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Near-Earth Asteroid Surface Roughness Depends on Compositional Class ∗ Lance A.M

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ARTICLE IN PRESS YICAR:8714 JID:YICAR AID:8714 /FLA [m5G; v 1.65; Prn:8/10/2008; 15:52] P.1 (1-11) Icarus ••• (••••) •••–••• Contents lists available at ScienceDirect Icarus www.elsevier.com/locate/icarus Near-Earth asteroid surface roughness depends on compositional class ∗ Lance A.M. Benner a, ,StevenJ.Ostroa, Christopher Magri b, Michael C. Nolan c, Ellen S. Howell c, Jon D. Giorgini a, Raymond F. Jurgens a, Jean-Luc Margot d, Patrick A. Taylor d, Michael W. Busch e, Michael K. Shepard f a Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA b University of Maine at Farmington, 173 High Street, Preble Hall, Farmington, ME 04938, USA c Arecibo Observatory, HC3 Box 53995, Arecibo, PR 00612, USA d Department of Astronomy, Cornell University, Ithaca, NY 14853, USA e Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA f Department of Geography and Geosciences, Bloomsburg University, Bloomsburg, PA 17815, USA article info abstract Article history: Radar observations of 214 near-Earth asteroids (NEAs) reveal a very strong correlation of circular Received 21 February 2008 polarization ratio with visible–infrared taxonomic class, establishing distinct differences in the centimeter- Revised 18 June 2008 to-several-decimeter structural complexity of objects in different spectral classes. The correlation may be due to the intrinsic mechanical properties of different mineralogical assemblages but also may reflect very different formation ages and collisional histories. The highest ratios are measured for groups Keywords: Near-Earth objects associated with achondritic igneous rocky meteorites: the E class, whose parent body may be 3103 Asteroids Eger, and the V class, derived from the mainbelt asteroid (and Dawn mission target) 4 Vesta. Asteroids, surfaces © 2008 Elsevier Inc. All rights reserved. Asteroids, composition Radar observations 1. Introduction faces, which are the outcome of complex collisional histories and reflect the mechanical properties of objects’ mineral assemblages. Meteorites are samples of small, Earth-orbit-crossing asteroids, A radar echo’s circular polarization ratio measures the wave- which in turn are primarily derived from the main asteroid belt length-scale roughness of the target’s surface (Ostro et al., 2002; (Burbine et al., 2002; Lipschutz and Schultz, 2007). The mineralogy Ostro, 2007). In almost all radar observations, a circularly polar- and meteoritic analogues of asteroids are constrained by narrow- ized signal is transmitted and the echo is measured simultaneously band optical reflectance spectra (Tholen and Barucci, 1989; Bus et in the same sense of circular polarization as transmitted (the SC al., 2002) and to a lesser extent by classification schemes based on sense) and the opposite (OC) sense. The handedness of a circu- broadband optical color indices and albedos (Burbine et al., 2002; larly polarized signal is reversed upon reflection from a smooth Lipschutz and Schultz, 2007). Asteroid classification schemes now dielectric interface, leading to dominance of the echo by the OC include some 26 spectral classes (Bus et al., 2002), in most cases polarization. Multiple scattering, subsurface reflection, wavelength- tentatively associated with meteorite types (Burbine et al., 2002; scale facets, and structure with radii of curvature near the wave- Lipschutz and Schultz, 2007). A dependence of the abundance length can produce SC echoes. Therefore, the SC/OC ratio is a of different spectral classes on heliocentric distance (Gradie and measure of the near-surface, wavelength-scale structural complex- Tedesco, 1982) probably reflects the influence of temperature on ity or “roughness.” Since the sensitivities of the two polarization the identity of condensates from the primitive solar nebula. How- channels are easily calibrated via measurement of