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The Orbits of Meteorites from Natural Thermoluminescence

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The Orbits of Meteorites from Natural Thermoluminescence NASMCR---- _7 _7-- 207733 ./:;;/:y :',v" • ii., (:- _ -_"Z_- Attachment 5 ICAm:S 125. 281-287 (1997) \RTICLE Nf). IS965622 : i .'i_.-' /..¢__.,_"_7. _,, _1, j The Orbits of Meteorites from Natural Thermoluminescence P. H. BENOIT AND D. W. G. SEARS Cosmochemistry (Troup, Department of Chemistry and Biochemistry. University of Arkansas. Fayetteville. Arkansas 72701 E-mail: [email protected] Received December 15, 1995; revised July 29, 1996 The possible existence of meteoroid streams, even as The natural thermoluminescence (TL) of meteorites reflects relatively minor contributors to the terrestrial flux of extra- their irradiation and thermal histories. Virtually all ordinary terrestrial material, is important in understanding the or- chondrites have been irradiated long enough to have reach bital evolution of meteoroid bodies, the rarity of potential saturation natural TL levels, and thus natural TL levels in asteroidal sources for ordinary chondrites, which account these meteorites are determined largely by thermal history. The for more than 60% of meteorite falls (Greenberg and Nolan primary heat source for most meteorites is the Sun, and thus 1989), and apparent differences between meteorites cur- natural TL levels are determined primarily by the closest ap- rently falling and those that fell over the last few hundred proach to the Sun, i.e., perihelion. By converting natural TL thousand years and have been stored in the ice sheet of levels to perihelia, using an assumed albedo typical of meteoroid Antarctica (Koeberl and Cassidy 1991, Benoit and Sears bodies, it is found that most ordinary chondrites had perihelia of 0.85 to 1.0 AU prior to reaching Earth. This range is similar 1993a, Wolf and Lipschutz 1995). to that calculated from meteor and fireball observations. All Precise orbits are known for only 4 meteorites (Wetherill common classes of ordinary chondrites exhibit similar perihelia and Chapman 1988, Brown et aL 1994). Orbital parameters distributions; however, H and LL chondrites that fell in the have been estimated for another 40 meteorites from eye- local morning differ in their natural TL distribution from those witness accounts of their fall (Simonenko 1975, Wetherill that fell in the local afternoon and evening. This is consistent and Chapman 1988), but these are of low accuracy. Limited with earlier suggestions that time of fall reflects orbital distribu- orbital information can also be obtained from the local tion. The data also suggest that the orbits of some of the H time of fall (Wetherill 1968). Meteorites with perihelia _1 chondrites cluster and may have come from a debris "stream" astronomical unit (AU) should usually strike the trailing of meteoroids. If meteorites can exist in "orbital groups," sig- side of the Earth, or the local PM, while meteorites with nificant changes in the types and number of meteorites reaching perihelia < 1 AU should be more evenly spread over the Earth could occur on the < 10S-year time scale. _ 1997AcademicPress leading and trailing sides. The large abundance of ordinary chondrites falling in the local PM relative to AM is interpre- INTRODUCTION ted as indicating that most of these meteorites had perihelia of _1 AU. Most meteorites are thought to be samples of asteroids In this study we use the natural thermoluminescence of (Wetherill 1985); however, it is unlikely that meteorites individual meteorites to estimate their closest approach to are random representative samples of their parent bodies. the Sun. Natural thermoluminescence is energy stored in Their passage to Earth depends on their original orbit and crystals of certain minerals by ionizing radiation, such as the stochastic nature of the collisional events that ejected high-energy galactic cosmic rays (Aitken 1985, McKeever them into space (Wetherill 1985. Greenberg and Nolan 1985). This energy can be released in the form of visible 1989, Binzel et aL 1993). It has been proposed that some light by heating. fraction of the meteorite flux derives from coherent orbital The natural thermoluminescence (TL) of meteorites re- "'streams" of meteoroids (Halliday 1987. Doddet aL 1993). flects both their thermal and irradiation history (McKeever This idea is indirectly supported by orbital clustering of 1980. Benoit et aL 1991). Virtually all ordinary chondrites meteor-producing bodies and temporal clusters of meteor- have been exposed to galactic cosmic radiation for millions oid impacts on the Moon (Olsson-Steel 1988, Halliday et of years (Crabb and Schultz 1981), far longer than the al. 1990, Drummond 1991. Oberst and Nakamura 1991). -105 years required to obtain TL equilibrium (Benoit et There is. however, little evidence that, most meteorites al. 1991). Depth effects on TL levels are minimal, TL were members of meteoroid streams at the time of their profiles generally exhibiting <30% variation in even the fall (Wetherill 1985). largest meteorites (Benoit and Chen 1996). The intense 281 0019.1035/97 $25,00 Copyright _ 1997 by Academic Press All rights of reproduction in any [orm reserved. 