Density of Asteroids

Density of asteroids B. Carry European Space Astronomy Centre, ESA, P.O. Box 78, 28691 Villanueva de la Cañada,Madrid, Spain Abstract The small bodies of our solar system are the remnants of the early stages of planetary formation. A considerable amount of information regarding the processes that occurred during the accretion of the early planetesimals is still present among this population. A review of our current knowledge of the density of small bodies is presented here. Density is indeed a fundamental property for the understanding of their composition and internal structure. Intrinsic physical properties of small bodies are sought by searching for relationships between the dynamical and taxonomic classes, size, and density. Mass and volume estimates for 287 small bodies (asteroids, comets, and transneptunian objects) are collected from the literature. The accuracy and biases affecting the methods used to estimate these quantities are discussed and best-estimates are strictly selected. Bulk densities are subsequently computed and compared with meteorite density, allowing to estimate the macroporosity (i.e., amount of voids) within these bodies. Dwarf-planets apparently have no macroporosity, while smaller bodies (<400 km) can have large voids. This trend is apparently correlated with size: C and S-complex asteroids tends 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. Keywords: Minor planets, Mass, Volume, Density, Porosity 1. Small bodies as remnants of planetesimals with specific locations in the Solar System constrains the formation scenarios of our Solar System. This requires the identi- The small bodies of our solar System are the left-overs of the fication of links between the meteorites and the different popu- building blocks that accreted to form the planets, some 4.6 Gyr lations of small bodies. ago. They represent the most direct witnesses of the conditions Indeed, if meteorites are samples from the Solar System, sev- that reigned in the proto-planetary nebula (Bottke et al. 2002a). eral questions are raised. Is this sampling complete? Is this Indeed, terrestrial planets have thermally evolved and in some sampling homogeneous? Some of the identified asteroid types cases suffered erosion (e.g., plate tectonic, volcanism) erasing (see Sect.2) lack of a terrestrial analog. The most flagrant ex- evidence of their primitive composition. For most small bod- ample are the O-type asteroids (3628) Boznˇ emcovˇ a´ and (7472) ies, however, their small diameter limited the amount of radio- Kumakiri that appear unlike any measured meteorite assem- genic nuclides in their interior, and thus the amount of energy blage (Burbine et al. 2011). Coupled mineralogical and dy- for internal heating. The evolution of small bodies is therefore namical studies have shown that meteorites come from specific mainly exogenous, through eons of collisions, external heating, locations. Other regions of the Solar System may therefore be arXiv:1203.4336v1 [astro-ph.EP] 20 Mar 2012 and bombardment by high energy particles. unrepresented in our meteorite collection (see the discussions in A detailed study of the composition of small bodies can be Burbine et al. 2002; Bottke et al. 2002b; Vernazza et al. 2008, achieved in the laboratory, by analyzing their terrestrial coun- for instance). terparts: meteorites. The distribution of elements, isotopes in Additionally, the current orbits of small bodies may be dif- meteorites, together with the level of heating and aqueous alter- ferent from the place they originally formed. For instance, it ation they experienced tell us about the temperature, elemen- has been suggested that the giant planets migrated to their cur- tal abundance, and timescales during the accretion stages (e.g., rent orbits (the Nice model, see Tsiganis et al. 2005), inject- Halliday and Kleine 2006). The connection of this information ing material from the Kuiper Belt into the inner Solar System (Levison et al. 2009). Similarly, gravitational interaction among planetary embryos may have caused outward migration of plan- Email address: [email protected] (B. Carry) Preprint submitted to PSS March 21, 2012 etesimals from Earth’s vicinity into the main belt (Bottke et al. 2006, for an overview of meteorite classes). This has made the 2006). Current distribution of small bodies may therefore not compositional study of these so-called primitive asteroids a pri- reflect the original distribution of material in the Solar System. mary