1
The New Worlds Beyond 30 AU

COMPLEX's 1994 report, An Integrated Strategy for the Planetary Sciences: 1995–2010, explains that studies of the objects in the solar system are directed toward two broad goals: explaining how planets work as complex interacting physical and chemical systems, and understanding how the planetary bodies and life originated.1 To make progress on the latter topic, it is necessary to document the chemical and physical makeup of the planetary bodies, particularly those thought to retain clues about primordial conditions and processes. These primitive materials are most likely found to be on relatively unaltered bodies such as comets, asteroids, meteoroids, and interplanetary dust grains. 2 Since the Integrated Strategy was written, new information has been gathered that has enhanced the scientific significance of studies of the trans-neptunian region with respect to questions related to the solar system's origins.

Several recent discoveries have opened up a new frontier for planetary science in the region beyond 30 astronomical units (AU). The Voyager 2 encounter with Neptune and its satellite Triton3 (Plate 1) and Earthbased observations of Pluto4 (Plate 2) have transformed our knowledge of the outer solar system. Moreover, recent observations have led to the identification of two new classes of objects, Kuiper Belt objects (KBOs) and Centaurs.5,6 The Kuiper Belt is thought to be a disk of icy objects that are confined to within ~10° of the ecliptic plane at distances between 30 AU and hundreds of astronomical units. Centaurs are relatively small bodies (with diameters less than ~200 km) whose eccentric orbits cross the paths of Saturn, Uranus, and Neptune. Some numerical studies and new observations suggest that an additional class of bodies may also exist.7,8 These objects, members of the so-called scattered disk, are characterized by eccentricities and/or inclinations greater than those of typical KBOs. Additional components of the distant outer solar system (e.g., dust and the Oort Cloud), as well as the planet Neptune itself, are not considered in this report.

Observations of the distant outer solar system conducted over the last few years have established a new framework in which to view the relationships among the ensemble of trans-neptunian objects: KBOs, Centaurs, Pluto, Charon, and Triton (Figure 1.1). As a class, the trans-neptunian objects are primitive compared with inner solar system objects, but they do show evidence for diversity9 and for evolutionary processes having occurred.10 These newly discovered objects and their relationships represent a fertile, relatively unexplored region for investigation of fundamental questions on the origin and evolution of the solar system.



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--> 1 The New Worlds Beyond 30 AU COMPLEX's 1994 report, An Integrated Strategy for the Planetary Sciences: 1995–2010, explains that studies of the objects in the solar system are directed toward two broad goals: explaining how planets work as complex interacting physical and chemical systems, and understanding how the planetary bodies and life originated.1 To make progress on the latter topic, it is necessary to document the chemical and physical makeup of the planetary bodies, particularly those thought to retain clues about primordial conditions and processes. These primitive materials are most likely found to be on relatively unaltered bodies such as comets, asteroids, meteoroids, and interplanetary dust grains. 2 Since the Integrated Strategy was written, new information has been gathered that has enhanced the scientific significance of studies of the trans-neptunian region with respect to questions related to the solar system's origins. Several recent discoveries have opened up a new frontier for planetary science in the region beyond 30 astronomical units (AU). The Voyager 2 encounter with Neptune and its satellite Triton3 (Plate 1) and Earthbased observations of Pluto4 (Plate 2) have transformed our knowledge of the outer solar system. Moreover, recent observations have led to the identification of two new classes of objects, Kuiper Belt objects (KBOs) and Centaurs.5, 6 The Kuiper Belt is thought to be a disk of icy objects that are confined to within ~10° of the ecliptic plane at distances between 30 AU and hundreds of astronomical units. Centaurs are relatively small bodies (with diameters less than ~200 km) whose eccentric orbits cross the paths of Saturn, Uranus, and Neptune. Some numerical studies and new observations suggest that an additional class of bodies may also exist.7, 8 These objects, members of the so-called scattered disk, are characterized by eccentricities and/or inclinations greater than those of typical KBOs. Additional components of the distant outer solar system (e.g., dust and the Oort Cloud), as well as the planet Neptune itself, are not considered in this report. Observations of the distant outer solar system conducted over the last few years have established a new framework in which to view the relationships among the ensemble of trans-neptunian objects: KBOs, Centaurs, Pluto, Charon, and Triton (Figure 1.1). As a class, the trans-neptunian objects are primitive compared with inner solar system objects, but they do show evidence for diversity9 and for evolutionary processes having occurred.10 These newly discovered objects and their relationships represent a fertile, relatively unexplored region for investigation of fundamental questions on the origin and evolution of the solar system.

