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Compositional differences between meteorites near-

Compositional differences between meteorites near-

Compositional differences between meteorites and near-Earth asteroids

P. Vernazza1 , R. P. Binzel2 , C. A. Thomas2 , F. E. DeMeo3 , S. J. Bus4 , A. S. Rivkin5 & A. T. Tokunaga6

Understanding the nature and origin of the asteroid population in Earth’s vicinity (near-Earth asteroids, and its subset of potentially hazardous asteroids) is a matter of both scientific interest and practical importance1 . It is generally expected that the compositions of the asteroids that are most likely to hit Earth should reflect those of the most common meteorites. Here we report that most near-Earth asteroids (including the potentially hazardous subset) have spectral properties quantitatively similar to the class of meteorites known as LL chondrites.

The prominent Flora family in the inner part of the asteroid belt shares the same spectral properties, suggesting that it is a dominant source of near-Earth asteroids. The observed similarity of near-Earth asteroids to LL chondrites is, however, surprising, as this meteorite class is relatively rare ( 8 per cent of all meteorite falls). One possible explanation is the role of a size-dependent process, such as the Yarkovsky effect, in transporting material from the main belt. Spectroscopic observations of more than 400 near-Earth asteroids (NEAs) in visible wavelengths show that 65% of NEAs have S- and Q-type spectral properties2 . When corrected for discovery biases3 , the near-Earth population of S- and Q-type asteroids is estimated to be 36% of the total NEA population.

Given the dominance of S- and Q-type NEAs and the dominance of ordinary chondrites in meteorite falls (,80%), we focus our analysis on the possible correlations between these two major groups. To explore the mineralogical composition of potentially hazardous asteroids (PHAs) and the larger population of NEAs, we used the SpeX instrument4 on the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. This instrument is capable of measuring the flux of reflected sunlight from asteroids over the near-infrared range (0.8– 2.5 mm), which can reveal diagnostic absorption bands due to olivine and pyroxene at 1 mm and pyroxene at 2 mm (Fig. 1).


These absorption features are characteristic for both ordinary chondrites and most NEAs. Through a joint observational campaign working cooperatively between MIT, the University of Hawaii and the IRTF (http://smass., we have collected near-infrared spectra of 38 S- and Q-type NEAs, of which 12 fall into the subset of PHAs, defined as the objects whose trajectories pass within 0.05 AU from the orbit of the Earth. For each of these, the visible wavelength portion of their spectrum was also available from previously published studies2 . We performed a detailed comparison of these telescopically measured asteroid spectra with analogous wavelength laboratory measurements of ordinary chondrite meteorites catalogued in the RELAB database (

Using RELAB measurements for 57 ordinary chondrites, Fig. 1 presents a direct comparison between meteorite and asteroid reflectance spectral properties, including their observational uncertainties. We applied a radiative transfer model5 to analyse and compare the mineralogies for both asteroid and meteorite samples. We first applied this model to the ordinary chondrite meteorite spectra to (1) constrain the composition of ordinary chondrites relative to petrographic measurements; and (2) define a robust tracer that would allow us to perform a direct comparison between these meteorites and their possible asteroid parent bodies. We used various end-member minerals found in ordinary chondrites (olivine, orthopyroxene and minor components such as clinopyroxene, chromite and iron). For olivine, orthopyroxene and clinopyroxene, we considered different chemistries, that is, different Mg numbers6 . In agreement with petrographic measurements, we found that olivine and orthopyroxene are the two main minerals in ordinary chondrites and that clinopyroxene is a minor component (,6%). The inclusion of chromite and iron neither improved our fits nor changed the abundance of olivine, orthopxyroxene and clinopyroxene by more than 3%. Thus, we used olivine (ol), orthopyroxene (opx) and clinopyroxene only, and defined as a tracer the ratio of inferred abundances for the two main minerals, ol/(ol1opx). This ratio varies up to ,40% for studied ordinary chondrites. For the three ordinary chondrite subgroups (H, L, LL) we obtain the following average ol/(ol1opx) ratios: H, 58.8 6 4.5%; L, 64.2 6 6.8%; and LL, 75.1 6 4.5%. Each of our model-fit average ratios is within 3% of its corresponding value derived from direct laboratory analysis7 .

We find an average ol/ (ol1opx) ratio of 63% for all ordinary chondrites in our sample. We applied the same radiative transfer model5 to our NEA sample (as well as to members of the Flora family, discussed below). To account for the spectral reddening (if present) due to space weathering processes8 , we used a space weathering model9 . We found similar ol/(ol1opx) ratios for both S- and Q-type NEAs, thus demonstrating that space weathering processes do not influence their interpreted mineralogies (see Supplementary Information). We obtain an average ol/(ol1opx) value of 69.6% for all of our S- and Q-type NEA sample. The distribution of inferred ol/(ol1opx) values for studied ordinary chondrites and NEAs are shown in Fig. 2. We weighted each ordinary chondrite subclass (H < 43%, L < 47% and LL < 10%) by their relative fall statistics among all ordinary chondrites. We find that in both their intrinsic spectral properties (Fig. 1) and in our mineralogical analysis (Fig. 2), our NEA sample does not have the same distribution as the overall population of ordinary chondrite meteorites. On average, NEAs appear more olivine-rich than ordinary chondrites. The best match occurs between the NEAs and the LL ordinary chondrites, where we find that most NEAs (,2/3) give a particularly predominant match to the LL chondrites.

We define compatibility between an NEA and an LL chondrite as occurring 1 Research and Scientific Support Department, European Space Agency, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands. 2 Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. 3 Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique, Observatoire de Paris, 5 Place Jules Janssen, Meudon, F-92195, France. 4 Institute for Astronomy, University of Hawaii, 640 North A’ohoku Place, Hilo, Hawaii 96720, USA. 5 Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, Maryland 20723, USA. 6 Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822, USA. Vol 454| 14 August 2008| doi:10.1038/nature07154 858 ©2008 Macmillan Publishers Limited. All rights reserved when the ol/(ol1opx) ratio for the former falls into the LL compositional range (70–85%). This result is surprising, because LL chondrites are the least abundant ordinary chondrites (they represent only 10% of all ordinary chondrites, and 8% of all meteorites). It may suggest that the NEAs we sample telescopically (with radii of 0.3 km to 10 km) are not the immediate parent bodies of smaller objects that fall to Earth as meteorites (that is, pre-atmospheric meteorite parent bodies having radii on the order of metres). Our interpreted mineralogical difference between 0.3–10 km NEAs and ordinary chondrite meteorites suggests that different dynamical mechanisms and/or main-belt source regions may be responsible for supplying these two sample populations (see Supplementary Discussion). Dynamical simulations predict that ,40% of all roughly kilometre-sized NEAs should be delivered from the asteroid belt to Earth-crossing orbits by the n6 resonance located on the innermost edge of the main belt10. Interestingly, the S-type Flora family, which accounts for 15–20% of the asteroids residing in the inner main-belt region between 2.1 and 2.5 AU, borders the n6 resonance and is expected to contribute substantially to the population of NEAs11.

Using our same mineralogical modelling technique to analyse spectra of the asteroid Flora and its fellow family members, we find they have ol/(ol1opx) values in the 76–82% range. This relatively high olivine abundance of the Flora family agrees with previous measurements12,13 and is in excellent agreement with the distribution of ol/ (ol1opx) ratios for our NEA sample. Band 1 Band 2 0.5 1.0 1.5 2.0 2.5 0.8 1.0 1.2 1.4 Wavelength (µm) Relative reflectance NEA average NEA de-reddened H average L average LL average Model fit Reddening Grain 0.0 0.5 1.0 1.5 2.0 2.5 0.96 0.98 1.00 1.02 1.04 1.06 1.08 Band area ratio Band 1 centre a b Figure 1 | Spectral comparisons of asteroids and meteorites. a, Comparison between the visible to near-infrared spectral signatures of the most common NEAs and ordinary chondrite meteorites. Both populations have similar spectral characteristics, displaying broad absorptions (band 1 and band 2) due to the presence of olivine and pyroxene. For NEAs, these bands are usually superimposed on a spectral continuum whose overall reflectance increases with wavelength. Asteroids showing this reddish continuum are classified as ‘S-types’ while those that match ordinary chondrites (neutral slope) are called ‘Q-types’19,20. Space weathering processes similar to those acting on the Moon8,21 can redden and darken the Q-type spectrum of a fresh asteroid surface, giving it the appearance of an S-type spectrum2,12,22,23. The majority (,2/3) of our NEA sample show a band 1 centre at wavelengths .1.02 mm, displayed as an average spectrum before (NEA average) and after (NEA de-reddened) applying a space weathering correction9 . The average spectra for the H, L and LL ordinary chondrite sub-classes (57 ordinary chondrite spectra in total) and the model fit of the average NEA spectrum (ol/(ol1opx) < 76%) are displayed. The majority of NEAs are spectrally similar to, and have modelled mineralogies within the range of, LL chondrite meteorites. b, A quantitative comparison and error analysis for the spectral properties for our NEA sample (38 objects; circles) and our meteorite sample (57 objects; H, L, L_L symbols). PHAs are inscribed with ‘1’. Band 1 centre is defined as the wavelength that bisects that band into two equal areas. The band area ratio is band 2/band 1, calculated using the modified Gaussian method24. (Differences in conventions for fitting the continuum25 do not allow these ratio values to be directly compared between techniques.) Typical 1s observational error bars (s.d. of the mean) for the asteroids are exemplified by PHA 1862 Apollo (coordinates 0.25, 1.05), based on multiple independent observations. (Meteorite measurements are consistently repeatable in the laboratory and have uncertainties small compared to their plotted symbols.)

Spectral band centres and areas are also subject to uncertainties arising from variations in grain size and space weathering; the points at lower left depict modelled uncertainties arising from extreme ranges of these effects. Within the span of all uncertainties, ,2/3 of sampled S- and Q- type NEAs (and equivalently for the PHAs) can be seen to overlap the spectral parameter space of LL chondrite meteorites. olivine/(olivine+orthopyroxene) Frequency 0.00 0.05 0.10 0.15 0.20 0.25 Ordinary chondrites 20 40 60 80 100 0.00 0.05 0.10 0.15 0.20 0.25 Flora PHAs S- and Q-type NEAs Figure 2 | Model results for the ratio of olivine to olivine1orthopyroxene for 57 ordinary chondrites and 38 S- and Q-type NEAs. a, Ordinary chondrites; b, S- and Q-type NEAs. We also depict the inferred compositional ranges for the Flora family and PHAs (8 of 12 observed PHAs have ol/(ol1opx) ratios in the range 73–79%; the 4 others are displayed with ‘1’ symbols; error bars are 1s). About 95% (64%) of H and L ordinary chondrites are on the left of the dashed line at ol/(ol1opx) 5 70 and ,95% (64%) of LL ordinary chondrites are on the right. The x position of the H, L and LL labels indicates their average ol/(ol1opx) value. We define compatibility between an NEA and an LL chondrite when the ol/(ol1opx) ratio for an NEA falls into the LL compositional range (70–85%). It appears that most ordinary chondrites and large S- and Q-type NEAs do not have compatible mineralogy. Most NEAs (,63%) are compatible with an LL mineralogy (ol/(ol1opx) in the 70–85% range), and account for 10% of the ordinary chondrite falls. NATURE| Vol 454|14 August 2008 LETTERS 859 ©2008 Macmillan Publishers Limited. All rights reserved Our results have two direct implications. First, our observed compositional link of LL chondrites with the Flora family appears inconsistent with Flora as the source for shocked L chondrites14 (,25% of all falls), assuming Flora is a relatively homogeneous parent body. Second, the particular predominance of PHAs (as a representative sample of all NEAs) falling into the LL chondrite category would suggest that a majority of terrestrial craters may have been produced by LL chondrite impactors. At present there are only sparse data showing a mix of L and LL terrestrial impactors15,16, but this type of analysis could serve as a geological test of our results. The present observational results also raise, but do not resolve, two questions. First, why do approximately 2/3 of all NEAs appear to have compositions most similar to a class of meteorites (LL chondrites) that comprise only 8% of all meteorite falls? And second, why does the Flora family (which we suggest dominates the kilometre-sized bodies) not also dominate the flux at the smaller (metre-scale) sizes that produce the greatest number of meteorites? Size range may be the key difference between observed NEAs and the bodies that frequently deliver meteorites (see Supplementary Notes). Our PHA and NEA sample comprises approximately kilometresized bodies, whereas meteorite precursor bodies are most frequently a few metres in size. We note that size is a key factor in the efficiency of the Yarkovsky effect10,17,18, which causes bodies (of diameter D # 20 km) to drift in orbital semi-major axis and encounter major resonances that speed their delivery to the inner Solar System. At the largest sizes (kilometres), the Yarkovsky drift is expected to be limited and relatively slow (0.01 AU over ,0.5 Gyr), which implies that the source regions of observable PHAs and NEAs most probably border on resonances.

Thus the Flora asteroid family, located at the belt’s inner edge, adjacent to the highly effective n6 resonance, is particularly suited as a predominant source for delivering kilometre-sized objects to the vicinity of Earth. If kilometre-sized objects are indeed predominantly delivered from the inner asteroid belt (and perhaps the Flora family), then Yarkovsky drift and the same n6 resonance will be even more effective for delivering an inbound flux of potential meteorite bodies in the metre-size range. At such sizes, Yarkovsky drift is predicted to be much more substantial and rapid (0.1 AU over ,50 Myr) throughout the entire inner asteroid belt. Thus metre-sized bodies throughout a much greater breadth of the asteroid belt may be drifted into a number of possible resonances for transport to the inner Solar System. If this is the case, the compositional distribution of meteorite falls is likely to be very different from that of a sample drawn only from the inner edge. Thus a heliocentric-distance-dependent and sizedependent process such as the Yarkovsky effect may account for the apparently different compositional distributions we find between the kilometre-sized and metre-sized populations of bodies intersecting the orbit of Earth. Received 3 March; accepted 27 May 2008. 1. Gehrels, T., Matthews, M. S. & Schumann, A. M. Hazards Due to Comets and Asteroids (Univ. Arizona Press, 1994). 2. Binzel, R. P. et al. Observed spectral properties of near-Earth objects: Results for population distribution, source regions, and space weathering processes. Icarus 170, 259–294 (2004). 3. Stuart, J. S. & Binzel, R. P. Bias-corrected population, size distribution, and impact hazard for the near-Earth objects. Icarus 170, 295–311 (2004). 4. Rayner, J. T. et al.A medium-resolution 0.8–5.5 micron spectrograph and imager for the NASA Infrared Telescope Facility. Publ. Astron. Soc. Pacif. 115, 362–382 (2003). 5. Shkuratov, Y., Starukhina, L., Hoffmann, H. & Arnold, G. A model of spectral albedo of particulate surfaces: Implications for optical properties of the Moon. Icarus 137, 235–246 (1999). 6. Lucey, P. G. Model near-infrared optical constants of olivine and pyroxene as a function of iron content. J. Geophys. Res. 103, 1703–1714 (1998). 7. Hutchison, R. Meteorites: A Petrologic, Chemical and Isotopic Synthesis (Cambridge Univ. Press, 2004). 8. Clark, B. E., Hapke, B., Pieters, C. & Britt, D. in Asteroids III (eds Bottke, W. F., Cellino, A., Paolicchi, P. & Binzel, R. P.) 585–599 (Univ. Arizona Press, 2002). 9. Brunetto, R. et al. Modeling asteroid surfaces from observations and irradiation experiments: The case of 832 Karin. Icarus 184, 327–337 (2006). 10. Bottke, W. F., Vokrouhlikcy, D., Rubincam, D. P. & Broz, M. in Asteroids III (eds Bottke, W. F., Cellino, A., Paolicchi, P. & Binzel, R. P.) 395–408 (Univ. Arizona Press, 2002). 11. Nesvorny´, D., Morbidelli, A., Vokrouhlicky´, D., Bottke, W. F. & Brozˇ, M. The Flora family: A case of the dynamically dispersed collisional swarm? Icarus157,155–172 (2002). 12. Chapman, C. R. S-type asteroids, ordinary chondrites, and space weathering: The evidence from Galileo’s fly-bys of Gaspra and Ida. Meteorit. Planet. Sci. 31, 699–725 (1996). 13. Gaffey, M. J. Rotational spectral variations of asteroid 8 Flora: Implications for the nature of S-type asteroids and for the parent bodies of ordinary chondrite meteorites. Icarus 60, 83–114 (1984). 14. Nesvorny´, D., Vokrouhlicky´, D., Bottke, W. F., Gladman, B. & Ha¨ggstro¨m, T. Express delivery of fossil meteorites from the inner asteroid belt to Sweden. Icarus 188, 400–413 (2007). 15. Maier, W. D. et al.

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Acknowledgements We acknowledge S. M. Slivan (Wellesley College) for data processing software development. Most data were acquired by the authors operating as Visiting Astronomers at the IRTF, which is operated by the University of Hawaii under cooperative agreement NNX08AE38A with NASA’s Science Mission Directorate, Planetary Astronomy Program. This Letter is based on work supported by the NSF (grant 0506716) and NASA (grant NAG5-12355). Any opinions, findings, and conclusions or recommendations expressed here are those of the authors and do not necessarily reflect the views of the NSF or NASA. Author Contributions P.V. performed the quantitative mineral analysis that solidified the results of this Letter, and led the formulation of possible explanations. C.A.T. provided the quantitative analysis of spectral properties. S.J.B. provided the comparison to the Flora region. R.P.B. and A.T.T. served as principal investigators for a joint observing programme to acquire the near-infrared data. Most data were acquired by R.P.B, S.J.B. and C.A.T., while F.E.D. performed most of the processing. Processing routines were developed by S.J.B., A.S.R. and R.P.B. P.V. and R.P.B. worked jointly to write the Letter. All authors discussed the results and commented on the manuscript. Author Information Reprints and permissions information is available at Correspondence and requests for materials should be addressed to P.V. (