Exploring the Trans-Neptunian Solar System (1998)

The amount of information available about individual objects in the trans-neptunian region is highly variable. The most extensive are available from the Voyager 2 spacecraft's encounter with Triton, but only telescopic observations are available for Pluto, Charon, the Centaurs, and the KBOs.

Triton is by far the best-explored icy body in the distant outer solar system¹ and, as such, sets the context for the discussion of the other bodies. Triton is in an inclined, retrograde orbit around Neptune, which suggests that it was captured. Triton is a rock-rich icy body, with the fraction of rock approaching 70% by mass. It is the only satellite other than Titan with a substantial atmosphere. It also has a complex seasonal cycle, leading to the possibility of climatic changes on a wide range of time scales. Only about 180 impact craters were detected at Voyager resolution (1 km/pixel), suggesting that Triton's surface is relatively young. Within the region observed in detail by Voyager 2's cameras, Triton exhibits a wide array of features produced by tectonic, volcanic, and atmospheric processes (see Plate 1). As with other icy satellites, the energy sources for the volcanic and tectonic activity on Triton are not well understood, but they may involve catastrophic tidal heating associated with Triton's capture from solar orbit in the distant past.

Voyager 2's observations revealed that Triton's mean density is 2.06 g cm^-3 (Table 2.1). This density, combined with surface spectroscopy and theoretical considerations, suggests that Triton is composed of silicates, water ice, organic materials, and other ices with low melting points (e.g., N₂, CH₄, CO₂). If Triton had suffered a major collision(s) with other neptunian satellites at the time of capture, ² then its interior would have been disrupted. It is not clear, however, whether a collision with a satellite would lead to a higher or lower bulk density. Triton probably is differentiated into a rocky core surrounded by a mainly water-ice mantle. The case for differentiation of Triton's interior is based both on the large rock fraction, which enhances radioactive heating of Triton's interior, and on tidal heating of the interior during the circularization of its orbit following capture from a heliocentric orbit. The icy mantle is probably about 400 km thick and may include a large organic fraction.³

About 30% of the surface of Triton was imaged by Voyager 2 at resolutions ranging from about a kilometer to tens of kilometers (see Plate 1). The geology of the imaged portion is among the most complex and varied of any of the solar system's icy satellites.⁴ There is no evidence for preserved ancient heavily cratered terrain, which implies that Triton was internally active for at least several hundred million years following its formation. The total crater population is low and indicates an average age for Triton's surface on the order of several hundred million years. This is a relatively short time and suggests that Triton has been internally active in the recent past and may still be active.

Most of the surface of Triton appears to be composed of materials that have erupted from the interior, and many of the landforms present are clearly of cryovolcanic (i.e., ice eruption) origin. This observation supports the interpretation of continued internal activity. A crude stratigraphy has been defined. The enigmatic “cantaloupe terrain,” a complex landscape of pits, ridges, and troughs, is the oldest part of the preserved surface. Superimposed on this terrain are smooth materials in the valley floors. The youngest materials, of presumed endogenic origin, make up walled plains. The bright surficial materials, generally considered to represent ephemeral frosts condensed from the atmosphere, are not thick enough to obscure the underlying geological material units. The dark materials are thought to be carbonaceous materials, possibly vented from the interior.

The degree of differentiation of the interior of Triton is a major issue. A generally accepted model for the history of Triton includes its capture from solar orbit by Neptune.⁵ The initial capture may have involved gas drag with the protoneptunian nebula (if captured early) or collision with one or more satellites (if captured after satellite formation).⁶ In either case, the subsequent circularization of Triton's orbit would cause significant tidal heating, melting, and differentiation. Alternatively, it is possible that Triton's youthful geology may have been powered by radiogenic heating alone if low-melting-point ices (e.g., ammonia) were sufficiently abundant to lower the viscosity of the upper layers and allow the crust to creep. A greater understanding of Triton's surface geology and interior structure could provide evidence of Triton's thermal history. It would be interesting to compare Triton with Pluto, which is not believed to have been heated tidally but has probably undergone a major collision with Charon.

Triton is one of three places (with Earth and Io) in the solar system where active eruptions have been seen. Voyager images revealed dark eruptive plumes—geysers—which were highly collimated and roughly 1 km in diameter, rising vertically from the surface.⁷ At an altitude of ~8 km, the ascending material was abruptly sheared

laterally, and the aerosols were carried horizontally for ~100 km by prevailing winds. The amount of mass injected into the atmosphere by the geysers at the time of the Voyager encounter was small relative to the seasonal interhemispheric mass transport, but very large by geological standards. If the geyser activity persists over geological time, it is likely to be one of the dominant processes controlling the nature of Triton's atmosphere and surface. Only two plumes were unambiguously identified by Voyager, although there was evidence for several more “spent” geysers. The observed plumes appeared to be clustered near the subsolar latitude, but this was also one of the best-observed regions of the planet, and the apparent correlation may be due to a selection effect.

