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Status of Planetary Science in 1995 OVERVIEW This chapter begins with two sections summarizing the ac- complishments of solar system exploration over the three decades from 1965 to 1995, and the expected scientific questions as of the end of that period. The remaining sections constitute a much more detailed status report for individual classes of objects, with further discussions of open questions. All this material supports, and leads to, the program of missions for the period 1995 to 2015 presented in Chapter 4. State of Planetary Exploration as of 1995 Among the high points already attained or anticipated for the first three decades of planetary study ending in 1995 are: . Mercury: Characterization of physiographic provinces for half the surface; discovery of a planetary magnetic field. 19
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20 . Venus: Establishment of atmospheric and cloud composi- tion; characterization of the high-temperature surface environ- ment; preliminary elemental analysis of surface material from landers; study of solar wind interaction; determination of global to- pography and gravitational field; characterization of physiographic provinces from radar images. . Moon: Determination of detailed geological history, chro- nology, and geochemistry of major geological provinces; detailed study of selected samples of surface material; investigation of cra- tering, regolith formation, and interaction of the surface with the solar wind for an airless body; discovery of remanent magnetic fields; seismic characterization; measurement of heat flow; de- termination of composition of the solar wind, both present and ancient. (By 1995, global surface mapping should be achieved or under way.3 . Mars: Near-global mapping of topography, gravity field, and thermal properties; establishment of geological diversity (vol- canoes, canyon lands, polar terrains, etch; discovery of evidence for former extensive surface water (e.g., valley and channel net- works); preliminary surface chemical analysis from landers; estate lishment of structure and chemical and isotopic composition of the atmosphere; determination of geological processes and a relative chronology; study of local and global meteorology over three mar- tian years from landers and orbiters; search for microbial life and organic compounds (yielding negative results). (By 1995, global characterization- morphology, elemental distributions, and some mineralogy—of surface units is expected.) . Jupiter system: Study of atmospheric composition and circulation; detailed composition and structure of atmosphere and clouds from direct entry probe; discovery of atmospheric lightning and auroras; detailed characterization of the magnetic field and the magnetosphere (sources and sinks, plasma processes); study of the lo plasma torus and of the interactions between this satellite and the magnetosphere; discovery and characterization of the To volcanoes and interior heat flow; discovery of the ring and several small satellites; comparative studies of icy and rocky planetary oh jects. (The Galileo orbiter will carry out detailed global mapping of the large Galilean satellites and continue efforts in many of the other areas mentioned above, especially the torus and magneto- sphere. The probe will carry out a detailed sounding of Jupiter's atmosphere and clouds.)
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21 ~ Saturn system: Initial global study of Saturn and its mag- netosphere; establishment of atmospheric composition differences between Jupiter and Saturn; detailed study of the ring system and investigation of new dynamical phenomena; discovery of several new satellites, including previously unknown orbital configura- tions; measurements of the composition and structure of the at- mosphere and clouds of Titan; low-resolution mapping of satellite surfaces, except Titan. ~ Uranus: Results of Voyager flyby (1986~. (Initial discov- eries include a strong magnetic field with a large inclination and remarkably diverse geology on several of the satellites.) Neptune: Results of Voyager flyby (1989~. Comets: Results of Halley flybys (1986), including imaging of the nucleus, and exploration of the proximate environment. Also, deployment of planned comet rendezvous missions. ~ Asteroids: Results of Galileo flyby of a selected asteroid and of planned flybys by the Comet Rendezvous mission. . Meteorites: Evidence for early magnetic field, late addi- tions of material with differing nucleosynthetic histories, wide- spread high-temperature events In the solar nebula; many exam- ples of core formation In small bodies, basaltic volcanism, extrater- restrial synthesis of arn~no acids; discovery of meteorites from the Moon and possibly Mars. Other Planetary Systems: Discovery that many stars are surrounded by dust clouds or disks, and imagery of one such disk; discovery of a star with a planet-like companion. (Many follow-up studies are expected by 1995.) Applications of these results to the study of planetary origin and evolution include: ~ Establishment of the age of the solar system as 4.6 bil- lion years by analysis of radioactive decay products in the Earth, meteorites, and lunar samples. ~ Dating of the late stages of accretion of the Moon (and presumably the other terrestrial planets) as 3.7 billion years ago, although most of the mass was probably accumulated within the first 107 or 108 years. ~ Determination of a geological chronology for the Moon, with the final major stages of lunar volcanism measured at 3 billion years ago; establishment of the current rate for impact cratering in the Earth-Moon system.
