Planetary and Space Science 73 (2012) 98–118 Contents lists available at SciVerse ScienceDirect Planetary and Space Science journal homepage: www.elsevier.com/locate/pss Density of asteroids B. Carry n European Space Astronomy Centre, ESA, P.O. Box 78, 28691 Villanueva de la Can˜ada, Madrid, Spain article info abstract Article history: The small bodies of our solar system are the remnants of the early stages of planetary formation. A Received 23 August 2011 considerable amount of information regarding the processes that occurred during the accretion of the Received in revised form early planetesimals is still present among this population. A review of our current knowledge of the 1 March 2012 density of small bodies is presented here. Density is indeed a fundamental property for the under- Accepted 6 March 2012 standing of their composition and internal structure. Intrinsic physical properties of small bodies are Available online 3 April 2012 sought by searching for relationships between the dynamical and taxonomic classes, size, and density. Keywords: Mass and volume estimates for 287 small bodies (asteroids, comets, and transneptunian objects) are Minor planets collected from the literature. The accuracy and biases affecting the methods used to estimate these Mass quantities are discussed and best-estimates are strictly selected. Bulk densities are subsequently Volume computed and compared with meteorite density, allowing to estimate the macroporosity (i.e., amount Density Porosity of voids) within these bodies. Dwarf-planets apparently have no macroporosity, while smaller bodies (o400 km) can have large voids. This trend is apparently correlated with size: C- and S-complex asteroids tend to have larger density with increasing diameter. The average density of each Bus-DeMeo taxonomic classes is computed (DeMeo et al., 2009; Icarus 202). S-complex asteroids are more dense on average than those in the C-complex that in turn have a larger macroporosity, although both complexes partly overlap. Within the C-complex asteroids, B-types stand out in albedo, reflectance spectra, and density, indicating a unique composition and structure. Asteroids in the X-complex span a wide range of densities, suggesting that many compositions are included in the complex. Comets and TNOs have high macroporosity and low density, supporting the current models of internal structures made of icy aggregates. Although the number of density estimates sky-rocketed during last decade from a handful to 287, only a third of the estimates are more precise than 20%. Several lines of investigation to refine this statistic are contemplated, including observations of multiple systems, 3-D shape modeling, and orbital analysis from Gaia astrometry. & 2012 Elsevier Ltd. All rights reserved. 1. Small bodies as remnants of planetesimals A detailed study of the composition of small bodies can be achieved in the laboratory, by analyzing their terrestrial counter- The small bodies of our solar System are the left-overs of the parts: meteorites. The distribution of elements, isotopes in building blocks that accreted to form the planets, some 4.6 Gyr meteorites, together with the level of heating and aqueous ago. They represent the most direct witnesses of the conditions alteration they experienced tell us about the temperature, ele- that reigned in the proto-planetary nebula (Bottke et al., 2002a). mental abundance, and timescales during the accretion stages Indeed, terrestrial planets have thermally evolved and in some (e.g., Halliday and Kleine, 2006). The connection of this informa- cases suffered erosion (e.g., plate tectonic, volcanism) erasing tion with specific locations in the Solar System constrains the evidence of their primitive composition. For most small bodies, formation scenarios of our Solar System. This requires the however, their small diameter limited the amount of radiogenic identification of links between the meteorites and the different nuclides in their interior, and thus the amount of energy for populations of small bodies. internal heating. The evolution of small bodies is therefore mainly Indeed, if meteorites are samples from the Solar System, exogenous, through eons of collisions, external heating, and several questions are raised. Is this sampling complete? Is this bombardment by high energy particles. sampling homogeneous? Some of the identified asteroid types (see Section 2) lack of a terrestrial analog. The most flagrant examples are the O-type asteroids (3628) Bozˇnemcovaˇ ´ and (7472) Kumakiri that appear unlike any measured meteorite assemblage n Tel.: þ34 91 81 31 233. (Burbine et al., 2011). Coupled mineralogical and dynamical E-mail address: [email protected] studies have shown that meteorites come from specific locations. 0032-0633/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pss.2012.03.009 B. Carry / Planetary and Space Science 73 (2012) 98–118 99 Other regions of the Solar System may therefore be unrepresented meteorite classes). This has made the compositional study of in our meteorite collection (see the discussions in, Burbine et al., these so-called primitive asteroids a primary goal in planetary 2002; Bottke et al., 2002b; Vernazza et al., 2008, for instance). science. Additionally, the current orbits of small bodies may be The VNIR spectra of asteroids in the X-complex are devoid of different from the place they originally formed. For instance, it strong absorption bands. However, several weak features (e.g., has been suggested that the giant planets migrated to their around 0:9 mm) have been identified and used to discriminate sub- current orbits (the Nice model, see, Tsiganis et al., 2005), injecting classes (Clark et al., 2004; Ockert-Bell et al., 2010; Fornasier et al., material from the Kuiper Belt into the inner Solar System (Levison 2011). Proposed meteorite analogs for X, Xc, Xe, and Xk asteroids et al., 2009). Similarly, gravitational interaction among planetary virtually cover the entire meteorite collection: the anhydrous embryos may have caused outward migration of planetesimals CV/CO carbonaceous chondrites (Barucci et al., 2005, 2012), from Earth’s vicinity into the main belt (Bottke et al., 2006). enstatite chondrites and aubrites (Vernazza et al., 2009b, 2011b; Current distribution of small bodies may therefore not reflect the Ockert-Bell et al., 2010), mesosiderites (Vernazza et al., 2009b), original distribution of material in the Solar System. It however stony-iron (Ockert-Bell et al., 2010), and iron meteorites (Fornasier tells us about the dynamical processes that occurred over history. et al., 2011). The mineralogy represented in the X-complex is Analysis of the composition of meteorites in the laboratory, of therefore probably more diverse than in the S- and C-complexes, small bodies from remote-sensing, and of their distribution in the due to the limits of the taxonomy based on spectral features only. Solar System are therefore pre-requisites to understanding the It is worth noting that in former taxonomies (e.g., Tholen and formation and evolution of our Solar System. Barucci, 1989), the X-complex was divided into three main groups, E, M, and P, distinguished by albedo. L-types have been suggested to be the most ancient asteroids 2. Linking small bodies with meteorites that currently exist. From the comparison of their VNIR spectra with laboratory material, a fraction of 30 710% of calcium- and Most of our knowledge on the mineralogy of asteroids has aluminum-rich inclusions was proposed (Sunshine et al., 2008). been derived by the analysis of their reflectance spectra in the This value is significantly higher than that of meteorites. This visible and near-infrared (VNIR). The shape of these spectra has suggests a very early accretion together with a low degree of been used to classify the asteroids into broad groups, following alteration while crossing the entire history of the Solar System. several classification schemes called taxonomies. In what follows, With a similar spectral shape, K-types have often be described as I refer to the taxonomy by DeMeo et al. (2009), based on the intermediates between S- and C-like material (DeMeo et al., largest wavelength range (0:422:4 mm). It encloses 15 classes 2009). Most of the K-type are associated with the Eos dynamical grouped into three complexes (C, S, and X), with nine additional family in the outer Main Belt. They have been tentatively linked classes called end-members (see, DeMeo et al., 2009, for a detailed with the anhydrous CO, CV, and CK, and hydrated but metal- description of the classes). Mineralogical interpretations and links rich CR carbonaceous chondrites meteorites (Bell et al., 1989; with meteorites have been proposed for several classes. Doressoundiram et al., 1998; Clark et al., 2009). Asteroids belonging to the S-complex (S, Sa, Sq, Sr, and Sv) and The mineralogy of the remaining end-members classes is more to the Q class have been successfully linked to the most common uncertain, owing to the apparent absence of strong spectral meteorites, the ordinary chondrites (OCs). This link had been features (D and T) or to the mismatch of features with any known suggested for years based on the presence of two deep absorption material (O and R). It has been suggested that T-types contain a bands in their spectra, around 1 and 2 mm, similar to that of OCs high fraction of metallic contents, and may be related to the iron and characteristic of a mixture of olivines and pyroxenes (see for cores of differentiated asteroids, hence iron meteorites (Britt instance, Chapman, 1996; Brunetto et al., 2006, among many et al., 1992). D-types are among the reddest objects in the Solar others). The analysis of the sample from the S-type asteroid System, not unlike that of comet nuclei and some transneptunian Itokawa returned by the Hayabusa spacecraft confirmed this link objects (Barucci et al., 2008). Their emission spectra in the mid- (Yurimoto et al., 2011).