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--> 2 The Role of Mobility in Solar System Exploration The six case studies selected to illustrate the role of mobility in the intensive study of planetary bodies address important goals relevant to atmospheric structure and dynamics, the composition of small bodies without atmospheres and with negligible gravity, the composition of larger bodies with significant gravity, tectonic processes on the terrestrial planets, and the search for evidence of present or past life in the solar system beyond Earth. As such, they address priority goals in a range of scientific disciplines. These questions derive directly from previous National Research Council (NRC) reports. Subsequent sections discuss the scientific importance of each of these questions, describe the observations that need to be made to answer them, and outline the associated need for mobility in obtaining the relevant observations. Circulation in the Lower Atmosphere of Venus Importance Among the most important parameters for understanding Earth-like environments are the physical and chemical properties of planetary atmospheres. Only by studying current conditions can we understand the origin and evolution of atmospheres. Thus, this case study directly addresses one of the specific objectives of the campaign, Evolution of Earth-like Environments, outlined in NASA's solar system exploration roadmap. Beginning with Earth-based ultraviolet observations of Venus's clouds1 and data from the former Soviet Union's Venera 8 mission in 1972,2 it has been established that the planet's entire atmosphere rotates at a faster speed (though in the same basic retrograde direction) than the underlying surface. At high levels in the atmosphere, the speed of this so-called atmospheric superrotation is very large: at cloud-top levels (~50 to 60 km) the east-west (zonal) wind speed is ~100 m/s near the equator. As a result the atmosphere completes one rotation every 4 days, but the planet itself rotates much more slowly. Although the fastest winds are found near the cloud tops, most of the atmosphere's angular momentum is concentrated in the very dense lower atmosphere well below the clouds. General theoretical considerations of atmospheric circulation show that superrotation at the equator can only be produced by eddy, or longitudinally asymmetric, motions that act to transport momentum either vertically or horizontally in a countergradient sense.3,4,5 Some possible eddy motions are suggested by theoretical and modeling
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--> studies, including planetary scale waves, thermal tides, and vertically propagating gravity waves. These are probably of greatest importance above the lowest one to two scale heights (~20 to 30 km), but gravity waves could have their dominant source at the ground as a result of winds blowing over surface topography. It is likely that a Hadley circulation also exists in the lower atmosphere, with mean flow toward the equator at lower levels and toward the pole at somewhat higher levels. Such a circulation would tend to transport angular momentum upward and poleward and could be important in governing the zonal wind structure. The circulation of the atmosphere in the lowest one to two scale heights and the maintenance of the atmospheric superrotation are among the outstanding problems in planetary atmospheric science.6 Venus is a very slowly rotating body, with a thick atmosphere in which considerable heating occurs at relatively high altitudes. Titan is the other example of such an atmosphere, and there is indirect evidence that its atmosphere may also superrotate.7,8 Recent modeling9 provides a suggestion that superrotation might be a general feature of slowly rotating bodies with thick atmospheres. Thus far, however, modeling has not been able to reproduce the very strong superrotation of Venus's atmosphere. By comparison, the atmospheres of Earth and Mars exhibit only very weak, if any, superrotations. Both are rapidly rotating bodies with relatively thin atmospheres in which the bulk of the solar heating occurs at the ground. Even in the case of Venus, where only a small fraction of the incident sunlight reaches the ground, the transfer of heat from the surface to the atmosphere contributes significantly to the forcing of lower atmospheric circulation. Similarly the transfer of momentum between the ground and atmosphere is very important. Both of these processes take place through a planetary boundary layer, of which essentially nothing is known at present for Venus. Observations of very high vertical resolution are crucial to resolving the boundary layer and the transfer processes that occur within it. The boundary layer can be strongly affected by the nature of the surface, and this may vary considerably on Venus from place to place. Necessary Observations The following measurements are needed to achieve a better understanding of the circulation in Venus's lower atmosphere: Multiple, geographically dispersed, simultaneous, and temporally extended measurements of pressure and temperature as a function of altitude and horizontal location to determine the basic structure of the lower atmosphere; Multiple, geographically dispersed, simultaneous, and temporally extended measurements of the winds in the lowest one to two scale heights, with sufficient accuracy and sampling rate to enable the definition of the circulation itself and the determination of horizontal-and vertical-momentum fluxes as a function of altitude and horizontal location; and Multiple measurements of radiative fluxes, both solar and infrared, as a function of altitude and at different locations to determine the radiative forcing of the atmospheric circulation. Need for Mobility A geographically dispersed series of entry probes could obtain valuable measurements, but would provide only a snapshot of the atmospheric circulation. Temporally extended measurements are essential because the atmospheric eddies involved in the superrotation are expected to vary on time scales of hours to days and longer. Remote-sensing techniques appear to be feasible for higher atmospheric levels, but probably would not reveal the atmospheric structure within one to two scale heights of the ground because of the extremely high density of the atmosphere. Mobility within the atmosphere provides an efficient approach, and the relevant measurements could, potentially, be made using the following modes of mobility: A number of balloons, capable of semi-autonomous flight for an extended period, to obtain simultaneous