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22 ~ Comparative studies of geological processes on the terres- trial planets and the icy satellites of the outer solar system, includ- ing impact cratering, volcanic and tectonic activity, and erosional and depositional processes. ~ Preliminary study of the development and evolution of planetary crusts in planets of different compositions and internal structures, with insight into the role of tectonics and magmat~m in the formation of the crust and interior of the Earth and other planets. ~ Inference that the great bulk of the atmospheres of Earth, Mars, and Venus are all secondary, that is, degassed from the interior or acquired late in accretion, and not remnants of the gas from the solar nebula. . Discovery of unique and as yet unexplained abundances of noble gases (total amounts, relative amounts, and isotopic ratios) on Earth, Mars, and Venus. Discovery of a large (100 times) enrichment of deuterium on Venus compared with Earth. Venus must have started out with much more water (or vapor) than it has now, and a Runaway greenhouses may have caused most of it to be lost. Discovery that all terrestrial bodies have experienced dif- ferentiation, with accompanying volcanism and tectonics, but wit differences in history from one planet to another. ~ Discovery of the uniquely high levels of volcanic activity on To, and preliminary characterization of volcanism based on different physical-chemical systems than had been encountered in the terrestrial planets. In the Saturn system, resurfacing on Enceladus represents yet another example of such volcanic activity. . Discovery of unexpected complexity in the rings of Saturn and Uranus (e.g., the presence of shepherd satellites, of spiral density waves, and of bending waves), providing important insights into the dynamics of self-gravitating spinning disks. ~ In situ investigation of plasma processes of wide astrophy~ ical application in the huge magnetospheres of Jupiter and Saturn. . The determination of the composition of Jupiter's atmo- sphere, which is expected to be representative of the composition of the solar nebula, especially for hydrogen and the noble gases. The abundances that will be determined by the instruments on the Galileo probe will probably become the standard for solar composition.
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23 In supporting future investigations, an essential contribution will be made by theorists who endeavor to mode! the natural evm lution of gas-dust disks into stars and their associated planetary bodies. Theoretical investigations of the early stages of this evolu- tion begin with numerical and analytic modeling of star formation, in particular, the conditions under which single stars like the Sun can form. Study of the later stages of this evolution emphasizes modeling the manner and time scale for the accumulation of dust into planetesimals, and the subsequent accumulation of these plan- etesimals into planetary cores of silicates, metal, and ices. In the case of at least Jupiter and Saturn, the final stage of formation involved the gravitational capture of massive envelopes from the gas of the disk. Between now and 1995 we can expect that continuing progress will be made in this field, most likely without the help of crucial observations or sudden theoretical breakthroughs. However, in the absence of a new generation of observational facilities that permit higher resolution imaging of other protostelIar systems, it is quite possible that in the next decade we will not address the first-order questions required to make substantive progress. On the other hand, we can look forward to a significant refinement ant] enhancement of theoretical understanding concerning many aspects of nebular evolution. Much of this progress in theoretical understanding is contingent upon the availability of computational resources of continually greater power. If the first asteroidal flybys occur during the next decade, we can expect to begin to be able to place the great wealth of me- teoritical data into a planetological context. We can also expect that basic information regarding early solar system history will continue to flow from laboratory study of meteorites and strato- spheric collection of interstellar particles. In this connection, it should be pointed out that, to a large extent, the current laW oratory instrumentation used in this work was obtained during lunar sample analysis during the 1960s and early 1970s, and that attention must be given to modernizing the laboratories in which this work is done. Scientific Questions as of 1995 Fundamental questions in planetary science will remain much the same in 1995 as they are today, but new knowledge and new
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24 capabilities will alter our view of how to approach them. First, the reconnaissance and exploration of the solar system will by no means be completed. Saturn and Titan are already ripe for in situ investigation and study of interactions among the magnetosphere, rings, and other satellites. Investigation of comets and asteroids will have begun, but intensive study and exploration of the wide diversity of asteroids will remain. In this area we will want to know the following: the overall structure of the asteroid belt and its radial variations of composition and physical characteristics, which are expected to reveal clues about the structure of the pro- toplanetary nebula; the mechanisms that powered the evolution of differentiated asteroids; and the chemical composition and phys- ical character of comet nuclei, in order to determine under what conditions these most primitive planetesimals formed. Internal structure of terrestrial bodies is a broad field for which, apart from the Earth, we still will have only the limited data for the Moon from Apollo, and the even more limited data for Mars from Viking. Even such basic information as crustal thickness will still be lacking. The absolute history of planetary bodies will not be understood without an unambiguous chronology based on radioactive clocks. For example, it is suspected that the martian channels and volcanoes were formed over a protracted period, even though the time scale is based only on crater counts and is very uncertain. There is little prospect of obtaining dates by other means than laboratory analysis of returned samples. Such samples remain valuable long after their acquisition and return to Earth: improved techniques can (and do for the Moon) continue to be applied to the original samples. Only one side of Mercury will have been imaged from space- craft, but all the other terrestrial planets are known to be asym- metric In the distribution of geological provinces. While the Galilean satellites of Jupiter will have been studied in some detail, only the most rudimentary reconnaissance will have been made of the other outer planet satellites. Only single flybys of Saturn, Uranus, and Neptune will have taken place, and the Pluto system will remain unvisited. Our ideas about the origin of this solar system lead us to believe that planet-forming processes occur commonly during star formation. We will want to determine the prevalence and the prop- erties of planetary systems around other stars accurately enough to compare them with one another, as well as with our own system.
