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From Kuiper Belt Object to Cometary Nucleus: the Missing Ultrared Matter

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From Kuiper Belt Object to Cometary Nucleus: the Missing Ultrared Matter FROM KUIPER BELT OBJECT TO comets up-close via spacecraft. To what extent COMETARY NUCLEUS can we use the results from missions such as Stardust, Deep Impact and Rosetta to understand David Jewitt fundamental properties of the Kuiper Belt Institute for Astronomy, 2680 Woodlawn Drive, Objects? Will we, in fact, measure only heavily Honolulu, HI 96822, USA processed remnants of the nucleus source E-mail: [email protected] population? In keeping with the flavor of the ACM2002 ABSTRACT meeting in Berlin, I will attempt an overview that is "sweeping" in style and will avoid We now realise that many small body belaboring the text by trying to include full populations are related through a common origin citations to all the related work in the literature. in the Kuiper Belt, notably the Centaurs, the I will, however, try to call attention to other talks Jupiter Family Comets and certain dead-comets. and presentations from ACM that are clearly But does primitive material from the Kuiper Belt connected to the subject of this overview (these survive the journey to the inner solar system and will be listed in the form Author ACM2002). In can we learn anything about it by studying the this way, I hope to emphasize broad connections comets? We discuss observational evidence and between the A, C and M parts of the subject. other considerations that relate to this question. 2. KUIPER BELT 1. GENERAL SPECIFICATIONS The Kuiper Belt is the subject of a number of In this paper I want to discuss the evidence for recent reviews, and so need not be discussed in physical relations between Kuiper Belt Objects detail here (see Jewitt 1999, Luu and Jewitt (KBOs) and comets, in the context of the largely 2002, Schulz 2002). Suffice it to say that there accepted dynamical flow-down of objects from are about 70,000 KBOs larger than 100 km the Kuiper Belt to the inner solar system. This diameter in the observable region of the Kuiper new picture, stemming from a conjecture by Belt. Extrapolated to kilometer scales, there may Julio Fernandez (1980) and made real by the be 1 to 10 billion objects with a combined mass discovery of the Kuiper Belt in the 1990's, has of a few tenths of an Earth mass (Jewitt et al. 1998, Trujillo et al. 2001). These seem to be the revolutionised our view of the comets. Before surviving members of a population that was once the Kuiper Belt, the nuclei of comets were seen ~100 times larger than now. as pristine relics from the accretion disk of the sun. Perhaps they could even be identified The Belt contains at least three distinct directly with the first generation planetesimals dynamical regimes and, while there is general (Goldreich and Ward 1973) or at least with agreement that the Kuiper Belt is a source of loosely bonded aggregates of these planetesimals comets to the inner solar system, there is no (Weidenschilling 1997). Now, though, the concensus about which regime supplies the most Jupiter- Family Comets, at least, seem to be comets. The resonant KBOs, which account for escaped collisional fragments produced by ∆V ~ a few to 10 percent of the Kuiper belt population 1.5 km/s collisions between parent bodies in the by number, have orbit periods in simple ratios of Kuiper Belt. They could be shocked, and they integers to that of Neptune. Most are in the 3:2 might carry interesting signatures of collisional resonance at 39 AU and are known as Plutinos, and thermal processing from their larger parent to honor Pluto which is also trapped at this bodies. Even after their collisional creation location. Chaotic zones at the boundaries of the beyond Neptune, the cometary nuclei should be resonances are one plausible source of the modified as they are scattered by the planets on comets. Collisional fragments from nearby their way towards eventual detection in the inner parent bodies could be injected into the chaotic solar system. So, interesting questions include zones and it has also been suggested that Pluto "how are objects modified as they make the might scatter other Plutinos into unstable orbits journey from Kuiper Belt to the region of the (Yu and Tremaine 1999). Their orbital inner planets?" and "what are the observational eccentricities would then be excited until the constraints on any such modification?". The perihelion dropped to ~30 AU, allowing answers to these questions are important in the context of well-developed plans to examine the involvement with and prompt scattering by definition seems unsatisfactory to me, in that it Neptune. includes Pluto (q = 26 AU) and many of the Plutinos as Centaurs. The Scattered KBOs occupy a thick torus with an inner edge near 35 - 40 AU and extending