The relationship of active comets, “extinct” comets,and dark asteroids

Spectrophotometric data on groups of asteroids in different types of orbits reveal different distributions of spectral properties, depending on whether the orbits are cometary or noncometary. In a list of 10 asteroids frequently suggested on purely dynamical grounds to be extinct or dormant comets, all have properties suggestive of spectral classes D, P, or C. Preliminary IRAS albedo results support this. Objects in these classes are very dark, reddish-black to neutral-black, and prevalent among the Trojans and outer belt. Two comets observed at low activity (visible nuclei) also have properties more consistent with D asteroids than any other class (very low reported geometric albedos of 0.02 and red colors). Consistent with these results are very low albedos reported for materials in more than a dozen comets; they average 0.05. Also, sampled cometary dust particles appear to consist of dark carbonaceous materials. Dramatically different are a control group of 13 Aten/Apollo/Amor objects selected from noncometary orbits. Most are in moderate-albedo classes: 8 or 9 appear to be of class S, and only 1 is in a low-albedo class (C). These are probably mostly objects perturbed out of the inner asteroid belt. The preponderence of S's in the noncometary group, together with the preponderence of ordinary chondrites among meteorites, may be evidence that such meteorites came from S asteroids. The data indicate that extinct, dormant, inactive, and minimally active comet nuclei have low albedos (p_v=a few percent) and very red to moderately red colors. As a group, their spectra are more similar to those of outer Solar System asteroids of classes D, P, and C, than to those of inner belt classes, though the observations are frequently not yet complete enough to assign definitively a spectral class. The results, taken together, support the view that dynamically identified “extinct comet candidates” are indeed outer Solar System objects probably of cometary origin. The results also support a scenario of Solar System formation in which dark carbonaceous dust dominated the spectrophotometric properties of planetesimals formed from about 2.7 AU out to at least the Trojan region at 5.2 AU. From 2.7 to at least 5.2 AU, and from class C to class D, the color of this dust reddens, apparently due to increasing amounts of red organic condensates. Comets are probably also colored to different degrees, by dust of this type, and may in some cases be even redder than D asteroids.     

The colors of cometary nuclei—Comparison with other primitive bodies of the Solar System and implications for their origin

We present new color results of cometary nuclei obtained with the Hubble Space Telescope (HST) whose superior resolution enables us to accurately isolate the nucleus signals from the surrounding comae. By combining with scrutinized available data obtained with ground-based telescopes, we accumulated a sample of 51 cometary nuclei, 44 ecliptic comets (ECs) and 7 nearly-isotropic comets (NICs) using the nomenclature of Levison [Levison, H.F., 1996. In: Rettig, T.W., Hahn, J.M. (Eds.), Completing the Inventory of the Solar System. In: ASP Conf. Ser., vol. 107, pp. 173-192]. We analyze color distributions and color-color correlations as well as correlations with other physical parameters. We present our compilation of colors of 232 outer Solar System objects—separately considering the different dynamical populations, classical KBOs in low and high-inclination orbits (respectively CKBO-LI and CKBO-HI), resonant KBOs (practically Plutinos), scattered-disk objects (SDOs) and Centaurs—of 12 candidate dead comets, and of 85 Trojans. We perform a systematic analysis of all color distributions, and conclude by synthesizing the implications of the dynamical evolution and of the colors for the origin of the minor bodies of the Solar System. We find that the color distributions are remarkably consistent with the scenarios of the formation of TNOs by Gomes [Gomes, R.S., 2003. Icarus 161, 404-418] generalized by the “Nice” model [Levison, H.F., Morbidelli, A., VanLaerhoven, Ch., Gomes, R., Tsiganis, L., 2008. Icarus 196, 258-273], and of the Trojans by Morbidelli et al. [Morbidelli, A., Levison, H.F., Tsiganis, K., Gomes, R., 2005. Nature 435, 462-465]. The color distributions of the Centaurs are globally similar to those of the CKBO-HI, the Plutinos and the SDOs. However the potential bimodality of their distributions allows to possibly distinguish two groups based on their (B−R) index: Centaur I with (B−R)>1.7 and Centaurs II with (B−R)<1.4. Centaurs I could be composed of TNOs (prominently CKBO-LI) and ultra red objects from a yet unstudied family. Centaurs II could consist in a population of evolved objects which have already visited the inner Solar System, and which has been scattered back beyond Jupiter. The diversity of colors of the ECs, in particular the existence of very red objects, is consistent with an origin in the Kuiper belt. Candidate dead comets represent an ultimate state of evolution as they appear more evolved than the Trojans and Centaurs II.     