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25 We will want to carry on detailed studies of protostars in order to ascertain the physical character of their accretion disks, thought to be the sites of planet formation. It seems likely that Earth is the only site of organic life in the solar system, but there is no dearth of organic molecules on or in such objects as meteorites, Titan, the jovian planets, and giant molecular clouds located in other parts of the galaxy. Mars, formerly the object of greatest interest, is now seen to be the site of destruction of organic compounds by an intensely oxidizing atmosphere and soil. Conditions, however, may have been more benign in the remote past. There is still much to be learned about the origin of life by study of the objects mentioned above, and perhaps others such as comets. If other planetary systems exist, they may be seats of organic evolution. PLANETARY GEOSCIENCES During a relatively short period of time, studies of planets made by earth-based telescopes have advanced to detailed in situ measurements from spacecraft of the planets' surfaces and at- mospheres. A complex view of the planets and their satellites continues to emerge. In late 1962, Mariner 2 the first interplanetary spacecraft- flew by Venus: the journey of Voyager 2 is still in progress. The 203-kg Mariner 2 had only six instruments, whereas the 81~kg Voyager 2 has two color TV cameras and ten other advanced instruments. These two spacecraft represent the simple beginning and the sophisticated continuation of solar system exploration. In the early years of exploration, missions were selected more by technical feasibility than by scientific priority. So little was known that any mission greatly increased our knowledge. Now, comparative study of the planets is a significant scientific en- deavor. Great advances in understanding the origin and evolution of the planets and properties of the solar system will come from comparisons of all planetary objects. Common features such as atmospheres, magnetic fields, and geologic processes can be un- derstood best by such comparison. In turn, these comparative planetary studies provide insight about the history and evolution of the Earth. Nevertheless, exploration has shown that each planet is unique and interesting in its own right.
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26 Scientific Objectives for Planetary Geosciences The following topics in planetary geosciences contribute to an understanding of the solar system: formation; interior structure, dynamics, and physical state; crustal evolution; and planet mor- phology and surface processes. These topics, and the measurement objectives for them, are discussed below. Formation One key to understanding the formation of the planets is the determination of their chemical and isotopic compositions and the timing of their accretion. The results can be compared for all the planets, satellit es, and meteorites in order to place constraints on models of chemical difl.erentiation as a function of heliocentric or planetocentric distance. The results also shed light on the potential for heat sources important for considerations of internal activity and to assess models of planetary accretion. Interior Structure, Dynamics, and Physical State Measurements of the seismic behavior of planets, the strength and nature of their magnetic and gravity fields, and the heat flow from their interior are critical for determining the characteristics of planetary interiors. When combined with knowledge of mass and composition, the results permit assessment of the nature of possible interior differentiation (core/mantIe/crust) and the pos- sibilities for an internal dynamo. Crustal Evolution A principal objective in planetary exploration is the determi- nation of the age, composition, and distribution of crustal mate- rials, including volatiles. The results allow refinement of models relating to planetary accretion, differentiation, and degassing. In addition, such determinations allow assessment of the style and timing of volcanism and tectonism and their relation to other geo- logical events, as well as the role of volcanism in the evolution of possible atmospheres.