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--> Figure 1.1 Map of the trans-neptunian region, October 1997. The circles indicate the orbits of the giant planets. The Kuiper Belt objects are shown in their current location with their designations (e.g., QB1, TG2, and so on). Note that on the scale of this map, the inner part of the Oort Cloud would be about 3 meters away from the figure. Map courtesy of Gareth V. Williams, Harvard-Smithsonian Astrophysical Observatory.

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--> Trans-Neptunian Objects The first KBO was discovered in 1992.11 As of April 1998, there are some 64 known KBOs,12 but the rate at which they are currently being discovered (10 to 15 per year) suggests that many more will be identified over the next few years. If the known KBOs have low albedos, they are typically a few tens to a few hundred of kilometers in diameter. In total there are probably tens of thousands of ~100-km bodies and many more much smaller objects yet to be discovered. Spectroscopy of a small subset of known KBOs suggests that they have diverse colors and, presumably, surface compositions.13 The Centaurs are an eclectic group: some members appear bluish and display cometary activity (e.g., Chiron), whereas others are very red (e.g., Pholus).14 All Centaurs have orbits that are unstable on short time scales (millions of years). Thus, they have only recently arrived at the locations at which they are found. Observations and theoretical calculations suggest certain relationships among trans-neptunian objects.15, 16 Roughly half of the KBOs are in stable orbits resonant with Neptune. This situation is similar to that of Pluto, whose dynamical relationship to Neptune is well established. It is therefore prudent to view Pluto and its satellite Charon as large members of the KBO population. It is also reasonable to view Triton as a KBO, considering its likely solar nebula origin, its similarity to Pluto, and the fact that it is also in a stable orbit with regard to perturbations by Neptune. Because the inner part of the Kuiper Belt is unstable with respect to gravitational perturbations by Neptune, KBOs are suspected to be the major source for short-period comets that now reside within a few astronomical units of the Sun. The Centaurs appear to be former KBOs.17, 18 That is, they are recently departed members of the Kuiper Belt, on their way to the inner solar system. As mentioned above, Chiron displays cometary activity. It seems that a Centaur may represent an intermediate stage of dynamical evolution between KBOs and short-period comets. The observed Centaurs are larger members of this intermediate class. Dramatic advances in our knowledge of the structure and composition of several trans-neptunian objects have taken place concurrently with advances in our knowledge of the dynamical relationships between the different classes of objects. Voyager observations of Triton and Earth-based observations of Pluto have provided information on the physical processes operating on these larger trans-neptunian objects. A suite of volatile ices has been discovered on Pluto and Triton, and it has been established that both bodies possess atmospheres. Moreover, the list of chemical compounds observed in cometary comae continues to grow, posing interesting questions about the chemical relationships between the largest denizens of the trans-neptunian region (Pluto and Triton) and the smallest (comets). The small, volatile-and organic-rich bodies of the outermost solar system represent the frozen leftovers from planet formation in the solar nebula. As such, they hold clues not only to the origin of the outer planets but also to the origin of Earth's inventory of volatiles and, possibly, to the origin of prebiological organic material on Earth. The ices and organic materials constituting Pluto, Charon, Triton, and the objects in the Kuiper Belt formed from solid material that originated in the molecular clouds of the interstellar medium. Chemistry on the icy grains of the interstellar medium is capable of producing the fundamental organic molecules from which life on this planet (and possibly other planets) arose. Because of the cold temperatures at trans-neptunian distances and because smaller objects are less likely to have undergone internal differentiation, the smaller KBOs are thought to have been relatively unmodified since their formation. It is therefore expected that studies of the chemical composition of KBOs will provide knowledge of the pathways of volatile and organic molecular materials from their interstellar origins to their disposition in Earth's hydrosphere, atmosphere, and biosphere. Such knowledge may open the window to our understanding of the deepest and most compelling issues of the origin of life and its possible presence elsewhere. Progress in the last decade on the relationships among trans-neptunian objects enables the formulation of specific questions about the formation of the outer solar system and suggests spacecraft and ground-based observations capable of answering these questions. It is in this light that COMPLEX reviews recent developments in this field and attempts to establish priorities for future research. COMPLEX has identified five major themes that characterize the importance of the new worlds beyond 30 AU:

