THE GLOBAL THERMAL INFRARED SPECTRUM OF BENNU: COMPARISON WITH SPITZER IRS SPECTRA Lucy F. Lim1, M. Antonietta Barucci2, Humberto Campins3, Philip R. Christensen4, Beth Clark5, Marco Delbo6, Joshua P. Emery7, Victoria E. Hamilton8, Javier Licandro9, Dante S. Lauretta10, and The OSIRIS-REx Team (1) NASA/GSFC (2) Paris Observatory Meudon (3) University of Central Florida, (4) Arizona State University, (5) Ithaca College, (6) CNRS, France (7) Univ of Tennessee- EPS, Knoxville, (8) Southwest Research Instute Boulder, (9) Instuto de Astrosica de Canarias, (10) University of Arizona, Lunar and Planetary Laboratory

SPITZER IRS ASTEROID SPECTRA BENNU AND LOW- AND BENNU: OTES VS. SPITZER IRS The Infrared Spectrograph (“IRS”; Houck et al. 2004) observed dozens of asteroids and other small bodies during Bennu (thermal model 1) Emery et al., LPSC 2010 its cryogenic mission, 2003 to 2009, in a thermal-IR spectral range (5.2 to 38 microns) similar to that covered by OSIRIS-REx OTES. Bennu (thermal model 2) Targets included asteroids with a variety of sizes, VNIR spectral Bennu (thermal model 1) types, and dynamical characteristics (e.g., Barucci et al. 2008). Here we discuss the initial OTES spectra of (101955) Bennu (Hamilton et Bennu (thermal model 2) Bennu (2007 Spitzer IRS data) al., this meeting) in the context of the Spitzer data set. Bennu (200% contrast) Many low-albedo main-belt asteroids such as 24 Themis and 65 Cybele (Hargrove et al. 2012, 2015; Licandro et al. 2011) show emissivity “plateau" maxima in the 10-micron region rather than the 3200 Phaethon minima typical of silicate minerals and meteorites in the laboratory. B-type NEA, D ≈ 5 km Bennu itself was observed with the Spitzer IRS in 2007. The OTES data Bennu, however, has distinct emissivity minima close to 10.2 and confirm that Bennu’s global spectral emissivity contrast in the thermal IR 22.7 microns. Its overall spectral contrast is also very low compared is too low to have been observable in the (pre-encounter) Spitzer IRS with that of main-belt C-complex asteroids. data.

BENNU VS. IRS ASTEROID SPECTRA WITH ANHYDROUS METEORITIC Fig. 4 from Hargrove et al. 2015 ANALOGUES B-type MBA Tempel-2 D ≈ 220 km nucleus, D ≈ 9 km pv ≈ 0.022 (Kelley et al. 2017)

1 Ceres, D ≈ 900 km KAO spectra (Cohen et al. 1998) Themis family member D ≈ 120 km

Bennu (thermal model 1)

624 Hektor REFERENCES 956 Elisa D-type , D ≈ 200 km Barucci, A., et al. (2008). A&A 477: 665. MBA (Emery et al. 2006) Cohen et al. (1998). AJ 115: 1671-1679. V-type, D≈10km Spectra have been offset for clarity Emery, J. P., Cruikshank, D. P., and van Cleve, J. (2006). Icarus 182: Dashed vertical lines mark the locations of minima in the Bennu spectra 496-512. Hargrove, K. D., et al. (2015). Icarus 254: 150-156. Houck, J.R. et al. [2004]. Astrophys. J. Suppl. 154, 18–24 5261 Eureka Kelley, M. S. P., et al. (2017). Icarus 284: 344-358. Sa-type Mars trojan, D≈2 km Licandro, J., et al. (2011). A&A 525: A34. • Bennu’s TIR spectrum is unlike those of “10-micron plateau” main-belt Licandro, J., et al. (2012). A&A 537: A73. and Trojan low-albedo objects, including the B-type asteroid 24 Themis Lim, L. F., Emery, J.P., and Moskovitz, N. (2011). Icarus 2013: 510-523. • Different mineralogy or different regolith structure? • Many Spitzer IRS spectra have insufficient S/N to detect Bennu-like emissivity features • Future observers and future investigations will need to set higher S/N targets to investigate these features

OSIRIS-REX | AGU FALL MEETING | POSTER SESSION | DECEMBER 2018