Radiometry of near-earth asteroids

We report 10 micrometers infrared photometry for 22 Aten, Apollo, and Amor asteroids. Thermal models are used to derive the corresponding radiometric albedos and diameters. Several of these asteroids appear to have surfaces of relatively high thermal inertia due to the exposure of bare rock or a coarse regolith. The Apollo asteroid 3103, 1982 BB, is recognized as class E. The Jupiter-crossing Amor asteroid 3552, 1983 SA, is confirmed as class D, but low albedos remain rare for near-Earth asteroids.     

Asteroids with Satellites: Inventory, Properties, and Prospects for Future Discoveries

The population of binary asteroids numbers over 160 systems, and they can be found amongst near-Earth asteroids (NEAs), Main-Belt asteroids (MBAs), Jupiter Trojans, Centaurs and trans-Neptunian objects (TNOs). The discoveries have been made with space missions, radar observations, photometric lightcurves, and high resolution imaging from the ground and space. The properties of each population are widely different due to varying formation mechanisms and discovery techniques for each group. Future large-aperture telescopes will be capable of imaging both components for nearly all known systems and will drastically improve prospects for discovery of smaller and more tightly bound systems throughout the Solar System. The study of binary asteroids has provided valuable estimates on asteroid density and structure, a better understanding of the radiative YORP-effect, insights on catastrophic collisions, and may prove to be a key diagnostic for understanding the formation and evolution of the Kuiper Belt population.     

Mineralogical-petrological characterization of near-Earth asteroids

Studies of near-Earth asteroids are aimed at determining their dynamical and structural history. The mineralogy and petrology of 17 near-Earth asteroids are characterized using reflectance spectroscopy with ground-based telescopes as one method to address their major issues. Implications for the origin and evolution are discussed in a separate paper. Assuming the surfaces are composed of cosmically abundant materials, the presence of certain mineralogical species can be determined from diagnostic absorption features and spectral characteristics which have been studied under known laboratory conditions and understood in terms of crystal field theory. With one possible exception, the surface composition of near-Earth asteroids consists of common rock-forming minerals such as olivine, pyroxene, and phyllosilicates. Opaque components are present but cannot be mineralogically identified with existing experimental data. The spectrum of 2201 Oljato cannot be interpreted in terms of common rock-forming minerals. This spectrum was examined for cometary features because its high orbital eccentricity suggests a possible relation to comets. No common cometary features are identified in its spectrum. The predominance of mafic silicate absorption features in spectra of near-Earth asteroids compared to the majority of main-belt asteroids may be a primary compositional feature or may be the signature of relatively fresher asteroid material.     

Radar detection of near-Earth Asteroids 1915 Quetzalcoatl, 3199 Nefertiti, 3757 (1982 XB), and 4034 (1986 PA)

We describe Arecibo (2380 MHz, 12.6 cm) Doppler-only radar detections of near-Earth Asteroids 1915 Quetzalcoatl, 3199 Nefertiti, 3757 (1982 XB), and 4034 (1986 PA) obtained between 1981 and 1989. Estimates of the echo spectral bandwidths, radar cross-sections, and circular polarization ratios of these objects constrain their sizes, radar albedos, surface roughnesses, taxonomic classes, rotation periods, and pole directions. Our radar constraints on the diameters of Quetzalcoatl and Nefertiti are most consistent with sizes determined using thermal-radiometry and the Fast Rotation Model (FRM); this consistency may indicate that these asteroids have surfaces of high thermal inertia (i.e., little or no regolith). Constraints on Quetzalcoatl's radar albedo rule out a “metallic M” classification. The radar constraints for Nefertiti are inconsistent with a rotation pole published by Kaasalainen et al. (2004, Icarus 167, 178). Our estimates of 1982 XB's size are consistent with previously published estimates. The radar bandwidth of 1986 PA places an upper bound of about 24 h on its rotation period.     