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--> Exploration of new territory, Reservoirs of primitive materials, Processes that reveal the solar system's origin and evolution, Links to extrasolar planets, and Prebiotic chemistry. A substantial body of observational studies and theoretical understanding already exists for the first three items on this list. They are all well-established areas of scientific inquiry. The last two, in contrast, are more speculative. They represent relatively new areas of inquiry where the interests of planetary scientists begin to overlap with those of astrophysicists and life scientists. Although much less is known about these themes than the other three, they suggest a number of interesting possibilities whose exploration will require an interdisciplinary approach. An additional theme, the possible effect of the trans-neptunian region on the inner solar system, is touched on briefly for completeness. It is not explored in detail in this report, because it is beyond the scope of the current study. This report includes a general discussion of the five themes that characterize the outstanding scientific issues of the outer solar system, a review of current understanding of objects in the distant outer solar system, a description of observations that could address the outstanding issues, and a discussion of the technological developments that are necessary for these measurements. COMPLEX closes this report with recommendations for further exploration. Exploration of New Territory Telescopic discoveries of new Kuiper Belt objects are being made monthly. With ongoing access to suitable telescopes, these discoveries will continue for many years since very little of the sky has been surveyed to date. While telescopes are showing us that these objects exist in the trans-neptunian region, spacecraft missions are necessary for exploring the detailed nature of these icy bodies. The pre-telescope solar system ended at Saturn. Uranus, Neptune, and Pluto were discovered over a period of 148 years (in 1782, 1846, and 1930, respectively). In the 62 years from Pluto's discovery to 1992, Pluto's moon (Charon) and two Centaurs (Chiron and Pholus) were found. Since 1992, some 60 objects have been discovered in the trans-neptunian region (see Figure 1.1). This explosive increase in the rate of discovery is due to the enhanced capabilities of electronic cameras with large, efficient detectors on modest (2-meter) ground-based telescopes to scan relatively large regions of the sky (e.g., 60 square minutes of arc) for faint (24th-magnitude) objects.19 Trans-neptunian objects have very slow orbital motion and take 160 to 800 years to orbit the Sun. By observing them at opposition, when Earth's motion creates greater apparent angular motion, a change in location relative to the background stars apparent in images taken a few hours apart can be simply related to their distance. The parallactic motion of KBOs tells us that most are located at a distance between 30 and 50 AU. Without knowledge of their albedo, it is not possible to directly determine the size of each object. But assuming they have an albedo similar to that observed for cometary nuclei (0.04), their sizes are estimated to range from 50 to 400 km, although smaller bodies may have escaped detection. Searches have covered only a small fraction of the ecliptic plane (<0.1%). If planetary scientists continue to have access to suitable telescopes with appropriate instrumentation, and the current rate of discovery (about 10 to 15 objects per year) continues, then approximately 100 KBOs will be known by the end of the century. Since the number of smaller objects generally exceeds the number of larger objects, access to more powerful facilities, particularly the Hubble Space Telescope (HST) and the new generation of 8-meter ground-based telescopes, will allow the discovery of fainter (i.e., smaller, farther, or darker) objects. Exploration of this new frontier should not be limited to telescopes. Voyager 2's flyby of Triton demonstrated the enormous value of getting close to a planetary object.20 One brief flyby and a dozen images told us about Triton's size, mass, surface geology, surface composition, active outgassing, atmosphere, and ionosphere, and its interaction with surrounding magnetospheric plasma. The close-up view of Triton (see Plate 1) has given us a

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--> glimpse of a large Kuiper Belt object that is believed to have been captured by Neptune. The value of this information about Triton will be magnified when we have a similar level of detailed information about a Kuiper Belt object that has not been captured, such as Pluto. While HST pictures of Pluto (see Plate 2) are impressive, it must be remembered that the resolution is comparable to that of the pre-Voyager images of the Galilean satellites of Jupiter. Full exploration of the trans-neptunian region of the solar system will require spacecraft missions. NASA's plans for a Pluto mission have undergone significant revision over the last few years. A Pluto mission was originally conceived as a Cassini-class project costing more than $1 billion. Budgetary pressures, combined with technical advances, have now pushed the estimated cost of such a mission down to such a point that the initiation of a Pluto mission in the early years of the next decade is now conceivable. Reservoirs of Primitive Materials While KBOs may not be pristine relics of the original solar nebula, it is in the outer solar system that we might expect to find the least-modified materials as well as samples that have suffered a range of degrees of modification. These bodies can provide the links for understanding the relationships among the interstellar medium, the