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--> measurements in the lowest one to two scale heights over a significant fraction of the planet; and Balloons with the ability to land and monitor surface and near-surface processes at multiple locations. Both mobility requirements could, in principle, be accomplished by using a single type of balloon. Practical considerations, however, suggest two types of balloons flown on sequential missions. One type of balloon would be designed to perform atmospheric measurements and, perhaps, be optimized for extended operation; the other type would be designed to conduct surface and near-surface studies. In either case, it is likely that the balloons must rise periodically to cloud-top levels in the atmosphere to cool instruments and hardware. This requirement would allow additional measurements at higher levels, which would extend the lower atmosphere profiles and aid in their interpretation. During vertical ascent and descent, the balloons would travel horizontally via the winds, which would greatly expand the observed regions of the planet. The length of the mission would be determined by the atmospheric rotation rate and the relevant time scales of thermal processes in the dense lower atmosphere. A mission lasting for, say, 30 days would provide measurements covering much if not all of the globe. Geographic coverage is very sensitive to the height to which the balloons must rise in the atmosphere to cool and to the amount of time they spend at high altitudes where the winds are stronger than they are at lower altitudes. Obtaining adequate latitude coverage might require 5 to 10 balloons (i.e., a minimum of one in each of the polar and mid-latitude regions and another at the equator; twice as many would give some degree of redundancy). This is so because the mean north-south winds in the lower atmosphere (probably associated with a Hadley Cell circulation) are likely to be relatively weak (less than ~1 to 2 m/s), and any given balloon is likely to spend its entire lifetime in a rather narrow band of latitudes. Careful tracking of the balloons will be necessary so that their locations are known with respect to those of gross atmospheric features. A positional accuracy of some 100 km may be sufficient. This could be achieved at least on Venus's Earth-facing hemisphere by using interferometric observations by Earth-based radio telescopes. This technique was successfully used to track the balloons deployed by the former Soviet Union's Vega 1 and 2 missions in 1985 (see Box 3.5 in Chapter 3). In summary, a small fleet of balloons could obtain the needed measurements, offering major advantages over multiple entry probes and remote sensing from orbiters. In particular, balloons could reveal the full extent of horizontal and vertical variations in composition, structure, and wind velocity, including the important but currently poorly understood planetary boundary layer. Tectonic Processes on Venus Importance A major theme of solar system exploration is to understand how planets work.10 One of the key processes is the conversion of thermal energy in planetary interiors to the mechanical energy that deforms their surfaces and creates geologic landforms. The geology of Earth is dominated by plate tectonics, a mode of surface deformation apparently unique to our planet. The other solid planets and moons display a variety of tectonic styles that operate to create a diversity of geological landscapes. Just as plate tectonics is probably unique to Earth, certain tectonic styles might also be unique to other bodies. COMPLEX has identified the determination of the nature and sources of stress responsible for the global tectonics of Mars, Venus, and several icy satellites of the outer planets as a primary objective for understanding planets.11 A specific objective is to determine why the tectonic histories of Venus, Earth, and Mars are so markedly distinct.12 With regard to Venus, the major puzzle remaining after exploration by the Magellan spacecraft is the identification of the tectonic process that resurfaced the planet 300 million to 600 million years ago.13 Can such an event occur again on Venus? Are similar tectonic upheavals possible in Earth's future? While theoretical speculations for the resurfacing of Venus abound, the real answer to what caused it lies in the geologic structures of the planet's surface. The unusual and possibly unique landforms on Venus, such as chasmata and coronae, must be studied thoroughly at sufficiently close range to understand their origin and significance for global-scale tectonics.
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--> The study of global tectonics is relevant to the goals of one of the campaigns, Formation and Dynamics of Earth-like Planets, in NASA's solar system exploration roadmap,14 because tectonic processes control crustal evolution. Although this campaign is concerned predominantly with atmospheres and climate, Venus's tectonic style and history are also important, because they exercise significant control over interactions between the planet's surface and its atmosphere. Rift-like features, called chasmata, were first imaged with Earth-based radar.15 Their dimensions, global extent, and possible tectonic significance became more apparent in Pioneer Venus altimetry data.16,17 Chasmata are long, narrow troughs; some are many thousands of kilometers long but only about 100 kilometers or less wide, and several to nearly 10 kilometers deep. Some chasmata are associated with major volcanic highlands, such as Atla and Beta Regiones. Many are associated with coronae (Figure 2.1), structures characterized by diverse and complex topography and quasi-circular rings of tectonic deformation. The margins of some very large coronae, such as Artemis and Latona, consist of segments of chasmata. Volcanism is commonly, but not universally, associated with coronae and chasmata. Key questions about these unique features include these: How do chasmata and the associated coronae form? Is crustal extension involved, making chasmata similar to rifts on Earth? Some chasmata appear to be associated with crustal compression. Are chasmata then more like oceanic trenches on Earth? FIGURE 2.1 Part of Diana Chasma (bright band from lower left to center right), Venus. The ovoidal structure along the band at center right is the corona Ceres. Magellan SAR image C1_MIDR15S146; radar-look direction from the left; horizontal dimension about 1,500 km. Courtesy NASA, Jet Propulsion Laboratory.
