are confined to a small fraction of the color-albedo plane, with surfaces that are on average less red and darker than the KBOs and Centaurs. The diagram reinforces the conclusion that the surfaces of the comets and of the Trojans, while similar to each other, are not the same as the surfaces of the Centaurs and KBOs. If this difference reflects an evolutionary trend, then the fact that the Centaur and KBOs overlap in Fig. 34 shows that the modification occurs after the Centaur phase. Most likely it is associated with the onset of sublimation on bodies whose perihelia have approached or crossed the orbit of Jupiter (the rough boundary outside which water does not appreciably sublimate [71]). The very high albedos of EL61, Pluto, and perhaps some other objects are clearly associated with the presence of surface ice and the cleanliness of this ice suggests that it has been recently emplaced, probably by frost deposition from an atmosphere. None of the Trojans or cometary nuclei possess surface ice in quantities sufficient to influence the albedo, because they are too hot (surface ice would quickly sublimate). However, the simple removal of ice cannot explain why the surfaces of many low albedo KBOs and

Centaurs are so much redder than any seen in the comet or Trojan populations (cf. Fig. 23). Some form of instability of the ultrared matter is required.

4.2 Kuiper Belt Physical Properties: Spectra

Only —10 KBOs are bright enough for useful spectra to be obtained. The spectra fall into three basic classes.

The Water Worlds (Fig. 35). KBOs (50000) Quaoar [76], 2003 EL61 [143], and others show strong absorptions at 2.0 |m and 1.5 |m that are diagnostic of water ice. Water ice is stable against sublimation at Kuiper belt distances and temperatures, and it is appropriate to think of it as "bed rock" for other, more volatile species. The ice on Quaoar and 2003 EL61 is known to be crystalline as it shows a narrow band at 1.65 | m that is absent in the spectrum of amorphous ice. This is a puzzle, because ice at the —40K to 50 K surface temperatures of the KBOs should be indefinitely stable in the amorphous form. Why should the ice instead be crystalline?

Crystallinity indicates that the ice has been raised above the critical temperature for transformation (roughly 100 or 110 K) at some point in its history. This heating could have occurred in the deep interiors of the KBOs provided that there is a way for heated ice at depth to be emplaced onto the surface.

Fig. 35. Near infrared reflection spectrum of (50000) Quaoar. The solid line is a crystalline water ice spectrum over-plotted (not fitted) to the data. Note the feature at 1.65 |m that is diagnostic of crystalline ice. From [77]

One way for this to occur is through the past action of cryovolcanism; liquid water (or slush) might have erupted onto the surfaces of these KBOs when they were still internally hot from the decay of trapped radioactive nuclei. Conceivably, heating by micrometeorites is responsible, although this possibility is difficult to test given that we do not know the flux of impacting dust particles within the Kuiper belt. A more serious problem is that crystalline ice exposed to the unimpeded bombardment of energetic particles from the Solar wind and the cosmic rays should be transformed back toward the amorphous state, as the bonds in the crystalline lattice are systematically demolished. The timescale for this process is uncertain but probably short 1-10 My). Hence, it appears that these KBOs must be resurfaced on a geologically very short timescale in order for the ice to have escaped back-conversion to the amorphous form. Again, the mechanisms for resurfacing are unknown. Comet-like outgassing (perhaps with CO playing the role of "volatile") is a possibility, but some effect related to micrometeorite "gardening" of the regolith, as is seen in the rocky fragmental layer on the surface of the Moon, seems more likely. The optically active surface layers may be continually churned together with buried crystalline ice that is protected from irradiation.