solar nebula, and current materials in the solar system. The material within some 30 AU of the Sun has been strongly modified by the interplay of physical and chemical processes responsible for the formation of the major planetary bodies and their evolution to their current state. The trans-neptunian region is by far the largest part of the solar system (see Figure 1.1), and it is likely that those expanses retain clues regarding the formation and evolution of our solar system.21 Materials originally formed and processed in the interstellar medium (ISM) are believed to have been incorporated into the early solar nebula (Plate 3, top right). The gases and dust of the early solar nebula probably accreted into relatively small planetesimals (<10 km) that then accreted to form larger bodies (>100 km). Some of the accreted bodies formed the cores of the giant planets. The gravity of these cores led to the capture of nebular gases by Jupiter and Saturn. In the inner solar system, the growth of the planetesimals to Earth-size bodies occurred in a largely gas-free environment over a period of approximately 100 million years. Formation of the cores of the giant planets, however, required the runaway growth of objects of some 10 Earth masses prior to the removal of the remaining gas from the solar nebula. Thus, accumulation must have taken place at a much faster rate (~106 to 107 years for Jupiter and Saturn and somewhat longer for Uranus and Neptune) in the outer solar nebula than in the inner solar nebula. Comets and the trans-neptunian objects are the relics of bodies in the outer solar system that failed to be incorporated into the giant planets during the runaway growth of their cores. During and after the accretion of Uranus and Neptune, most of the remaining planetesimals and larger objects in this region were ejected from the solar system or stranded in the Kuiper Belt (Plate 3, bottom). Beyond Neptune a fraction of the initial planetesimals and smaller protoplanets probably remain in the region in which they formed. Various physical and chemical processes operated within the solar nebula, such as the mixing of nebular material with infalling gas and grains, ultraviolet processing of materials, and condensation of volatiles (Plate 3, upper left). The condensed grains must have formed aggregates, but the process whereby the grains stuck together is poorly understood. Low-velocity micron-and submicron-size particles readily stick to each other or to surfaces. They stick because their large surface-area/mass ratios enhance the effectiveness of surface forces (e.g., van der Waals, electrostatic forces, adhesion, and so on). The aggregation of centimeter-size and larger particles is more of a mystery. It has been suggested that either carbonaceous or siliceous grains with a mantle of carbon compounds could have been “sticky” enough for accretion, but this is unlikely in the context of the cold environments in the outer solar system. An alternative model suggests that the growth of fluffy, fractal-like structures is the primary accumulation process for solids. Porous particles yield inelastic collisions where fine-grained components absorb the kinetic energy of the impact. Figure 1.2 illustrates the processing and ultimate fates of the interstellar material that was incorporated into the solar nebula. Dynamical processes such as accretion, gravitational perturbations, and collisions resulted in the planetesimals ending up in very different parts of the solar system. Those objects likely to retain some information related to the initial materials incorporated into the solar nebula are the icy planetesimals represented by comets, Kuiper Belt objects, Centaurs, and possibly some asteroids.

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--> Figure 1.2 An evolutionary tree for planetesimals. At the top of the tree are the materials that originally formed in the interstellar medium. As one goes downward along the tree, the materials become incorporated into the solar nebula and eventually the growing planetesimals. At the bottom of the tree, the planetesimals undergo several processes that can result in their being incorporated into various objects currently recognized in the solar system. Figure courtesy of Dale Cruikshank, NASA Ames Research Center. Most of the inner solar system is devoid of remnants of these planet-building bodies. Some remnants remain in the asteroid belt, but because of their relative proximity to the Sun their surfaces may have been altered by external processes (e.g., collisional events, incident solar energy, and so on). Internal processes (e.g., differentiation) may also have occurred due to their relatively rock-rich composition. Moreover, the relatively high temperatures experienced by bodies in the inner solar system imply that fewer volatile components of the early solar system are retained in asteroids. In contrast, the Kuiper Belt's greater distance from the Sun implies minimal processing and dynamical perturbation, which leads to confidence that KBOs formed fairly close to where they are found today. Thus it is possible that the Kuiper Belt holds a sample of planetesimals that have suffered less modification. Comparison with asteroids is important because KBOs are believed to contain volatiles that have been largely lost from asteroids. Comparison with Centaurs and comets is important because KBOs are thought to have remained more or less in place in the outermost regions, whereas Centaurs and comets have undergone large variations in their distance from the Sun.

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--> Processes that Reveal The Solar System's Origin and Evolution. The observable characteristics of objects tell us about the processes they have experienced. The distribution of a population of objects in orbital phase space provides clues about their origins and the dynamical processes that control them over long periods. The distribution of sizes within a population reveals the relative importance of accretion versus collisional erosion. The wide range of sizes and different collisional histories among objects in the trans-neptunian region implies varying degrees of internal differentiation. Surface geology provides important constraints on an object's thermal history. Surface chemistry and atmospheric properties reveal processes of outgassing, photochemistry, transport, and redeposition of volatiles. The physical and chemical characteristics of objects in the outermost region of the solar system provide clues about the physical and chemical processes they have experienced. The smaller bodies of the outer solar system pose an interesting problem: Are they basically primitive, or have they been significantly modified? Spectro-scopic evidence suggests that the KBOs are not pristine and have suffered some surface processing, and the diversity of their colors implies a wide range of degree of modification.22 Since researchers believe that the accretion of these objects was arrested in the early stages of solar system formation, studies of the dynamics, geology, surface chemistry, and atmospheres of KBOs may provide the best opportunity to probe the processes that occur during this phase. Dynamics KBOs are widely believed to be the source of short-period comets.23, 24 Many of the detected KBOs seem to be, like Pluto and Charon, in the “safe havens” of orbital resonance with Neptune. The Centaurs are a dynamically separate family of objects in unstable orbits whose semimajor axes fall between Jupiter and Neptune. Centaurs may be bodies that have been dislodged from the Kuiper Belt region and are in the process of being scattered by the gravitational effects of the giant planets. Their dynamical lifetimes are measured in millions of years, clearly separating them from the much more stable objects in the Kuiper Belt. The degree of orbital evolution of the trans-neptunian objects tells us about their orbital history, their relationship to Neptune, and possibly Neptune's orbital evolution. The size distribution of KBOs is expected to be the net result of a combination of accretion into larger objects and collisional erosion into smaller objects.25 Recent observations already suggest that the size distribution of KBOs is complex and has a different form for the smaller objects (radius <100 km) as compared to the larger objects (radius >100 km). For objects with a radius of ~100 km, the distribution is relatively flat, suggesting that objects smaller than 100 km across are eroding whereas larger objects may still be accreting. Moreover, there is a jump from the largest KBOs (radius <400 km) to Pluto and Triton (radius >1,000 km), which may reflect a real gap in the sizes of these objects or may instead be owing simply to the incompleteness of observations.26 If the shortage of objects with a radius of >400 km is real, then this suggests that Pluto and Charon may be the result of runaway accretion, i.e., that they accumulated much of the available material, denying other planetesimals the chance to grow to comparable size. The size distribution of objects in the trans-neptunian region provides vital information about the collisional processes that lead to erosion or accretion of material and how these processes vary with size of the object, its location, and time. Geology Voyager images of the icy satellites of the giant planets suggest that the level of geological activity depends on composition, silicate content (which leads to radioactive heating), and the presence or absence of tidal heating.27 Further complications arise if relatively large impacts have disrupted the satellites in question: a situation argued to occur for moons close to their parent giant planets whose strong gravity focuses impactor trajectories or for satellites that may have undergone major collisions during capture. For Kuiper Belt objects, low-melting-point ices are expected, but the amounts and types are uncertain. Internal reworking may have been powered by radioactive heating alone if low-melting-point ices (e.g., ammonia)

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--> were sufficiently abundant to lower the viscosity and allow the ice-rock mixture to creep. However, the amounts of radiogenic elements and low-melting-point ices in the KBOs are not known. Tidal heating is less important for KBOs, but collisions have affected the dynamically evolved, inner portion of the Kuiper Belt. Thus, the larger KBOs may have been sufficiently heated during earlier epochs to be at least partially differentiated. Collisions may even have broken up the objects, exposing these interiors to view in much the same way that the asteroids today reveal the separation of rock and metal in an earlier generation of inner solar system planetesimals. Thermal stresses caused by internal heating and by subsequent exothermic crystallization of ices may have resulted in fractures that vented volatiles to the surface and to space. Loss of surface volatiles during impacts might have increased the concentration of rocky or organic materials. The strong magnetic field associated with the Sun in its T Tauri phase may have generated substantial currents in planetesimals, heating the interiors by electromagnetic induction. It is possible, therefore, that even the smaller objects in the trans-neptunian region could be differentiated. Comparison of the mean density and surface composition of these objects would provide clues about their interiors and their thermal histories. Surface Chemistry Having formed in the outer regions of the solar nebula at temperatures of only 40 to 50 Kelvin, trans-neptunian objects are thought to possess