Lunar and Planetary Science XLVIII (2017) 1968.pdf

HOW ABUNDANT ARE DIFFERENT METEORITE GROUPS AMONG S-COMPLEX AND Q-TYPE NEAR-EARTH ASTEROIDS? T. H. Burbine1, R. P. Binzel2 and B. J. Burt2, 1Department of Astronomy, Mount Holyoke College, South Hadley, MA 01075 ([email protected]), USA, 2Department of Earth, At- mospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.

Introduction: This study analyzes the reflectance Table 2. Percentage of meteorite spectra with spectra of ~100 meteorites (primarily ordinary chon- different asteroid taxonomic designations. drites) that have spectral features due to olivine and Asteroid Ordinary Acapulcoite/ pyroxene (ordinary chondrites, acapulocites/lodranites, Class Chondrite Lodranite Ureilite ureilites) and use the results to determine the best me- K 3% 0% 53% teoritic analogs for 155 S-complex and Q-type NEAs. L 3% 0% 29% Band centers, which are the best spectral parameters Q 41% 17% 0% for deriving the mineralogies of objects, were deter- S 9% 0% 6% mined for all the spectra. All meteorite spectra were Sq 26% 0% 6% also given an asteroid taxonomic designation. Sr 17% 0% 0% Data: Meteorite spectra obtained at the Sv 1% 0% 0% Keck/NASA RELAB (Reflectance Experiment Labor- Svw 0% 17% 0% atory) were analyzed. The meteorite spectra were tak- V 0% 17% 0% en at a resolution of 0.01 µm. Most of the spectra were Vw 0% 50% 0% obtained on samples that had been powdered and Xk 0% 0% 6% sieved to grain sizes of approximately 150 µm or less. The Band I and II centers and uncertainties for the To determine which asteroids had band centers that meteorites and asteroids were determined using a spe- are consistent with particular meteorite groups, the cially written MATLABTM program that was slightly asteroid band centers were compared to the found for modified from the one used in Burbine [1]. The main each meteorite type. The range for each meteorite revisions were to normalize the uncertainties when the group was assumed to be its mean Band I and Band II reflectance values are normalized to unity during the center plus or minus two sigma, which would encom- fitting and to do the resampling 9,999 times. pass 95% of the variation for a range of values that The near-infrared NEA spectra were primarily ob- have a Gaussian distribution. tained at the NASA IRTF (Infrared Telescope Facility) A number of assumptions have been made. The [2] while the visible data (which usually included some range of band centers for each meteorite is assumed to spectral coverage in the near-infrared) were obtained be Gaussian and represents the range of band centers using various telescopes. found for asteroids with that mineralogy. No tempera- Results: Mean band centers and uncertainties were ture corrections are done to the asteroid spectra since calculated for all the meteorite groups (Table 1). The NEAs have surface temperatures that are roughly room analyzed meteorite spectra were also given an asteroid temperature [3]. Space weathering is assumed not to taxonomic designation using the web-based Bus- alter the position of the band centers. Phase angle is DeMeo classifier. The range of classifications for each assumed not to affect the position of the band centers. meteorite group (Table 2) shows that each asteroid A meteoritic match is made when an asteroid has a taxonomic class most likely contains a wide variety of Band I and a Band II center (plus or minus one sigma) meteorite types. But ordinary chondrites appear to that falls within the range of a particular meteorite type dominate among S-complex and Q-type bodies. (Table 3). Sixty-nine percent of the analyzed S- complex and Q-type NEAs had band centers (within Table 1. Mean band centers and uncertainties uncertainties) consistent with those of ordinary chon- for different meteorite groups. drites. This percentage includes objects that also have Band I Band II matches with acapulcoites/lodranites and/or ureilites. Meteorite Group Center (µm) Center (µm) Due to the large band center ranges found for ureilites H 0.933±0.005 1.925±0.016 (Table 1), all of the NEAs that matched ordinary L 0.948±0.010 1.943±0.028 chondrites also had band centers consistent with those LL 0.990±0.008 1.961±0.026 of ureilites. It should be noted that the only definitive- acapulcoite/ 0.932±0.005 1.888±0.023 ly known asteroid with a ureilite composition is C- lodranite complex body 2008 TC3, which was spectrally ob- ureilite 0.955±0.028 1.956±0.036 served right before it impacted the Earth’s atmosphere. Lunar and Planetary Science XLVIII (2017) 1968.pdf

Asteroid 2008 TC3 is the parent body of the Almahata resonances due to the relatively old age of the Flora Sitta ureilite. family, its relatively small semi-major axis, and its proximity to the ν6 secular resonance [4]. Table 3. Band center matches with different combina- Using laboratory measurements and fireball data, tions of meteorites for S-complex and Q-type NEAs. Cotto-Figueroa et al. [14] finds that the strengths of LL Meteorite Groups Match chondrite bodies are greater than the strengths of H and H-L-LL-acapulcoite/lodranite-ureilite 1% L chondrite bodies for bodies of similar sizes. Maybe H-L-acapulcoite/lodranite-ureilite 14% impacts preferentially break down H and L chondrite H-ureilite 1% bodies to smaller sizes compared to LL chondrites. It H-L-ureilite 3% is also possible that the H chondrite and L chondrite L-ureilite 17% source bodies were disrupted in near-Earth space, L-LL-ureilite 3% which could substantially increase the flux of H and LL-ureilite 30% LL chondrite material to Earth. acapulcoite/lodranite 1% Conclusions: Almost all ordinary chondrites have ureilite 9% reflectance spectra consistent with S-complex and Q- none 21% type bodies. LL-like mineralogies are much more abundant than H- and L-like mineralogies among Ordinary chondrites are the most common meteor- NEAs compared to fall statistics. However, ureilites ites to fall on Earth today with L chondrites being 35% and acapulcoites/lodranites cannot be ruled out as be- of all falls, H chondrites being 32%, and LL chondrites ing present within the population of S-complex and Q- being 8%. The high proportion of interpreted LL-like type NEAs. mineralogies among NEAs (Table 3) compared to fall References: [1] Burbine T. H. (2014) LPS XLV, statistics is similar to the results determined previously Abstract #1646. [2] Binzel R. P. et al. (2006) LPS [4,5,6,7]. This high proportion of LL-chondrite miner- XXXVII, Abstract #1491. [3] Hinrichs J. L. et al. alogies is likely due to the observed NEAs and the (1999) Geophys. Res. Lett., 26, 1661-1664. [4] meteorite source bodies having very different diame- Vernazza P. et al. (2008) Nature, 454, 858–860. [5] de ters. The NEAs tend to have km-sized diameters while León J. et al. (2010) Astron. & Astrophys., 517, A23. the ordinary chondrite source bodies have diameters of [6] Dunn T. L. et al. (2013) Icarus, 222, 273-282. [7] tens of meters or less. Thomas C. A. et al. (2014) Icarus, 228, 217-246. [8] To try to understand this high proportion of LL-like Gaffey M. J. and Gilbert S. L. (1998) Meteoritics & mineralogies among NEAs, we must look at the possi- Planet. Sci., 33, 1281–1295. [9] Vernazza P. et al. ble parent bodies of these meteorites. H chondrites (2014) Astrophys. J., 791, 120. [10] Nesvorný D. et al. have been linked with (6) Hebe [semi-major axis (a) of (2009) Icarus, 200, 698–701. [11] Korochantseva E. 2.43 AU)] [8] due to spectral similarities and Hebe’s V. et al. (2007) Meteoritics & Planet. Sci., 42, 113- location near the 3:1 meteorite-supplying resonance 130. [12] Dykhuis M. J. et al. (2014) Icarus, 243, 111- (2.50 AU). However, a number of other asteroids near 128. [13] Bottke W. F. Jr. et al. (2002) In Asteroids III, the 3:1 and 5:2 (2.82 AU) resonances have also been pp. 395-408. [14] Cotto-Figueroa D. et al. (2016) Ica- identified as having H-like mineralogies [9]. rus, 277, 73-77. L chondrites have been linked [10] with the (1272) Acknowledgements: THB would like to thank the Gefion family (a = 2.70-2.82 AU), which is located Remote, In Situ, and Synchrotron Studies for Science near the 5:2 resonance. Many L chondrites have 39Ar- and Exploration (RIS4E) Solar System Exploration 40Ar ages of ~470 Ma [11], indicating a major impact Research Virtual Institute (SSERVI). RPB and THB at that time on the L chondrite parent body. would also like to thank NASA grant NAG5-12355 for LL chondrites have been linked [4] to the (8) Flora support in this research. All (or part) of the data uti- family (a = 2.16-2.40 AU) due to the interpreted min- lized in this publication were obtained and made avail- eralogical similarities between Flora family members able by the MIT-UH-IRTF Joint Campaign for NEO and LL chondrites. The Flora family has an estimated Reconnaissance. The IRTF is operated by the Univer- formation age of ~950 Ma [12]. sity of Hawaii under Cooperative Agreement NCC 5- The Yarkovsky effect, which can cause semi-major 538 with the National Aeronautics and Space Admin- axis drift, decreases in strength with increasing size for istration (NASA), Office of Space Science, Planetary km-sized bodies and increasing distance from the Sun Astronomy Program. The MIT component of this work [13]. If the Flora family is the source of the LL chon- is supported by NASA grant 09-NEOO009-0001 and drites, a higher proportion of NEA-sized LL-like bod- by the National Science Foundation under grants ies may have been able to reach meteorite-supplying 0506716 and 0907766.