On the Light-Absorbing Surface Layer of Cometary Nuclei: I. Radiative Transfer

A small but increasing volume of observations of cometary nuclei has accumulated during the past two decades. This development is accelerating with upcoming space missions such as Stardust, Contour, and Rosetta. In response to the growing need for a theoretical understanding of optical properties of cometary nuclei, we have calculated synthetic reflectance spectra in the wavelength region 0.2-2.0 μm, photometric colors in the Johnson-Kron-Cousins UBVRI system, and visual geometric albedos for a large number of porous ice-dust mixtures with differing composition, regolith grain sizes, and grain morphologies, such as core-mantle grains, dense clusters of such grains, and large irregular particles with internal scatterers. The calculations are based on Mie theory, the discrete dipole approximation, Hapke theory, and a numerical solution to the equation of radiative transfer in particulate media. In addition, wavelength-integrated directional-hemispherical albedos and flux attenuation profiles in the regolith as functions of depth have been calculated in order to improve the energy budget and treatment of energy boundary conditions in thermal models of cometary nuclei.Our results are compared with spectra and colors of observed cometary nuclei. Our main conclusions are that only regolith consisting of relatively large core-mantle grains, or clusters of smaller core-mantle grains, is capable of reproducing the red colors seen in comets; that ice-dust mixtures actually can be darker than ice-free regolith in certain circumstances; and that solar radiation sometimes penetrates to a depth that is comparable to the region in which diurnal temperature variations occur.     

A survey of Karin cluster asteroids with the Spitzer Space Telescope

The Karin cluster is one of the youngest known families of main-belt asteroids, dating back to a collisional event only 5.8±0.2 Myr ago. Using the Spitzer Space Telescope we have photometrically sampled the thermal continua (3.5-22 μm) of 17 Karin cluster asteroids of different sizes, down to the smallest members discovered so far, in order to make the first direct measurements of their sizes and albedos and study the physical properties of their surfaces. Our targets are also amongst the smallest main-belt asteroids observed to date in the mid-infrared. The derived diameters range from 17.3 km for 832 Karin to 1.5 km for 75176, with typical uncertainties of 10%. The mean albedo is pv=0.215±0.015, compared to 0.20±0.07 for 832 Karin itself (for H=11.2±0.3), consistent with the view that the Karin asteroids are closely related physically as well as dynamically. The albedo distribution (0.12?pv?0.32) is consistent with the range associated with S-type asteroids but the variation from one object to another appears to be significant. Contrary to the case for near-Earth asteroids, our data show no evidence of an albedo dependence on size. However, the mean albedo is lower than expected for young, fresh “S-type” surfaces, suggesting that space weathering can darken main-belt asteroid surfaces on very short timescales. Our data are also suggestive of a connection between surface roughness and albedo, which may reflect rejuvenation of weathered surfaces by impact gardening. While the available data allow only estimates of lower limits for thermal inertia, we find no evidence for the relatively high values of thermal inertia reported for some similarly sized near-Earth asteroids. Our results constitute the first observational confirmation of the legitimacy of assumptions made in recent modeling of the formation of the Karin cluster via a single catastrophic collision 5.8±0.2 Myr ago.     

High albedos of low inclination Classical Kuiper belt objects

We present observations of thermal emission from fifteen transneptunian objects (TNOs) made using the Spitzer Space Telescope. Thirteen of the targets are members of the Classical population: six dynamically hot Classicals, five dynamically cold Classicals, and two dynamically cold inner Classical Kuiper belt objects (KBOs). We fit our observations using thermal models to determine the sizes and albedos of our targets finding that the cold Classical KBOs have distinctly higher visual albedos than the hot Classicals and other TNO dynamical classes. The cold Classicals are known to be distinct from other TNOs in terms of their color distribution, size distribution, and binarity fraction. The Classical objects in our sample all have red colors yet they show a diversity of albedos which suggests that there is not a simple relationship between albedo and color. As a consequence of high albedos, the mass estimate of the cold Classical Kuiper belt is reduced from approximately 0.01 M_⊕ to approximately 0.001 M_⊕. Our results also increase significantly the sample of small Classical KBOs with known albedos and sizes from 21 to 32 such objects.     