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27 Planet Morphology and Surface Processes The types and distributions of landforms and other geological units on planetary surfaces can be determined through geological mapping using remote-sensing data. The results allow assessment of the processes, such as volcanism and tectonism, that have led to the formation and modification of planetary surfaces. Some landforrns, such as dunes and valleys, are indicative of processes associated with wind and water, and thus contribute to models of atmospheric evolution. Assessments must therefore be made of the distribution and exchange of volatiles among the crust, regolith, poles, and atmosphere. Knowledge of the geological processes- volcanism, tectonism, impact cratering, and surficial modifications can be combined with relative and radiometric age determinations of the features associated with those processes to derive geological histories of the planetary surfaces. An important aspect of comparative planetology relates to the origin and evolution of life. Knowledge of the geological environ- ments permits assessment of the likelihood for the evolution and sustenance of organic life, at least in comparison to Earth. The images of Earth taken from space with its thin skin of oceans and clouds help us begin to appreciate the uniqueness of our planet and the fragile balance that makes life here possible. An additional impetus for planetary exploration is the poten- tial for using space resources. In a period when natural resources are being depleted rapidly on Earth, no detailed assessment has been made of the resources that exist in space. The Moon and asteroids may hold significant potential as sources of metals and minerals for utilization in space. The initial utilization of such re- sources may be to support space missions that would travel farther into space, or permanent bases on the Moon or Mars. Measurement Objectives The goals outlined above guide the definition of a set of general scientific objectives as follows: ~ Characterize the internal structure, dynamics, physical state, and bulk composition of the planet of interest; . Characterize the planet's chemical composition and min- eralogy of surface materials on a regional and global scale;
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28 ~ Determine the planet's chemical composition, ~runeralogy, and absolute ages of rocks and soil for the principal geologic provinces; ~ Characterize the processes that have produced the land- forms of the planet; ~ Determine the chemical and isotopic composition, distri- bution, and transport of probative compounds that relate to the formation and chemical evolution of the planet's atmosphere, and their incorporation in surface and crustal rocks and polar ice; ~ Characterize the planetary magnetic field and its interac- tion with the upper atmosphere, solar radiation, and the solar wind; ~ Determine the extent of organic chemical and possible bio- logical evolution on Mars and Titan, and explain how the history of the planet constrains these evolutionary processes. The Inner Solar System The inner planets- Mercury, Venus, Earth and its Moon, and Mars range from 0.4 to 1.5 AU in distance from the Sun and are smaller and denser than the outer planets. These terrestrial plan- ets are composed chiefly of rock and metal, are poor in volatiles, and have few satellites. Their densities range from S.4 g/cm3 for Mercury to 3.9 g/cm3 for Mars. The variation in density with so- lar distance has been discussed in the context of a thermodynamic mode} for the proto-solar nebula in which temperature and pres- sure decrease with distance from the nebular center and control the chern~stry of condensed material. However, it may be that primary differences in planetary density are due to accidental variations in fractionation and reaggregation from collisions. After their for- mation, all inner planet surfaces were significantly modified by a wide variety of internal and external processes. Nevertheless, each planet has followed its own evolutionary path. By exploring this diverse family of planets and by comparing their features with those of the Earth, we seek to characterize the evolution of the inner solar system and the causes of the unique aspects of each planet. We also seek to gain insights into the history, as well as the future, of Earth and the life that has evolved on it. Further insights into the terrestrial planets will come from study of the large satellites of the outer solar system as well.
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29 The Moon Apollo yielded an enormous advance in the understanding of planets by providing samples of the Moon and a wealth of other information. The tune scale of the Moon's evolution has been established, and several first-order questions have been answered. Equally important, a basis has been established for interpreting the evolution of other planetary bodies, including the Earth. Dur- ing the accretionary phase of continuous planetesimal in-fall, the Moon appears to have melted to depths of at least a few hundred kilometers. The ancient crust developed during tints maelstrom, with segments repeatedly fragmented and reincorporated into the evolving magmas until a thickness was established that could with- stand the waning bombardment. The larger craters on the Moon record a period of intense bom- bardment that ended about 3.7 billion years ago, a phenomenon that presumably affected all of the inner planets at about the same fume. This bombardment provides a chronological reference, ac- curately measured in the case of the Moon by radioisotope dating techniques, that ~ the basis for constructing the geologic history of Mars, Mercury, and (presumably) Venus. The evolution of the crust of the Moon is known from remote sensing, from instrument data provided by landers, and from study of returned samples. Remote-sensing data show that two major provinces constitute the lunar crust: young (sparsely cratered), low-albedo mare terrains, and old (heavily cratered), high-albedo highlands. The oldest reliably dated rocks on the Moon (from the highlands) are radiometrically dated at about 4.5 billion years old. The youngest mare basaltic lava flows are estimated to be about 2.3 billion years old. The lunar highlands appear to be the result of differentiation at 4.5 billion years and consist of minerals that floated in the melt. However, there is controversy as to whether the upper crust of the Moon was generated in a "magma ocean," whether the whole planet was molten, or whether local areas were successively molten over a long period of time. Geophysical data show that the Moon had a strong magnetic field early in its history, but the field has since disappeared. Al- though most scientists consider the Moon to have a small, partly molten core, its presence is the subject of intense debate due to differences in interpretation of the seismic record. Because all the data provided by Surveyor, Apollo, Soviet