An unresolved controversy is the energy source for production of the geysers. Some believe that the geysers represent true volcanism, with the heat coming from the interior of Triton. Others have suggested that the features are surficial, with most of the energy being solar heating. In the later case, infrared radiation is trapped under an ice “lid” by a solid-state greenhouse effect. In either case, the geysers' existence depends on the presence of highly volatile ices beneath the surface. Determination of the extent, vigor, duration, geographic distribution, and energy source for the geysers is of fundamental importance. Knowledge of this process is essential to understanding the evolution of the atmosphere, and the process may represent the link between Triton's interior and its surface-atmosphere system.

Earth-based spectroscopic observations reveal that Triton's surface is covered by a variety of volatile ices.⁸ The spectra imply that N₂ is the dominant ice species with trace amounts (<1% by mass) of CO, CH₄, and CO₂, if the surface grains are mixed at the granular level. Greater amounts of CO₂ may be present if the CO₂ occurs as a discrete component not mixed with the other ices at the granular scale. Water ice has been detected, but its relative abundance is currently unknown. The CH₄ is present as a dilute contaminant within N₂ ice. Unfortunately Voyager 2 did not have the instrumentation to identify these molecules, and consequently their geographic distribution can only be guessed at.

Voyager did discover extensive deposits of bright material covering most of Triton's southern hemisphere. Because ground-based spectroscopy indicates that N₂ is the most abundant ice, the extensive bright deposits may well be an N₂ polar cap. Curiously, Triton's summer hemisphere was in the midst of a so-called major summer at the time of the Voyager encounter, and so the existence of extensive frost deposits at this location is difficult to understand.⁹ Many theories have been offered, but with the information at hand none of the explanations is truly compelling.

Voyager 2's flyby provided the first accurate measurement of Triton's surface temperature (38 ± 3 Kelvin) and definitive evidence of a thin atmosphere with a surface pressure of 14 microbars.¹⁰ Voyager observations showed that Triton's atmosphere is complex. Combined Voyager and ground-based spectroscopic data suggest that Triton's atmosphere is predominantly N₂ with trace amounts of CH₄, H₂, and CO, consistent with the detected surface ices. Recent ground-based spectroscopic observations have yielded an N₂ ice temperature of 38.3 ± 1 Kelvin, in agreement with the surface temperature measured by Voyager.

Discrete clouds and pervasive hazes as well as aerosols from the geysers were observed by Voyager 2. The cloud motions and features on the surface interpreted as wind streaks indicate a circulation pattern that is consistent with the expected seasonal transport at low altitudes but that abruptly changes direction within the first 10 km above the surface. The implication is that the equatorial atmosphere is warmer than the polar atmosphere, but there are no observations that test this hypothesis.

The following properties of Triton's atmosphere were derived from Voyager data:

• The lower atmosphere at summer latitudes has a saturated adiabatic lapse rate (about -0.1 Kelvin km^-1);
• N₂ clouds exist; and
• A 37-Kelvin tropopause exists at about 8 microbars (8 to 12 km).

Recent ground-based stellar occultation measurements suggest that part of Triton's lower atmosphere may be isothermal at 0.1- to 1-microbar pressures.¹¹ Above this level, Triton has been inferred to have a thermosphere with temperature rising to an isothermal value near 96 to 102 Kelvin at ionospheric heights (200 to 400 km) due to

solar extreme ultraviolet radiation and magnetospheric energetic electron heating.¹² Heat is conducted down to the tropopause or surface and also partially radiated away by CO. Based on the temperature profile, Triton's extended atmosphere is inferred to have an exobase located at 750 to 800 km, above which thermal escape of light species occurs.

The hazes in Triton's atmosphere are thought to be the result of photochemistry. Photolysis of methane and molecular nitrogen produces complex hydrocarbons and nitriles. These photochemical products are produced at a rate that results in partial pressures large than their vapor pressure at the ambient atmospheric temperature; consequently, the photochemical products condense, creating atmospheric hazes. Voyager observations and inferences concerning photolysis rates, haze properties, sedimentation rates, and geographical and vertical distributions are consistent with this general picture.

