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Appendix B
Excerpts from the Final Report of the
Jupiter Polar Orbiter Workshop
B.1 MOTIVATION, GOALS, AND CONCLUSIONS
AND 1tECOMMENDATIONS
The inner jovian system comprises the inner Galilean satel-
lites, lo's heavy-ion torus, the radiation belts, Amalthea, Jupiter's
rings, and Jupiter itself. This region contains the solar system's
most intriguing collection of phenomena and processes relating
to planetary and satellite interiors, surfaces, atmospheres, ions
spheres, magnetospheres, and rings.
Believing that no spacecraft could survive the radiation en-
vironment there long enough to make sufficient measurements,
NASA has scheduled no mission to the inner javian system. Ret
cently the Committee on Planetary and Lunar Exploration, chaired
by Donalc} Hunten, and the Space Science in the Twenty-First
Century T - k Group on Solar and Space Physics, chaired by Fred
Scarf, independently questioned whether the radiation problem
was insurmountable. With polar orbits designed to minimize ra-
diation exposure and with realistically conceivable improvements
in radiation hardening, a mission to mine Jupiter's wealth of plan-
etary information might be possible.
79
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On July 18, 1985, a two-day, NASA-sponsored workshop con-
vened at UCLA to assess the scientific value and the likely fea-
sibility of a Jupiter Polar Orbiter (3PO) mission. The inner jo-
vian system is of interest to a broad range of planetary science;
therefore, the workshop participants and contributors represented
diverse disciplines.
This report describes two mission designs that the workshop
participants considered. These are referred to as the high-periaps~s
and the low-periapsis options. Both mission scenarios have great
scientific value and widespread support. The most important re-
sult to surface from the discussions about the orbital requirements
is the realization that a well-designed hybrid mission, with reason-
able capability to adjust orbit parameters, will very likely be able
to satisfy the varied scientific needs to be outlined in the main
body of this report.
The report gives the results of prelirn~nary calculations of the
radiation hazard for sample missions. The hazard from ring par-
ticles Is also considered. It presents the workshop's determination
of the science that a post-Galileo JPO mission should address and
discusses the inner Jovian system objects individually. The novelty
of the mission concept presents opportunities to use instruments
new to planetary exploration and to use conventional instruments
in new ways. The report separately explores these possibilities.
The final section lists the workshop's conclusions and makes a
number of recommendations. The most important of these are
given below.
Mayor C~cInsione
. It appears likely that of the two considered mission de-
signs and possible hybrid designs there exist versions that have
sufficiently long lifetimes against radiation damage to meet major
· · · ·
mission o Electives.
. A JPO mission designed to meet a substantial portion of
the objectives detailed in -this report has very strong scientific
justification.
While either the high-periapsis design, which emphasizes To
and plasma science, or the low-periapsis- design, which emphasizes
Jupiter science, has sufficient scientific merit to justify a mission, a
hybrid design that combines both would yield the greatest return
and have far broader support.
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Major Recommendations
~ NASA should authorize a thorough radiation hazard eval-
uation in coordination with these recommendations.
. NASA should authorize a parameter study of the two mid
sion designs considered and hybrids of these designs for use in
future discussions of mission options.
~ In support of the above recommendations, NASA should
undertake an advanced study project to determine achievable ra-
diation hardening to set total flux limits for mission designs.
~ In view of JPO's likely technical feasibility and its potential
for rich scientific yields as assessed by the workshop, NASA should
organize a science working team to explore more fully the issues
of feasibility and science yields and, if the workshop's preliminary
assessment is confirmed, to determine an optimum mission design.
B.2 MISSION DESIGN OPTIONS: ORBITS, LIFETIMES,
AND SCIENCE CAPABIIITIES
Understanding that the subject mission requires polar orbits
to minirn~ze radiation exposure, the workshop considered low-
periapsis and high-periapsis mission designs. These are illustrated
in Figure B.1. Table B.1 summarizes design parameters relevant
to a discussion of each option's merits.
The low-periapsis case (orbit ~ in Figure B.~) optimizes Jupiter
science by providing in situ aeronomy data, high-resolution remote
sensing measurements of the atmosphere, and high-order gravi-
tational and magnetic field moments. It allows low-altitude in
situ observations of the ionosphere, the thermosphere, the aurora-
producing particles, and plasma wave and radio emissions gen-
erated under conditions never before sampled in space or in the
laboratory. The 11° to 22° apsidal rotation during the estimated
nominal rn~ssion lifetime, combined with the torus's 14° ~daily"
wobble, gives adequate torus coverage. The orbit provides o~-
equatorial, high-altitude passes to observe in situ the waves that
scatter the aurora-producing energetic particles.
