high-priori missions in leer decades' the following ~hnologica1 goals need to be pursued
vigorously in ~e present decade:
Implement nuclear-electric propulsion;
~ Obtain enhmced~lecommunic~ions, including large microwave arrays mdior optimization ofthe NASA,s
pep Space Network (USE); md
~ ~~rmine the feasibility of implementing aerocapture in gist plmet atmospheres.
In addition, ou~r-plmet missions require nuclear-electric power sources such as radioisotope power systems
(l?P8s). Although ~e Ethnology is well in h~d, procurement sups must be taken to ensure availability.
Enrth-Bnsed Facilities
Importmt Earth-based facilities include large ground-~ased telescopes, survey telescopes' space ~lemopes'
md large radio arrays. All of these have general ashonomiea1 applications. The study of Mint plme~ around
other stars is ~ booming md important field, which will rely increasingly on solar system gist plme~ for
calibration. Here the panel emphasizes what em be learned about our own gimt planets from these Earth-based
facilities.
O~t ~0- to SO-m) Segmented Mirror Telescope
The adaptive optics (AO) eapabili~ will provide diffraction-limited imaging of solar system objects. The
large light-ga~ering power of Me telescope allows high-resolution spee~oseopy of Me outer planets, which is
critical for determining altitude derisions of their atmospheric properties. The planets eommunily needs to help
define Me capabilities of Me AO system md Me specific instrument ~~ will be developed for this telescope. The
ability to track moving targets is important for solar system studies in genera.
~~ PLANETS
De~d Paltry Telescope Blot R~d IRTF
~5
Some area of ply - science are best mrved by long-~rm monitoring. Examples include d~amic features
in diary plmet atmospheres' ~e joviar~ magnetosphere arid id response to volcar~ic outburst from Io' arid space-
cr~ missions thy need Ear~-based support. Them activities need large blocks of observing time. The solution
is to have ~ dedicated planetary ~lemope such as NASA's Infrared Telescope Facility (IRTF), although thy
telescope needs refurbishment to keep up win modem demands arid to utilize modern Ethnology.
spat Telescopes
Although ground-based telescopes with AO systems cart surpass ~e diffrwlion-limi~d imaging capabilities
of spa~-based telescopes, ~e lair allow one to observe in the ultraviolet md far infrared where ground-b~ed
telescopes carmot. Space-based telescopes also have ~ more nearly continuous dub cycle. Plar~ry scientists
mud ~ included in early plarming arid development to ensure thy space ~lemopes have instrument md tracking
capabilities thy serve solar system research objectives.
Sq~e Or Array
The Square-Kilome~r Array (SKA) is ~ proposed interrmliona1 radio astronomy venture ~~ parallels enhmce-
men~ that are being discussed for the Deep Space Network. The DSN is primarily for telemetry md the SKA is
primarily for listening, but it would be desirable if the two arrays were compatible md could be arrayed together
for special events md unusual seientif~e opportunities.
llECOh~lEN~ATIONS OF THE GIANT PLANETS PANEL TO THE SiTEEllING GROUP
The study of gist plme~ stands ~ He threshold of ~ new era. Even as we complete He first systematic
explorations of Jupiter md Saturn with orbiting spacecraft we are witnessing ~ explosion in He number of known
gist plme~ around other Mars. As we struggle to comprehend He diversity of plmet~ systems discovered
elsewhere in our galaxy' we are reminded of certain fundamental Dings ~~ we do not yet know about our own
impressive system of four gimt plme~. For example' we do not know if Jupiter he ~ solid core, or if it contains
enough wear to support standard theories of solar system formation md ev olution. We do not fully undersold He
meehmism that produces md sustains the boded atmospheric structure or how deep ~~ structure goes, or how it
might affect He distant spectral signature of anoint planet under He most general rime of possible conditions. We
are just begirming to probe the complexities of He mmy-body gravi~tiona1 interactions thy shape He rings, md
the magnetohydrod~amie interactions that shape the magnetospheres, although both are likely to be important
during He course of stellar md planetary evolution. On the over h~d' we know far more than we did two decades
ago. The legacy of He Voyager, Galileo' md (soon) Cassini missions md concurrent ground-based work is that
we now know how to formulae the questions presented above in ~ precise manner, md indeed, we know how to
find the answers.
As far as we know, there are two generic types of gist planet the gas aims like Jupiter md Saturn md He
fee gists like Uranus md Neptune. A better underbidding of the nature of bow types is needed, both to answer
fundamental questions concerning the formation of He solar system md to guide He interpretation of observations
of other plme~ry systems.
In assigning priorities for future missions to the gist planets, ~ variely of factors must be md have been
considered. Long travel times md tight mass md eommunie~ion eons~ain~ are ~ given with missions to the outer
plme~. The need for near-term development of enabling technologies for longer-term missions must be eon-
sidered. Above all, it mustbe recognized that extrasolar plmets will increasingly become ~ focus of bow scientific
md popular attention, as evidenced by the selection of Kepler ~ Discovery mission designed to search for
extrasolar plmets by looking for He luminosity derisions Hey may cause ~ Hey Posit the disks of Heir parent
stars. hazy more extrasolar planets will be deleted in the next decade. Some will be imaged' md Heir spectra
HEW FR0~ IN =E 50~R HIM
will ~ partially resolved. To provide critical ground-truth for these exciting discoveries' NASA should pursue ~
parallel program of close-up exploration md analysis of our own gimt plus. These two lines of investigation
cm be, md should ~' s~ergi~ic.
