Physical Properties of Comets

Physical Prop erties of Comets K. J. Meech Institute for Astronomy, University of Hawai`i, 2680 Wood lawn Drive, Honolulu, HI 96822, USA ABSTRACT There have b een several recent reviews of the physical prop erties of cometary nuclei, most concentrating on the sp eci cs of the rotation p erio ds, shap es, sizes and the surface prop erties. This review will presentan up dated summary of these prop erties based on recent observations. There are nucleus parameters known for 26 nuclei. These comet nuclei are relatively small prolate ellipsoids of low alb edo. The axis ratios range between 1.1{2.6 which when combined with rotation p erio ds constrains the densitylower limits. Typically the nuclei are found to havevery small fractional active areas. Little direct observational evidence exists for the internal physical prop erties, however, the results to date suggest a strengthless agglomeration of 3 gravitationally b ound planetesimals with a bulk densitybetween 0.5-1.0 gm cm . INTRODUCTION With recent advances in theoretical mo dels of the early solar system it is b ecoming increasingly imp ortant to have a go o d understanding of the physical prop erties of cometary nuclei, so that they may b e used to constrain mo dels of solar nebula evolution. Early mo dels of the comet formation environment suggested that most comets formed in the Uranus-Neptune zone at temp eratures b etween 25{60K. Our current understanding of the early solar system suggests that the proto{planetary disks are much more massive than previously b elieved. Lissauer 1987 shows that the accumulation of Jupiter's core required a solar nebula surface density many times larger than the minimum mass solar nebula. Furthermore, observations show that 25-50 of pre{main sequence stars have massive circumstellar disks which extend to b eyond 100 AU from the central star with masses up to 1 M Beckwith and Sargent, 1993. In these more massive mo dels, the innermost region for comet formation o ccurs b etween the helio centric distances r = 14{16 AUi.e. near 60K as determined by the volatile comp osition of comets, and the most distant region b etween 80{110 AU Yamamoto, 1985. As summarized byWeissman 1995, the current comet formation paradigm is that the comets which originated in the Uranus{Neptune zone were dynamically ejected out to the Oort cloud, while comets forming further out in the region now known as the Kuip er Belt KB, remained in{situ. As shown by Levison and Duncan 1993 and Holman and Wisdom 1993, the KB comets are the most predominant source of the observable short{p erio d comets. While there is nowa much b etter understanding of the general comet formation environment, there is presently no self{consistent scenario leading from the coagulation of the m{sized interstellar dust grains to the km{scale planetesimals. Lunine et al. 1991 have argued that with the thicker disks it is probable that sho ck heating during infall to the disk mid{plane will cause some sublimation of the primordial ices the extent to which this o ccurs will b e a function of radial distance from the protostar. Furthermore, gas turbulence in the nebula will not allow planetesimal formation via simple Van der Waals sticking and gravitational collapse Weidenschilli ng, 1988. Weidenschilli ng and Cuzzi 1993 have shown that turbulent mo dels for m-sized to m{sized planetesimal growth may b e develop ed that pre- dict p ossible radial and vertical mixing in the nebula. These e ects might b e observable as comp ositional di erences in meteorites or comets. However, the stage of planetesimal formation from m{sized to km{sized is not understo o d at all, and is sensitive to the disk turbulence hence disk mass and convection within the disk. The still p o orly understo o d km{scale planetesimal stage of evolution is accessible through observations of to day's comets, and by assessing their physical prop erties as a function of formation lo cation, we can place some invaluable constraints on the solar nebula mo dels. The rate of the protoplanetary growth 1 as a function of r dep ended on the size and mass distribution of the km-sized planetesimals whichhave survived as to day's comets, their surface density in the nebula and their velo city distributions Lissauer and Stewart, 1993. Unfortunately, as observations and detectors are b ecoming more sophisticated, we are nding that comets exhibit activity and dust comae out to much larger distances than previously b elieved Meech, 1994, so that probably most historical comet observations do not p ertain to the nuclei. In fact, until recently,we