The rapid latitude sweep of the low-periapsis option precludes
a large number of close To encounters. The low-radiation envi-
ronment required for visible, ultraviolet, ant] x-ray wavelength
imaging restricts such measurements to an orbit's transpolar seg-
ment. While the transpolar segment of the low-altitude option
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82
11-lo ED —
-
-
~'T
FIGURE B.1 The Jovian environment showing the low-periapsis option (I)
and the high-periapais option (II).
aids high-resolution auroral imagery, it limits synoptic aurora and
torus studies.
The high-periapsm mission design (orbit IT in the figure) opti-
m~zes To and torus science. With relatively small velocity change
maneuvers at apoapsis, periapsis can step radially through the
torus, giving complete radial and oE-equatorial coverage (less than
~ km/s ~ V required). Holding periapsis at lo's orbit permits mul-
tiple close lo encounters, enabling in situ atmosphere-aeronomy
measurements in lo's leading and trailing, day and night hemi-
spheres. The orbit allows one or more vertical plunges through
the northern and southern Alfven wings and To's wake. The or-
bit thoroughly probes the energetic particle populations and the
off-equatorial region where plasma waves grow. The long nomi-
nal mission lifetime permits changing spacecraft and instrument
parameters in response to analysis of in-orbit measurements. The
high altitude of the transpolar segment allows extensive pole-on
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83
TABLE B.1 Low- and High-Periapsis Mission Design Parameters
Periapsis
Altitude
Apospais at Pole Period
Number of
Orbits to
Galileo Time to
Dosage (BIG) Galileo
(prelimin- Dosage
ary estimate) (BIG)
Line of
Apaides
Latitude
Swing in
JIG Orbits
Low Periapsis 1.014 RJ
(Orbit I) used ~
example.
Fixed
through
mission.
High Periapsis 5 to 8 RJ
(Orbit II) used as
example.
Swept
through
during
. .
mlaelon.
8 RJ used Viable 1.2 10 to 20
as example. 0.S to earth days
Fixed 2.0 RJ
through
. .
melon.
60 R';, 6 R 23 20 to 40
used as (notch) earth days
acunple. 12 RJ
Fixed
through
ITiiasion.
12to24 11to22
earth days degrees
460 to 920 1 to 2
earth days degrees
visible, ultraviolet, x-ray, and neutral particle synoptic studies of
Jupiter's aurora and To's torus.
The high-periapsis option fails to provide in situ Jupiter aeron-
omy data, high-resolution Jupiter atmosphere data, high-order
magnetic and gravitation field data, and low-altitude auroral par-
ticle, radio, and plasma wave data. Remote sensing cannot supply
these.
Table B.2 summarizes the main strengths and deficiencies
of the two mmsion designs. While either option by itself could
probably be justified in terms of its scientific yield, both omit
significant scientific objectives. A follow-on study of a JPO mission
should consider a hybrid design that is based on active orbit change
maneuvers during the primary phase of the mission.
B.3 SCIENCE ISSUES
Introduction
The workshop participants agreed that while Pioneer and Voy-
ager flybys, Galileo orbiter, and earth-based remote sensing have
provided and will provide a wealth of valuable data on the inner
jovian system, they leave untouched or unresolved a host of major
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84
TABLE B.2 Comparison of Strengths and Failings of Low- and High-Periapais
Mission Designs
Low High
Periapsis (I) Periapsis (II)
In situ Io torus G G
Off-equatorial plasma waves G G
Close Io encounters F G
Io-torus coupling F G
Torus-Jupiter coupling G F
Radiation belts G F
"Ring currents" G G
Europa G G
Amalthea G F
Ring G F
Low-altitude auroral G B
Particles and plasma and radio waves G ~ B
Birkeland currents G B
Jupiter aeronomy G B
High-resolution Jupiter atmosphere G B
High-order magnetic field moments G B
High-order gravity moments G B
Synoptic UV, x-ray aurora and torus F G
Imaging fast neutrals F G
Reaction time and mission lifetime F G
NOTE: G = Good; F = Fair; B = Bad.
first-generation scientific problems. In addition, they have estate
lashed and will establish a host of important second-generation
questions. As a result, a Jupiter Polar Orbiter mission can be
designed to address a large, well-focused set of highly significant
science issues. Because of the unprecedented variety and the un-
matched intensity of the planetary processes in the inner jovian
system, the science yield from a comprehensive data-gather~ng
mission to this region could be enormous.
The jovian system may be unique qualitatively as well as
quantitatively. According to present thinking its components are
remarkably coupled to one another, causing them to behave in
many ways like a single interconnected unit. For example, tidal
flexing melts parts of 10'8 interior, forming volcanoes that contin-
ually resurface To with volatile material. These volatiles feed Io's
torus by sputtering. From an initial seed population, the torus
grows because each torus ion impacting to sputters much more
mass than its loss removes. Centrifugal outflow increaser as the
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torus mass grows until all further- addition flows centrifugally out-
ward, and the torus stabilizes. ' The torus acquires its substance
from lo, but it receives its motion from the rotating magnetic field
of Jupiter. Jupiter imposes its rotation on the torus' with electri-
cal currents, the strength of which' is limited by the ionosphere's
conductivity. This lo~v-aItitude current 'valve In turn is governed
at least partially by aurora-produc~ng particles precipitating from
the high-altitude radiation belts. These conductivity-producing
particles diffuse inward to their precipitation point by the same
electric field that carries torus matter out. Since the outflow ~
conductivity controlled, this completes a major system-wide feed-
back loop. After Galileo many of this loop's main components will'
remain unmeasured and unmapped.
