3
Key Measurement Objectives

In its past reports, COMPLEX has advocated a strategy for the planetary sciences that emphasizes a balanced program of ground-and space-based telescopic observations, laboratory measurements, theoretical studies, and remote-sensing and in situ measurements by spacecraft.1 There are good reasons to believe that such an approach is called for to further our knowledge of Kuiper Belt objects. In this chapter COMPLEX indicates the priority of the scientific measurements and studies that, based on current knowledge, are needed to advance understanding of objects at the fringe of the outer solar system.

Orbits.

Distribution of Orbits

A meaningful comparison between the distribution of orbital properties of KBOs and dynamical models of their orbital evolution requires accurate observations of a large population of objects. Furthermore, the full range of orbital properties—specifically, semimajor axis, inclination, and eccentricity—must be derived to understand the effects of orbital resonances with the giant planets on the relationships among these parameters. Because of the very long orbital periods of KBOs—typically more than 200 years—and faintness, typically 23rd to 24th magnitude, high-precision astrometric measurements are required over an extended period. A temporal baseline of a few months is barely sufficient to ensure that an object can be recovered at the next opposition. Observations of 1992 QB1, the first KBO detected, suggest that astrometric studies over a period of at least 5 years are necessary to establish a good orbit. Even after 5 years of study, the orbit of 1992 QB1 is still not secure enough for this KBO to be given a permanent designation.

Although almost 60 KBOs have been found to date, 30 to 40% have been lost because of the lack of timely follow-up observations. Of the remainder, roughly 40% reside in the 3:2 resonance at 39 AU. The rest are dispersed between 35 and 50 AU, but their spatial distribution cannot be characterized because their population density is so small. Thus, predictions that resonances other than the 3:2 should be populated cannot be verified because not enough objects are known to give statistically significant results. In short, we are far from having a large enough sample of KBOs to map out the dynamical properties of the trans-neptunian region. Additional search and astrometric programs are necessary. The rarity of KBOs necessarily means that search programs are long-term undertakings requiring significant amounts of telescope time.



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--> 3 Key Measurement Objectives In its past reports, COMPLEX has advocated a strategy for the planetary sciences that emphasizes a balanced program of ground-and space-based telescopic observations, laboratory measurements, theoretical studies, and remote-sensing and in situ measurements by spacecraft.1 There are good reasons to believe that such an approach is called for to further our knowledge of Kuiper Belt objects. In this chapter COMPLEX indicates the priority of the scientific measurements and studies that, based on current knowledge, are needed to advance understanding of objects at the fringe of the outer solar system. Orbits. Distribution of Orbits A meaningful comparison between the distribution of orbital properties of KBOs and dynamical models of their orbital evolution requires accurate observations of a large population of objects. Furthermore, the full range of orbital properties—specifically, semimajor axis, inclination, and eccentricity—must be derived to understand the effects of orbital resonances with the giant planets on the relationships among these parameters. Because of the very long orbital periods of KBOs—typically more than 200 years—and faintness, typically 23rd to 24th magnitude, high-precision astrometric measurements are required over an extended period. A temporal baseline of a few months is barely sufficient to ensure that an object can be recovered at the next opposition. Observations of 1992 QB1, the first KBO detected, suggest that astrometric studies over a period of at least 5 years are necessary to establish a good orbit. Even after 5 years of study, the orbit of 1992 QB1 is still not secure enough for this KBO to be given a permanent designation. Although almost 60 KBOs have been found to date, 30 to 40% have been lost because of the lack of timely follow-up observations. Of the remainder, roughly 40% reside in the 3:2 resonance at 39 AU. The rest are dispersed between 35 and 50 AU, but their spatial distribution cannot be characterized because their population density is so small. Thus, predictions that resonances other than the 3:2 should be populated cannot be verified because not enough objects are known to give statistically significant results. In short, we are far from having a large enough sample of KBOs to map out the dynamical properties of the trans-neptunian region. Additional search and astrometric programs are necessary. The rarity of KBOs necessarily means that search programs are long-term undertakings requiring significant amounts of telescope time.

