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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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80 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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81 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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85 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 myion 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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88 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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89 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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98 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 200K. 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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99 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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101 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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102 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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103 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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