out A second source of confusion is based on to distances of at least many 100's of AU. These physical attributes. It seems to be widely objects are so-called because of their weak accepted that some Centaurs (e.g. 2060 Chiron) perihelic involvement with Neptune and the show coma and can also be labelled "comets". likelihood that they have been progressively On the other hand, some comets that meet scattered out by that planet into their present plausible dynamical definitions of Centaurs (e.g. orbits on timescales of order 1 Gyr. The P/SW1 q = 5.7 AU, a = 6.0 AU) are traditionally Scattered KBOs may constitute a separate source and inexplicably regarded differently from of cometary nuclei (Duncan and Levison 1997), Centaurs. Another object, C/2001 T4, is both if the population is large enough. While very cometary and a Centaur (q = 8.6, a = 13.9 AU). uncertain, the number of Scattered KBOs has Given that the definition of "Centaur" is been estimated from observations as 3 x 104 dynamical, there is no basis for excluding objects (Trujillo et al. 2000). There is evidence that based on the presence of absence of coma. some KBOs have perihelion distances too large to permit substantial interaction with Neptune on Some 58 Centaurs (defined by 5 < q < 30 AU) timescales of the solar system age (Gladman et are known (as of 2002 August 20). Of these, al. 2002, Emel'yanenko ACM2002). These about half have q > 5 AU AND a < 30 AU, objects may vastly outnumber the known Kuiper meaning that they have completely decoupled Belt Objects but they constitute an unlikely from the Kuiper Belt. Extrapolated down to source of comets in the absence of an agent to kilometer radius scales, the number of Centaurs make these bodies Neptune-crossing. (This is is of order 10 million, while the number that are not to say that no such agent exists: it is possible larger than 50 km in radius is of order 100 that massive scatterers in the outer Belt, even (Sheppard et al. 2000). The number of Centaurs undetected planets, could inject bodies to reflects the relatively short lifetime of these Neptune-crossing orbits at a rate sufficient to bodies to ejection by the gas giant planets, supply the Jupiter-Family comets). variously estimated as 1 to 100 Myr (Dones et al. 1995). Some Centaurs are removed by striking The majority of the known KBOs belong to the the planets or, more rarely, their satellites. third, ‘Classical’, group (population ~ 40,000 About half are ejected from the solar system, larger than 100 km diameter), with nearly principally by Jupiter. The remainder are circular orbits of small inclination (i ~ 0.1 rad) injected into orbits that cross the paths of the and semimajor axes 42 ≤ a ≤ 47 AU. These terrestrial planets. Those that develop comae due Neptune-avoiding orbits are stable on long to thermally induced outgassing are relabelled timescales, so that the Classicals are not a likely "comets". source of comets. 3 CENTAURS 4 COMETS AND DEAD COMETS Objects scattered out of the Kuiper Belt and Comets whose motion is strongly controlled by whose dynamics are controlled by strong Jupiter are known as Jupiter Family Comets (the scattering by the major planets are known as formal definition, which for our purposes is little Centaurs. Unfortunately, there is no universal more than a distraction, is that comets with accepted definition of the Centaurs, with Tisserand Invariants 2 ≤ T < 3 are Jupiter Family confusion arising on two levels. First, Jewitt and Comets). About 200 JFCs are known and Kalas (1998) define objects with q => 5 AU and lifetime considerations suggest that they are a <= 30 AU (corresponding to the orbits of supplied to the inner solar system at a rate of Jupiter and Neptune, respectively) as Centaurs. order 1 per 1000 yrs (from the Kuiper Belt). The Such objects, unless stabilised in resonances like sentiment is often expressed in the literature and the 1:1 Jovian Trojans, are guaranteed to be at meetings that the population of JFCs is short-lived to gas giant planet encounters. A observationally well established. My view, given more liberal definition that is sometimes used is that cometary activity may cycle on and off as that a Centaur is any object with 5 <= q <= 30 surface mantles grow and are disrupted, is that AU, regardless of the semimajor axis. This such optimism is unfounded and that future, deep all-sky surveys like Pan-Starrs (Tholen where κ [m2 s-1] is the thermal diffusivity of the ACM2002) are needed to reliably assess the material, given by κ = k/(ρ cp), where k, ρ and cp population. are the conductivity, density and specific heat capacity of the bulk material of the nucleus. Dead (or dormant) comets are observationally With nominal values k = 0.1 W m-1 K-1, ρ = 1000 -3 -1 -1 difficult to identify, since they look just like kg m and cp = 1000 J kg K , and expressing asteroids.
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