interiors rich in molecular ices (H2O, CO2, CO, and N2). The composition of icy planetesimals can be used to identify and understand the degree of chemical heterogeneity of the solar nebula and the chemical evolutionary processes that have acted on these objects. Currently, although the color of some of the KBOs has been measured,28 there is little information about their surface compositions, and it is useful to consider the Centaurs Pholus and Chiron as illustrations of how comparison of surface composition may provide information about the different surface processes experienced by trans-neptunian objects. If it is assumed that both Pholus and Chiron began with similar complements of surface materials, then the observed strong difference in color and near-infrared spectral features suggests that these bodies experienced very different processes that produced their current surface compositions. As illustrated in Plate 4, light hydrocarbons (e.g., methane and methanol) can be converted to materials that are relatively enriched in carbon. This chemical transformation tends to eliminate spectral features associated with certain molecular vibrations, and it results in materials that have low albedos and spectra relatively devoid of features. Thus, for example, the observed spectral signature of light hydrocarbons on Pholus indicates that its surface has been less chemically processed than that of Chiron. Dynamical analysis of the chaotic orbit of Chiron suggests that it has been relatively close to the Sun for much longer than Pholus, and may once have been a short-period (Jupiter family) comet.29 This would have exposed the surface of Chiron to higher thermal and solar ultraviolet flux, providing a mechanism for transforming any original light hydrocarbons to more spectrally neutral organic solids. An alternative explanation would be that Chiron and Pholus were originally formed in different parts of the solar system from different materials. The few measurements of KBOs show a wide dispersion in optical color, indicating non-uniform surface properties. Statistical studies of the surface characteristics of many objects, combined with understanding of their dynamical histories from orbital studies, will help to distinguish between variations in original composition and different degrees of surface processing. Atmospheres Pluto and Triton have tenuous atmospheres with surface pressures of about 10 microbars.30, 31 The compositions of these atmospheres are related to these objects' surface compositions, an important clue to their origin. These atmospheres tell us that the objects are venting and losing volatiles, an important evolutionary process. Trans-neptunian objects may have tenuous atmospheres because the cold temperatures in the outer solar system permit the existence of volatile ices on the surfaces. A cometary (i.e., gravitationally unbound) atmosphere has, for example, been detected on the Centaur, Chiron. Claims of the existence of a bound dust atmosphere about Chiron remain controversial.32 Such a feature is both difficult to explain theoretically and difficult to observe,

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--> being beyond the resolution of all telescopes other than HST. The extent of a bound dust atmosphere will depend on Chiron's mass and on the density/size ratio of the dust grains (i.e., how much solar radiation pressure can perturb them). If such an atmosphere is real, careful measurements of its extent and the colors of Chiron's coma (to infer grain sizes) may give a direct estimate of the Centaurs density. The smaller KBOs cannot possess a gravitationally bound gaseous atmosphere, but outgassing events are possible. For the larger KBOs, bound dusty atmospheres may be a possibility, given the similarity in size to Chiron. The atmospheres of trans-neptunian objects are an important aspect of volatile evolution. Although the low temperatures beyond Neptune have permitted trans-neptunian objects to retain many ices, the more volatile species still can be lost through atmospheric escape. It is likely that atmospheric escape has affected the budget of volatiles on Pluto and Triton. Moreover, as the orbits of some trans-neptunian objects evolve inward, increased outgassing at higher temperatures (as on Chiron) will alter the inventory of volatiles. Understanding these processes is essential to investigations of the chemical compositions of objects in the trans-neptunian region and to understanding the effects of volatiles delivered to the inner solar system by comets. Atmospheres present a unique opportunity for the study of the composition of trans-neptunian objects. Studies of cometary comae show how the composition of the parent body can be measured from the evolved gases to a degree that is not possible with spectroscopy of the surfaces. Moreover, chemical processes occurring in an atmosphere influence the surface composition of a body. The simplest example of this process is the photochemical destruction of methane in the atmospheres of Pluto and Triton. Although photochemical by-products have yet to be found, there is every reason to believe that destruction of methane leads to the creation of more complex organic molecules, implying that atmospheres can play a role in the inventory and form of organic material in the trans-neptunian region. Links to Extrasolar Planets Some of the nearby stars similar to the Sun are surrounded by disks of dust that are thought to be derived from collisions between comets. It is natural, therefore, to relate such dust disks to the Kuiper Belt. Applying knowledge of the Kuiper Belt to stellar dust disks suggests that the inner boundary exhibited by some disks may be an indication of the existence of planets. Comparisons of the Kuiper Belt with these dust disks is an important component of the new field of comparative studies of solar systems. It has been known for more than a decade that some stars, including ones similar to the Sun, are surrounded by extended disks of dust. These dust disks are distinguished by their extra infrared emissions as observed by