Discrete cloud formation is more difficult to understand. Because the clouds are confined to the troposphere where N₂ is near its saturation point, the clouds are generally thought to be composed of N₂ ice; however, too little is known about the temperature, distribution of volatiles, and wind fields on Triton to develop a detailed understanding of the generation and geographical distribution of the clouds. What is clear is that the properties of the atmosphere are dependent on surface conditions and that the atmosphere is an essential element of the volatile cycle: to understand the distribution and transport of volatile ices, researchers need to understand the atmosphere.

Voyager radio occultation measurements showed a well-developed ionosphere on Triton with peak electron densities of about 2 to 5 × 10⁴ cm^-3 at 340 to 350 km and, above the peak, plasma scale heights of about 260 to 300 km. Identification of the major ion is still a subject of debate; it may consist of one or more of three abundant atomic ions: H⁺, C⁺, and N⁺.¹³

Although we have little detailed knowledge and a small number of observational constraints on the photo-chemistry of Triton's atmosphere, we do know enough to recognize that Triton's atmosphere is in an interesting regime. Its atmospheric temperatures are much lower than those in other planetary atmospheres. As a consequence, many neutral chemical reactions, which typically have energy barriers, are inhibited so that ion-neutral reactions and reactions on aerosol surfaces assume increased importance. The dominance of ion-neutral reactions is well established in the chemistry of the interstellar medium, where they play an important role in helping to shape the composition of molecular clouds, the raw material for solar system formation. Thus, Triton presents us with a nearby environment in which researchers can study these processes.

A major source of plasma in Neptune's magnetosphere is thought to be thermal escape of neutral H, H₂, and N from Triton's upper thermosphere (10²⁵ nitrogen and 10²⁶ hydrogen atoms sec^-1). Because Neptune's magnetic field is highly tilted with respect to the planet's spin axis, Triton encounters a large range of magnetic latitudes as it moves through Neptune's magnetosphere. Each pass through the magnetic equator subjects Triton to an energetic electron energy flux that is about 20 times larger than the solar extreme-ultraviolet energy flux. Some of these electrons enter the upper atmosphere and deposit power estimated to be as large as 10⁸ watts.¹⁴ Significant magnetospheric energetic electron power input is suggested by the high thermospheric temperature (~100 Kelvin), the large inferred nitrogen atom escape rate, and the high peak electron densities. At the same time, the magnetospheric plasma bombards and alters the satellite's lower atmosphere and surface.

Pluto remains the only planet not visited by a spacecraft. Nonetheless, some information is available from telescopic observations, including detailed studies of the 1985 to 1990 series of mutual eclipses and transits (“mutual events”), stellar occultations, radiometric measurements, near-infrared spectra, and ground-based and HST images (see Plate 2). These observations provide reasonably accurate estimates of size, albedo, system mass, surface composition, atmospheric pressure (upper limits) and composition, and surface temperature, as well as crude albedo maps.¹⁵

Models of the interiors of Pluto and its satellite Charon are based mainly on knowledge of their radii and masses.¹⁶ Radii have been determined by observations of stellar occultations and mutual events, and from study of HST images (see Table 2.1). The relative masses of Pluto and Charon can be determined from observations of their motions about the system's center of mass.¹⁷ Such observations reveal that Pluto is approximately twice the diameter of Charon and eight times more massive. The uncertainties in the radii and masses suggest that the mean densities of Pluto and Charon are in the range from 1.79 to 2.17 g cm^-3 and 1.33 to 1.87 g cm^-3, respectively (see Table 2.1).

Based on cosmochemical relative abundances and these densities, Pluto and Charon are inferred to be mixtures of mainly water ice and rock (i.e., silicates). Pluto's density corresponds to that of a rock-rich body, while the uncertainty in Charon's density allows for either rock-rich or ice-rich composition. The large uncertainties in the densities of Pluto and Charon mean that the percentage of rock is much less constrained for these bodies than for Triton (see Table 2.1). An additional major unknown about the interiors of Pluto and Charon is whether the ice and rock are essentially homogeneously mixed (i.e., no differentiation) or the rock has separated from the ice to form a rocky core surrounded by an ice mantle (i.e., differentiation has occurred). A collisional capture and subsequent orbital evolution would imply substantial melting of the interior. An intriguing possibility for the interior of Pluto is that it might contain amounts of relatively refractory organic solid.¹⁸

Some 12 major “regions,” where the surface is either bright or dark, can be discerned in HST images of Pluto (see Plate 2). These images show that Pluto is an unusually complex object, with more large-scale contrast than any planet except Earth. Some of the variations across Pluto's surface may be caused by topographic features such as basins or fresh impact craters. However, most of the surface features unveiled by HST, including the prominent northern polar cap, are likely produced by frosts that migrate across Pluto's surface in response to its orbital and seasonal cycles and chemical by-products deposited by Pluto's atmosphere.