probably had very little direct knowledge of the nucleus prop erties. Techniques for Measurement of Nucleus Prop erties Not only are the global physical prop erties imp ortant for the understanding of the early solar system accre- tion mo dels, but the nucleus size, alb edo and rotational p erio ds are critical parameters which a ect the solar energy distribution on the surface which will dictate the nature of a comet's activity.Even more dicult to observe, but more imp ortant to the understanding of cometary activity and ultimately to the origins of the comets, are the internal physical prop erties, including the volatile comp osition, dust{to{gas mass ratios, and the thermo{physical prop erties of the nucleus: thermal conductivity, p ore size, p orosity and nucleus bulk density. The evolution of activity in the nucleus is closely tied to the rate at which heat p enetrates into the interior, as dictated by the thermal parameters. The p orosity, tensile strength and nucleus density play critical roles in outbursts, splitting and tidal disruption, as well as in the observed non{gravitational motions. However, as will b e shown b elow, information ab out the internal prop erties of nuclei is much less well{constrained than for the external prop erties. In this review, the discussion will b e restricted to the external physical prop erties and the thermo{physical prop erties. Nucleus Size Distributions Nucleus sizes were rst estimated from photographic measurements at large r when the nucleus was b elieved to b e inactive Ro emer, 1966. By making assumptions ab out the visual alb edo, p , the nucleus radius could v b e estimated from the observed magnitude, m: 2 22 2 2 0:4m m p R =2:24 10 r 10 [1] v N where R is in [m], r andin[AU] and m is the sun's magnitude. The disadvantage of this technique N was that the photographic plates are not very sensitive to the extremely low surface{brightness comae which might b e present. Often the nucleus radii so determined were upp er limits Sekanina, 1976. Additionally, the unknown nucleus alb edo created a range of size estimates. The rst application of the technique using mo dern linear CCD detectors was with the recovery of comet P/Halley Jewitt & Danielson, 1984. This technique was subsequently used extensively by Jewitt and Meech 1985, 1987, 1988a. This is the most reliable easily applied means of remote determination of R when the nucleus is inactive. Nevertheless, it is N dep endent up on the unknown nucleus alb edo. The sensitivity of CCD detectors is required to measure the scattered radiation of these relatively small low{alb edo ob jects when they are far from the sun. Radiometry, a direct metho d for determining the Bond alb edo of an ob ject was rst applied to asteroids in the 1970s Allen, 1970, and a second technique utilizing thermal ux measurements and optical photometry of active comets was used to determine the alb edo of the dust grains O'Dell, 1971. The rst technique allows both the alb edo and diameter to b e determined from a simultaneous IR and optical detection if a comet is large enough and suciently close to the sun that it can b e detected in the IR, but b efore the observations are contaminated by dust. These techniques have b een applied to only a few nuclei. A summary of alb edos of coma material Meech, 1987, shows that the alb edo of the dust is generally low 0.03 <p <0.14. v Finally, Lamy and Toth 1995 have recently develop ed an indirect technique to mo del and remove the coma contribution from high{resolution HST images of active comets in order to infer the nucleus size. In total, wehave reasonable physical size estimates for 2 dozen comet nuclei cf. Table 1. The nuclei are quite small, and in fact, estimates of sizes have continued to decrease as b etter techniques emerge, and it is likely that we are seeing cometary activity at larger distances than previously b elieved Meech, 1994. This forces the observations to b e made at large r where they are more dicult. This evolution of nucleus size estimates, which is leading to a decrease in the inferred sizes is shown in Fig. 1. The top panel plots the directly measured radii from Table 1. Recently, large telescop e time has b een dedicated to obtaining upp er limits on the radii of p erio dic comets with well{known orbits Hainaut et al., 1994. This technique uses a non{detection or a measurementofnucleus plus coma as an upp er limit to the nucleus ux. The distribution for these directly measured upp er limits is shown in Fig. 1b. While the distribution is wider than 2 Fig.

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