the Infrared Astronomical Satellite (IRAS). The dust is probably derived from colliding and sublimating comets. It is only natural, therefore, to draw parallels between the dust disks around other stars and the Kuiper Belt.33 From our vantage point on Earth, looking out through the Kuiper Belt, we are not yet able to detect the disk of dust that has presumably been produced by collisions between KBOs. Space-based instruments being designed for detection of extrasolar planets from orbits beyond Mars (where there is less interference from zodiacal light) might be capable of detecting our own dust disk.34 Furthermore, it is interesting to note that many of the dust disks around other stars appear to have an inner edge, similar to the inner edge of the Kuiper Belt. 35 Since the inner boundary of the Kuiper Belt is thought to be carved out by the dynamical influence of Neptune, it seems reasonable to infer that extrasolar dust disks with inner boundaries indicate the presence of planets. Prebiotic Chemistry. As remnants of the early solar system, trans-neptunian objects can provide critical clues about processes of prebiotic chemistry and about the materials that would have been delivered to the early Earth. Such material may have been the source of volatile materials from which life arose here and possibly on other planets of this and other solar systems.36 The evolutionary process from primordial chemical systems to current biological systems is poorly understood. The process must, however, encompass the transition from simple chemical reactions involving a wide range of simple chemicals to a morphological chemical diversity antecedent to biological systems. Current life on

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--> Earth is characterized by a relative uniformity of biochemical processes, i.e., domination by a few elements—carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur (CHNOPS)—and the same processes being continuously repeated in many places. Before life began, there must have been a period of chemical evolution in which the primordial chemical diversity underwent a process of selection that eventually led to self-organizing chemical systems. The low temperatures found in the outermost regions of the solar system inhibit chemical interactions, and such regions thus are likely to preserve evidence of this diversity. Kuiper Belt objects may be repositories of evidence about the process of solar system formation and may offer insights on how the biogenic elements (CHNOPS) and their compounds can affect the formation of the planets themselves. The Kuiper Belt should be the place to find evidence of carbonaceous materials that may have played a role in the aggregation of grains in the early solar nebula. Even though a direct relationship between these carbon compounds and the chemistry of life has not been demonstrated and remains controversial, it is possible that some of the compounds essential for life may have been essential to planetary formation as well. As such, it may be that the existence of Kuiper Belt objects about a star could be an indication that prebiotic chemical evolution might also be occurring in that solar system. Observational programs designed to detect extrasolar planets might someday provide information that could be correlated with the distribution of life elsewhere in the galaxy. Effects of the Trans-Neptunian Region on the Inner Solar System Objects from the trans-neptunian region are periodically perturbed into the inner solar system, where they are likely to play an important role as a source of volatiles and as potential impactors. There is mounting evidence, both from modeling and from measurements of terrestrial samples carrying mantle-derived gases (e.g., basalts from mid-ocean ridges), that the initial noble-gas inventories on and in Earth were similar to those of the Sun.37 If so, where did these gases come from? The noble gases trapped in most meteorites, for example, have a nonsolar isotopic composition. Many possible sources have been suggested. These include accreted material irradiated by an early solar wind; direct adsorption of solar-composition nebular gases on planetary accretion cores or on preaccretion dust; and low-temperature occlusion of nebular gases in icy bodies from the outer solar system which were later perturbed inward and rained down on the inner planets. While the concept of an icy-planetesimal source has many supporters, it is somewhat ad hoc because the noble-gas abundances of trans-neptunian objects have not been observed directly. In addition to their role as possible conveyors of volatile materials to the inner solar system, the trans-neptunian objects perturbed into the inner solar system (i.e., comets) have a finite probability of colliding with planetary surfaces. In doing so, these impactors not only influence the formation and evolution of planetary surface features, but also may possibly influence the evolution of living organisms. A prime example of this is the theory that the mass extinction of dinosaurs and other species at the end of the Cretaceous period was triggered by a cometary impact. References 1. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010, National Academy Press, Washington, D.C., 1994, pp. 33–34. 2. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010, National Academy Press, Washington, D.C., 1994, pp. 13–14. 3. D.P. Cruikshank, ed., Neptune and Triton, University of Arizona Press, Tucson, Arizona, 1995. 4. For a current review see, for example, S.A. Stern and D.J. Tholan, eds. Pluto and Charon, University of Arizona Press, Tucson, Arizona, 1997. 5. For a recent review of the Kuiper Belt see, for example, P. Weissmann, “The Kuiper Belt,” Annual Reviews of Astronomy and Astrophysics 33:327, 1995. 6. P. Weissman and H.F. Levison, “The Population of the Trans-Neptunian Region: The Pluton-Charon Environment,” Pluto and Charon, S.A. Stern and D.J. Tholen, eds., University of Arizona Press, Tucson, Arizona, 1997, p. 559. 7. I.P. Williams et al., “The Slow-Moving Objects 1993 SB and 1993 SC,” Icarus 116:180, 1995.

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--> 8. D.C. Jewitt, J.X. Luu, and J. Chen, “The Mauna Kea-Cerro Tololo (MKCT) Kuiper Belt and Centaur Survey,” Astronomical Journal 112:1225, 1996. 9. J. Luu and D.C. Jewitt, “Color Diversity Among the Centaurs and Kuiper Belt Objects,” Astronomical Journal 112:2310, 1996. 10. M.J. Duncan, H.F. Levison, and S.M. Budd, “The Dynamical Structure of the Kuiper Belt,” Astronomical Journal 110:3073, 1995. 11. D.C. Jewitt and J.X. Luu, “Discovery of Candidate Kuiper Belt Object 1992 QB1,” Nature 362:730, 1993. 12. See the Minor Planet Center home page on the World Wide Web for an updated list: <http://cfa-www.harvard.edu/iau/lists/TNOs.html>. 13. J. Luu and D.C. Jewitt, “Color Diversity Among the Centaurs and Kuiper Belt Objects,” Astronomical Journal 112:2310, 1996. 14. J. Luu and D.C. Jewitt, “Color Diversity Among the Centaurs and Kuiper Belt Objects,” Astronomical Journal 112:2310, 1996. 15. For a recent review of the Kuiper Belt see, for example, P. Weissmann, “The Kuiper Belt,” Annual Reviews of Astronomy and Astrophysics 33:327, 1995. 16. P. Weissman and H.F. Levison, “The Population of the Trans-Neptunian Region: The Pluto-Charon Environment,” Pluto and Charon, S.A. Stern and D.J. Tholen, eds., University of Arizona Press, Tucson, Arizona, 1997, p. 559. 17. For a recent review of the Kuiper Belt see, for example, P. Weissmann, “The Kuiper Belt,” Annual Reviews of Astronomy and Astrophysics 33:327, 1995. 18. P. Weissman and H.F. Levison, “The Population of the Trans-Neptunian Region: The Pluto-Charon Environment,” Pluto and Charon, S.A. Stern and D.J. Tholen, eds., University of Arizona Press, Tucson, Arizona, 1997, p. 559. 19. D.C. Jewitt, J.X. Luu, and J. Chen, “The Mauna Kea-Cerro-Tololo (MKCT) Kuiper Belt and Centaur Survey,” Astronomical Journal 112:1225, 1996. 20. D.P. Cruikshank, ed., Neptune and Triton, University of Arizona Press, Tucson, Arizona, 1995. 21. J.I. Lunine, “Primitive Bodies” in The Formation and Evolution of Planetary Systems, H.A. Weaver and L. Douly, eds., Space Telescope Science Institute Symposium, Cambridge University Press, 1989. 22. J. Luu and D.C. Jewitt, “Color Diversity Among the Centaurs and Kuiper Belt Objects,” Astronomical Journal 112:2310, 1996. 23. For a recent review of the Kuiper Belt see, for example, P. Weissmann, “The Kuiper Belt,” Annual Reviews of Astronomy and Astrophysics 33:327, 1995. 24. P. Weissman and H.F. Levison, “The Population of the Trans-Neptunian Region: The Pluto-Charon Environment,” Pluto and Charon, S.A. Stern and D.J. Tholen, eds., University of Arizona Press, Tucson, Arizona, 1997, p. 559. 25. D.C. Jewitt, J.X. Luu, and J. Chen, “The Mauna Kea-Cerro-Tololo (MKCT) Kuiper Belt and Centaur Survey,” Astronomical Journal 112:1225, 1996. 26. P. Weissman and H.F. Levison, “The Population of the Trans-Neptunian Region: The Pluto-Charon Environment,” Pluto and Charon, S.A. Stern and D.J. Tholen, eds., University of Arizona Press, Tucson, Arizona, 1997, p. 559. 27. G. Schubert, T. Spohn, and R.T. Reynolds, “Thermal Histories, Compositions and Internal Structures of the Moons of the Solar System,” in Satellites, J.A. Burns and M.S. Mathews, eds., University of Arizona Press, Tucson, Arizona, 1986. 28. J. Luu and D.C. Jewitt, “Color Diversity Among the Centaurs and Kuiper Belt Objects,” Astronomical Journal 112:2310, 1996. 29. K. Oikawa and E. Everhart, “Past and Future Orbit of 1977 UB, Object Chiron,” Astronomical Journal 84:134, 1979. 30. R.V. Yelle et al., “Lower Atmospheric Structure and Surface-Atmosphere Interactions on Triton,” Neptune and Triton, D.P. Cruikshank, ed., University of Arizona Press, Tucson, Arizona, 1995, p. 1060. 31. R.V. Yelle and J.L. Elliot, “Atmospheric Structure and Composition: Pluto and Charon,” Pluto and Charon, S.A. Stern and D.J. Tholen, eds., University of Arizona Press, Tucson, Arizona, 1997, p. 347. 32. K. Meech et al., “Observations of Structures in the Inner Coma of Chiron with the HST Planetary Camera,” Astronomical Journal 113:844, 1997. 33. P. Artymowicz, “Beta Pictoris: An Early Solar System?,” Annual Reviews of Earth and Planetary Sciences 25:175, 1997. 34. For a review of the technological requirements for the detection of extrasolar planets, see, for example, “Exploration of Neighboring Planetary Systems (ExNPS) Team,” A Road Map for the Exploration of Neighboring Planetary Systems, JPL-96-22, C.A. Beichman, ed., Jet Propulsion Laboratory , Pasadena, California, 1996. 35. For a discussion of the relationship between the Kuiper Belt and protostellar and stellar disks see, for example, P.R. Weissman, “The Kuiper Belt,” Annual Reviews of Astronomy and Astrophysics 33:327, 1995. 36. Astrobiology Workshop Preliminary Report, D. DeVincenzi, ed., NASA Ames Research Center, Moffett Field, California, 1996. 37. R.O. Pepin, “Origin of Noble Gases in the Terrestrial Planets,” Annual Reviews of Earth and Planetary Sciences 20:389–430, 1992.