goal in planetary science. It however tells us about the dynamical processes that occurred The VNIR spectra of asteroids in the X-complex are devoid over history. Analysis of the composition of meteorites in the of strong absorption bands. However, several weak features laboratory, of small bodies from remote-sensing, and of their (e.g., around 0.9 µm) have been identified and used to discrim- distribution in the Solar System are therefore pre-requisites to inate sub-classes (Clark et al. 2004; Ockert-Bell et al. 2010; understanding the formation and evolution of our Solar System. Fornasier et al. 2011). Proposed meteorite analogs for X, Xc, Xe, and Xk asteroids virtually cover the entire meteorite collec- 2. Linking small bodies with meteorites tion: the anhydrous CV/CO carbonaceous chondrites (Barucci et al. 2005, 2012), enstatite chondrites and aubrites (Vernazza Most of our knowledge on the mineralogy of asteroids has et al. 2009b, 2011b; Ockert-Bell et al. 2010), mesosiderites been derived by analysis of their reflectance spectra in the vis- (Vernazza et al. 2009b), stony-iron (Ockert-Bell et al. 2010), ible and near-infrared (VNIR). The shape of these spectra has and iron meteorites (Fornasier et al. 2011). The mineralogy been used to classify the asteroids into broad groups, follow- represented in the X-complex is therefore probably more di- ing several classification schemes called taxonomies. In what verse than in the S- and C-complexes, due to the limits of the follows, I refer to the taxonomy by DeMeo et al.(2009), based taxonomy based on spectral features only. In is worth noting on the largest wavelength range (0.4–2.4 µm). It encloses 15 that in former taxonomies (e.g., Tholen and Barucci 1989), the classes grouped into three complexes (C, S, and X), with 9 ad- X-complex was divided into three main groups, E, M, and P, ditional classes called end-members (see DeMeo et al. 2009, for distinguished by albedo. a detailed description of the classes). Mineralogical interpreta- L-types have been suggested to be the most ancient asteroids tions and links with meteorites have been proposed for several that currently exist. From the comparison of their VNIR spec- classes. tra with laboratory material, a fraction of 30 ± 10% of Calcium- Asteroids belonging to the S complex (S, Sa, Sq, Sr, and Sv) and aluminum-rich inclusions was proposed (Sunshine et al. and to the Q class have been successfully linked to the most 2008). This value is significantly higher than that of meteorites. common meteorites, the ordinary chondrites (OCs). This link This suggests a very early accretion together with a low de- had been suggested for years based on the presence of two deep gree of alteration while crossing the entire history of the So- absorption bands in their spectra, around 1 and 2 microns, sim- lar System. With a similar spectral shape, K-types have often ilar to that of OCs and characteristic of a mixture of olivines be described as intermediates between S- and C-like material and pyroxenes (see for instance Chapman 1996; Brunetto et al. (DeMeo et al. 2009). Most of the K-type are associated with 2006, among many others). The analysis of the sample from the Eos dynamical family in the outer Main Belt. They have the S-type asteroid Itokawa returned by the Hayabusa space- been tentatively linked with the anhydrous CO, CV, and CK, craft confirmed this link (Yurimoto et al. 2011). The two end- and hydrated but metal-rich CR carbonaceous chondrites mete- member classes A and V have a mineralogy related to the S- orites (Bell et al. 1989; Doressoundiram et al. 1998; Clark et al. complex. A-types are asteroids made of almost pure olivine, 2009). which possible analogs are the achondrite meteorites of the Bra- The mineralogy of the remaining end-members classes is chinite and Pallasite groups (see, e.g., Bell et al. 1989; de Leon´ more uncertain, owing to the apparent absence of strong spec- et al. 2004). In opposition, V-types are made of pure pyroxenes tral features (D and T) or to the mismatch of features with any and are related to the HED achondrite meteorites (e.g., McCord known material (O and R). It has been suggested that T-types et al. 1970). A- and V-types are believed to correspond to the contain a high fraction of metallic contents, and may be related mantle and the crust of differentiated parent bodies (Burbine to the iron cores of differentiated asteroids, hence iron mete- et al.

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