Earth-based spectroscopic observations show that Pluto's surface, like Triton's is covered with ices and relatively volatile compounds. ¹⁹ Current models of the reflectance spectra suggest that N₂ is the dominant species on Pluto, with trace amounts (<2% by mass) of CO and CH₄, if the surface grains are mixed at the granular level. Water ice has been detected but is not yet incorporated into models; thus its relative mass fraction at the surface is currently unknown. As on Triton, the positions of the CH₄ absorption bands do not correspond to those of pure CH₄. However, the bands appear broader than those seen on Triton and suggest two reservoirs of CH₄. One CH₄ reservoir is perhaps a solid solution within the N₂ ice (as is suspected for Triton); the other reservoir may consist of discrete locations on Pluto's surface composed of relatively pure CH₄. Unlike the case for Triton, no evidence has been found for CO₂ ice on Pluto or Charon.

Earth-based spectroscopic observations indicate that H₂O ice is present on Charon. There also appears to be a relatively dark, spectrally neutral material that lowers the overall albedo throughout the visible and near infrared. Due to the low spectral resolution of the existing data, other volatile species such as N₂, CO, and CO₂ may be present on Charon and yet remain undetected.

Pluto's atmosphere was first detected during a stellar occultation observed at various ground-based observatories and from the Kuiper Airborne Observatory.²⁰ Observations of the occultation yielded a ratio of temperature to mean molecular mass (T/µ) of ~3.6 Kelvin amu^-1. In striking contrast, stellar occultations by Triton at the same microbar pressure levels yield temperatures at least a factor of 2 lower, T/µ ~ 1.6 Kelvin amu^-1. If Pluto has an N₂ atmosphere, the implied isothermal temperature is ~100 Kelvin.

A remarkable feature of the Pluto occultation data is the “knee or kink” (change in slope) in the light curves

at a radius of 1,215 ± 11 km (about 2 microbars). This feature has been variously interpreted as evidence for the following:²¹

1. A haze layer with an extinction optical depth of >0.15,
2. A steep temperature inversion with a gradient of 10 to 30 Kelvin km^-1 at the surface, or
3. The same steep temperature gradient on top of an underlying troposphere of moderate depth (<40 km).

In other words, researchers cannot ascertain whether Pluto has a thin atmosphere (e.g., 3 microbars surface pressure) or a thicker one (albeit only 100 microbars). An increased surface pressure would correspond to a smaller radius and thus a higher mean density for Pluto.

The density and thermal structure of Pluto's upper atmosphere and ionosphere are unknown. At the microbar level, it is probably the least gravitationally bound N₂ atmosphere in the solar system, and it could, potentially, be a hydrodynamically escaping atmosphere. At the very least, light constituents such as H, H₂, C, CH₄, and N are probably escaping rapidly, and some of these gases may be captured by Charon.

Knowledge of Pluto's atmospheric composition is limited. From its spectrum in the 1.6-micron region, a CH₄ column density of about 3 × 10¹⁹ cm^-2 was inferred.²² Other atmospheric species, in particular N₂ and CO, must be estimated from vapor pressure equilibrium considerations. By analogy with Triton, N₂ is expected to be the dominant atmospheric gas. The spectroscopic N₂ ice temperature is 40 ± 2 Kelvin on Pluto, ²³ which implies surface N₂ atmospheric pressures in the range from 18 to 157 microbars, assuming vapor pressure equilibrium. From the observed abundance of CO ice, the atmospheric CO mixing ratio is inferred to be about 5 × 10^-4.²⁴

The interaction of the tenuous solar-wind plasma with Pluto critically depends on the flux of material escaping from the planet's atmosphere. ²⁵ If the escape rate is greater than about 10²⁷ molecules sec^-1, then Pluto acts like a comet with the solar wind ionizing the outflowing material upstream, slowing down the solar wind, and pulling the mass-loaded solar wind into a downstream ion tail. If the atmospheric escape rate is lower, then the interaction will be more like that of Mars and of Venus, where the solar wind induces currents in each planet's ionosphere that deflect the solar wind around the planet. The Galileo spacecraft's discoveries of magnetic signatures at the asteroids Gaspra and Ida (as well as the Galilean satellites) raise the possibility that Pluto might also be magnetized. Because the solar-wind pressure is weak in the outer heliosphere, even a weak (20-nT) surface magnetic field would produce a magnetosphere around Pluto.