For the next decade, the Diary Plmets Parted recommends ~e following initiatives' in ranked order:
I . Cad t~ gas g`~t Jo. The cen~rpiwe of this effort should be ~ dedicated mission such ~ ~e
polar orbiter win three entry probes descried above. Ear~-based observ~iona1 arid theoretical efforts would, ~
always' be es~ntia1 complements to his spacecraft mission. The mission is scientifically focused arid ~chnologi-
cally fusible with only modest improvements to available Ethnology. It addresses several outriding questions
thy are fundamental bow for under~ar~ding ~e solar system arid for calibrating observations of other perry
systems.
2. Exploit t~ cape of t~ Cassin' orator at $~. Every effort should ~ made to maximize ~e
scientific yield from the Cassini orbiter mission. As ~e mission progresses' the quality of the dam should be
assessed arid the 1~1 of science support within ~e instrument Cams should be enhanced accordingly. Funding
should be provided to the research community for dam paralysis, arid ~ Flare for extending ~e primary mission
should be developed on ~e basis of new science ~~ cart be achieved.
3. ~~e t~ pro of ~~-~ In. Mmy of our key questions cm ~ addressed effwlively
with E~h-orbiting telescopes md ground-based telescopes utilizing adaptive optics. Plar~etary scientists should
be actively engaged in defining He capabilities md scheduling of these advanced observing platforms. Some
questions require ~ dedicated telescope for systematic long-term monitoring. The maintenance md refurbishing of
the I1lTF for this purpose is ~ continuing priority. Maximizing the science return from both in situ md Earth-b~ed
observations requires ~ robust concurrent program of dam Physic md modeling efforts.
4. ~~epareforfumre exploration of ~e `~e gi~t I. Preparations must begin in this decade to enable
~ future mission to Neptune' as described above. Technology needs for such ~ mission include nuclear-elmtric
propulsion md advanced power sources' enhanced telecommunications, md lightweight (a few kilograms) mass
spec~ome~rs md hem shields for entry probes.
FIEF Elf EN C ES
~ . T. Guillot' > PA FEZ Space Sc~ce A: ~ ~ 33- ~ 200> ~ 999.
2. T. Owen' P.R. h5ahaffy' H.~. Niem~' S.K. Eureka' T. Donahue' A. Bar-Nur~' Ad I. ~ Peers <~' Nature 402: 269-~0' ~ 999.
3. D. Gautier' F. overset' md O. hfousis' ~>
Astroph~[Jour~ers 550: 2~-230' 2001.
4. A. Burrows' W.~. Hubbard' J.I. Lur~ir~' Ad J. Liebert' >' Anew of
Mourn PI 73: 7 ~ 9-765> 200 ~ .
5. D. Sudarsky' A. Burrows' Ad P.A. Pir~o' IAsiJroph~l Jo arm! 538:
SSS 903) 2000.
6. W.E . Hubb lard' A. Burrows' Ad J.I. Lur~ir~> I~> AN ~~ of Astronomy a~A~troph~ 40: ~ 03- ~ 36>
2002.
7. D. Gubbins arid J. ~ loxham' llhiorpholo~ of the Geomagnetic Field Ad Implic~iorls for the ~od~amo>~' Nature 325: 509-5 ~ 1>
1987.
S. For ~ rearm review' see' for e xample' R .A. West' 1lAtmospheres of the Gist PA in P .R. We issued L. -A. hi~Fad~r~' Ad T A.
Johr~sor~ (eds.~' E~ope~a of ~ ~ So~r~> Academic Pre ss' Sari Diego' Calif.' ~ ?~' pp . 3 ~ S-337.
?. For ~ rearm roil review' ~~> for example' A.P. ~~rsoll' I~> ire J.K. Be Bye C.~. Pe~r~r~>
Ad A. Chaikin ~ds.~' ~e A7ew So~r~' Sky Publishir~g' Cambridge' h5~.' ~ 999' pp. 201-~20.
10. For ~ recent review' see' for ex:~mple' h5.S. Harley' >' ire P.R. Weissm~> L.-A. hdoFad~r~' Ad T.Y.
Johr~sor~ (eds.~> E~ope~a of ~ ~ So~r~) Academic Pre ss' Sari Diego' Calif.> ~ ?~> pp . 339-355.
~1. For ~ recent r~or~chnical review' ~~' for example' W.~. Hubbard' ll~eriors of the Gist Pl~ets>~' ire J.K. Be~' C.~. Peter~r~' Ad
A. Chalk ir1 (eds.) ~ ~e New Sour ~~> Sky Publishirlg' ~ Arid h4~.> ~ ?? ?' pp. ~ 93-200.
12. W.~. Hubbard' ~> ir1 J.K. Beauty' C.~. Peterserl' Ad A. Chaikin (eds.~> ~e Hew So~r~> Sky
Publishirlg' Cambridge' h] ass.' 1999) pp. 193-200 .
~ 3. T. Guillot' PA Comparison of the Warriors of Jupiter Ad S~um>~' PA a~ Space $~e Hi: ~ ~ 83- ~ 200' ~ 999.
PLANETS
~7
14. A.P. ~~rsoll~ I~' in J.K. Be~> Cog. Pe~r~r~> Ad A. Chaikir~ ~ds.~~ ~e New Sour ~~~ Sky
Publishir~g' Cambridge' h4~.' 1999' pp. 201-~20.
~ S. R.A. Wash I~> ire P.R . Weissmm' L .-A. he oFad~r~> arid T A. Johr~or~ feds.) ~ Cope of ~e
$~'Aca~mic Press' Sari Diego' Calif.' 1999' pp. 315-337.
16. B. Rakers' D.S. Colbum' K.A. Rages' T.~.D. freight' P. Arvm' G.S. Ortor~' P.A. Y~am~dra-Fisher' Id G.W. Grams' 1