The cool surfaces of binary near-Earth asteroids

Here we show results from thermal-infrared observations of km-sized binary near-Earth asteroids (NEAs). We combine previously published thermal properties for NEAs with newly derived values for three binary NEAs. The η value derived from the near-Earth asteroid thermal model (NEATM) for each object is then used to estimate an average thermal inertia for the population of binary NEAs and compared against similar estimates for the population of non-binaries. We find that these objects have, in general, surface temperatures cooler than the average values for non-binary NEAs as suggested by elevated η values. We discuss how this may be evidence of higher-than-average surface thermal inertia. This latter physical parameter is a sensitive indicator of the presence or absence of regolith: bodies covered with fine regolith, such as the Earth’s moon, have low thermal inertia, whereas a surface with little or no regolith displays high thermal inertia. Our results are suggestive of a binary formation mechanism capable of altering surface properties, possibly removing regolith: an obvious candidate is the YORP effect.We present also newly determined sizes and geometric visible albedos derived from thermal-infrared observations of three binary NEAs: (5381) Sekhmet, (153591) 2001 SN₂₆₃, and (164121) 2003 YT₁. The diameters of these asteroids are 1.41 ± 0.21 km, 1.56 ± 0.31 km, and 2.63 ± 0.40 km, respectively. Their albedos are 0.23 ± 0.13, 0.24 ± 0.16, and 0.048 ± 0.015, respectively.     

Keck observations of near-Earth asteroids in the thermal infrared

We present the results of thermal-infrared observations of 20 near-Earth asteroids (NEAs) obtained in the period March 2000-February 2002 with the 10-m Keck-I telescope on Mauna Kea, Hawaii. The measured fluxes have been fitted with thermal-model emission continua to determine sizes and albedos. This work increases the number of NEAs having measured albedos by 35%. The spread of albedos derived is very large (p_v=0.02−0.55); the mean value is 0.25, which is much higher than that of observed main-belt asteroids. In most cases the albedos are in the ranges expected for the spectral types, although some exceptions are evident. Our results are consistent with a trend of increasing albedo with decreasing size for S-type asteroids with diameters below 20 km. A number of objects are found to have unexpectedly low apparent color temperatures, which may reflect unusual thermal properties. However, the results from our limited sample suggest that high thermal-inertia, regolith-free objects may be uncommon, even amongst NEAs with diameters of less than 1 km. We discuss the significance of our results in the light of information on these NEAs taken from the literature and the uncertainties inherent in applying thermal models to near-Earth asteroids.     

Ion Irradiation of Asphaltite: Optical Effects and Implications for Trans-Neptunian Objects and Centaurs

Trans-Neptunian Objects (TNOs) and Centaurs show remarkable colour variationsin the visual and near-infrared spectral regions. Surface alteration processes such asspace weathering (e.g., bombardment with ions) and impact resurfacingmay play an important role in the colour diversity of such bodies. Ion irradiation ofhydrocarbon ices and their mixtures with water ice transforms neutral (grey) surfacecolours of ices to red and further to grey. Along with the ices, TNOs and Centaursprobably contain complex carbonaceous compounds, in particular, complexhydrocarbons. Unlike ices, such refractory organic materials have originally lowvisual albedos and red colours in the visible and near-infrared ranges. Here wepresent the first results of ion irradiation experiments on asphaltite. Asphaltite isa natural complex hydrocarbon material. The reflectance spectra of asphaltite inthe 0.4–0.8 μm range have been recorded before irradiation and after eachirradiation step. We demonstrate that irradiation of this red dark material with30 keV H⁺ and 15 keV N⁺ ions gradually transforms its colour from redto grey as a result of carbonization. A moderate increase in the visual albedo hasbeen observed. These results may imply that the surfaces of primitive red objectsoptically dominated by complex refractory organics may show a similar spaceweathering trend. Our laboratory results were compared with published coloursof TNOs and Centaurs. A broad variety of spectral colours observed for TNOs andCentaurs may be reproduced by various spectra of irradiated organics correspondingto different ion fluences. However, such objects probably also contain ices and silicatecomponents which show different space weathering trends. This fact, together with alack of information about albedos, may explain difficulties to reveal correlations between surface colours within TNO and Centaur populations and their other properties, such as absolute magnitudes and orbital parameters.     