The issue of the crystalline state of water ice in small bodies deserves further exploration. Ice in comets is rarely directly detected, but in comets C/Hale-Bopp [26] and C/2002 T7 (LINEAR) [83], the absence of the 1.65 |m band shows that the ice is amorphous. Both objects are long-period comets, and it is possible that the amorphous nature of the ice is a result of energetic particle bombardment, rather than primordial in nature. The outgassing activity of some comets at heliocentric distances beyond the ^5 AU water sublimation zone (e.g. Fig. 24) is often interpreted as evidence for internal heating by the (exothermic) amorphous ^ crystalline phase transition [126]. An interesting question to be addressed observationally is the state of the ice in Jupiter family comets: is this ice crystalline as in the large KBOs or amorphous, as in the two measured long-period comets?

The Methanoids (Fig. 36). KBOs Pluto, 2003 UB313 [153], and 2005 FY9 [99], show evidence for surface methane, with distinct bands in the near infrared spectral region. (Triton, likely to be a large KBO captured by Neptune, also shows a methane-rich spectrum).

Methane is interesting from two perspectives. First of all, methane is unstable to sublimation on long timescales at the distances and temperatures of most Kuiper belt objects. This instability has been explored in detail for Pluto, where it is found that the escape of methane is limited by the flux of energetic (EUV) Solar radiation [65], but can still exceed several kilometers equivalent thickness over the age of the Solar system. The escape from smaller bodies will be dramatically faster, perhaps explaining why the known Methanoids are large (but not explaining why ~ 1200 km diameter Quaoar is methane-free). Second, the origin of the methane is problematic. Low temperatures and pressures in the solar nebula are thought to

Fig. 36. Optical and near infrared reflection spectra of large KBOs Pluto (line) and 2003 UB313 (points). The principal absorptions in both spectra are due to methane. From [13]

favor the incorporation of carbon atoms in the oxidized form as CO and CO2, rather than in the reduced form of CH4 [125]. Therefore, it seems unlikely that the methane was delivered to these bodies from the nebula. One possibility is that CH4 arrived as a clathrate (a physical cage in crystalline water ice in which sufficiently small "guest molecules" can be trapped). In my mind, it seems more likely that the CH4 is produced in the interiors of these bodies, probably from hydrogen released by serpentinization followed by Fischer-Tropsch reactions and then outgassed on to the surface. The lack of methane on small KBOs could then reflect a lack of production, because only bodies large and hot enough to sustain liquid water can experience serpentinization.

Featureless Class. Objects in this class have sloped but otherwise featureless near infrared spectra. Obviously, all spectra are featureless when observed at sufficiently low signal-to-noise ratio, so here "featureless" is probably a relative term and many objects labeled as such will resolve into the other classes once better spectra are secured. By analogy with the featureless spectra of many mantled objects already observed at decent signal-to-noise ratios, including the nuclei of dead comets and the Jovian Trojans (e.g., [37,42,101]), however, it is likely that a subset of the featureless objects will remain so even under more intense scrutiny.

4.3 Kuiper Belt Physical Properties: Shapes, Spins

The shapes and spins of Kuiper belt objects are studied from their rotational lightcurves [86,134]. The most informative way to present these data is in a plot showing the photometric range as a function of the rotational frequency (rotations per day), as here in Fig. 37, from [134]. The range-frequency plane is divided into three regions, based on the original prescription of Leone et al. [93].

Region A shows lightcurves of small range and any period, for which the lightcurve could be affected by surface albedo variations and for which, in any case, the interpretation is likely to be highly ambiguous. Strictly, any lightcurve can be produced by a surface albedo distribution of arbitrary complexity. However, studies of the lightcurves of hundreds of asteroids show few examples where albedo variations are important, perhaps because regolith transport is efficient and albedo differences are quickly smeared out by the redistribution of dust. Those examples are confined to rotational ranges Am ~ 0.1-0.2 mag. To be conservative, in Fig. 37, we have marked Region A as extending up to Am = 0.3mag. The most notable exception to this empirical rule is Saturn's 1460 km diameter satellite Iapetus, which shows a hemispherical albedo asymmetry, with the leading hemisphere being ^6 times darker yi

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