45th Lunar and Planetary Science Conference (2014) 1428.pdf

Potentially Hazardous 2007 LE: Probable Parent Body of the Black Rose City and Daughter of Asteroid (6) Hebe. S, K. Fieber-Beyer a, 1, * M. J. Gaffey a,1, and P. S. Hardersena,1 a Dept of Space Studies, University Stop 9008, Univ.of North Dakota, 58202. 1Visiting astronomer at the IRTF under contract from NASA, which is operated by the Univ. of Hawai’i Mauna Kea, HI 96720. [email protected]

Introduction: Impacts by Near Earth Objects 20] and reinvented by [21]. The albedo and H allows (NEOs) are the only potentially preventable natural for the diameter (~0.54 km) to be calculated using hazard that could threaten human existence as we equations developed by [22, 23]. The derived albedo & know it. Although the probability of such an event is diameter are consistent with those reported by [24]. remote, the consequences are so severe that we are Analysis and Interpretation: Based on the meas- well justified investing in a modest effort to minimize ured Band I center, one can constrain a composition the probability. The impact of Shoemaker-Levy for the pyroxene surface component. Using the equa- 9 into Jupiter triggered intensive NEO search pro- tions of [17], the pyroxene and olivine compositions grams. Of particular interest are the potentially hazard- would be Fs20 and Fa23. Asteroid 2007 LE’s spectrum ous (PHAs) which approach Earth’s orbit. most resembles that of the black . The black However, detailed knowledge on the physical proper- chondrites are OCs that have experienced strong shock ties of NEOs lags far behind the discovery rate. Under- events that dramatically lowered their albedo. The standing the nature of the near-Earth asteroids is criti- mineralogy comprising black chondrites is essentially cal to assessing potential impact hazards. that of the OCs as far as the metal content and oli- Analysis of NEO data from WISE has provided di- vine/pyroxene compositions [25-27]. rect size determinations for a few percent of the NEO Fig. 1 plots 2007 LE and the black chondrite Rose population [e.g. 1,2]. Taxonomic classifications are City [28]. The Band I absorption feature is consistent also available for a few percent of the population [e.g. in position and depth with Rose City (7-9% deep). 3-5]. Actual compositional determinations and/or iden- Rose City is a complex H-chondrite, which experi- tified analogs are available for less than 1% enced shock, brecciation, and recrystallization [29]. of the population. Although most NEOs are believed to The H-type parent material was shocked to between 45 originate from collision events on asteroids in the and 90 GPa [30] at either ~380 or ~2300 Myr ago [31]. mainbelt, the pathways of these fragments into Earth- Rose City has mineral chemistries ranging from crossing orbits are known only in a statistical way. Fa20-22 and Fs15-17 [29], while 2007 LE‘s mineral chem- The PHA-NEO 2007 LE was discovered on June 2, istries are Fs20±3 and Fa23±3. The comparable composi- 2007 by the LINEAR NEO survey. At the time, noth- tions of H-chondrites are Fs14.5-18. The chemistry calcu- ing but (H) was known, which provided a rough esti- lated from the Band I center of 2007 LE using equa- mate of diameter. tions of [17] may be high because the equations were Methodology: Near-infrared (NIR) spectral ob- produced from spectra of unshocked OCs and 2007 LE servations of 2007 LE and standard stars were obtained is shock blackened. at the NASA IRTF on June 3, 2012 using the SpeX PHA 2007 LE’s albedo of 0.08 and Rose City’s al- instrument [6]. Data reduction was done using previ- bedo of 0.07 [28] are consistent within their uncertain- ously outlined procedures [7,8]. The position of the ties and further support the proposed link. 2007 LE is absorption features and the relative areas of the fea- from a region of highly shocked material on the H- tures are diagnostic of the compositions and abundanc- chondrite parent body. Compared to other shock es of mafic silicates [e.g. 9-17). The NIR spectrum of blackened chondrites, the relatively slow cooling rate PHA 2007 LE exhibits a single the absorption feature of Rose City [32] is consistent with a greater depth and in the 1 µm region (Fig. 1). The band center was a larger volume of shocked material on the parent measured to be 0.96 ± 0.02 µm. body. Conversely, the size of 2007 LE is small enough Several calibrations have been developed to derive that it need not sample a spectroscopically abundant mafic mineral compositions and abundances from lithology on its parent body [33]. The measured cosmic spectral parameters [e.g., 13-18]. These formulas pro- ray exposure age of Rose City (~39 Myr [34]) would duce comparable results within their uncertainties. represent the time since the meter-scale Rose City me- There is no discernable Band II in the 2007 LE spec- teoroid was ejected from its immediate parent body. trum. The increase in apparent reflectance beyond ~2.2 Discussion: 2007 LE was identified as a binary as- µm is the short wavelength edge of the thermal emis- teroid system [24]. Isotopic and mineralogical studies sion curve from the relatively warm asteroid. This of the H-chondrites suggest they all derive from a sin- thermal emission was used to derive the surface albedo gle parent body, or more precisely from a single chem- (~8%) of 2007 LE using a technique pioneered by [19, ical reservoir in the solar nebula. Rose City is a frag- 45th Lunar and Planetary Science Conference (2014) 1428.pdf