The Meudon Multicolor Survey (2MS) of Centaurs and trans-neptunian objects: extended dataset and status on the correlations reported

We present here the latest B−V, V−R, and R−I color measurements obtained with the CFH12K mosaic camera of the 3.6-m Canada-France-Hawaii Telescope (CFHT). This work is the latest extension of the Meudon Multicolor Survey (2MS) and extends the total number of Centaurs and trans-neptunian objects (TNOs) in the dataset to 71. With this large and homogeneous dataset, we performed relevant statistical analyses to search for correlations with physical and orbital parameters and interrelations with related populations (cometary nuclei and irregular satellites). With a larger dataset, we confirm the correlations found for the Classical TNOs in our previous survey: some colors are significantly correlated with perihelion distance and inclination. The only exception is with the eccentricity. However, results strongly depend on which objects are considered Classicals, and with a dynamically more restricted definition these correlations are no longer present. We also find that strongly significant trends with orbital parameters are not detected for Centaurs, Plutinos or scattered disk objects (SDOs). We also make for the first time reliable statistical comparison between TNOs and related populations (e.g., Centaurs, irregular satellites, short period comets—i.e., SPCs). We find that (1) the colors of SPCs do not match either their TNO or Centaur precursors, and this suggests that some process modifies the surface of SPCs at entry into the inner Solar System. The only exception concerns colors of SDOs from which we could statistically assess that SPCs and SDOs could be drawn from a same single parent distribution. (2) Not surprisingly, Centaurs are compatible with each of the Edgeworth-Kuiper belt dynamical groups at a highly significant level except with the SDOs. (3) Centaurs' colors still present a strong dichotomy between a neutral/slightly red group (e.g., Chiron) and a very red group (e.g., Pholus). (4) The irregular satellite population is not compatible with any of the Centaur, Plutino or Classical populations; however, the similarity of their color properties with SDOs suggests that both groups can be extracted from the same parent distribution. However, due to the small number of Centaurs and SDOs these conclusions cannot be taken as definitive.     

Periodic variation of Oort Cloud flux and cometary impacts on the Earth and Jupiter

Time variation in impact probability is studied by assuming that the periodic flux of the Oort Cloud comets within 15 au arises from the motion of the Sun with respect to the Galactic mid-plane. The periodic flux clearly shows up in the impact rate of the captured Oort Cloud cometary population, with a phase shift caused by the orbital evolution. Depending on the assumed flux of comets and the size distribution of comets, the impact rate of the Oort Cloud comets of 1 km in diameter or greater is from 5 to 700 impacts Myr⁻¹ on the Earth and from 0.5 to 70 impacts per 1000 yr on Jupiter. The relative fractions of impacts are 0.09, 0.11, 0.26 and 0.54 for long-period comets, Halley type comets, Jupiter family comets and near-Earth objects, respectively. For Jupiter, the corresponding fractions in the first three categories are 0.18, 0.31 and 0.51. If we consider physical fading of comet activity that is compatible with the observations, then the impact rates of active comets are two orders of magnitude smaller than the total impact rates by all kinds of comets and cometary asteroids of size 1 km or greater.     

Note on the structure of comet nuclei

The recent developments in cometary studies suggest rather low mean densities and weak structures for the nuclei. They appear to be accumulations of fairly discrete units loosely bound together, as deduced from the observations of Comet Shoemaker–Levy 9 during its encounter with Jupiter. The compressive strengths deduced from comet splitting by Öpik and Sekanina are extremely low. These values are confirmed by theory developed here, assuming that Comet P/Holmes had a companion that collided with it in 1892. There follows a short discussion that suggests that the mean densities of comets should increase with comet dimensions. The place of origin of short-period comets may relate to these properties.     

Structure and density of cometary nuclei

Abstract— Understanding the nature of the cometary nucleus remains one of the major problems in solar system science. Whipple's (1950) icy conglomerate model has been very successful at explaining a range of cometary phenomena, including the source of cometary activity and the nongravitational orbital motion of the nuclei. However, the internal structure of the nuclei is still largely unknown. We review herein the evidence for cometary nuclei as fluffy aggregates or primordial rubble piles, as first proposed by Donn et al. (1985) and Weissman (1986). These models assume that cometary nuclei are weakly bonded aggregations of smaller, icy‐conglomerate planetesimals, possibly held together only by self‐gravity. Evidence for this model comes from studies of the accretion and subsequent evolution of material in the solar nebula, from observations of disrupted comets, and in particular comet Shoemaker‐Levy 9, from measurements of the ensemble rotational properties of observed cometary nuclei, and from recent spacecraft missions to comets. Although the evidence for rubble pile nuclei is growing, the eventual answer to this question will likely not come until we can place a spacecraft in orbit around a cometary nucleus and study it in detail over many months to years. ESA's Rosetta mission, now en route to comet 67P/Churyumov‐Gerasimenko, will provide that opportunity.     

Dynamics and Origin of Comets: New Problems Appeared after the <Emphasis Type="Italic">Rosetta</Emphasis> Space Mission

The data obtained in the recent Rosetta space mission to comet 67P/Churyumov–Gerasimenko have had a profound impact on the understanding of the nature of comets. In addition to revising the notions on the physical properties and structure of comets, this addresses dynamical aspects of the formation of the observed cometary populations (short- and long-period comets, Centaurs, trans-Neptunian objects, and Oort-cloud objects). In the review, we discuss new problems that have appeared in the theory of dynamical evolution and origin of comets due to the Rosetta mission.     

Comets in the near-Earth object population

Because the lifespan of near-Earth objects (NEOs) is shorter than the age of the Solar System, these objects originate elsewhere. Their most likely sources are the main asteroid belt and comets. Through physical observations we seek to identify potential dormant or extinct comets among “asteroids” catalogued as NEOs and thereby determine the fraction of “comet candidates” within the total NEO population. Both discovery statistics and dynamical models indicate that candidate cometary objects in near-Earth space are predominantly found among those having a jovian Tisserand parameter Tj<3. Therefore, we seek to identify comet candidates among asteroid-like NEOs using three criteria: Tj<3, spectral parameters (C, D, T, or P taxonomic types), and/or low (<0.075) albedos. We present new observations for 20 NEOs having Tj<3, consisting of visible spectra, near-infrared spectra, and/or albedo measurements obtained using the NASA Infrared Telescope Facility, the Kitt Peak National Observatory 4 m, and the Magellan Observatory 6.5-m. Four of our “asteroid” targets have been subsequently confirmed as low activity comets. Thus our sample includes spectra of the nuclei of Comets 2002 EX12 = 169P (NEAT), 2001 WF2 = 182P (LONEOS), 2003 WY25 = D/1891 W1 (Blanplain), and Halley Family Comet 2006 HR30 = P/2006 HR30 (Siding Spring). From the available literature, we tabulate physical properties for 55 NEOs having Tj<3, and after accounting for possible bias effects, we estimate that 54±10% of NEOs in Tj<3 orbits have “comet-like” spectra or albedos. Bias corrected discovery statistics [Stuart, J.S., Binzel, R.P., 2004. Icarus 170, 295-311] estimate 30±5% of the entire NEO population resides in orbits having Tj<3. Combining these two factors suggests that 16±5% of the total discovered “asteroid-like” NEO population has “comet-like” dynamical and physical properties. Outer main-belt asteroids typically have similar taxonomic and albedo properties as our “comet candidates.” Using the model of Bottke et al. [Bottke, W.F., Morbidelli, A., Jedicke, R., Petit, J.M., Levison, H., Michel, P., Metcalfe, T.S., 2002. Icarus 156, 399-433] to evaluate source region probabilities, we conclude that 8±5% of the total asteroid-like NEO population have the requisite orbital properties, physical properties, and dynamical likelihood to have originated as comets from the outer Solar System.     

The provenance and evolution of comets

The process of comet formation through the hierarchical aggregation of originally submicron-sized interstellar grains to form micron-sized particles and then larger bodies in the protoplanetary disc, culminating in the formation of planetesimals in the disc extending from Jupiter to beyond Neptune, is briefly reviewed. The ‘planetesimal’ theory for the origin of comets implies the existence of distinct cometary reservoirs, with implications for the immediate provenance of observed comets (both long-period and short-period) and their evolution as a result of planetary perturbations and physical decay, for example splitting and sublimation. The principal mode of cometary decay and collisional interaction with the terrestrial planets is through the formation and evolution of streams of cometary debris and hitherto undiscovered ‘families’ of cometary asteroids. Recent dynamical results, in particular the sungrazing and sun-colliding end-state for short-period comet and asteroid orbits, are briefly discussed.