Since the first Kuiper Belt object²⁶ was discovered in 1992,²⁷ the number of KBOs directly detected has increased to almost 60,^28–31 and more will undoubtedly be revealed. In addition, a team using HST has developed statistical arguments to infer the existence of a large population of smaller objects in the trans-neptunian region. ³²

The radial distances and azimuthal location of all objects discovered as of October 1997 are shown in Figure 1.1. The full orbital elements have been determined for many of these objects.³³ The orbital inclination (i) can be determined to within 0.5° after a few nights of observations, whereas the eccentricity (e) and semimajor axis (a) can take months to converge.³⁴ Figure 2.1 shows these orbital parameters for the detected objects plus the locations and widths of the main orbital resonances with Neptune.³⁵

The orbits of the so-called classical Kuiper Belt objects fall into two main categories:³⁶

1. Objects with a < 41 AU and e > 0.1 (e.g., Pluto and Charon) that are in mean motion resonances with Neptune; and
2. Objects with 41<a < 50 AU and e < 0.1 (e.g., 1992 QB1) that are not in resonant orbits.

Theoretical studies and observations of the recently discovered object 1996 TL₆₆ suggest the existence of an

Figure 2.1

Orbital eccentricity and inclination versus semimajor axis for Kuiper Belt objects (KBOs). Also indicated from theoretical studies are the orbital resonances with Neptune. Many of the KBOs lie within the 4:3 and 3:2 resonances with Neptune that tend to stabilize their orbits. A significant number of KBOs fall outside these resonances and are prone to perturbation by Neptune and Uranus. Adapted from 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; and R. Malhotra, “The Origin of Pluto's Orbit: Implications for the Solar System Beyond Neptune,” Astronomical Journal 110:420, 1995.

additional component to the trans-neptunian region, the so-called scattered Kuiper Belt.^37,38 These objects are characterized by highly eccentric orbits extending to ~130 AU. They may have been planetesimals that were scattered out of the Uranus-Neptune region into eccentric orbits. Their existence poses the question of whether the Kuiper Belt extends as far as the Oort Cloud. Dynamical studies of the trans-neptunian region show that orbits with a < 35 AU and with 40 < a < 42 AU are very unstable to gravitational perturbations by Neptune and Uranus. ^39,40 These studies show that a small fraction of KBOs continue to stray into these unstable zones where they are likely to suffer major perturbations, ^41,42 confirming an earlier suggestion that the disklike Kuiper Belt is the more probable source of low-inclination, short-period, Jupiter-family comets than is the isotropically distributed Oort Cloud.⁴³

Although the absence of KBOs within 35 AU can be explained by Neptune's perturbations, the lack of objects in the dynamically stable region of low-eccentricity orbits between 36 and 39 AU remains an important mystery.⁴⁴ Malhotra has proposed that Neptune's orbit has evolved outward, sweeping up objects into the stable 3:2 resonance and clearing the inner Kuiper Belt.⁴⁵ Others have proposed the presence of as-yet-undetected massive perturbers that have cleared the 36- to 39-AU gap.^46,47

Jewitt and colleagues have argued that the inclination distribution of the trans-neptunian objects is important because it controls the velocity dispersion among these objects and hence determines whether the collisional regime is erosive or agglomerative.⁴⁸ The upper part of Figure 2.1 suggests that objects located in resonant orbits have higher inclinations, consistent with the dynamical studies.^49,50 Malhotra's work also shows that the fraction of KBOs whose orbits are pumped up into higher inclinations as they are swept into resonances depends on the time scale for outward migration of the giant planets. She also points out that these resonant orbits tend to put the

Figure 2.2

Sketch of the size distribution of trans-neptunian objects. The horizontal axis is the object radius, R. The vertical axis is the cumulative number of objects with radii greater than R. The references for the 5 points are given in the text. Power-law functions with slopes of b = -3 and b = -5 are shown by the solid curve. The dashed line shows how the distribution might be modified if the slope were flatter in the 50- to 500-km region as suggested by simulations of the KBO surveys. Adapted from P.R. 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.

objects farthest from the ecliptic at perihelion (when they are brightest) so that searches need to cover a broad band of latitudes in order to avoid a selection bias in sampling the KBO population.⁵¹

The size distribution of a population of objects has been a useful diagnostic for understanding the processes that lead to the erosion and/or accretion of planetary bodies. From recent observations and theoretical studies, it is emerging that objects in the trans-neptunian region probably follow a complex size distribution (Figure 2.2). Apart from Triton, the only firm observations of size are those of Pluto and Charon, and these still have substantial uncertainties (see Table 2.1). All other sizes are derived from brightness values and assume that the objects have an albedo of 0.04, comparable to the albedos of observed comets. Thus, there are at least factor-of-two uncertainties in estimates of the sizes of KBOs.

Estimates of the number of comet-sized objects (1 to 5 km in radius) in the Kuiper Belt are based on the supply of short-period comets to the inner solar system and have large error bars (see Figure 2.2). From HST observations, Cochran and colleagues inferred the presence of 10^8–9 objects each 5 to 10 km in diameter.⁵² This claim remains controversial.⁵³ An upper limit on the number of objects in the 20to 330-km size range was derived by Levison and Stern based on limits to perturbations of Charon's orbit.⁵⁴ Recent ground-based surveys have produced the recent detections of 50- to 300-km objects and estimates of their size distribution.^55,56 Earlier surveys put limits on the number of objects in the 1,000-km range.^57,58

The diameter distribution for comets within the solar system is estimated to have a power-law slope of b = -3.⁵⁹ If this power law is also applicable and if there are 5 × 10⁹ comet-sized objects (as inferred from the supply of small-period comets) in the Kuiper Belt, then the b = -3 distribution predicts two orders of magnitude more 50to 200-km objects than observed. This discrepancy suggests that the slope in the distribution must steepen at larger sizes (e.g., b = -5, as shown by the solid line in Figure 2.2). Simulations of the ground-based

Figure 2.3

The visual color of trans-neptunian objects, defined as the visual magnitude minus the R magnitude (V–R) versus semimajor axis. Larger values of V–R indicate redder color. The horizontal line near 0.27 indicates the intrinsic solar color. It appears that there is a trend of increasing redness with increasing heliocentric distance. These differences in color may be due to heterogeneity among the bodies or differences in their degree of surface modification. Adapted from 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.

surveys of KBOs suggest a much flatter slope for 50- to 300-km objects (e.g., b ~ -1, as shown by the dashed line in Figure 2.2). This shallow slope cannot persist over a wide range of sizes without predicting too few comets and too many Pluto-sized objects. Thus, it is possible that the size distribution in the Kuiper Belt is steep at large and small sizes and flat in between. The flatter distribution is consistent with a suggestion by Stern that larger objects are still accreting while smaller objects are eroding due to collisions.⁶⁰ A flat slope at ~100 km would indicate the boundary between these two regimes.

Very little is known about the physical characteristics or compositions of KBOs. Their sizes are only very roughly estimated. In 11 cases, broadband colors have been determined (Figure 2.3), revealing a diversity of color even within this small sample, although the bodies tend to be red. This wide range of colors, exceeding the range of colors exhibited by asteroids or comets, either may suggest diversity of composition (e.g., the red color suggests the presence of organic compounds), or may provide evidence for further surface modification of the bodies. Luu and Jewitt argue that the range of colors suggests surface inhomogeneity due to collisional resurfacing.⁶¹ They predict that newer areas will be lighter, that objects with lower albedo will be redder, and that the color variation will be less among larger objects.

A handful of bodies (seven as of December 1997) have been identified with orbits that cross those of Saturn, Uranus, and Neptune. These bodies are referred to as Centaurs. The importance of the Centaurs is that, based on dynamical calculations, they are located in orbits that are not stable over the lifetime of the solar system.⁶² This suggests that the Centaurs formerly resided in the Kuiper Belt and only relatively recently have been delivered into their current orbits. Another argument for a common origin of Centaurs and KBOs is that the color diversity and redness of the Centaurs match those of KBOs but are not comparable to those of either asteroids or comet nuclei. ⁶³ A more recent study has, however, challenged this view.⁶⁴ It suggests that the color diversity of cometary nuclei is just as great as that of Centaurs and KBOs, the principal difference between the objects being that cometary nuclei are not as red as Centaurs and KBOs. This claim is limited by the sparsity of published data on the colors of cometary nuclei. If additional observations establish a common origin for Centaurs and KBOs, then the Centaurs can provide compositional information on the more distant Kuiper Belt objects and information about their subsequent processing.

The characteristics of the known Centaurs include the following:

1. Perihelia between 8.5 and 10.6 AU;
2. Aphelia between 19 and 36 AU;
3. Orbital eccentricities ranging from 0.38 to 0.58;
4. Orbital inclinations between 5.4 and 25 degrees; and
5. Estimated diameters between 20 and 200 km, based on Earth-based telescopic observations at thermal wavelengths and assumed low visual albedo (Chiron and 5145 Pholus have independent assessments of their diameters).

Three of these objects (Chiron, Pholus, and 1993 HA2) are large enough and bright enough for Earth-based telescopic spectral observations to be obtained at visual and near-infrared wavelengths (0.4 to 1.0 microns). With larger telescopes spectral observations could be made of fainter objects.

Chiron is a uniquely complex body. It apparently undergoes sporadic outbursts of activity that may be driven by the sublimation of CO. Its orbit is chaotic, and it may have spent a considerable amount of time in the inner solar system. If this is the case, understanding its inventory of volatiles is a major issue—if aging effects are clearly visible in short-period comets, why not in Chiron? At visible wavelengths, Chiron has a flat reflectance spectrum similar to that of the C-type asteroids. Thermal-infrared observations suggest that its visual albedo appears to be low (0.04 to 0.1), further indicating a similarity to the C-type asteroids. The similarity in reflectance to the C-type asteroids extends to the near infrared, where measurements show a flat, featureless continuum.

Chiron's cometary outbursts are both short-lived (on time scales of hours) and long term (large peaks of up to a year or more). The outbursts are clearly not related to perihelion passage, since Chiron was actually less active at perihelion than it was at 12 AU shortly after discovery of its variability. The duration of the long-term outbursts may be related in a complex way to modulation from the bound-dust atmosphere suggested by recent HST observations. This makes Chiron an interesting laboratory for studying the physics of exospheres. Although the existence of the bound coma remains controversial, its confirmation would provide a means of estimating Chiron's density. Preliminary estimates suggest a low density (~0.5), which is consistent with recent models of planetesimal accretion.⁶⁵

Several observations of Chiron appear to be in conflict in terms of brightness and color. Indeed, Chiron's color seems to vary with its state of activity. At times of outburst Chiron's coma clearly becomes bluer. Although chemical surface processing may be thought to play a role in this behavior, it is more likely to result from the complex effects that the gravity has on the particle-size distribution in the inner and outer coma. The coma becomes bluer during an outburst because of the injection of a large number of small grains that subsequently escape into space. Very recent observations hint at a possible color effect in Chiron's rotational light curve. Preliminary dynamical atmospheric models suggest that surface albedo features on Chiron would not be completely covered by fallback from the bound-dust atmosphere.

In marked contrast to Chiron's bluish color, the visible reflectances of 5145 Pholus and 1993 HA2 are extremely red (see Figure 2.3) and exhibit steep upward slopes toward longer wavelengths.⁶⁶ This spectral behavior is not characteristic of ice, rock, or minerals, but a variety of organic solids exhibit a range of red slopes throughout the visible region. Unfortunately, the red slope alone is insufficient to identify a specific solid material. Several near-infrared spectral observations have been obtained of Pholus. The presence of several distinctive absorption features has led to suggestions that the surface of Pholus is composed of a mixture of H₂O ice and a variety of organic materials (newer high-resolution spectra indicate NH₃ is not a detectable constituent). The organic materials suggested to date include light hydrocarbons or methanol ice, Titan tholin, polymeric HCN, and carbon black. The presence of the light hydrocarbons suggests that Pholus has been less chemically processed than comets and asteroids. Intriguingly, extrapolations of orbital calculations suggest that Pholus's current orbit is dynamically new, suggesting that it may have arrived recently from the Kuiper Belt.

1. D.P. Cruikshank, ed., Neptune and Triton, University of Arizona Press, Tucson, Arizona, 1995.

2. P. Goldreich et al., “Neptune's Story,” Science 245:500, 1989.

3. W.B. McKinnon, D.P. Simonelli, and G. Schubert, “Composition, Internal Structure, and Thermal Evolution of Pluto and Charon,” Pluto and Charon, S.A. Stern and D.J. Tholen, eds., University of Arizona Press, Tucson, Arizona, 1997, p. 295.

4. S.K. Croft et al., “The Geology of Triton,” Neptune and Triton, D.P. Cruikshank, ed., University of Arizona Press, Tucson, Arizona, 1995, p. 879.

5. W.B. McKinnon, J.I. Lunine, and D. Banfield, “Origin and Evolution of Triton,” Neptune and Triton, D.P. Cruikshank, ed., University of Arizona Press, Tucson, Arizona, 1995, p. 807.

6. P. Goldreich et al., “Neptune's Story,” Science 245:500, 1989.

7. R.L. Kirk et al., “Triton Plumes: Discovery, Characteristics, and Models,” Neptune and Triton, D.P. Cruikshank, ed., University of Arizona Press, Tucson, Arizona, 1995, p. 949.

8. R.H. Brown et al., “Surface Composition and Photometric Properties of Triton,” Neptune and Triton, D.P. Cruikshank, ed., University of Arizona Press, Tucson, Arizona, 1995, p. 991.

9. 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.

10. 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. 1031.

11. C.B. Olkin et al., “The Thermal Structure of Triton's Atmosphere: Results from the 1993 and 1995 Occultations,” Icarus 129:1780, 1997.