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--> Are the mineral and chemical compositions of associated volcanic rocks different from those of the lavas that cover most of the planet, suggesting different mantle sources? Is there any correlation between topographic features of chasmata and the mineral and chemical compositions of rocks? For instance, do chasmata expose intrusive rocks of deep origin? Necessary Observations An understanding of the nature of chasmata on Venus will require a combination of geological, geochemical, and geophysical observations similar to those required for studies of terrestrial geologic features, including these: Determinations of elemental compositions, mineralogy, and physical properties of rocks; Measurements of isotopic abundances; Identification of deformational features, such as folds and faults, and determination of the motion sense across faults; Measurements of seismic velocity to determine parameters such as crustal thickness; Measurements of gravity and topography to constrain internal structure and mantle processes; and Measurements of remanent magnetization and electrical properties to constrain internal structure and thermal history. Geological, geochemical, and geophysical studies should be carried out within and near chasmata along transects perpendicular to their trends and extending one or two chasma widths beyond their margins. Moreover, chasmata should be studied at many locations along their lengths because they change character with distance along trend. These observations are achieved only by surface measurements and by low-altitude reconnaissance over horizontal distances of hundreds of kilometers. The many parameters one must measure require the determination of crustal characteristics at scales of 10 km or smaller. This, in turn, implies spacing ground stations 10 km or less apart, and obtaining aerial data at altitudes of 10 km, or at most, a few tens of kilometers. An added benefit of images collected by rovers or by low-altitude balloons is the ability to ''calibrate" the radar data that underlie all of our current interpretations of Venus's structure and crustal history. Need for Mobility The global distribution, long-wavelength gravity signatures, and general topographic and structural characteristics of chasmata and coronae have been determined by Pioneer Venus, Venera, and Magellan missions. The Venera and Vega landers provided valuable data concerning rock compositions at specific sites, but none of them landed on a chasma or corona, and each provided only a point datum. High-resolution geophysical data and structural analysis are beyond the resolution currently available from orbit. Moreover, mineralogical and compositional data cannot be obtained by spectroscopy from orbit because Venus is continuously covered by clouds. Thus, collecting the relevant high-resolution geological, geochemical, and geophysical measurements from within and near chasmata will involve highly capable mobile platforms. Specifically: Collection of gravity, topography, and mineralogical data will require an aircraft or controllable balloon to fly at low altitude (a few kilometers) along traverses across chasmata for distances greater than 100 km; and Deployment of seismic stations, measurements of rock compositions, and sample collection for compositional analysis could be accomplished by rovers capable of traversing tens of kilometers, or by an aircraft or balloon that can touch down at intervals while conducting traverses across chasmata. These requirements pose significant technical challenges because of the hostile surface environment of Venus (pressures greater than 90 bars and temperatures near 730 K) and the demands for range and controllability imposed on the mobile platforms. These factors suggest that this case study will have to be addressed in an
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--> incremental manner, using a sequence of missions. The sequence would, possibly, begin with the use of balloons (perhaps related to those designed for surface and near-surface operations, as discussed in the previous case study). Extinct or Extant Life on Mars Importance The possible existence of extraterrestrial life is one of the most fascinating issues being addressed by the scientific community and a topic of great public interest. Given the likely environmental requirements for the origin of life on a planet—access to the necessary biogenic elements, the presence of liquid water, and the availability of a source of energy that can drive chemical disequilibrium—Mars is one of the most plausible places within our own solar system where life might exist or have existed. Mars shows geologic evidence for the existence of liquid water (at the surface early in its history and beneath the surface throughout its history), for the presence of the biogenic elements, and for the availability of abundant geochemical or geothermal energy that can drive chemical reactions. The search for evidence of past or present life on Mars has become one of the major scientific goals of Mars exploration and of the ongoing Mars Surveyor program. A recent NRC report18 summarized one of the specific objectives for Mars exploration as "searching . . . [Mars] for extinct or extant life, including evidence of the accumulation of a reservoir of prebiotic organic compounds and the extent of any subsequent prebiotic chemical evolution." This objective is a direct outgrowth of recommendations and conclusions reached in previous reports19,20,21 and of the inference that the putative martian meteorite ALH84001 may contain evidence for past life on Mars.22 This case study is also directly relevant to one of the campaigns that NASA's solar system exploration roadmap outlines, the Evolution of Earth-like Environments, and is indirectly relevant to another, the Formation and Dynamics of Earth-like Planets. These connections exist because of the sensitivity of life to climate, which is, in turn, controlled by the totality of processes involved in Mars's origin and evolution: atmospheric, surface, and interior. It is widely accepted that determining whether or not life existed on Mars will require a return of samples to Earth for laboratory analysis. In turn, this will require a careful selection of rocks and other materials from appropriate scientifically relevant sites on the martian surface. Necessary Observations Specific observations and measurements that are pertinent to studies of life on Mars include the following: Determination of the geologic context of regions to be sampled, including the geologic history of the terrain and the elemental and mineralogical composition of the surface at regional and local scales; Determination of the composition (at meter scale) of promising areas to search for minerals that might be indicative of the existence (past or present) of liquid water; A search for evidence of organic molecules within specific samples. Of particular interest are biochemicals such as amino acids, purines and pyrimidines, and sugars; Determination of stable isotope ratios within individual rocks or mineral components of rocks for signatures thought to be indicative of biological processes; Determination of mineral types, abundances, and petrographic relationships within individual samples that can be used to indicate the nature of the environment in which the material has existed and the likelihood of living organisms having been associated with it; Analyses of textures and structures of rocks and soils on scales from 1 to 10 microns in order to search for features that would indicate possible fossil biota; and Determination of the radiometric ages of samples and/or deposits from sites for which the geological history can be deciphered.