This picture of a microcosm of'tightly coupled satellite, torus,
energetic magnetospheric particles,-and ionospheric plasma on the
planet itself has evolved from- Goldreich and Lynden-Bell's 1969
unipolar induction mode! of the Jupiter interaction, as new
information was' obtained and further research undertaken. As
has been noted repeatedly, the interaction between lo and Jupiter
is an example of a fundamental type of astrophysical interaction
that has a stellar analog in pulsars, accretion disks,' and so on. JPO
can map and systematically examine the most striking example
of this class of interaction In the solar system. The potential to
acquire transferable knowledge in this area is great.
A Jupiter Polar Orbiter mussion will substantially advance
comparative planetology by providing in situ data from the atmo-
spheres and ionospheres of Jupiter and lo and by greatly increasing
the coverage and resolution of the unaging and remote sensing of
these objects. Supplying these data ~s certainly- one of the' remain-
ing major tasks and major challenges confronting NASA. -
In this section the post-Galileo science issues relating to the
various objects within the inner jovian-system are treated gem
arately. The order progresses from To's and Europa's surfaces
inward to Jupiterts interior.
lo and Europa
The two inner Galilean satellites differ markedly Tom the
outer two. Both are essentially rocky objects' (Io's density is 3.5
g/cm3 and Europa'9 is 3.0 g/cm3), and both have apparently
undergone extensive and complex geological modification.
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80
Spacecraft exploration of these fascinating worlds began with
Voyager and win continue with the upcoming Galileo mission.
However, major areas of investigation will remain even after the
Galileo rn~ssion.
Europa's cracked, icy surface was observed only poorly by
Voyager, and major questions remain concerning the age of the
surface, the nature of the processes causing the tectonic patterns
evident In the surface, the extent and depth of the ice layer,
and whether liquid water can exist beneath the visible surface.
The Galileo orbiter wall make several close (<1000 km) flybys of
Europa, and wait undoubtedly answer, at least partially, many of
these questions. There ~ a high probability, however, that Galileo
discoveries about this satellite wiD lead to many new questions
and the need for future investigation.
To is one of the strangest bodies in the solar system. Its sur-
face is dominated by volcanic activity, including violent, geyser-
like eruptions sending material hundreds of kilometers above the
surface. The ultimate energy source for this level of activity in
such a small object is believed to be tidal dissipation, but current
estunates of Io's total energy output are still somewhat higher
than theoretical estimates based on tidal theory and orbital evo-
lution. To has a tenuous atmosphere, probably spatially and tem-
porally variable, including contributions from both an ambient
atmosphere and the injection of gases by the volcanoes. Both the
atmosphere and the surface are believed to interact strongly with
the magnetosphere, supplying about one metric ton per second
of oxygen =d sulfur to the magnetosphere by poorly understood
processes. The Galileo mission will study lo, but has limitations:
the orbiter wall make only one close (~IOO~km minimum altitude)
equatorial flyby at the beginning of the my—ion and drill thereafter
make synoptic observations from much greater distances. This
means that high-resolution imaging and spectroscopy data will be
obtained only for a limited region on one hemisphere of To, which
does not contain some of the largest volcanic features observed by
Voyager.
To make major contributions to post-Galileo studies of To,
a JPO mission must make several very close encounters with the
satellite, at least one within the atmosphere/ionosphere itself. The
opportunity to achieve at least one close Europa flyby is also highly
desirable. From such flybys, high-resolution visible imaging and
surface spectroscopy could greatly extend Galileo's coverage and
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87
could make possible study of geological changes covering the period
from the Voyager encounters. Multiple close flybys would also
allow stuclies of the higher order gravitational field components,
probing the interior and searching for crustal heterogeneities.
lo's A=~
lo's atmosphere is believed to be primarily sulfur dioxide in
vapor-pressure equilibrium with deposits on the surface; the pres-
sure may vary from 10-8 bar near noon to 10-~6 ban at night. Of
several photochem~cal products, it is possible that O2 knight build
up to a pressure of 10-~° bar. All these numbers are uncertain,
based as they are on extensive theoretical modeling of two elec-
tron density profiles from Pioneer 10 and the ground-based and
Voyager identifications of sulfur dioxide frost and vapor. Many of
the remarks about Jupiter's upper atmosphere apply to lo as well.