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--> Key Measurements Relating to Orbits Conducting an extensive survey of the ecliptic plane at optical wavelengths (increasing coverage from ~15 to 100 square degrees should reveal ~400 objects). Surveying regions up to 40° away from the ecliptic plane to investigate if ecliptic surveys underestimate the number of objects with high inclinations. Performing precise astrometric observations of ~100 KBOs over a period of at least 5 years for accurate orbit determinations. Bulk Properties Measuring Bulk Properties To estimate the density and, hence, bulk composition of a planetary body requires separate measurements of its mass and size. Of all trans-neptunian objects, this has been accomplished only for Triton, where Doppler tracking of the gravitational deflection of Voyager 2 yielded its mass and Voyager 2 images gave accurate measurements of size. The lack of precise radii and masses for Pluto and Charon makes it difficult to reach a robust conclusion on their densities. Improved values of the masses of Pluto and Charon will probably be provided by more precise measurements of the barycentric wobble. To achieve the necessary accuracy in measurements of their sizes will require a spacecraft flyby with a solar occultation. To determine the degree of differentiation of these bodies it will be necessary to measure high-order moments of their gravitational fields via Doppler tracking of a close flyby. While the current location and orbital parameters are useful information for dynamical studies of the Kuiper Belt, better estimates of the sizes of KBOs are vital for estimating the total mass of the Kuiper Belt. The size distribution of the population (i.e., the number of objects as a function of their size) is a key indicator of collisional and accretional processes. Currently, the sizes of KBOs are inferred from their brightness by assuming a value for their albedo. Another way to measure size is to compare an object's thermal output with the amount of sunlight that it receives. At trans-neptunian distances from the Sun, equilibrium temperatures are 30 to 50 Kelvin so that thermal emissions from KBOs peak, according to Wein's law, in the infrared region at wavelengths between 10 and 100 microns. Accurate size measurement therefore requires a radiometric observation of a KBO's thermal flux at wavelengths of ~10 to 100 microns. The smaller KBOs are at current lower limits for detectability with moderate-aperture telescopes. Deep searches have, in the past, been confined to the ecliptic plane and have covered only a small region of the sky. Statistical results imply that many bodies lie just at the edge of detectability by HST. For larger KBOs, search efforts on 2-meter-class telescopes must be continued to answer the question of the existence of objects with diameters between 200 and 2,400 km, objects apparently missing from the current population-number distribution. Key Measurements Relating to Bulk Properties Determining the sizes and masses of Pluto and Charon to constrain their densities. Measuring the magnetic and gravity fields of Pluto and Triton during a spacecraft flyby to constrain the internal structure. Performing precise radiometric observations of KBOs in the far-infrared (100-micron) region. Conducting further searches for the smaller objects suggested by the HST statistical study. Searching for objects with diameters in the range of 200 to 2,400 km.

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--> Surfaces and Chemical Compositions. The Importance of Surface and Chemical-Composition Measurements Of the trans-neptunian objects, only Triton has been imaged at a resolution sufficient to allow interpretation of some of its geological processes and history (70% of Triton's surface remains unmapped). Because of the possibility that Triton was captured from solar orbit by Neptune, its geological history may not be an adequate model for Pluto even though the two bodies are of roughly similar size and mean density. Large areas of different albedo are known to occur on Pluto, suggesting that a heterogeneous surface geology is possible. Images obtained by a future mission would permit several types of geological analysis relevant to inferring internal activity and surficial processes. From geological mapping of material units, combined with application of basic stratigraphic principles, it is generally possible to arrange material units in a chronological sequence. If these units are characterized by very different densities of impact craters, it will be possible to place approximate age limits on them. Thus, the combination of geological mapping and crater studies can provide a first-order estimate of Pluto's geological history. Additional motivations exist for studies of the chemical compositions of objects in the distant outer solar system. The particular volatiles contained in these small bodies and their relative proportions are sensitive thermometers of the conditions in the solar nebula. In addition, compositional information can contribute to greater understanding of how much processing the interstellar material underwent prior to incorporation into planetesimals. The first-order indicator of the identity of the volatiles present is the distances at which there is evidence of activity. If objects are bright enough, spectroscopic observations at radio wavelengths may be able to identify the active molecules. Repeated observations of the same object at different heliocentric distances will be important because deviations from the inverse-square law of brightness are a particularly sensitive indicator of possible activity, even when a coma is not detectable. Such studies will, of course, require knowledge of any rotational modulation of the object's light curve. Existing ground-based near-infrared spectroscopy of Pluto, Triton, and Pholus illustrates the potential for gaining surface compositional information about the relative abundances of volatile components (N2, CO, CH4, CO2, CH3OH, H2O, and more evolved hydrocarbons) on the other Centaurs and KBOs. However, the other known Centaurs and KBOs are fainter and smaller, and often more distant, such that similar spectroscopic measurements are currently not feasible with existing 4-meter-class ground-based telescopes and instruments. Thus, obtaining compositional information on their surfaces requires access to large-aperture ground-based telescopes (e.g., the existing 10-meter Keck telescopes or the 8-meter, infrared-optimized Gemini telescope now under construction) or future space-based facilities with appropriate spectroscopic capabilities in the infrared and millimeter regions of the spectrum. Additionally, near-infrared spectroscopic observations can be made by remote sensing from robotic spacecraft that could identify spatial variation in composition over the surface of these objects. This knowledge could be used to identify physical and/or chemical processes that have been active on the surfaces. The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) recently installed on HST may provide the ability to determine compositional information for some of the brighter KBOs. Future space-based telescopes, such as the 4-meter-class facility recommended by the report of the HST & Beyond (Dressler) Committee2 or the 6- to 8-meter Next Generation Space Telescope,3 will be suitable for compositional studies in the 1- to 5-micron regions if they have the capability to observe moving targets. Key Measurements Relating to Surfaces and Chemical Compositions Imaging the unmapped 70% of Triton's surface. Obtaining visual and near-infrared images of Pluto-Charon during a spacecraft flyby to enable geological mapping and crater population studies, as well as a search for evidence of tectonics, indicators of the distribution of frosts, and evidence of cryogenic volcanism. Performing multispectral observations of a statistically useful number (tens) of KBOs to determine their surface composition and variability over a rotation period.

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--> Obtaining deep images to search for comas and/or other evidence of cometary activity in the Centaurs, KBOs, and distant comets. Gathering visible and infrared images of Centaurs and KBOs with flyby missions. Atmospheres Probing Atmospheres The vertical density and temperature structure of Pluto's lower atmosphere needs to be measured to determine whether there is a troposphere. Likewise, a measurement of the vertical temperature profile at the sub-microbar level can yield a signature of a hydrodynamically escaping atmosphere. The former measurement can be accomplished by a radio-occultation experiment, and the latter measurement can be obtained from an ultraviolet solar-occultation experiment on a robotic spacecraft passing behind Pluto and looking at Earth and the Sun, respectively. A more fundamental but inherently more difficult set of measurements would be needed to determine what controls Pluto's surface pressure and temperature. A similar set of occultation measurements should be performed at Charon, whose atmosphere is probably much thinner than Pluto's. A solar-occultation measurement can yield the composition, density, and temperature profiles on Charon at sub-microbar pressures. Ground-based stellar-occultation measurements should continue to be made when the opportunity arises in order to determine the time evolution of the density and thermal structure of Pluto's and Triton's atmospheres at the microbar level. It is preferable that these measurements be recorded at multiple wavelengths, which may lead to a better understanding of the “knee” structure in Pluto's light transmission curve.4 The thermal structure of the atmospheres of Pluto and Triton, and perhaps Charon, depends critically on the vertical density distributions of minor constituents that absorb either incident sunlight or thermal infrared radiation emitted by these objects themselves. The most important species are CH4, CO, and HCN, the last photochemically produced from N2 and CH4. Measurements of their height distributions can be made by solar occultations, near-infrared remote sensing, and possibly millimeter spectroscopy. The latter two types of measurements may be accomplished by ground-based telescopes. Technology developments in near-infrared detectors may allow sufficiently high spectral resolution measurements of outer solar system objects to detect atmospheric CO absorption features from reflected sunlight. Atmospheric methane absorption features near 1.6 microns have been detected on Pluto, but not on Triton. The development of interferometric capability at millimeter wavelengths may allow the detection of rotational line emission from HCN and CO from Pluto, Charon, and other KBOs. The fact that a single radio telescope can routinely measure rotational line emission from these molecules in Triton's much denser atmosphere suggests that this technique may be extended to thinner and more distant atmospheres. Key Measurements Relating to Atmospheres Performing solar and radio occultations of Pluto's atmosphere with ultraviolet and radio instruments during a flyby mission. Conducting stellar-occultation measurements of Triton, Pluto, and Charon. Obtaining infrared and millimeter-wave spectroscopic measurements of molecular species in the atmospheres of Triton and Pluto. Searching for atmospheres around the larger known KBOs and Centaurs using observations of stellar occultations. Plasma Interactions. Particles and Fields in the Trans-Neptunian Region Plasmas in the outer solar system are tenuous, and magnetic fields are weak. Over the lifetime of objects in