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--> Need for Mobility Although the global- and regional-scale surveys of mineralogical and elemental compositions that are a prerequisite for any assessment of Mars's potential as an abode of life can be determined from orbit, the detailed characterization of local sites of particular exobiological interest requires in situ measurements.23 Most researchers do not expect that evidence for past or present life will be so abundant or widespread that it will be available in the immediate vicinity of landing sites. This is particularly true given that landings may occur up to tens of kilometers from the desired aim point. Without the mobility necessary to conduct in situ exploration, it may not be possible to identify a target location uniquely. The required mobility could, potentially, be achieved by use of the following technologies: Balloons or aircraft to act as mineralogical "eyes," i.e., obtain compositional information by performing spectroscopic assessments of rock and soil units at spatial scales smaller than can be obtained practically from orbital vehicles; Highly capable rovers to traverse from a landing site to sites of specific exobiologic interest and to explore the geologic history and context of the intervening region. These rovers must be capable of both autonomous navigation and real-time traverse planning from Earth. Given the current uncertainties in landing at a specific location, traverses of up to several tens of kilometers over complex terrain will be likely (demonstration of a precision-landing capability may, however, reduce this traverse distance by a considerable factor); An articulated arm capable of positioning analytic or imaging devices against rock and soil surfaces, oriented at any angle from the horizontal, to an accuracy and precision of better than 1 cm. A device to manipulate and move rocks from the surface, including picking them up, turning them over, or pushing them out of the way, to allow characterization or examination of all sides of a rock and of the underlying surface, or to place samples in a container for eventual return to Earth; Devices for crushing, breaking, or abrading rocks in order to expose unweathered surfaces for analysis or to create fragments small enough to be collected for in situ analysis or sample return; Devices for digging or coring into the subsurface to depths of at least a meter, and perhaps as much as several tens of meters, to search for evidence of biota or relevant organic chemistry; and Techniques to prevent biological or chemical contamination during these activities. This variety of mobi remote-sensing observations from balloons or aircraft. Many of the initial scientific and technological steps necessary to perform these studies have either already taken place or will be accomplished by missions in the near future. The Viking landers demonstrated the use of robotic arms to dig trenches, manipulate surface materials, and deliver samples to analytic instruments. Additional experience in performing such tasks will be gained in the near future with the operation of the robotic arm on the Mars Polar Lander. Sojourner provided initial experience with rover operations on Mars, and subsequent rovers in the Mars Surveyor program will, according to current plans, perform more complex activities, such as extracting core samples from rocks and soil and caching them for later return to Earth. Physical and Chemical Heterogeneity within Small Bodies Importance Comets and chondritic asteroids are thought to consist of relatively primitive materials and, thus, their study is relevant to addressing issues encompassed by the campaign, Building Blocks and Our Chemical Origins, outlined in NASA's solar system exploration roadmap. However, many of the small bodies in the solar system have apparently been modified by thermal and impact processes. Imaging of asteroids during spacecraft flybys and studies of meteorites derived from asteroidal bodies reveal that asteroids have experienced both internal and external modifications to varying degrees. This includes thermal metamorphism, aqueous alteration, melting, core
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--> formation, impact cratering with attendant shock effects, regolith formation, and space weathering. The internal structure and composition, geophysical attributes, and surface geology of such bodies remain largely unknown. Their characterization represents a key objective remaining to be met.24,25 Perhaps the best-documented example of an asteroid exhibiting diverse surface properties is 4 Vesta, the third-largest known asteroid. For many years26 it has been known that Vesta exhibits spectral variations with rotation,27 and the heterogeneity of Vesta's surface has been confirmed by geologic mapping based on observations performed with the Hubble Space Telescope.28 The spectral properties of Vesta have long been recognized as being similar to those of HED (howardite, eucrite, diogenite) meteorites, an important class of achondrites. The igneous nature of these samples implies that their parent body was highly differentiated. Geographic variations in Vesta's topography determined from Hubble Space Telescope images29 reveal the presence of a very large impact crater that apparently excavated to depths of many kilometers into the crust and possibly the mantle, in the process ejecting kilometer-sized fragments that now can be dynamically linked to Vesta through their orbital properties.30 Reflectance spectra from the floor of this large crater suggest that there are significant mineralogical differences between the surficial crust and deeper units.31 The small Vesta-like asteroids liberated from the larger parent body also have spectral properties that link them to the HED meteorites and to Vesta itself.32 Understanding the heterogeneity within individual bodies such as Vesta provides a geologic context for these meteorites. Documenting the diversity among small solar system bodies, both heavily processed like Vesta and those that may have experienced less severe processing, can address the following objectives previously identified in NRC reports:33,34,35 Determination of the record of early solar system processes and history retained by small bodies; Constraining the nature and composition of