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77 images of about 20 km resolution at 2 AU from the Earth and will determine the shape of many asteroids. In addition, the high spatial resolution will allow reflectance spectra to be taken at many spots to test for surface heterogeneity. The capabilities of ST will also allow reflectance spectra to be taken in the ultraviolet region, and perhaps in the future, in more of the infrared region. These regions may show more diagnostic features than have been available from ground-based studies. Many asteroids have been grouped according to spectral re- fiectance features, and many of these features have been related to known meteorite types. One difficulty in the link between asteroids ant} meteorites is that the reflectance spectra of the most common meteorites, the ordinary chondrites, are not precisely matched by the main-belt asteroid spectral reflectance groups. The closest match, some of the S asteroids, may actuary be the source of the ordinary chondrites if the surficial features of the asteroids have been modified by exposure to the space environment. By 1995 these processes may be better understood as a result of labora- tory studies, facilitating a closer observational link between the meteorites and asteroids. There are good dynarn~cal reasons for believing that most ordinary chondrite meteorites are derived from a limited (~0.05 AU) region of the asteroid belt in the vicinity of the 3:1 Kirkwood gap at 2.5 AU. (Asteroids with values of their sern~major axes in this region wiD have periods in resonance with the orbital period of Jupiter). Except for the questions of spectrophotometric inter- pretation mentioned above, the known larger S asteroids, in the vicinity of 2.5 AU are prone candidate sources for ordinary chon- drites. Most of the meteorite population is clearly derived from the main-belt asteroids, and their study provides our most detailed understanding of the nature and properties of the asteroids. Because of the diversity of studies on meteorites, it is dif- ficult to predict the state of knowledge in 1995. Prior to 1995, we can expect much new information will be obtained through the NSF-sponsored Antarctic meteorite collection program and the de- velopment of new analytical techniques. This collection represents a les~biased sample of the small meteorite end of the terrestrial meteorite flux and a substantial increase in the number of stony meteorites available for study. For this reason it may provide a look at meteorites derived from the asteroid belt at velocities so high that a significant yield of large meteorites ~ not expected.
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78 This possibility appears to have already been realized by iden- tification of several meteorites of lunar origin In the Antarctic collection. A large group of meteorites, the achondrites, have clearly undergone igneous differentiation processes dated at 4.5 billion years nearly contemporaneous with the formation of the solar system. Besides giving evidence that at Idast some asteroids were once geologically active, the meteorites allow the study of igneous processes that occurred under a set of conditions unlike those of Earth or Moon, and thus broaden our understanding of the effects of different parameters that cannot otherwise be studied in the laboratory. In addition, most of the iron meteorites formed very early in solar system history in cores of many (up to 50) different asteroids. These data provide strong evidence for the presence of a substantial heat source capable of producing primordial melting of the asteroids (and possibly planetary planetesimals as well). Many of the meteorites are breccias (made up of fragments of other preexisting rock) and provide evidence for the nature of collisional processes at an early epoch. The meteorites, and hence the asteroids, also retain a record of the cosmic-ray intensity in the past, as well as a record of significant (up to a few gauss) magnetic fields that were present before the asteroids accumulated. Several of the carbonaceous chondrites have been found to contain amino acids and other complicated organic molecules, which are clearly of extraterrestrial origin. Carbonaceous chondrites also contain calcium and aluminum-rich inclusions that appear to be among the earliest objects to have formed in our solar system. These inclusions provide evidence of very complicated processes in the early solar nebula involving multiple episodes of high temperature (~1400K). They also contain a record of the nucleosynthesis of elements that were added to our solar system just before the inclusions formed. A problem associated with all of these meteorite studies, how- ever, is the lack of a geological context for the interpretation of the observations. An asteroid sample return mission would go a long way toward providing this context. It would also provide the in situ data that would substantially increase the value of the remote studies of the asteroids. Similarly, the SNC meteorites can answer a number of questions about Mars, but they are clearly derived from a lava flow, whose composition is unlikely to represent that of the surface as a whole.