ment of the H-OC parent body which has been identi- T. H. et al. (2009). MAPS 44, 1331–1341. [17] Dunn fied as the mainbelt asteroid (6) Hebe [36]. T.L., et al. (2010). Icarus 208, 789-797. [18] Burbine Dynamical models indicate the majority of T. H, et al (2007). Lunar and Planetary Science asteroidal material delivered to the inner solar system XXXVIII. Abstract 2117. [19] Abell P. A. (2003) originates from the 3:1 and the ν6 secular Ph.D. Diss., Rensselaer Polytechnic Inst. [20] Abell P. resonances [37-43]. [39] predicted that (6) Hebe would A. et. al (2002). ACM 2002, Abstract 18-070. [21] deliver 2.3% of its fragments into the 3:1 resonance Rivkin A. S. et. al (2005). Icarus 175, 175–180. [22] and 83.9% of its fragment into the ν6 resonance. Hebe Fowler J. W. Chillemi J. R. (1992). JPL, Pasadena, fragments are frequently delivered into Earth-crossing California, pp. 17–43. [23] Pravec R. and Harris A. W. orbits, consistent with the fact that H-chondrites con- (2007). Icarus 190, 250–259. [24] Johnston, W. R., stitute approximately a third of all meteorite falls. BINARY MINOR V6.0. EAR-A- Asteroid (6) Hebe most likely suffered a major col- COMPIL-5-BINMP-V6.0. NASA Planetary Data Sys- lisional event ~380 or ~2300 Myr ago that shocked a tem, 2013. [25] Gaffey M. J. 1976. JGR 81, 905-920. significant volume and/or ejected the ~0.5 km shock [26] Britt D. T. and Pieters C. M. 1991. Meteoritics 26, blackened PHA 2007 LE. Perturbations by the giant 279-285. [27] Britt D. T. and Pieters C. M. 1994. GCA planets transferred 2007 LE into an Earth-crossing 58, 3905-3919. [28] Gaffey M. J. 1976. JGR 81, 905- orbit. The CRE age of Rose City [~39 Myr, 34] would 920. [29] Fruland R.M. 1975. M.S. Thesis University suggest that 2007 LE was delivered into an Earth- of Houston, Houston. [30] Stöffler D. et al. (1991) crossing orbit through the slower ν6 resonance. GCA 55, 3845-3867. [31] Bogard D. D. (1995) Once in near-Earth space, the Rose City Meteoritics 30, 244-268. [32] Yolcubal I. et al. (1997). (and possibly 2007 LE’s secondary) could have been JGR, 102(E9), 21589–21611. [33] Keil K. et al. 1992. liberated from 2007 LE ~39 Myr ago by either 1) a Icarus 98, 43-53. [34] Graf T. and Marti K. (1995). small impact, 2) a gravitational encounter with the JGR 100, 21247-21263. [35] Mason, B., and H. B. Earth during which tidal forces pulled loose fragments Wiik 1966. Am. Mus. Novit. 2272, 1-24. [36] Gaffey off [e.g. 44,45], or 3) spinning the asteroid by YORP M. J. and Gilbert S. L., 1998. MAPS 33, 1281-1295. effect until it underwent rotational fission [e.g., 46]. [37] Yoshikawa M. 1990. Icarus 87, 78-102. [38] This is the first case where a specific meteorite has Hadjidemetriou J. D. 1993. Celest. Mech. Dyn. been linked to a near-Earth Object for which a Astron., 56, 563-599. [39] Farinella P. et al. 1993. Ica- mainbelt parent body has been identified. In this case rus 101, 174-187. [40] Morbidelli A. and M. the pathway leads from asteroid (6) Hebe through 1995. Icarus 115, 60-65. [41] Moons M. and NEO-PHA 2007 LE to the meteorite Rose City. Morbidelli A. 1995 Icarus 114, 33-50. [42] Bycova L. Acknowledgements: This work was supported by E. and Galushina T.Yu 2001. . Space Sci 49, the NASA Near-Earth Objects Observations / Plane- 811–815. [43] Bycova L. E. and Galushina T. 2002. tary Astronomy Program grant NNX12AG12G and by Cel. Mech. Dyn. Ast. 82, 265–284. [44] Richardson the NASA Planetary Geology and Geophysics Pro- D. C. et al. (1998). Icarus 134, 47–76. [45] Walsh K. J. gram grant NNX11AN84G. and D. C. Richardson (2006) Icarus 180, 201–216. [46] References: [1] Mueller M. et. al (2011) Astron. J. Walsh K. J. et al. (2008) Nature 454, 188-191. 141, 109-117. [2] Mainzer A. et al. (2012) Ap J Let- ters 760, L12. [3] Binzel R. P. et al. (2004) Icarus 170, 259–294. [4] Fevig R. A. and Fink U. (2007) Icarus 188, 175-188. [5] DeMeo F. E. et al. (2009). Icarus 202, 160–180. [6] Rayner J.T. et al.. Pub Atron Soc. Pac. 115:362-382. [7] M.J. Gaffey et al. (2002), Aster- oids III, 183-204. [8] P.S. Hardersen et al. (2005) Ica- rus 175, 141-58. [9] Adams J.B. (1974). JGR, 79, 4829-4836. [10] Adams J. B. (1975). In Infrared and Raman Spectroscopy of Lunar and Terrestrial Minerals (ed. C. Karr), pp. 91-116. Academic Press, NY. [11] Cloutis E.A. et al. (1986). JGR 91, 11641-11653. [12] Gaffey M. J. 2003. LPSC XXXIV. #1602. [13] Gaffey M. J. et. al (2002). In Asteroids III, Univ. of Arizona Figure 1: Normalized reflectance spectrum of PHA- Press, pp. 183-204. [14] Gastineau-Lyons H. et. al NEO 2007 LE (black symbols) compared to the spec- (2002). MAPS 37, 75-89. [15] Burbine T. H. et al. trum of the black H-chondrite Rose City (red). (2003). Antarct. Met. Res. 16, 185-195. [16] Burbine