planetesimals, such as those that accreted to form the planets; and Recognition of relationships among asteroids, comets, and extraterrestrial samples (meteorites and interplanetary dust particles). COMPLEX cites Vesta as an example because enough is known about it to anticipate the measurements and mobility techniques likely to be required to explore less-well-known small bodies. Many other asteroids and comets would be attractive targets. Judging from the prevalence of metamorphosed chondrites among meteorites, many chondritic asteroids must have experienced significant heating, although not necessarily to the point of melting as in the case of Vesta. Interplanetary dust particles thought to have been derived from comets show minimal thermal processing, but even these objects have been affected by heating during atmospheric transit. Thus, the abundances of volatile elements and organic compounds in their parent objects may only be understood from measurements made in situ on cometary nuclei. Necessary Observations The horizontal and vertical variations in mineralogy and mineral abundances can be estimated in a rudimentary way from rotational spectra,36 but the data may not be interpretable in terms of unique, individual minerals. The following observations are needed: Optical measurements at high resolution, essentially equivalent to viewing through a geological hand lens or microscope, to determine the texture of rocks and soils, as well as particle-size distributions in regolith materials (these observations provide a context for interpreting chemical data and for calibrating remote sensing measurements); Direct measurements of the mineralogy of surface and subsurface units, using methods such as x-ray diffraction and various spectroscopic techniques (e.g., thermal infrared, Raman, Mössbauer), to help constrain the processes that produce the rocks and soils and the conditions under which they operate; Chemical analyses of surface and subsurface units, which can be combined with mineralogy data to estimate mineral relative abundances and provide ground truth for calibrating spectra;
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--> Determination of isotopic and trace-element abundances to quantify the chronology of various events and to constrain and model petrogenetic processes such as partial melting, fractional crystallization, or impact melting; Seismic data to determine the asteroid's interior structure; Heat flow measurements, which can be used to estimate the inventory of long-lived radioactive isotopes in the whole body; Measurements of the magnetic field to provide information on the body's differentiation and thermal history; and Small-scale gravity measurements to help define the moment of inertia of complexly shaped and compositionally diverse objects. Local variations in the gravity field can also be modeled to reveal the existence of subsurface plutons and other heterogeneities. Need for Mobility Chemical variations for minor and trace elements that carry much information on interior source regions and on melting and crystallization processes cannot be measured remotely. Remote measurements also cannot determine radiogenic isotope compositions and absolute ages of various units, data that provide essential detailed information on the evolutionary history of the body.37,38 A more fundamental understanding of the geologic context for the HED meteorite samples from Vesta's ancient volcanic surface will first require measurements from orbit to identify both ancient flow sites and units exhibiting the most extreme diversity either through a complex volcanic history or from deep impact excavation and mixing. Observations of the context and petrology of rock types at multiple locations will provide a basis for fully tapping the potential of linking in situ measurements with the wealth of laboratory information available from HED meteorites. Through this link, a more fundamental understanding of the geology, stratigraphy, and internal structure of Vesta's preserved ancient planetary surface will be achieved. The observations identified above require mobility, either for sampling of multiple surface and subsurface units on a heterogeneous asteroid such as Vesta, or for positioning geophysical instruments that must work in tandem. Horizontal mobility also could provide vertical mobility by sampling blocks excavated by impact craters. Whether mobility is provided by rovers, touch-and-go orbiters, or some other platform will depend in large part on the size of the body under study. The specific mobility requirements for this case study are the following: Mobility of meters to hundreds of meters for kilometer-sized bodies, and of tens of kilometers for larger objects such as Vesta, to measure the horizontal variations of mineralogy and chemistry of materials, or to collect samples for return to Earth; Vertical sampling to depths on the order of tens of meters of regolith on asteroids or devolatilized crust on comets. Although recoverable penetrators might provide a means to sample the subsurface, coring devices are probably superior choices; Mobility of meters to hundreds of meters for kilometer-sized bodies, and of tens of kilometers for larger objects such as Vesta, to deploy geophysical instruments; and Devices capable of collecting samples for return to Earth. Some important measurements, such as abundances of radiogenic isotopes and trace elements, will likely require sample return to Earth-based laboratories unless future instrument development advances significantly in remote sample handling, processing, and analysis. Returning samples from small asteroids and comets, with escape velocities of less than a few meters per second, may well be technologically easier and involve less cost than performing complex sample manipulation and analysis on the bodies themselves. As with the other case studies, it is highly unlikely that a single mission will embody all of these mobility modes. A number of current or approved missions will demonstrate how some of the necessary technological challenges will be addressed. If, as currently planned, the Near-Earth Asteroid Rendezvous (NEAR) spacecraft lands successfully on Eros at the end of its mission, it will demonstrate many facets of the precision navigation necessary for the operation of a touch-and-go orbiter.