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79 Questions in Asteroid Science. Accessing the information that asteroids contain about the early solar system will require much more detailed understanding of the asteroids' surface composi- tions, their internal structure, the degree to which they are het- erogeneous, and their dynamical and collisional evolution. At present our knowledge of the composition of specific aster- oids is almost entirely founded on earth-based spectrophotometric data. Although we may expect this situation to change some- what between now and 1995, for almost all asteroidal bodies it will still prevail. Earth-based data represent a surficial average over the entire observed disk of the asteroid, and heterogeneity is at present only crudely exhibited as a consequence of aster- oid rotation. When looked at carefully enough, every asteroid appears spectrometrically unique, and different in detail from bad oratory spectra of meteorites. Are these differences fundamental or are they primarily the result of heterogeneity or exposure to the surface environment or regolith phenomena? Making these distinctions requires remote sensing data of higher resolution. In addition, corroborating "ground truth" data are lacking, and will be required to ensure that unanticipated mineralogical differences are not overlooked or misinterpreted. It is likely that asteroidal collisions have exposed interior re- gions of asteroidal bodies. When combined with higher resolution spectral and imaging data, this affords an opportunity to overcome the apparent irritation to surficial composition. This may facil- itate addressing such questions as the fragmentation history and internal structure of differentiated asteroids, and the identification of specific asteroids as sources of particular meteorite classes. A better understanding of this asteroid collisional evolution is also needed In order to learn which asteroids are primitive objects, as opposed to collision products of larger bodies, and whether they are best thought of as "rubble piles," megaregoliths, or simply as solid rocks. In this same connection, present observations of Hi- rayama families serve as a key source of information concerning as- teroidal collision phenomena. Their apparent spectrophotometric heterogeneity calls into question the basic assumptions supporting these inferences. With these more detailed compositional data it will also be possible to address the relationship between the heliocentric dis- tance of asteroids and their chemical composition. Understanding this possible relationship is central to examining the conventional 1
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80 assumption that the formative solar system had a marked radial temperature dependence, related to the composition of planetes- imals and planets formed at different locations in the early solar system. COMETS General Characteristics Comets are thought to be small conglomerates of rock, ice, and dust several kilometers in diameter and 10~5 to 10~8 g in mass formed during the early years of the solar system's history. Today most reside in the so-called Oort cloud, in loosely bound orbits at tens of thousands of astronomical units from the Sun. Perturbations induced by the gravity of passing stars and inter- stelIar clouds occasionally alter a comet's orbit and send it near the Sun, where solar heat evaporates the ice. The subsequent outflow of gas and entrained dust, illuminated by sunlight, produces the comets. Some of these comets that venture into the solar system are further influenced by the gravity of planets, becoming trapped in periodic orbits near comets. Some of these comets that ven- ture into the solar system are further influenced by the gravity of planets, becoming trapped in periodic orbits near the Sun. Such periodic comets appear regularly for thousands of years until their volatile material is depleted. Several schools of thought hold that comet-like objects were among the fundamental building blocks of some larger planetary bodies. As a result of their small sizes and their large average distances from the Sun, evolutionary processes that differentiated the planets are thought to have been insignificant for many comets. They may have played a role in later states of planetary evolution, perhaps by providing volatile constituents for some atmospheres. It has been speculated that some of these cometary constituents were essential to the origin of life. A bright comet appears as a roughly spherical coma or at- mosphere composed of comparable quantities of dust and volatile species such as neutral gases and ions. The curved, relatively featureless dust tad! directed almost exactly away from the Sun shows considerable temporal and spatial structure. Depending on a comet's distance from the Sun and the light in which it is oW served, its coma can be quite large, in the range of 104 to 107 km
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81 when a comet ~ at 1 AU. The plasma and dust tails are even larger in the case of a bright comet, some 107 to 108 km in extent. All of these phenomena result from the gas and dust that emanate from the nucleus; the nucleus itself is small, of the order of 1 to 10 km in diameter for typical comets. The ice and snow in comet nuclei are composed of condensed gases and other volatile materials, including water and probably carbon monoxide, carbon dioxide, HCN, CH3CN, and uniden- tified complex organic molecules. These species are the parent molecules of the molecules and ions observed in the coma and tail. The nonvolatile material is in the form of grains ranging from subrn~crometer-sized dust to sand grains and perhaps pebbles and boulders, containing silicates and possibly metals, oxides, sulfides, and organic compounds. When solar heat vaporizes the volatile material, the outflowing gas carries smaller solid particles with it. Since the comet's gravity is finite, though weak, any larger peW bles and boulders are likely to remain bound on the surface-of the nucleus possibly leading to the formation of an "extinct comet of asteroidal appearance. Some of the earth-approaching "asteroids" may be highly evolved comets of this kind. Since many comets show evidence of directional emission of gas and dust, it appears that the surfaces of their nuclei are inhomogeneous and may have localized active regions. A tentative estimate of the volatile fractions of four recent comets has been made from their apparent production rates of carbon, oxygen, and nitrogen. Although the parent molecules are uncertain, they seem to be composed mainly of hydrogen, carbon, nitrogen, and oxygen. The mass ratio of the dust to gas liberated from a nucleus has been estimated in two cases to be 0.5 and 1.7 within a factor of 2. By comparison, the ratio of volatile to nonvolatile components is about 100 for solar material. These results imply that hydrogen and helium are depleted in comets. Nevertheless, comets seem to contain 3 to 10 times as much volatile material as the most volatile-rich meteorites. Thus comets appear to have formed from material at temperatures much lower than that characteristic of meteorites, at about 150K as opposed to more than 400K. This suggests that cometary material is the least- differentiated and best-preserved product of the preplanetary solar nebula that is known to remain in existence. It is speculated that some fraction of comet dust may be unaltered interstellar material. It may be possible to illuminate
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82 this question by establishing some elemental isotopic ratios. Rel- ative isotopic abundances of the elements reflect their formation processes. Carbon Is a good example. Bodies within the solar system, including the Sun, Moon, terrestrial planets, meteorites, and Jupiter, exhibit a common value of about 90 for the i2C/~3C isotopic ratio. Red giant stars show a range from 12 to 50 for this ratio. In carbon stars the ratio falls in a wide range from 2 to 100. Although observed values in the interstellar medium also span a wide range from 13 to 105, some investigators have argued that a value of 40 Is representative. Many of the so-called "BrownIee particles," which are collected in the stratosphere, are suspected to originate In comets. At best, however, they are necessarily deprived of their most volatile components. State of Enowledge In 1995 Our ideas about comets are constructed from lirn~ted remote observations as well as from our more general notions about solar system bodies. Significant advances in our knowledge of comets are expected during the decade preceding 1995, as a result of ini- tial spacecraft missions carried out by a number of different space agencies. The NASA fly-through of Comet Giacobini-Zinner con- ducted the first in situ measurements of a comet. The instrument suite carried by that spacecraft was designed for solar wind and magnetospheric studies. The primary contribution of the mission was to characterize the particle and field distribution around the comet and to establish the basic features of that comet's ~nter- action with the solar wind. Several important physical problems were addressed by the Giacobini-Zinner mission, including the medium-energy, nonthermal particle distribution, the morphology of the magnetic field, the character of the interaction between cometary gas and the solar wind plasma, and plasma instabil- ities excited by the interaction. Although the Giacobini-Zinner investigation was the first in situ comet investigation, neither the encounter orbit nor the instruments were chosen with the comet in mind. The same is true of the Pioneer Venus orbiter, which obtained images in radiation scattered by atomic hydrogen. The three scientific spacecraft that encountered Halley's Comet were specifically designed for the purpose. The ESA Giotto project aimed for the closest approach to the comet's nucleus- passing as close as 500 km. Giotto passed through all of the outer
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83 comet/solar wind interaction layers and into the inner cometary coma. The instruments carried out a first-order characterization of the solar wind interaction, a crude characterization of the den- sity and composition of cometary gases, and a crude analysis of the elemental composition of cometary dust. In addition, a limited number of medium-resolution pictures of the comet's nucleus were obtained. The Soviet Halley project sent two spacecraft through the outer regions of the comet's interaction with the solar wind. Gen- erally speaking, the information from the Soviet mission was sim~- lar to that from Giotto. The Japanese Suisei and Sakigake missions provided coordinated information on the solar wind flow upstream of the comet. Altogether, the investigations of Comets Halley and Giacobini- Zinner achieved a gross characterization of the morphology of large-scale cometary phenomena. As valuable as these investiga- tions were, most of the highest-priority questions that challenge our understanding of comets and that promise to reveal clues about the early solar system remain unanswered. Detailed understanding of comets will require more intimate and extensive measurements than are accessible to flyby investi- gations. Comet rendezvous and comet nucleus sample return will be the means by which the major objectives of cometary science will be realized. The United States will carry out a comet ren- dezvous mission by the middle of the l990s, which will follow a comet through most of its inner-solar-system passage. Analyses of cometary gas and dust, energetic particles, and magnetic fields and plasmas, as well as detailed investigations of the structure and gross composition of the comet nucleus will be carried out by the comet rendezvous mission. Successful completion of the comet rendezvous should an- swer many outstanding questions about the gross characteristics of cometary features and phenomena. The next obvious step in the study of comets will be comet sample return. A crude sample return may be accomplished by flying a collector, at high velocity, through a cometary coma. However, the material returned in this way will retain only the information about its basic elemental and isotopic composition. In some ways, this situation resembles that of the stratospheric "BrownIee particles," which do in fact retain