unpolarized ra- ever, there remains great uncertainty in the relationships between dio sources, the uncertainty in SC/OC estimates is limited to the meteorite types and asteroid classes. The rare exceptions involve propagation of statistical fluctuations in receiver noise. To place asteroids visited by rendezvous spacecraft: the ∼15-km-diameter the results below in context, Mercury, Venus, and the Moon have ∼ NEA 433 Eros (Veverka et al., 1997) and the several-hundred- SC/OC 0.1 and Mars has a ratio of about 0.3, although for each meter-long NEA 25143 Itokawa (Fujiwara et al., 2006)areboth object, there are local variations. mineralogically akin to ordinary chondrites. Similarly, there is only limited information about the physical characteristics of NEA sur- 2. Results Here we report 3.5- and 13-cm-wavelength radar observations * Corresponding author. Fax: +1 818 354 9476. of 214 near-Earth asteroids (Figs. 1, 2; Tables 1, 2, and 3) that re- E-mail address: [email protected] (L.A.M. Benner). veal a very strong correlation between circular polarization ratio 0019-1035/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2008.06.010 Please cite this article in press as: Benner, L.A.M., et al. Near-Earth asteroid surface roughness depends on compositional class. Icarus (2008), doi:10.1016/j.icarus.2008.06.010 ARTICLE IN PRESS YICAR:8714 JID:YICAR AID:8714 /FLA [m5G; v 1.65; Prn:8/10/2008; 15:52] P.2 (1-11) 2 L.A.M. Benner et al. / Icarus ••• (••••) •••–••• Table 1 Summary of asteroid SC/OC distributions by spectral class. Spectral Near-Earth Main-belt class SC/OC SC/OC Mean SD Range Number Mean SD Range Number C 0.285 0.120 0.40 17 0.098 0.056 0.22 25 E 0.892 0.079 0.23 6 0.67 0.226 0.32 2 F 0.15 – – 1 0.058 0.101 0.26 6 G 0.102 0.080 0.20 5 M 0.143 0.055 0.11 3 0.153 0.097 0.23 6 O0.21––1 PD 0.188 0.019 0.04 4 0.096 0.062 0.18 9 S 0.270 0.079 0.40 70 0.198 0.094 0.35 27 V 0.603 0.088 0.23 6 0.28 – – 1 X 0.674 0.436 0.90 5 ? 0.339 0.284 1.48 101 Notes. Main-belt asteroid results are from Magri et al. (2007a) except for the two E-class MBAs, which are from Shepard et al. (2008b). “?” indicates objects with un- known taxonomies. “X” refers to the degenerate E, M, and P objects (Tholen and Barucci, 1989; Bus et al., 2002). Table 2 lists SC/OC estimates in decreasing order Fig. 1. Distribution of NEA SC/OC versus absolute magnitude. Spectral classes are and Table 3 lists SC/OC for objects ordered by catalog number and provisional des- indicated with different letters and colors. We adopt the classes described in Tholen ignation. A histogram for the NEAs appears in Fig. 3. NEA SC/OC estimates range and Barucci (1989), which identifies 14 groups based on the shape of their visible from zero to 1.5, with a mean of 0.34 ± 0.25 and a median of 0.26. spectra and their albedos. S, Q, K, and L subclasses within the taxonomy in Bus et al. (2002) have been grouped into the S class. Dark C and B objects are labeled as “C.” Of the 214 objects in the radar sample, estimates of VIS/IR spectral class are are associated with the enstatite achondrites (Zellner et al., 1977; available for 113. Gaffey et al., 1992; Clark et al., 2004), also known as aubrites, whose composition is predominantly the orthopyroxene enstatite (MgSiO3). Aubrites contain white enstatite crystals up to ten cen- timeters in diameter and small amounts of olivine, plagioclase, clinopyroxene, sulfides, and nickel–iron metal (Zellner et al., 1977; Clark et al., 2004), which suggests that they cooled underground under highly reducing conditions (McSween, 1999). Most aubrites consist of angular pieces or fragments in a softer matrix (Sears, 1995) and some are fragile (Hutchison, 2006). The bulk of classified targets in the tables are C or S, which include carbonaceous and ordinary chondrite analogues (Burbine et al., 2002; Lipschutz and Schultz, 2007), respectively, as well as stony-iron analogues. Chondrites are undifferentiated, consti- tute the vast majority (∼80%) of meteorite falls, and are derived from asteroids that never melted (Lipschutz and Schultz, 2007). Our C and S targets have SC/OC from ∼0.1 to ∼0.5. The lowest SC/OC targets include our sample’s handful of objects in the M spectral class (SC/OC from 0.1 to 0.2), which includes iron me- teorite and enstatite–chondrite analogues (Burbine et al., 2002; Rivkin et al., 2002), and four members of the P class, which are Fig. 2. Distribution of NEA SC/OC versus absolute magnitude including estimated standard errors on SC/OC. Standard errors are 1σ and are estimated using methods thought to be dark, primitive, carbonaceous objects from the outer given in Ostro (2007). For objects observed on multiple dates, errors give the range main belt (Bell et al., 1989). The X group is a degenerate class of SC/OC. comprised of E, M, and P objects that have nearly indistinguish- able VIS/IR spectra and which can be separated primarily by their and visible-infrared (VIS/IR) taxonomic class, as well as unexpected optical albedos (Tholen and Barucci, 1989). Our five X objects in- diversity in the decimeter-scale roughness of NEA surfaces. clude two with SC/OC = 1.1, one with SC/OC = 0.7, and two with NEA SC/OC estimates range from zero to 1.5, with a mean of SC/OC ∼ 0.2. 0.34 ± 0.25 and a median of 0.26, and 12% (26/214) have SC/OC How can the striking correlation of SC/OC with spectral class exceeding 0.6, indicating that decimeter-scale surface roughness be understood? One possibility is that mineralogy has played a among NEAs is relatively common.
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    EPSC Abstracts Vol. 6, EPSC-DPS2011-297, 2011 EPSC-DPS Joint Meeting 2011 c Author(s) 2011 Twenty Years of Toutatis M.W. Busch (1), L.A.M. Benner (2), D.J. Scheeres (3), J.-L. Margot (1), C. Magri (4), M.C. Nolan (5), and J.D. Giorgini (2) (1) Department of Earth and Space Sciences, UCLA, Los Angeles, California, USA (2) Jet Propulsion Laboratory, Pasadena, California, USA (3) Aerospace Engineering Sciences, University of Colorado, Boulder, Colorado, USA (4) University of Maine at Farmington, Farmington, Maine, USA (5) Arecibo Observatory, Arecibo, Puerto Rico, USA Abstract Near-Earth asteroid 4179 Toutatis is near a particularly if the moments of inertia are 4:1 orbital resonance with the Earth. consistent with a uniform internal density. Following its discovery in 1989, Toutatis Toutatis’ last close-Earth-approach for was observed extensively with the Arecibo several decades will be in December 2012, and Goldstone radars during flybys in 1992, when it will be 0.046 AU away. We will 1996, 2000, 2004, and 2008. The 1992 and make predictions for what radar 1996 data show that Toutatis is a bifurcated observations at that time should see. object with overall dimensions of 4.6 x 2.3 x 1.9 km and a surface marked with prominent impact craters. Most significantly, Toutatis References is in a non-principal-axis tumbling rotation [1] Hudson, R.S. and Ostro, S.J.: Shape and non-principal state, spinning about its long axis with a axis spin state of asteroid 4179 Toutatis, Science 270 84-86, period of 5.41 days while that axis precesses 1995.
  • Earth's Recurrent Quasi-Satellite?

    Earth's Recurrent Quasi-Satellite?

    2002 AA: Earth’s recurrent quasi-satellite ? Pawel Wajer To cite this version: Pawel Wajer. 2002 AA: Earth’s recurrent quasi-satellite ?. Icarus, Elsevier, 2009, 200 (1), pp.147. 10.1016/j.icarus.2008.10.018. hal-00510967 HAL Id: hal-00510967 https://hal.archives-ouvertes.fr/hal-00510967 Submitted on 23 Aug 2010 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Accepted Manuscript 2002 AA29: Earth’s recurrent quasi-satellite ? Paweł Wajer PII: S0019-1035(08)00381-3 DOI: 10.1016/j.icarus.2008.10.018 Reference: YICAR 8801 To appear in: Icarus Received date: 11 April 2008 Revised date: 20 October 2008 Accepted date: 23 October 2008 Please cite this article as: P. Wajer, 2002 AA29: Earth’s recurrent quasi-satellite ?, Icarus (2008), doi: 10.1016/j.icarus.2008.10.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.