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--> Other missions under development that are relevant to this case study are Japan's MUSES-C asteroid sample-return mission and NASA's Champollion/Deep Space 4 comet-nucleus lander. MUSES-C will deploy a NASA-developed microrover on 4660 Nereus in 2003. Champollion will land on the nucleus of Comet Temple 1 in 2005, drill into the surface to the depth of approximately 1 meter, extract some 100 cc of material, and package it for return to Earth. Zonal Winds in the Jovian Atmosphere Importance The recent Galileo mission provided strong evidence that the composition39,40 and structure41,42 in some regions of the jovian atmosphere differ greatly from those that exist in the bulk of the planet.43 The Galileo probe appears to have descended into a "desert"—a region known as a 5-µm hot spot44—in which the temperature increases with depth along a dry adiabat (neutral stability). Such regions contain relatively little water or other condensables, such as ammonia (Figure 2.2). Data from Galileo's Near-Infrared Mapping Spectrometer (NIMS) observations of hot spots and their surroundings show a gradient in water concentration, increasing outward from the dry centers of hot spots to their peripheries.45 The hot spot sampled by the Galileo probe may be an area in which downward motions associated with convection are occurring.46 In surrounding areas, rising air loses its minor constituent condensables, such as water, through condensation and cloud formation. Thus, descending air in the hot spot is depleted of condensable species, accounting for the dryness. However, dry air on Jupiter is lighter than wet air, and the mechanism for forcing dry air downward in the 5-µm hot spots is uncertain. FIGURE 2.2 A near-equatorial hot spot (elongate dark patch) in the atmosphere of Jupiter. The upper image was taken in the 727-nm methane band, and the lower image was taken in the 889-nm methane band. The images cover an area of some 34,000 by 11,000 km and are centered on about 5° North, 336° West. Courtesy NASA, Jet Propulsion Laboratory.
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--> Other Galileo observations have shown that the zonal winds at the probe's entry site increase with depth and approach a constant velocity of about 200 m/s at the largest depths studied.47,48,49,50 The increase of zonal wind speed with depth suggests that these winds are driven by deep-seated dynamical processes, such as the global thermal convection of jovian internal heat.51,52,53 If solar energy were driving the zonal winds, their speed should decrease with depth below the solar-energy deposition level in the atmosphere. The detailed dynamical processes involved in the forcing of the zonal winds at depth are, however, not well understood. Whether or not the zonal winds persist to pressure levels greater than about 20 bars, and whether the special characteristics of 5-µm hot spots (dryness, downflow, neutral stability) continue to similar depths, are questions of vital importance to understanding Jupiter's atmosphere. A better understanding of the dynamics of Jupiter's atmosphere is one of the key elements of NASA's solar system exploration roadmap's campaign, Astrophysical Analogs in the Solar System. Necessary Observations To gain an improved understanding of the dynamics and compositional variations of 5-µm hot spots and the dynamics of the zonal winds in the jovian atmosphere, the following measurements are needed: Abundances of condensable species (e.g., H2O, NH3, NH4S) as functions of altitude and horizontal location in 5-µm hot spots and in the background atmosphere; Compositions, abundances, and altitudes of cloud layers; Temperatures, pressures, and winds as a function of depth and horizontal location; and Radiative fluxes versus depth and horizontal position. Need for Mobility Some of the required measurements could be obtained with multiple, widely separated atmospheric entry probes, but this would provide data at only a handful of particular locations and of short time duration. Remotesensing by instruments such as Galileo's NIMS could also acquire relevant data, but not below the clouds, and with only limited horizontal and vertical spatial resolution above the clouds. Mobility is required to obtain the structure, composition, and wind velocity data below the clouds with the appropriate spatial and temporal characteristics. Observations from balloons would be extended in time and would include simultaneous (synoptic) measurements, which is very desirable. The requisite measurements could be made by the following: The deployment of a number of balloons capable of vertical ascent and descent to probe the atmosphere to great depth. Vertical ascent and descent capability may be limited by communication difficulties and the high temperatures and pressures to be found at depth. If so, very deep levels could be probed with the use of specially designed and equipped dropsondes released from the balloons. The deployment of balloons at various latitudes to sample different belts and zones. The balloons would be carried in the prevailing zonal winds and, thus, could sample longitudinal and altitudinal variations in atmospheric conditions within a belt or zone for the duration of their lifetimes. The deployment of balloons in special atmospheric locations such as the 5-µm hot spots or selected storm systems. These mobility requirements may best be addressed by a number of missions specifically designed to deploy different types of balloons in Jupiter's atmosphere. The initial mission would release balloons in a number of different latitude bands. Subsequent missions would emplace more sophisticated balloons capable of descending deep into Jupiter' s atmosphere and/or balloons with the ability to release dropsondes. In either case, it is likely that the latter balloons or dropsondes would need to survive to pressure depths of some 100 bars, that is, four to five times that experienced by Galileo's probe. The primary factors determining the required lifetimes of the balloons are the wind speeds. These vary greatly
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--> from place to place on Jupiter but tend to be strong (~50 to 100 m/s). At these speeds, a lifetime of less than a week is probably sufficient to traverse a hot spot. Considerably longer lifetimes would, however, be required to sample a sizable portion of the planet. Knowledge of the locations of the balloons with respect to gross atmospheric features will be required. Radio tracking with interferometric techniques may be sufficient to achieve the requisite accuracy, at least while the balloons are on Jupiter's Earth-facing hemisphere. In summary, a fleet of balloons could obtain the needed measurements, offering major advantages over multiple entry probes. In particular, balloons could reveal the full extent of horizontal and vertical variations in composition, structure, and wind velocity in the jovian atmosphere. Europa's Internal Structure Importance The nature of Europa's internal structure and, in particular, the possibility of a liquid water ocean beneath the ice cover are crucial to the past or present existence of life on this satellite of Jupiter.54 The surface of Europa is composed primarily of water ice, with only a small amount of contaminating material.55,56 The scarcity of impact craters in many areas implies a relatively young age for the surface and suggests that there must have been active resurfacing in geologically recent epochs. Because the actual age of this relatively recent resurfacing depends on models of the flux of objects striking Europa's surface,57,58 there is some disagreement about the meaning of "recent." The most likely model, however, predicts surface ages as young as 10 million years, with an uncertainty of a factor of five.59 In addition, the surface is riven by cracks and faults, suggesting that tectonic movement of the ice has occurred. Tidal heating similar to that driving the extreme volcanic activity on lo provides some heating of the European interior. The deeper parts of the ice cover may have reached the melting point, and thus, the ice may be underlain by a local or global ocean. Recent Doppler measurements from the Galileo spacecraft yield a moment-of-inertia factor indicating that the outer water ice/liquid layer of Europa is at least 125 km thick.60 Images of the surface show faulting and movement of the ice at all scales down to a few tens of meters; the physical appearance is similar to that of terrestrial sea ice. The surface ice has broken apart and moved in blocks (Figure 2.3), either on warm, soft ice very near the melting point or on a subsurface ocean.61 Although not proven, the possibility of liquid water near the surface, perhaps globally distributed and very recent in time, is strong. Because Europa has no appreciable atmosphere, any liquid at the free surface would immediately freeze by evaporative self-cooling and by thermal radiation. If liquid water is present in the near subsurface, there are substantial ramifications for Europa, as both the ice tectonics and the interior heating change dramatically. Dissipation of tidal energy increases in the presence of a liquid water ocean because the surface ice shell experiences greater deformation. Movement of the ice at the surface and resurfacing of the planet, either by liquid or ice emplacement, are more efficient in the presence of liquid. The presence of liquid water enhances the possibilities for the origin and evolution of life on Europa because liquid water is generally thought to be required for life. In addition, access to biogenic elements and a usable source of energy to drive chemical reactions are necessary. These may be available at, for example, the interface between the liquid water and the rocky material that underlies the ice/water surface layer. These possibilities imply that Europa is a natural laboratory for studies of the processes leading to the origins of life. As such, this case study is directly relevant to NASA's solar system exploration roadmap campaign, Pre-Biotic Chemistry in the Outer Solar System The history of the surface and interior of Europa, along with the prebiological and possible biological nature of the interior, may be revealed by determining the structure of the surface layers at scales from global to local (i.e., subkilometer) to establish whether liquid water is present today or has been present in geologically recent epochs. If liquid water is present today, the distribution of liquid throughout the ice layer, the nature of the liquid region, and the potential for biological activity need to be determined.