[o's orbit is enveloped in a plasma torus containing atoms
and ions of sulfur, oxygen, and sodium. Maintenance of the torus
requires a supply of 1028 to 1029 atoms per second, undoubtedly
from To's atmosphere, which therefore must be completely replaced
every few days. The current view is that the atmospheric atoms
and molecules are ejected from To's gravity by the impacts of
torus ions co-rotating with Jupiter and therefore passing lo at 55
km/s. This raises the question of what determines the density,
because a different density would create a proportionally different
source strength. The torus has been stable for several years, but
the Pioneer measurements unply a much lower brightness at that
earlier tune.
Progress in understanding this system requires direct measure-
ment of the atmosphere and its positive ions, such as are readily
made by a standard aeronomy package on a spacecraft making a
close pass. Several passes at different local solar times would be
even better, but even a single one would create a breakthrough in
our understanding. Except for a slightly higher mass range for the
neutral and ion mass spectrometer, the requirements are identical
to those for the similar Jupiter passes (or for many earth missions,
Pioneer Venus, and the future Mars Aeronomy Orbiter). Although
the torus is discussed elsewhere, its close coupling to the lo atmo-
sphere and ionosphere must be stressed. Its role in atmospheric
loss has already been mentioned. It probably also supplies fluxes
of electrons that create the ionosphere. Higher energy positive ions
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from the magnetosphere are believed to play a role in sputtering
material from the surface into the atmosphere and from there into
the torus.
lo-Torus Coupling
As early as 1965, ground-based observations showed that To
controls radio ern~ssions from Jupiter's ionosphere. Thereafter,
theory and observation revealed additional ways in which To
an unusual satellite. Its orbit is embedded in a-dense heavy-
ion plasma, localized in a torus ringing Jupiter. The torus plasma,
whose source is To, corotates with Jupiter and consequently sweeps
by To at a relative velocity of more than 50 km/~.
The interaction between a flowing plasma and a conducting
body can take many forms, as we know from the diverse ways in
which planets and the Moon interact with the solar wind. The
nature of the interaction depends on properties of the satellite and
of the plasma, and in the case of lo may also depend on properties
of the Jovian ionosphere. The relevant satellite properties are its
heigh~integrated conductivity, its efficiency in providing neutrals
that can be ionized in the nearby plasma, adding "pickup ions" to
the flow, and the strength of any intrinsic magnetic field. Further
complexity may be added if the satellite is spatially nonuniform
(as, for example, if its ionosphere Is absent on the dark side).
In addition to flow velocity, important plasma properties include
density, temperature, and field strength, which, in turn, determine
the speed with which perturbations of pressure flow velocity and
magnetic field strength and orientation propagate in the interac-
tion region. Jupiter's ionosphere reflects waves back toward the
torus with an efficiency determined by its conductivity. The re-
flected wave returns to the interaction region (thereby modifying
the interaction) if the wave travel tone is shorter than the time for
an element of plasma to flow by lo.
Further discussion focuses principally on the interaction with
To. There the upstream plasma perturbations are small because the
flow speed Is small compared with wave speeds and no upstream
shock Is expected. Very near To, the plasma slows and ~ diverted,
pulling the field with it. Perturbations travel away from lo along
the field toward Jupiter's ionosphere, but at the same time, plasma
flow sweeps the perturbed region downstream. Thus perturba-
tions are principally found downstream behind a front at angle
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GA = ~tan-~(V/vA), where V is the relative flow velocity and
VA is the Alfven wave velocity. The strongly perturbed region
is bounded by ~Alfven winged that extend away from To toward
the northern and southern ionospheres of Jupiter. Details of the
plasma and field configurations within the Alfven wings depend
critically on the above-noted satellite parameter. Furthermore, as
Jupiter rotates, To's magnetic latitude varies periodically, and it
moves up and down through the torus. Both plasma properties
and wave transit time to Jupiter's ionosphere vary periodically
as a result. Long-term temporal variability may also occur as
volcanoes on To become active or shut off.
Thus, To's interaction with the plasma in its neighborhood
depends on parameters of the satellite ill-constrained by data and
on parameters of the ambient plasma that may vary with position
and time.
In its single flyby through the region close to To and down-
strearn in the flow, the Galileo spacecraft wall characterize some
features of To. A meaningful upper limit on an intrinsic magnetic
field will be obtained. Mass pickup in the vicinity of To will be
characterized, and the mechanisms limiting ionization of neutrab
in the vicinity of To may be defined.
As noted above, the Galileo spacecraft anti have only one close
flyby of To. While this will give the first observations of To's plasma
wake and help determine whether To has an intrinsic magnetic field,
we will sample only a small portion of the region of the To-jovian in-
teractions. For instance, even if Galileo passes through or near the
Alfven wing, it will make only a singI - point observation. Galileo
will not provide any information about the structure of the Alfven
wind with latitude. It is important to determine the shape of the
Alfven wing by measuring its spatial dependence. For instance, if
mass loading is important, the structure of the wake region will
provide information about mass pick-up currents. Observations of
Alfven waves that have been reflected from Jupiter's ionosphere
could give us information about the interaction between To and
the jovian ionosphere. In particular, multiple passes would help
to determine how often the reflected Alfven waves can be reflected
before they decay.
It is widely believed that neutrab sputtered from To or its at-
mosphere are the source of torus plasma. These sputtered neutrals
are expected to form a cloud extending well away from To. Par-
ticles in this cloud are ionized to form the plasma torus. Galileo
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information on the absolute densities of the major constituents in
Jupiter's upper atmosphere. These measurements yielded the den-
sities of molecular and atomfic hydrogen and a number of impor-
tant hydrocarbons at pressure levels In the range of lo-6 to 10-7
bar. The helium abundance was measured only at much higher
pressure levels, and thus only extrapolated values are available for
the upper atmosphere. The Voyager ultraviolet experiment also
provided information on the exospheric neutral gas temperature;
this solar occultation measurement gave a temperature of 1100
200°K.
It needs to be emphasized that all this information is merely
a snapshot of conditions at low latitudes at one particular time.
Ground-, Pioneer-, and Voyager-based information (e.g., Lyman
emission) clearly indicates that there must be significant temporal,
latitude, and maybe longitude variations in the physical processes
controlling the behavior of Jupiter's upper atmosphere. Unlike the
Earth, Jupiter receives only a very small amount of energy from
the Sun; the measured high exospheric temperature implies the
presence of some important energy sources (e.g., gravity waves,
Joule heating, particle precipitation), but the lack of information
on the relevant parameter, such as temperature variations, bulk ve-
locities, conductivitie~, ion composition, and precipitating particle
fluxes, precludes the deterrrunation of the dormnant processtes).
Each of the two Pioneer and Voyager spacecraft provided
two radio occultation profiles of the ionospheric electron densities.
This information does indicate that the peak electron densities are
likely to be of the order of 105 to 106 cm3, consistent with theoret-
ical predictions, but does not allow any meaningful determination
of the altitude of the main ionospheric peak or the magnitude of
the diurnal and latitudinal variabilities. In situ measurements are
required in order to really understand the ionosphere of Jupiter, or
even to interpret the radio occultation results. Galileo will not pro-
vide any in situ measurements of the thermosphere or ionosphere
of Jupiter.
In order to make meaningful advances in our understanding of
the physical and chemical processes controlling the behavior of the
upper atmosphere and ionosphere, extensive measurements of the
composition, structure, dynamics, and energetics of the neutral gas
and plasma are necessary. Specifically, neutral temperature and
density distributions need to be obtained as functions of height,
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latitude, and local time. This anti require both in situ mass spec-
trometer measurements of deuterium, hydrogen, and helium at
the higher altitudes and remote sensing of these species as well as
the hydrocarbons in the lower thermosphere using an ultraviolet
spectrometer and/or imaging devices. Neutral wind measurements
can be made in situ using a mass spectrometer and/or remotely by
Fabry Perot interferometer; such information would be very useful
in studying the dynamics and energetics of the thermosphere.
Determination of the properties of the ionospheric thermal
plasma is another major area of study that needs to be carried
out. [on composition and temperature can be measured In situ
as a function of height, latitude, and local time using ion mass
spectrometry. Langmuir probes can be used to measure electron
temperatures and densities at high temporal and spatial resolu-
tion. Electron density profiles can also be obtained using the
radio occultation technique. These measured parameters will help
to determine the electrical conductivity of the ionosphere, which
must be known in order to understand magnetosphere-ionosphere-
atmosphere coupling processes. The ionosphere measurements
used in conjunction with the in situ neutral data wiD permit a
consistent picture of the chemistry, energetics, and dynamics of
the exosphere/ionosphere to be attempted.
Measurements of the higher energy (nonthermal) particle
fluxes are also required, both in the ionosphere and in the inner
magnetosphere. The Voyager UVS exper~rnents observed intense
I,yman and Werner band emissions at higher latitudes, implying
the precipitation of large auroral particle fluxes into the upper
atmosphere of Jupiter. Energetic particle precipitation (whether
electrons, protons, or heavy ions) undoubtedly drives much of
the aeronomy of the thermosphere and ionosphere, even at low
latitudes, where airgiow observations imply the presence of an
unknown energy source. Therefore detailed measurements of the
intensity, pitch angle, and energy distribution and composition of
these precipitating particles as a function of location and time are
of high priority.
Jovian Atmosphere
The Voyager spacecraft encountered Jupiter nearly in the
equatorial plane, limiting imaging to the region within 50° of
the equator. The Galileo spacecraft encounter will also be nearly
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100
in the equatorial plane. In addition, the resolution obtained with
the infrared spectrometers strongly limits correlative studies uti-
lizing the visual and infrared data sets. The scale of variations
in temperature and composition is too small to be resolved. The
Galileo probe will provide just one isolated profile through the
atmosphere.
Although little is known of the Jovian atmosphere in Dolar
~ e ~ ~ - ~ , ~ ~ . ~
_ ~
regions, ~ong~tuc~nal coverage at lower latitudes reveals cons~der-
able longitudinal homogeneity. The desirable approach ~ to seek
detailed concurrent infrared and visual maps that characterize
the abundance of trace atmospheric components and dynamical
parameters (temperatures, winds) of the visible cloud deck as a
function of latitude. The goal should be to resolve latitudinally as
many atmospheric parameters as possible within limited longitu-
dinal regions.
This mission offers new opportunities to obtain the following:
1. Cloud structure spatial coverage of the cloud deck at 1~
km resolution (comparable to a pressure scale height) in visible
light.
2. Winds temporal sampling optimized to observe rapidly
evolving eddies at highest resolution and slowly varying, larger
areas at lower resolution.
3. Temperature and composition spatial maps with 100 km
resolution at cloud tops and above.
4. Deep structure microwave sounding of temperature, and
abundances of ammonia and water below the cloud deck.
5. Interior structure increased information concerning ~rrav-
itY harmonics providing constraints for interior m,`,lPla
~ __ _ __ O O _ _ .
6. Chemistry ratio maps [such as H2(para)/H2(ortho),
PH3/H3PO4)CO/CH4, and H2S/NH4SHi, vertical motions, so-
lar ultraviolet, precipitation, lightning auroras, and chemical pros
cesses in the atmosphere.
Fundamental questions that can be addressed concern the
following:
1. Poleward heat flux.
2. Large-scale rising and sinking in belts and zones.
3. Deep versus shallow global circulation.
4. Internal heat flux versus latitudinal heat flow.
5. The nature of the chromophores.
6. The abundance of water and hydrogen sulfide.
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7. Temperature gradients vertical, longitudinal, equator-tin
pole, belt-to-zone, and so on.
8. Energy balance between long-lived features and their sur-
roundings.
Jupiter's Interior Magnetic Field
Jupiter, the largest planet In the solar system, ~ endowed with
a powerful and complex magnetic field generated by fluid motion
in its interior much like Earth's. Magnetic field measurements suf-
ficient to characterize only the lowest order moments of Jupiter's
magnetic field have been obtained by the Pioneer and Voyager
spacecraft flybys of the 1970~. These observations demonstrate
that while Jupiter's magnetic field ~ much stronger than Earth's,
they are intriguingly similar. Jupiter's dipole is tilted by 9.6°
with respect to its rotation axis. For Earth the value is 11.5°. It
also appears that Jupiter's dynamo is characterized by the same
quadrupole deficit that ~ evident at Earth. It has been suggested
that the quadrupole deficit ~ a general charactermtic of the dy-
namo process; perhaps the same dynamo dynamics occur in both
planets.
Jupiter's dynamo is far more accessible to study than any
other planetary dynamo, owing to the large radius of its dynamo
generation region. Jupiter's dynamo may occur in a conducting
volume bounded by the pre~ure-induced transition of molecular
to metallic hydrogen, which occurs at #0.8 Jovian radii. However,
dynamo generation may also occur further out in serfficonducting
molecular hydrogen. Spacecraft observations of the magnetic field
can in the case of Jupiter be obtained relatively close to the core.
This is important because the magnetic field rapidly attenuates
away from the core (~/,N+2, where N is the order), and causes
large uncertainty when the measured magnetic field is projected
downward onto the core. Thus, detailed and globally Attributed
observations of Jupiter's magnetic field obtained at close-in radial
distances would advance immeasurably our understanding of the
dynamo problem.
The low-altitude polar orbit envisioned for JPO can provide a
realistic and achievable mapping sequence for the study of Jupiter's
magnetic field. Measurement accuracies of =50 ppm or better, cou-
pled with pointing knowledge of =10 arcsec, would be sufficient
to provide a detailed description of the Jovian field, approaching
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the present knowledge of Earth's field, and more than a thou-
sandfold ~rnprovement in current knowledge of Jupiter's low-order
moments. For the first time in the century-old endeavor to under-
stand Earth's dynamo, a second example would be available for
study.
In addition to providing a vastly improved description of
Jupiter's magnetic field, it may be possible to detect the secu-
lar variation of Jupiter's field In orbit, should sufficient time be
available to complete several mapping cycles. Since Jupiter has
strong zonal flows that might extend into the interior and couple to
the magnetic field, there may be some rapid secular variation and
the possibility of correlating the wind pattern to the field d~tribu-
tion. This would allow the precise determination of the radius of
Jupiter's dynamo-generating region, using the frozen flux theorem
of Bondi and Gold. At the Earth, application of the frozen flux
theorem to the secular variation observations leads to a determined
core radius within a few percent of that determined seismically.
At Jupiter, this information would determine the pressure and
temperature of the molecular to metallic hydrogen transition. In
the absence of a detectable secular variation the core radius may
possibly be obtained from studies of the spatial harmonic content
of the field above the core.
A detailed description of the Jovian field ~ essential to an
improved understanding of Io, the torus, and To-magnetosphere
interactions, since Jupiter's ionosphere ~ an unportant ~valve" in
the tightly coupled {o-magnetosphere-ionosphere system. Present
knowledge severely limits the accuracy with which the foot of To's
field line can be located in the ionosphere. Only low-order mag-
netic field harmonics are known at present, and surface magnetic
field intensities are known to about 1 gauss. To understand the
variations of the torus with longitude and local time, it is first nec-
essary to map the Jovian magnetic field to sufficient accuracy. The
low-altitude, polar orbit of JPO can provide a satisfactory maw
ping sequence and detailed knowledge of Jupiter's magnetic field,
the coordinate system that organizes the diversity of phenomena
of interest in the Jovian system. Polar field-aligned currents are
an important diagnostic of the Jovian magnetosphere and an es-
sential tool in the study of mass and momentum transport in the
magnetosphere. The polar field-aligned currents carry anulIar my
mentum from Jupiter to the torus and outward-flowing plasma.
The systematic study of polar field-aligned currents in the Earth's
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magnetosphere by low-altitude polar satellites and ground obser-
vatories has been the key to understanding the dynamics of the
Earth's magnetosphere and its interaction with the solar wind.
Similarly, the systematic study of polar Birkeland currents at
Jupiter is essential ~ monitoring the dynamic interaction between
Juipter and lo (Io torus). The low-altitude polar orbit proposed
for JPO is ideal for the measurement and synoptic mapping of
these polar Birkeland currents.
Jovian Interior: Gravitational Field
The best models of Jupiter's gravitational field were derived
through Doppler tracking of Pioneers 10 and 11 during their re-
connaissance of the Jovian system. Although the two Voyager
spacecraft carried improved radio equipment, they did not ap-
proach sufficiently close to Jupiter that large improvements in the
harmonic coefficients were possible. Currently, ]2 iS known to ~
part in 105, 34 iS known to 1 percent, 36 hm3 been marginally
detected, and the nonhydrostatic harmonics are consistent with
zero. The Galileo orbiter wiB remain relatively far from Jupiter
but may provide small improvements on the field model, indi-
rectly, through precise (optical) measurements of the precession of
natural satellite orbits.
The current field model is based primarily on tracking at
1.75 Rj and beyond (since Pioneer 11 was in occultation at its
closest approach). As a consequence, the absolute sensitivity in
detecting a high-order harmonic, J2n, was degraded by (~.75~-2~.
For example, the absolute error in 36 ~ currently over an order
of magnitude greater than the absolute error in 32. The proposed
mission, if it includes even one close encounter (1.05 Rj) will
reduce the absolute error in each ]2n to an essentially constant
value, independent of n. If there are N close encounters (with
N ~ 20), then the error will be reduced by a further factor of
at least Nt/2. Even more importantly, good spatial coverage will
be achieved by multiple close approaches, leading to very tight
constraints on nonhydrostatic (especially tesseral) harmonics of
the field. The proposed mission should therefore give a value of
J6 to several percent, a detection of Jo, and a detection of any
tesseral components exceeding 10-7 of the total field.
There are at least three reasons why an improved field mode!
of this accuracy is highly valuable. First, an improved value of J6 iS
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104
"determined" by J4, so the interior models provide rather precise
(10 percent error) predictions of 36. A test of these models ~ an
incremental but nevertheless important step in improving our un-
derstanding of giant planet composition and structure, especially
of the outermost layer.
Second, the higher harmonics (especially J~ and higher) are
substantially affected by differential rotation, if the observed zonal
flow extends deep into the planet. The effect is much larger in
Saturn than in Jupiter but may be large enough (at the few
percent level in 36) to be detectable in Jupiter. This is a marginal
but potentially very import ant test of the hypothesis for deem
seated zonal flows.
Third, and potentially most exciting, is the possibility of
"semmologyn the detection of tesseral harmonics or tune-depen-
dent (i.e., nonsynchronous) contributions to the 32n caused by
normal modes of the planet, excited by convection. (Tidal effects
are also present.) The likely amplitude of these effects ~ largely
unknown but could be at the level of 1 part in 107 and hence
marginally detectable. The science of solar seismology ~ greatly
aiding our understanding of the Sun, and the corresponding detec-
tion in Jupiter would be a comparable breakthrough for the giant
planets. Clearly, it is of importance to establish the capabilities
of this mission, especially the effects of such improvements as the
use of a k-band signal (32 GHz) in the radio link, which would
greatly reduce plasma interference, the use of VERB! techniques to
augment conventional Doppler tracking, and the possibility that
extremely precise ephemerides of the satellites could further re-
duce the errors. One possible area of concern is atmospheric drag
during closest approach.
B.4 CONCLUSIONS AND RECOMMENDATIONS
Scientific J=t~fication
· A Jupiter Polar Orbiter mission is scientifically exciting
for the number of planetary system processes it will elucidate
and because of the basic data it will provide for comparative
planetology.
· The value to planetary science that measuring [o's activity
and associated phenomena adds to a mission to measure Jupiter's
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105
planetary parameters cannot be overemphasized. The combina-
tion of low- and high-altitude measurements gives a mission to the
inner jovian system extraordinary scientific interest.
Mission Design FeasibUit~
. I.ow-periapsis, high-periapsis, and hybrid orbits capable of
meeting all major mission objectives are achievable with currently
available propulsion systems.
. Through orbit design and radiation hardening, mission
lifetimes appear achievable that will enable completion of major
· · · ~
mission o Electives.
The ring particle hazard also appears surmountable through
orbit design.
· A Mariner Mark IT class spacecraft could accommodate
the full suite of scientific objectives and engineering requirements,
especially power.
.
Major Measurement Objectives
(Mo~mg Tom Outermost to Innermost)
. At least one but very preferably more than one encounter
close enough to To to enable in situ atmospheric measurements
and to determine lo's magnetic characteristics.
At least one pass through Io's Alfven wings and wake.
. A thorough synoptic map based on in situ measurements
of the density contours of the constituents of lo's plasma torus
and its thermal structure. (All present maps are theoretically
derived from single cuts through the torus as will be true also
after Galileo.) The plasma ribbon separating the cold and warm
tori especially needs to be investigated.
A synoptic map of the amplitudes of plasma waves that
scatter the aurora-producing particles.
. Measurements of the species, energy spectra, charge state,
and angular distributions of energetic particles to determine ac-
celeration and transport mechanisms, sources and sinks over the
energy range 10 keV < E < I~0 MeV per nucleon for ions and 10
keV < E < 50 MeV for electrons.
. Determination of the source locations of the magneto
spheric radio components and simultaneous measurement of the
wave polarization. Measurement of the radio frequency signals
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106
Jlovlan aurora.
from atmospheric lightning and use of these signals to determine
the peak ionospheric electron density along the su~]PO track.
Remote monitoring of solar wind conditions external to Jupiter's
magnetosphere.
~ To resolve problems concerning the nature of the three pow
ulations of ring particles and their interrelationships; the factors
controlling ring structure; and the origin of this ring system.
Synoptic-scale polar-view images of the to torus and the
~ Fast neutral particle images of the radiation belts and
neutral particle torus.
~ Determination of pattern and strength of Birkeland cur-
rents at low altitudes.
Direct measurements of the particle fluxes precipitating
into Jupiter's upper atmosphere, in order to establish the mecha-
nism~s) responsible for the observed auroral phenomena.
~ Direct measurements of ionospheric flow velocities and
field-aligned fluxes to establish whether the ionosphere is a sig-
nificant source of plasma for the magnetosphere.
~ In situ measurements of the thermal ion and electron tem-
peratures to establish the major plasma energy sources and sinks
in the ionosphere.
. Remote sensing of the thermospheric winds for a study of
upper atmospheric dynamics.
~ Synoptic measurements of the thermospheric temperature
variations to aldow a meaningful study of the energetics of a unique
planetary upper atmosphere not controlled directly by solar radi-
ation.
.
data.
.
High-resolution remote sensing atmospheric data.
Global low-altitude, high-order moment magnetic field data.
Global low-altitude, high-order moment gravitation field
Installment Requirements
A full complement of optical imagers, high-resolution inter-
ferometer magnetometers, plasma and radio wave detectors, and
plasma ion and energetic particle detectors.
~ Monochromatic imagers at x-ray, ultraviolet, and visible
wavelengths.
. Fast neutral particle imagers.
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107
.
Iyzers.
In situ aeronomy, ion, electron, and neutral particle ana-
Recommendations
. NASA should authorize a thorough radiation hazard eval-
uation of the considered mission designs, and hybrids of these
designs.
. NASA should authorize a parameter study of the two con-
siderec] mission designs and hybrids of these designs for use in
future discussions of minion options.
· NASA shouIc} undertake an advanced stucly project to cle-
term~ne achievable radiation hardening to set total flux limits for
· · -
mlsslon c .eelgns.
. NASA should organize a science working team to explore
more fully the issues of feasibility and science yields, and, if the
workshop's preliminary assessment is confirmed, to deterrn~ne an
optimum mission design.
APPENDIX: LIST OF PARTICIPANTS
Workshop Organizers
Robert A. Brown, NASA Marshall Space Flight Center
Andrew F. Nagy, University of Michigan
H. Warren Moos, Johns Hopkins University
Fred Scarf, TRW Defense and Space Systems
George L. Siscoe (Host), University of California
Workshop Sponsors
Henry Brinton, NASA Headquarters
William Quaide, NASA Headquarters
JPO Workshop Participants and Post-Workshop
Contributors
Ab~alla, M., UCI,A
Bagenal, F., Imperial College
Barbosa, D., UCLA
Beebe, R., NMSU
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Representative terms from entire chapter:
mission designs