12. D.F. Strobel and M.E. Summers, “Triton's Upper Atmosphere and Ionosphere,” Neptune and Triton, D.P. Cruikshank, ed., University of Arizona Press, Tucson, Arizona, 1995, p. 1107.

13. D.F. Strobel and M.E. Summers, “Triton's Upper Atmosphere and Ionosphere,” Neptune and Triton, D.P. Cruikshank, ed., University of Arizona Press, Tucson, Arizona, 1995, p. 1107.

14. D.F. Strobel and M.E. Summers, “Triton's Upper Atmosphere and Ionosphere,” Neptune and Triton, D.P. Cruikshank, ed., University of Arizona Press, Tucson, Arizona, 1995, p. 1107.

15. For a current review see, for example, Pluto and Charon, S.A. Stern and D.J. Tholen, eds., University of Arizona Press, Tucson, Arizona, 1997.

16. W.B. McKinnon, D.P. Simonelli, and G. Schubert, “Composition, Internal Structure, and Thermal Evolution of Pluto and Charon,” Pluto and Charon, S.A. Stern and D.J. Tholen, eds., University of Arizona Press, Tucson, Arizona, 1997, p. 295.

17. J.A. Foust et al., “Determination of the Charon/Pluto Mass Ratio from Center-of-Light Astrometry,” Icarus 126:362, 1997.

18. W.B. McKinnon, D.P. Simonelli, and G. Schubert, “Composition, Internal Structure, and Thermal Evolution of Pluto and Charon,” Pluto and Charon, S.A. Stern and D.J. Tholen, eds., University of Arizona Press, Tucson, Arizona, 1997, p. 295.

19. D.P. Cruikshank et al., “The Surfaces of Pluto and Charon,” Pluto and Charon, S.A. Stern and D.J. Tholen, eds., University of Arizona Press, Tucson, Arizona, 1997, p. 221.

20. For a review, see 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.

21. 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.

22. L.A. Young et al., “Detection of Gaseous Methane on Pluto,” Icarus 127:258, 1997.

23. K.A. Tryka et al., “The Temperature of Nitrogen Ice on Pluto and Its Implication for Flux Measurements,” Icarus 112:513, 1994.

24. 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.

25. F. Bagenal et al., “Pluto's Interaction with the Solar Wind,” Pluto and Charon, S.A. Stern and D.J. Tholen, eds., University of Arizona Press, Tucson, Arizona, 1997, p. 523.

26. 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.

27. D.C. Jewitt and J.X. Luu, “Discovery of Candidate Kuiper Belt Object 1992 QB1,” Nature 362:730, 1993.

28. D.C. Jewitt and J.X. Luu, “The Solar System Beyond Neptune,” Astronomical Journal 109:1867, 1995.

29. M. Irwin, S. Tremaine, and A.N. Zytkow, “A Search for Slow-Moving Objects and the Luminosity of the Kuiper Belt,” Astronomical Journal 110:3082, 1995.

30. I.P. Williams et al., “The Slow-Moving Objects 1993 SB and 1993 SC,” Icarus 116:180, 1995.

31. 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.

32. A.L. Cochran et al., “The Discovery of Halley-Sized Kuiper Belt Objects Using Hubble Space Telescope,” Astrophysical Journal 455:342, 1995.

33. 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>.

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36. P. Weissman and H.F. Levison, “The Population of the Trans-Neptunian Region: The Pluto-Charon Environment,” Pluto and Charon, D.J. Tholen and S.A. Stern, eds., University of Arizona Press, Tucson, Arizona, 1997, p. 559.

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47. P. Weissman and H. 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.

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50. M.J. Duncan, H.F. Levison, and S.M. Budd, “The Dynamical Structure of the Kuiper Belt,” Astronomical Journal 110:3073, 1995.

51. R. Malhotra, “The Origin of Pluto's Orbit: Implications for the Solar System Beyond Neptune,” Astronomical Journal 110:420, 1995.

52. A.L. Cochran, H.F. Levison, S.A. Stern, and M.J. Duncan, “The Discovery of Halley-sized Kuiper Belt Objects Using Hubble Space Telescope,” Astrophysical Journal 455:342, 1995.

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55. D.C. Jewitt and J.X. Luu, “The Solar System Beyond Neptune,” Astronomical Journal 109:1867, 1995.

56. M. Irwin, S. Tremaine, and A.N. Zytkow, “A Search for Slow-Moving Objects and the Luminosity of the Kuiper Belt,” Astronomical Journal 110:3082, 1995.

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66. J. Luu and D.C. Jewitt, “Color Diversity Among the Centaurs and Kuiper Belt Objects,” Astronomical Journal 112:2310, 1996.