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84 much of their original structure. Valuable as they are, these par- ticles cannot be identified with any particular comet, and in fact may not even arise from comets at all. By 1995, the next major step in cometary science will be to return an intact sample of comet nucleus material. This material can then be analyzed in detail in laboratories in order to carry out the mineralogical, chemical, and isotopic analyses that are needed to unravel the formation processes and evolutionary history of comets. Questions in Cometary Science It is expected that as a result of spacecraft missions and earth- based studies, our knowledge of comets will increase significantly during the next decade. It ~ clear that those results will represent only the beginning of our quest for understanding these most primitive and unevolved aggregates of matter assembled during the birth of the solar system. An important set of questions concerns the description of the present state of cometary nuclei. These include descriptions of the chemical, isotopic, and mineralogical composition of comets, their internal structure, and the range of variation of these characteris- tics between different comets. Questions of another class address cometary processes. As oW served, comets are complex systems of neutral gas plasma, dust, and larger solid bodies. We must understand the physical pro- cesses that determine the loss of material from the cometary nu- cleus and the resulting short-term evolution of its structure, and those that produce the elaborate extended coma and tail of the comets. Some of these same physical processes determine the nongrav- itational evolution of cometary orbits, that is, a Rocket effect" caused by the asymmetrical emission of gases from the nucleus. These altered orbits can have a substantial effect on the probabil- ity that a comet will impact planets and are central to the question of the evolution of active comets into Apollo objects of asteroidal appearance. This knowledge of the present state of comets and the physi- cal processes to which they are subject is required to understand the properties of earth-impacting material derived from comets. It is known that meteors, meteorites, and cosrn~c dust represent
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85 the impact of cometary and asteroidal materials, but the relation- ship of this material to their sources is imperfectly understood. Like returned samples, laboratory study of this material provides important information regarding these bodies and their origin. Characterization and identification of possible cometary material are required. The resulting knowledge of present-day cometary composition, structure, and processes is fundamental to understanding where, when, and how the comet nuclei formed. In particular, we need to understand the age and any alteration of the various components of cometary nuclei. To what extent do they represent evolved solar system material, early solar nebula condensates, or solids of interstellar origin? This extrapolation back to the time of the origin of the solar system is also needed to interpret the extent to which comet-like objects contributed to the formation of the giant planets and the volatile inventories of the terrestrial planets. Comet Measurements and Technical Requirements Addressing the scientific questions cited above will require detailed investigations of the comet's nucleus and the emitted dust, as well as the gas, plasmas, and fields in the comet. Comelary Nucleus. Investigations of the comet nucleus should be aimed at establishing its composition and its physical and structural characteristics. Compositional measurements should determine the atomic, molecular, and mineralogical content of the refractory and the volatile solids. Together with measurements of the physical and structural features, this information will help in ascertaining the history of cometary matter and the processes responsible for its formation and for the assembly of cometary nuclei. Accurate isotopic measurements should be carried out on both the refractory and volatile constituents to explore the nucleosyn- thetic history and establish a time scale for major events in the history of cometary material. Both the compositional and struc- tural investigations should be extended over the variety of physical scales that characterize cometary nucleus material; this covers a range from the microscopic grains to the full size of major macro- scopic components of cometary nuclei. It is desirable to identify the major mineral assemblages of the nucleus for those constituents
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86 that make up 5 percent or more of the comet's composition, and with a resolution of better than 10 percent of the nuclear diameter. On the small scales, complete determination of the structure and composition of cometary material requires detailed analysis of the dust component to discern the dust's character and origin. Cometary Atmosphere. Measurements of the gaseous component of cometary effluent will reveal information about the most volatile materials in the nucleus. Spacecraft should have the capability to identify and determine the abundances of all molecular species in the mass range 1 to several hundred, and to determine the isotopic ratios for the important species at an accuracy sufficient to identify deviations from solar composition and other anomalous isotopic variations. A major purpose of the cometary atmospheric mea- surements is to ascertain the composition of the so-called parent molecules of cometary effluent and their evolution as they leave the comet. Both the neutral and ionized components of the cometary atmosphere should be analyzed in detail, including the variations in composition with distance from the comet's nucleus. Solar Wind Interaction. The structure of the large-scale cometary phenomena should be deterrn~ned. Measurements of the electro- magnetic fields, plasma, energetic particles, and neutral gas should be made in a volume surrounding the nucleus and encompassing the upstream coma and solar wind interaction region, as well as a significant volume of the tail. Measurements should be made with sufficient spatial and temporal completeness to allow identi- fication of the major dynarn~cal physical processes, including tran- sients, that play important roles in shaping the overall cometary structure and providing its sources of energy. Technical Capabilities. Cometary studies require a sophisticated complement of spaceborne and laboratory instrumentation that will not be discussed in detail here. Scientific investigations of comets need a launch system able to reach a variety of orbits and to maneuver for an extended period of time near the comet. At present, low-thrust electric propulsion seems to provide, by a wide margin, the best propulsion system for such missions. Achieving the scientific objectives of comet exploration will require missions of months' duration designed to carry out ex- tended investigations of a single comet during both its most active
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87 and its more quiescent times. Return of high-~ntegrity samples of cometary nucleus material to Earth for detailed analysm in ter- restrial laboratories will be essential to realize the overall goals of comet science, as well as of solar system science in general. However, adequate analyses should be carried out in situ, because these materials may well undergo significant changes once they are removed from their natural environment.
Representative terms from entire chapter: