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Observational Constraints on Surface Characteristics of Comet Nuclei

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Observational Constraints on Surface Characteristics of Comet Nuclei Observational Constraints on Surface Characteristics of Comet Nuclei Humberto Campins ([email protected] u) Lunar and Planetary Laboratory, University of Arizona Yanga Fernandez University of Hawai'i Abstract. Direct observations of the nuclear surfaces of comets have b een dicult; however a growing number of studies are overcoming observational challenges and yielding new information on cometary surfaces. In this review, we fo cus on recent determi- nations of the alb edos, re ectances, and thermal inertias of comet nuclei. There is not much diversity in the geometric alb edo of the comet nuclei observed so far (a range of 0.025 to 0.06). There is a greater diversity of alb edos among the Centaurs, and the sample of prop erly observed TNOs (2) is still to o small. Based on their alb edos and Tisserand invariants, Fernandez et al. (2001) estimate that ab out 5% of the near-Earth asteroids have a cometary origin, and place an upp er limit of 10%. The agreement between this estimate and two other indep endent metho ds provide the strongest constraint to date on the fraction of ob jects that comets contribute to the p opulation of near-Earth asteroids. There is a diversity of visible colors among comets, extinct comet candidates, Centaurs and TNOs. Comet nuclei are clearly not as red as the reddest Centaurs and TNOs. What Jewitt (2002) calls ultra-red matter seems to be absent from the surfaces of comet nuclei. Rotationally resolved observations of b oth colors and alb edos are needed to disentangle the e ects of rotational variability from other intrinsic qualities. New constraints on thermal inertia of comets are consistent with previous indep endent estimates. The thermal inertia estimates for Centaurs 2060 Chiron and 8405 Asb olus are signi cantly lower than predicted by thermal mo dels, and also lower than the few upp er limits or constraints known for active, ordinary nuclei. Keywords: Comets, Nuclei, Surfaces 1. Intro duction The nucleus is where cometary activity originates. However, direct ob- servations of the nuclear surfaces of comets have been dicult. This diculty is due to the gas and dust coma generally present when comets are close to the Sun, and due to the faintness of comet nuclei when at large helio centric distances. A growing number of studies are overcoming these observational challenges and yielding new information on cometary surfaces. In this review, we fo cus on recent determinations of the alb edos, re ectances, and thermal inertias of comet nuclei. We also compare these surface characteristics to those of related p opula- c 2002 Kluwer Academic Publishers. Printed in the Netherlands. finalversion.tex; 14/07/2002; 20:40; p.1 2 H. Campins and Y. Fernandez tions such as extinct comet candidates, Centaurs, near-Earth asteroids (NEAs), transneptunian ob jects (TNOs) and Tro jan asteroids. The size distribution of cometary nuclei is discussed elsewhere by Fernandez et al (1999). and by Weismann and Lowry (2001). Successful observations of comet surfaces have used observational techniques that fall into three categories, (a) observations of comets at large helio centric distances, (b) observations of comets near Earth and (c) imaging by visiting spacecraft. Observations of distant comets aim to identify the nucleus in the absence of a coma; however, many comets remain active at large distances. For example, photometry of comet Encke throughout its orbit reveals a p eculiar behavior, with an actual increase in Encke's intrinsic brightness near aphelion (e.g., Meech et al. 2001, Licandro et al. 2001, Sekanina 1991, Barker et al. 1981). Hence, it is often necessary to estimate and subtract a remnant coma, which is dicult to characterize due to the low spatial resolution. One of the main uncertainties asso ciated with observations of distant comet nuclei is the p ossibility that an unresolved coma can go undetected. Neverthe- less, recent studies have rep orted apparently successful observations of comet nuclei at visible (e.g., Jewitt 2002) as well as mid-infrared wave- lengths (Fernandez et al. 2002). These studies allow estimates of the nucler size, assuming an alb edo in the case of visible wavelengths only, and measuring the alb edo, in the case of simultaneous mid-infrared and visible detections. At smaller geo centric distances, the increased spatial resolution al- lows a better characterization and subtraction of a coma. This tech- nique was initially applied to low activity comets Neujmin 1, Arend- Rigaux and Temp el 2 (Campins et al. 1987, Millis et al. 1988, A'Hearn et al. 1989). More active comets with very close approaches to Earth have yielded useful but more limited information ab out their nuclei. For example, estimates of the sizes of comets IRAS-Araki-Alco ck, Sugano- Saigusa-Fujikawa and Hyakutake resulted from mid infrared observa- tions and radar observations near closest approach (Hanner et al. 1985, Hanner et al. 1987, Harmon et al. 1989, Harmon et al. 1997). The Hubble Space Telescop e has brought many more comets within the reach of the coma subtraction technique. The sizes, and approximate shap es of some 15 comet nuclei have b een estimated so far based on HST imaging, in a few cases the nuclear colors have also been extracted (Lamy et al. 1998, 1999, 2001). Finally, imaging of the resolved nuclei of comets Halley and Bor- relly have b een obtained by visiting spacecreaft. Results from the 1986 encounters with comet Halley have been summarized in a number of publications, including Huebner (1990) and references therein. On September 22, 2001, NASA's Deep Space 1 (DS1) spacecraft encoun- finalversion.tex; 14/07/2002; 20:40; p.2 Surface Characteristics of Comet Nuclei 3 tered comet Borrely. Initial results on Borrelly's nuclear characteristics have b een rep orted (e.g., Britt et al. 2001, Buratti et al. 2001) and additional details are exp ected as the science teams have more time to analyze the observations. Figure 1, is the DS1 image of comet Borrelly at closest approach. One of the most notable features of the surface of comet Borrelly is the low value of the alb edo (average geometric alb edo 2.2%, which is somewhat dep endent on the phase curve assumed; Bu- ratti et al. 2001) and its variability across the surface (at least a factor of two). When considering alb edo values obtained from ground based ob- servations, it is imp ortant to keep in mind the range of values observed in comet Borrelly. The ground based values are averaged over the side of the comet nucleus that faces Earth. Rep orts of rotational variability of color and sp ectral shap e in a comet and a Centaur, suggest that even with the coarse sampling achievable from Earth based telescop es, rotational variability of the alb edo of comet nuclei could be detectable photometrically (see section 3). 1.1. Related Populations Several p opulations of minor solar system bodies may be linked to comet nuclei. Jupiter-family comets, Centaurs and TNOs are b elieved to be closely related. More sp eci cally, the low inclination of Jupiter- family comet orbits led Fernandez (1980) to prop ose that these ob jects come from an ecliptic p opulation of icy ob jects in the transneptunian region. Since then, a number of authors have studied how gravita- tional interactions and collisions can bring TNOs (e.g., Duncan Quinn and Tremaine 1988, Levison and Duncan 1997) and their fragments (Farinella and Davis 1996) into orbits contained within those of the giant planets (which is our de nition of a Centaur orbit). Interactions with the giant planets can reduce the p erihelion distance of some of these ob jects to the point where they are observed as active comets. In addition, the Tro jan asteroids could be a source of some of the Jupiter- family comets (e.g., Marzari et al. 1995). In turn, extinct or dormant comets have been prop osed as one source of near-Earth asteroids (e.g., Bottke et al. 2002). Information ab out the surface comp osition of these related p opulations is also increasing and helping us understand the links between them. Oort cloud comets are b elieved to have formed near the giant planets 4 and were gravitationally scattered into orbits with aphelia in the 10 to 5 10 AU range (e.g., Stern and Weissman 2001 and references therein). As we discuss b elow, most observations of the nuclear surfaces made to date are of Jupiter-family comets. Detailed observations of Oort cloud finalversion.tex; 14/07/2002; 20:40; p.3 4 H. Campins and Y. Fernandez Figure 1. Comet Borrelly is the rst Jupiter-family comet to be imaged by a visiting spacecraft. This image was obtained 160 seconds b efore closest approach by NASA's Deep Space 1 spacecraft. The image resolution was approximately 48 meters per pixel. A variety of terrain and surface features are apparent. Smo oth rolling planes containing brighter regions are present near the middle of the image and seem to be the source of the dust jets observed in the coma. Darker and rougher terrain is also observed and may represent older surface material. Alb edo variations of at least a factor of two across surface have been identi ed (Britt et al. 2001, Buratti et al. 2001). Stereo analysis shows that the smaller end of the nucleus (lower left) is tipp ed toward the viewer (out of the frame). Sunlight is coming from the left of the frame (image and caption information courtesy of NASA and the Jet Propulsion Lab oratory). finalversion.tex; 14/07/2002; 20:40; p.4 Surface Characteristics of Comet Nuclei 5 comet nuclei are an imp ortant missing element necessary to test current views of the origin of comets.
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