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--> FIGURE 2.3 A portion of the surface of Europa showing prominent deformation bands, a complex fracture pattern, and jumbled blocks of elevated crust. The solar illumination is from the right in this Galileo image, which is centered on 8° North, 275° West. The field of view covers an area 100 by 140 km across. Courtesy NASA, Jet Propulsion Laboratory. Necessary Observations The goals outlined above for the characterization of Europa require the following observations: Determination of the geologic structure and tectonic history of the ice crust; Estimation of the ages of different segments of crust based on differences in impact-crater densities; In situ analysis of young, near-surface ice to determine its chemistry, including salts, particulates, organic constituents, and possible isotopic indicators of biological activity; Measurements of local and global values of the geothermal gradient in the ice crust within boreholes and by remote sensing using microwave techniques; Geodetic measurements of the response of the crust to tidal forces; Analysis of the chemical and physical properties of near-surface liquid, if present; and Identification of the geological processes occurring at the interface between a liquid layer, if it exists, and the surrounding ice or rock. Need for Mobility Detailed measurements of Europa's shape and gravitational field are priority goals of NASA's Europa Orbiter mission.62 These data should provide the critical evidence needed to determine if a subsurface ocean actually exists. Other, follow-on measurements, such as those pertaining to chemical composition, crustal processes, and detailed internal structure, require in situ measurements and will necessitate substantial mobility on the satellite's surface. In particular, in situ measurements of the composition of the ice or of the non-ice portion of the surface,
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--> and measurements that pertain to the possible presence of liquid water and its properties, would need to be made from a variety of locations on and beneath Europa's surface. As these locations cannot be determined a priori, but will require analysis of data from the surface, the ability to move from one place to another is required. Thus, in addition to acquiring global remote-sensing measurements from a low-altitude orbiting spacecraft, the following mobility modes are potentially required: A surface rover capable of moving over long distances (perhaps the tens of kilometers necessary to cross the landing-error ellipse and reach areas of interest) to make pertinent geological, geochemical, and geophysical measurements, and to identify regions that might have a locally thin crust; A multifunctional arm on a rover or lander to deploy and position instrument detectors or sampling devices; Drilling and coring devices capable of penetrating to shallow depths (meters) beneath the surface. Mobility of this form would allow the deployment of instruments to measure subsurface temperature gradients, in situ composition measurement, and the collection of samples for analysis on board a lander or rover; Devices for collecting coherent samples, and facilities to maintain them in a pristine thermal environment for eventual return to Earth; A cryobot for melting into the ice shell of Europa, to depths on the order of kilometers, to deploy instruments either within the ice or within the underlying water, if it is present; and A small submarine, deployed by the cryobot, to explore the subsurface water ocean, if it is present. The range of mobility modes required suggests a progression of missions that each collect data relevant to the feasibility of later activities. Such a progression could begin with a relatively simple lander equipped with either an arm to collect samples or a similarly equipped rover. Additional surface missions, such as landers capable of performing more complex activities (e.g., drilling to relatively shallow depths), may be required before the deployment of cryobots capable of melting their way through a considerable thickness of ice and, possibly, penetrating the ice/water interface. Even the simplest of these activities is likely to present unique technological challenges due to Europa's extreme radiation environment and low surface temperatures. References 1. C. Boyer and P. Guerin, "Étude de la Rotation Retrograde, en 4 Jours, de le Couche Exterieure Nuageuse de Venus," Icarus 11: 338, 1969. 2. M.Ya. Marov et al., "Venera 8: Measurements of Temperature, Pressure, and Wind Velocity on the Illuminated Side of Venus," Journal of Atmospheric Science 30: 1210, 1973. 3. R. Hide, "Dynamics of the Atmospheres of the Major Planets with an Appendix on the Viscous Boundary Layer at the Rigid Boundary Surface of an Electrically-Conducting Rotating Fluid in the Presence of a Magnetic Field," Journal of Atmospheric Science 26: 841, 1969. 4. G. Schubert, "General Circulation and the Dynamical State of the Venus Atmosphere," in Venus, D.M. Hunten, L. Collins, and T.M. Donahue, eds., University of Arizona Press, Tucson, Ariz., 1983, pp. 681–765. 5. P.J. Gierasch, "Meridional Circulation and the Maintenance of the Venus Atmospheric Rotation," Journal of Atmospheric Science 32: 1038, 1975. 6. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010, National Academy Press, Washington, D.C., 1994, p. 125. 7. F.M. Flasar, R.E. Samuelson, and B.J. Conrath, "Titan's Atmosphere: Temperature and Dynamics," Nature 292: 693, 1981. 8. D.D. Wenkert and G.W. Garneau, "Does Titan's Atmosphere Have a 2-Day Rotation Period?," Bulletin of the American Astronomical Society 19: 875, 1987. 9. A.D. Del Genio, W. Zhou, and T.P. Eichler, "Equatorial Superrotation in a Slowly Rotating GCM: Implications for Titan and Venus," Icarus 101: 1, 1993. 10. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010, National Academy Press, Washington, D.C., 1994, pp. 70–173. 11. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010,National Academy Press, Washington, D.C., 1994, p. 5. 12. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010,National Academy Press, Washington, D.C., 1994, p. 93. 13. R.G. Strom, G.G. Schaber, and D.D. Dawson, "The Global Resurfacing of Venus," Journal of Geophysical Research 99: 10899, 1994.
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Xu, "Chips Off of Asteroid 4 Vesta: Evidence for the Parent Body of Basaltic Achondrite Meteroites," Science 260: 186, 1993. 31. P.C. Thomas et al., "Impact Excavation on Asteroid 4 Vesta: Hubble Space Telescope Results," Science 277: 1492, 1997. 32. R.P. Binzel and C. Xu, "Chips Off of Asteroid 4 Vesta: Evidence for the Parent Body of Basaltic Achondrite Meteroites," Science 260: 186, 1993. 33. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010, National Academy Press, Washington, D.C., 1994. 34. Space Studies Board, National Research Council, Strategy for the Exploration of Primitive Solar-System Bodies—Asteroids, Comets, and Meteoroids: 1980–1990, National Academy of Sciences, Washington, D.C., 1980. 35. Space Studies Board, National Research Council, The Search for Life's Origins, National Academy Press, Washington, D.C., 1990. 36. M.J. 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Representative terms from entire chapter: