Icarus 269 (2016) 62–74 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus The Veritas and Themis asteroid families: 5–14 μm spectra with the Spitzer Space Telescope Zoe A. Landsman a,∗, Javier Licandro b,c, Humberto Campins a, Julie Ziffer d, Mario de Prá e, Dale P. Cruikshank f a Department of Physics, University of Central Florida, 4111 Libra Drive, PS 430, Orlando, FL 32826, United States b Instituto de Astrofísica de Canarias, C/Vía Láctea s/n, 38205, La Laguna, Tenerife, Spain c Departamento de Astrofísica, Universidad de La Laguna, E-38205, La Laguna, Tenerife, Spain d Department of Physics, University of Southern Maine, 96 Falmouth St, Portland, ME 04103, United States e Observatório Nacional, R. General José Cristino, 77 - Imperial de São Cristóvão, Rio de Janeiro, RJ 20921-400, Brazil f NASA Ames Research Center, MS 245-6, Moffett Field, CA 94035, United States article info abstract Article history: Spectroscopic investigations of primitive asteroid families constrain family evolution and composition and Received 18 October 2015 conditions in the solar nebula, and reveal information about past and present distributions of volatiles in Revised 30 December 2015 the solar system. Visible and near-infrared studies of primitive asteroid families have shown spectral di- Accepted 8 January 2016 versity between and within families. Here, we aim to better understand the composition and physical Available online 14 January 2016 properties of two primitive families with vastly different ages: ancient Themis (∼2.5 Gyr) and young Ver- Keywords: itas (∼8 Myr). We analyzed the 5 – 14 μm Spitzer Space Telescope spectra of 11 Themis-family asteroids, Asteroids including eight previously studied by Licandro et al. (2012), and nine Veritas-family asteroids, for a to- Asteroids tal of 20 asteroids in our sample. We detect a broad 10-μm emission feature, attributed to fine-grained composition and/or porous silicate regolith, in all 11 Themis-family spectra and six of nine Veritas-family asteroids, Asteroids with 10-μm spectral contrast ranging from 1% ± 0.1% to 8.5% ± 0.9%. We used thermal modeling to de- surfaces rive diameters, beaming parameters and albedos for our sample. Asteroids in both families have beaming Infrared observations parameters near unity and geometric albedos in the range 0.03 – 0.14. Spectral contrast of the 10-μm Spectroscopy silicate emission feature is correlated with beaming parameter and rotation period in the Themis family, and may be related to near-infrared spectral slope for both families. We see no correlations of 10-μm emission with diameter or albedo for either family. Comparison with laboratory spectra of primitive me- teorites suggests these asteroids are similar to meteorites with relatively low abundances of phyllosili- cates. Overall, our results suggest the Themis and Veritas families are primitive asteroids with variation in composition and/or regolith properties within both families. © 2016 Elsevier Inc. All rights reserved. 1. Introduction and even water ice (e.g., Campins et al., 2010; Rivkin and Emery, 2010; Licandro et al., 2011a). The study of primitive asteroids Asteroid families are the remnants of catastrophic collisions. provides constraints on models of solar system formation and Family members share similar orbits and cluster in the parameter evolution and is relevant to the origin of Earth’s water and life. space of their proper orbital elements. Spectroscopy of asteroid Themis and Veritas are two compositionally primitive families families can be diagnostic of composition and degree of aqueous residing in the outer main belt, with low inclinations and low alteration, and spectral hetero- or homogeneity within a family eccentricities (Fig. 1). The Veritas asteroid family is dynamically can reveal the state of differentiation of the parent body (Cellino young, 8.3 ± 0.5 Myr (Nesvorný et al., 2003). In a study of visible et al., 2002). Primitive asteroid families are of particular interest (0.4–0.9μm) spectroscopy from large surveys, Mothé-Diniz et al. because they represent a record of conditions in the early solar (2005) reported homogeneity among the Veritas asteroids, with all system. These bodies contain organic materials, hydrated minerals, Veritas members in the study falling within the C taxonomic com- plex. Previously, Di Martino et al. (1997) hadreportedevidencefor both C and D types among the Veritas asteroids. At ∼2.5 Gyr, the Themis family falls at the opposite dynamical ∗ Corresponding author. Tel.: +1 4078232325. E-mail address: [email protected] (Z.A. Landsman). age extreme (Marzari et al., 1995; Nesvorný et al., 2003). Themis http://dx.doi.org/10.1016/j.icarus.2016.01.008 0019-1035/© 2016 Elsevier Inc. All rights reserved. Z.A. Landsman et al. / Icarus 269 (2016) 62–74 63 Fig. 1. The Themis and Veritas asteroid families in the parameter spaces of the sine of inclination vs. semimajor axis (left) and the sine of inclination vs. eccentricity (right). family members studied with visible spectroscopy are typically B- vibrational modes in silicates, is sensitive to grain size, porosity types and C-types, with a smaller number of X-types, consistent and mineralogy (e.g., Emery et al., 2006; Vernazza et al., 2012). with a parent body mineralogy similar to CM meteorites (Florczak Additionally, the mid-IR may be diagnostic of degree of aque- et al., 1999; Mothé-Diniz et al., 2005). A visible and near-infrared ous alteration. In a study of primitive meteorites, McAdam et al. spectroscopic study of the Themis family and a young ( < 10 Myr) (2015) found that an emissivity minimum around 12 μm is cor- sub-family, Beagle, shows a range of spectral slopes within the related with the amount of phyllosilicates contained in the mete- Themis family, while the Beagle asteroids tend to be bluer, brighter, orites; the feature occurs at shorter wavelengths (near 11.4 μm) for and with less variation in their spectral slopes (Fornasier et al., more aqueously altered meteorites and longer wavelengths (near 2016). These trends are attributed to heterogeneity in the Themis 12.3 μm) for less altered meteorites. Interestingly, they found that parent body, space weathering effects, or both. Kaluna et al. (2016) the 0.7-μm absorption band, one typical diagnostic measure of the also studied visible spectra and albedos of Themis and Beagle as- presence of aqueous alteration in asteroids, did not correlate to teroids, and they, too, report that the Beagle asteroids are bluer the measured amount of phyllosilicates in the primitive meteorites. and brighter than the other Themis members, suggesting space They compared the Spitzer spectrum of (24) Themis to the mete- weathering results in the reddening and darkening of C-complex orite spectra and found it was similar to less-altered meteorites. asteroids. 2–4 μm spectroscopy of (24) Themis, the largest mem- Here, we present the results of our analysis of the mid-infrared ber of the Themis family, has revealed the presence of water Spitzer Space Telescope spectra of twenty Themis and Veritas as- ice and organics on its surface (Campins et al., 2010; Rivkin and teroids. Our sample (Table 1) consists of 9 Veritas asteroids and Emery, 2010). Recently, the signature of water ice was detected in spectra (90) Antiope, another large Themis member (Hargrove et al., 2015). Table 1 Studies of primitive asteroid families in the visible and near- Analytic proper orbital elements and taxonomic classification. infrared (NIR; 0.8 – 2.4 μm) have shown relationships between a Object Family ap (AU) ep ip (deg) Taxonomic class spectral properties and family age. Nesvorný et al. (2005) reported a trend of SDSS colors with family age for primitive asteroids, with (24) Themis Themis 3.13393 0.1525 1.0829 Cb,Cc,Bd,Ce b c d e the young Veritas asteroids at one end of the trend line and the (90) Antiope Themis 3.14543 0.1552 0.0231 C ,Ch ,C ,C (222) Lucia Themis 3.13457 0.15520 1.0486 BUb ancient Themis family at the other end. Ziffer et al. (2011) com- (223) Rosa Themis 3.08982 0.13470 1.5128 Xb,Xcc pared NIR spectra of the Themis and Veritas families. They found (316) Goberta Themis 3.17127 0.1370 1.3465 Cc that the Themis family members have red (positive) NIR slopes, (379) Huenna Themis 3.13564 0.1569 0.0307 Bb,Cd c d while the Veritas family members have blue (negative) NIR slopes. (383) Janina Themis 3.13374 0.15330 1.4096 B ,B (468) Lina Themis 3.14105 0.15970 1.2205 Xkd Observed differences between the Themis and Veritas family may (492) Gismonda Themis 3.11169 0.15220 1.2663 - be due to the large difference in the ages of the two families, com- (515) Athalia Themis 3.12074 0.15620 1.0428 Cbd positional differences, differences in thermal evolution, or a com- (526) Jena Themis 3.12237 0.15580 1.4211 Bb,Chc bination of these (Nesvorný et al., 2005; Ziffer et al., 2011). (1086) Nata Veritas 3.16534 0.06260 9.2998 Chc,Chd (2428) Kamenyar Veritas 3.16967 0.06340 9.3056 Chd We aim to investigate whether similar spectral trends exist d μ (2934) Aristophanes Veritas 3.16615 0.06230 9.1895 Ch in the mid-infrared (mid-IR; 5–14 m) by comparing spectra (3090) Tjossem Veritas 3.16909 0.06300 9.2301 Cgd for the Themis and Veritas families acquired with the Spitzer (5592) Oshima Veritas 3.16806 0.05990 9.3869 Chc Space Telescope. This wavelength range is ideally suited to study (5594) Jimmiller Veritas 3.16823 0.06040 9.3114 – both the composition and the physical properties of primitive (7231) Porco Veritas 3.16583 0.06280 9.2301 – (8726) Masamotonasu Veritas 3.17264 0.06030 9.5147 – asteroids.