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--> the trans-neptunian region, the particle bombardment of icy surfaces and erosion of their thin atmospheres can be significant. Measurement of the interaction of the solar wind with an object's surface, atmosphere, and/or ionosphere requires in situ measurements of the plasma environment by robotic spacecraft. For example, an accurate measure of the escape rate of Pluto's atmosphere can be obtained only by measuring the deceleration of the solar wind resulting from ionization of the escaping neutral material. Detection of an object's intrinsic magnetic field from a spacecraft requires a magnetometer that can measure <1-nT fields. Key Measurements Relating to Plasma Interactions Determining the magnetic fields of Pluto, Triton, and the interplanetary medium. Studying the plasma density and flow velocity of the 30-AU solar wind. Measuring the velocity distributions and composition of particles from Pluto's atmosphere that have been ionized and picked up by the solar wind. Laboratory Studies The Importance of Laboratory Studies Observational and theoretical programs depend heavily on the results of laboratory studies to make advances in our understanding. Thus a vigorous program of cryogenic laboratory measurements and related studies are required to provided a balanced, broad study of KBOs. For example, spectroscopic measurements of the atmospheric and surface composition of KBOs require supporting laboratory data on the optical properties of ices at ~30- to 50-Kelvin temperatures in order to interpret the observational data and identify composition. Vapor pressures for pure ices and ices of varying composition are poorly understood, or unknown, at these temperatures. Studies of photochemistry in the atmosphere and on the surface require data on kinetic-reaction rates at low temperatures. Although they are important for modeling geological processes, the mechanical properties of the identified materials at these low temperatures are as yet poorly determined. For example, it has been argued that Pluto's atmosphere will diminish at aphelion, because the surface temperature in radiative equilibrium will decrease with increasing distance from the Sun and, based on vapor-pressure equilibrium, the nitrogen surface pressure will drop precipitously. Laboratory studies suggest that nitrogen ice undergoes a dramatic alteration in its physical properties (including its emissivity) when it undergoes a phase change that occurs at 35.5 Kelvin.5, 6 Thus it is possible that Pluto's surface temperature never drops below 35.3 Kelvin over a plutonian year. Further laboratory research is needed to quantify the emissivity of different phases of nitrogen ice. Other areas in which laboratory investigations may provide useful data include studies relating to the photochemical production of complex organic molecules and to reactions between ions and neutrals and ions and the surfaces of aerosols. The surface composition of planetary bodies can be influenced by chemical processes occurring in their atmospheres. The photochemical destruction of methane in the atmospheres of Pluto and Triton could, for example, lead to the creation of complex organic compounds on the surfaces of these bodies. Although such compounds have not yet been identified, laboratory studies of relevant reaction rates could provide useful guidance on the inventory and form of complex organic materials likely to exist in the trans-neptunian region. Such reactions assume increased importance when neutral chemical reactions are suppressed, as they are at the extremely low temperatures found in the atmospheres of Pluto and Triton. Since neutral reactions are also suppressed in interstellar molecular clouds, laboratory studies could, potentially, provide a synergistic link between the material from which the solar system formed and the remnant material existing today in the trans-neptunian region. Investigation of the adhesion of gases to ices at low temperatures is another area in which laboratory studies are likely to be important. Such studies may yield important insights into problems relating to the abundances of inert gases and the deuterium-hydrogen ratios observed in comets.

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--> Key Laboratory Studies Determining the physical and chemical properties of ices of various compositions at temperatures of the outer solar system (30 to 50 Kelvin). Studying the photochemical reactions leading to the creation of complex organic compounds. Investigating ion-neutral reactions and reactions of ions with aerosol surfaces. Measuring the adhesion of gases to ices at low (30 to 50 Kelvin) temperatures. Because some of these laboratory studies are of interest to communities broader than the space sciences, they may be appropriate topics for interdisciplinary cooperation between NASA, the National Science Foundation, and other relevant agencies. Theoretical Studies. Researchers need the complementary efforts of theoretical and computational studies to analyze and interpret observational data and provide a framework for understanding its significance. Theory also plays an important role in suggesting future directions for observational and laboratory research. Key Theoretical Studies Studying orbital dynamics (e.g., the evolution of KBO orbits and the processes responsible for the sunward migration of the Centaurs). Understanding the physics and chemistry of the solar nebula (e.g., modeling the temperature evolution of the solar nebula and the acquisition and/or removal of its volatiles). Investigating the evolution of ices exposed to radiation and low temperatures. Analyzing prebiotic chemical processes. Determining the thermal evolution of objects <2,000 km in diameter. Studying major collisions involving bodies such as Triton and Pluto-Charon. Modeling the process or processes by which Triton was captured by Neptune and Charon was captured by Pluto. Comparing the effects of possible collisional histories and tidal heating on the interior structures of Triton, Pluto, and Charon. References 1. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010, National Academy Press, Washington, D.C., 1994, p. 186. 2. A. Dressler, ed., Exploration and the Search for Origins: A Vision for Ultraviolet-Optical-Infrared Space Astronomy, report of the HST & Beyond Committee, Association of Universities for Research in Astronomy, Washington, D.C., 1996. 3. H.S. Stockman, ed., Next Generation Space Telescope: Visiting a Time When Galaxies Were Young, report of the NGST Study Team, Association of Universities for Research in Astronomy, Washington, D.C., 1997. 4. 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. 5. T.A. Scott, “Solid and Liquid Nitrogen,” Physics Reports 27 (3):85–157, 1976. 6. J.A. Stansberry, J.R. Spencer, B. Schmitt, A. Benchkoura, R.V. Yelle, and J.I. Lunine, “A Model for the Overabundance of Methane in the Atmospheres of Pluto and Triton,” Planetary and Space Sciences 44:1051, 1996.