50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 1124.pdf

THE GLOBAL THERMAL INFRARED SPECTRUM OF (101955) BENNU IN THE CONTEXT OF 1 SPITZER IRS ASTEROID SPECTRA. Lucy F. Lim , A. Barucci2, H. Campins3, P. Christensen4, B. E. Clark5, M. Delbo6, J.P. Emery7, V.E. Hamilton8, J. Licandro9, D. S. Lauretta10, and the OSIRIS-REx Team. 1NASA God- dard Space Flight Center (lucy.f.lim@nasa.gov), 2Paris Observatory Meudon, 3University of Central Florida, 4Arizona State University, 5Ithaca College, 6CNRS, France, 7University of Tennessee–EPS, Knoxville, 8Southwest Research Institute Boulder, 9Instituto de Astrofísica de Canarias, 10University of Arizona, Lunar and Planetary Laboratory.

Introduction: The Spitzer Space Telescope Infrared parts in the thermal-IR as it does in the VNIR. How- Spectrograph (IRS; [1]) observed dozens of asteroids ever, rather than a Bennu-like pattern of an emissivity and other small bodies during its cryogenic mission, maximum just shortward of 9 µm followed by a sili- 2003 to 2009, in a thermal-IR spectral range (5.2 to cate minimum at 10.2 µm, the Themis family mem- 38 µm) similar to that covered by the OSIRIS-REx bers show a broad 10 µm emissivity maximum of ap- Thermal Emission Spectrometer (OTES). Targets in- proximately 3% spectral contrast [8]. This “plateau” cluded asteroids with a variety of sizes, VNIR spectral is similar, although lower in spectral contrast, to that types, and dynamical characteristics. Here we discuss observed in comet nuclei [9] and Trojan asteroids [10] the initial OTES spectra of (101955) Bennu (Hamil- and often attributed to underdense layers of fine- ton et al., this meeting) in the context of the Spitzer grained silicate particles in the uppermost layer of the data set. regolith [10, 11]. If Bennu’s parent body had a similar Many low-albedo main-belt asteroids such as (24) regolith structure, it must have been lost. Whether this Themis and (65) Cybele [2, 3] show emissivity “plat- took place at the time of Bennu’s formation as a sub- eau” maxima in the 10-µm region rather than the min- km body or during Bennu’s transport into the inner ima typical of silicate minerals and meteorites in the solar system is an open question. laboratory. Bennu, by contrast, has distinct emissivity Additionally, (261) Prymno, a B-type asteroid in minima close to 10.3 and 23 µm similar to those the inner Main Belt (a = 2.33 AU) but outside the found in hydrated meteorites. Its overall spectral con- Polana/Eulalia complex, shows a similar “10-µm plat- trast is also very low compared with that of main-belt eau” pattern. Note that like the Themis family mem- C-complex asteroids. bers observed with the IRS, Prymno is a large asteroid Bennu itself was observed with the Spitzer IRS in with a WISE diameter of 50 km [12]. 2007 [4]. The new OTES data confirm that Bennu’s B-type NEA 3200 Phaethon: (3200) Phaethon is a 5- global spectral emissivity contrast in the thermal IR is km B-type near-Earth object that was observed by the too low to have been observable at the S/N of the IRS [Cruikshank GTO program; see also ref. 13]. Ra- (pre-encounter) Spitzer IRS data. ther than a Themis-like “plateau” feature, Phaethon Main Belt analogues for Bennu: Dynamically, shows a different non-Bennu-like emissivity pattern (101955) Bennu has been considered to be most likely with minima longward of 11 µm and 20 µm. We note to have originated in the families of (495) Eulalia (C- that Phaethon has been proposed to have originated in type, a = 2.49 AU) or (142) Polana (B-type, a = 2.42 the high-inclination Pallas family [14] rather than in AU) in the inner Main Belt [5]. Regrettably, neither the low-inclination Polana/Eulalia region. Based on Eulalia nor Polana was observed with the Spitzer IRS. VNIR spectral data, the Pallas-family B-type asteroids VNIR spectral surveys of the two families [6, 7] have have been distinguished from the Themis-family B- found a mixture of B- and C-type asteroids and a dis- types and the wider B-type population [15, 16, 17] by tribution of spectral slopes in both, suggesting that their steeper spectral slopes and higher albedos. Bennu may have had either a B- or C-type parent Phaethon has also been exposed to much more in- body. tense insolation than Bennu due to Phaethon’s low perihelion. Thus, both its region of origin and its B-type Main Belt asteroids: 5–14 µm IRS spectra of thermal weathering history remain as candidate hy- the B-type outer-main-belt asteroid (24) Themis (a = potheses for its spectral differences from Bennu. 3.13 AU) and 10 of its Band C-type family members in the 40–100 km size range have been published [2, Other C-complex and low-albedo asteroids: Emis- 3, 8]. Although Themis is dynamically unlikely to be sivity spectra for several low-albedo binary asteroids the parent body of Bennu, if the B taxonomic type were published by [18]. Among these, the spectra of corresponded to a single characteristic rock type, one (4492) Debussy (a = 2.77 AU; D = 17 km) and the might expect Bennu to resemble its B-type counter- sub-kilometer NEO 2000 DP107 are notable for the 50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 1124.pdf

absence of a strong “plateau” feature. The spectrum of Debussy resembles that of Bennu in the 20–30 µm region but is distinct in the 8–14 µm region, with a narrow emissivity minimum at 12 µm that is absent from the spectrum of Bennu. Transparency behavior, thermal inertia, and the result- ing inferred particle sizes will also be discussed.

References [1] J. R. Houck,et al., ApJS154, 18 (2004). [2] J. Licandro, et al., A&A 537, A73(2012). [3] K. D. Hargrove, J. P. Emery, H. Campins, M. S. P. Kelley, Icarus 254, 150 (2015). [4] J. P. Emery, et al., Lunar and Planetary Science Conference Abstracts (2010), #2282. [5] W. F. Bottke, et al., Icarus 247, 191 (2015). [6] J. de León, et al., Icarus 266, 57 (2016). [7] N. Pinilla-Alonso, et al., Icarus 274, 231 (2016). [8] Z. A. Landsman, et al., Icarus 269, 62 (2016). [9] M. S. P. Kelley, C. E. Woodward, R. D. Gehrz, W. T. Reach, D. E. Harker, Icarus 284, 344 (2017). [10] J. P. Emery, D. P. Cruikshank, J. van Cleve, Icarus 182, 496 (2006). [11] P. Vernazza, et al., Icarus 221, 1162 (2012). [12] J. R. Masiero, et al., ApJ 791, 121 (2014). [13] M. McAdam, M. Mommert, D. Trilling, AAS/Division for Planetary Sciences Meeting Ab- stracts #50 (2018), #312.01. [14] J.de León, H. Campins, K. Tsiganis, A. Mor- bidelli, J. Licandro, A&A 513, A26 (2010). [15] B. E. Clark, et al., Journal of Geophysical Re- search (Planets) 115, 6005 (2010). [16] J. de León, N. Pinilla-Alonso, H. Campins, J. Licandro, G. A. Marzo, Icarus 218, 196 (2012). [17] V.Alí-Lagoa, et al., A&A 591, A14 (2016). [18] F.Marchis, et al., Icarus 221, 1130 (2012).