282 BENOIT AND SEARS I I l I I I I heating of atmospheric passage drains natural TL in the outermost few millimeters of meteorites (Sears 1975, Be- noit and Chen 1996), but we have deliberately avoided 1000 /S" sampling these regions in this study. Therefwe, the natural /., ,'/i,"" TL of modern falls is controlled largely bv the maximum /,. temperature experienced in space, which occurs at perihe- lion. The only exceptions are very large meteoroid bodies 8 and meteoroids that have experienced shock heating within ('- 100 o ,." the last lip years, and both these exceptions appear to be {/) ,,." very rare among the ordinary chondrites. t'- g /.." Following the treatment of Garlick and Gibson (1948), I.: natural TL can be considered as the filling of "traps" bv o g electrons put into the conduction band as a result of inter- _ 10 Dose Rate e.- action with energetic particles, such as high-energy cosmic .." [ (Gy/year) rays. The exact nature of these "'traps" is presently un- ] -- 001 known, but is generally considered to involve crystallo- [ ....... 0.005 Z graphic defects and impurities (Prescott and Fox 1993). In .." ] ........ 005 crystals, populations of these "'traps" are the source of TL. I I I I 1 I I I I the electrons being freed from the traps and emitting a 0.6 0.8 1 12 photon of light as the sample is heated. A typical TL Perihelion (AU) experiment resulting in a "'glow curve" involves heating a sample up to about 500°C in an inert atmosphere at a FIG. l. Equilibrium natural thermoluminescence ol ordinary, chon- constant rate, while monitoring the amount of light given drites at 250°C in the glow curve as a function of closest approach to the Sun {perihelion). Perihelion is given in astronomic units (AU). off by the sample as a function of temperature. In the present study we rearrange the basic equilibrium TL equa- tion to solve for temperature (McKeever 1980, Benoit et al. 1991), a blackbody approximation in this case is appropriate be- -E cause no asteroids, including those observed closely by T_. (1) space probes (Helfenstein et al. 1994), have atmospheres and the abundances of radioactive elements are too low in klnI([N/n]-7._l) ln(2)r I ' ordinary chondrites for these bodies to have any significant degree of internal heating during the last few million years. where n/N is the measured TL level relative to saturation. As a simplifying assumption, we set e to unity and estimate R is the radiation close required to fill half the TL "traps." a value of a of 0.85 from the albedos of meteorite samples k is Boltzmann's constant, and r is the radiation dose rate. in the laboratory and remote-sensing measurements of as- We use a dose rate of 0.01 Gy/year (l rad/year) in our teroid surfaces (Chapman and Salisbury 1973, Helfenstein calculations: the actual dose rate experienced by samples et al. 1994). In Fig. 1 we show the relationship between will depend on the depths of the samples in the meteoroid. natural TL level and perihelia for ordinary chondrites de- a factor we discuss below. E and s are characteristic of rived from Eqs. (1) and (2). This calculation shows that the TL phosphors, sodic feldspar in the case of ordinary equilibrium natural TL levels exhibit a strong dependence chondrites, at a particular portion of the glow curve and on perihelia for perihelia <1.1 AU. We show curves for they are essentially constants for ordinary chondrites three dose rates, ranging from 0.005 to 0.05 Gy/year. In (McKeever 1980). We can then calculate the distance from the following discussion we quote perihelion values using the Sun (X) from the calculated temperature assuming the 0.01-Gy/year curve, an apparently typical value for blackbody behavior. ordinary chondrite meteoroid bodies (Benoit and Chen 1996. and references therein). The 0.005- and 0.05-Gy/year curves are more appropriate for samples from very small (2) and very large meteoroid bodies, respectively, and it is apparent that differences in meteoroid size have fairly min- imal effect on our interpretation. where _ and e are the absorptivity and emissivity of the When we give a perihelion value for a particular meteor- surface of the meteoroid body, H is the solar constant, and ite. numerous uncertainties are included, some of which er is the Stefan-Boltzmann constant (Melcher 1981). Using cannot be estimated with accuracy. Equation (2) is suscep- ORBITS OF METEORITES 283 Perihelion (AU) tible to uncertainties in albedo. Meteorite samples do ex- 06 07 08 0.9 1.0 hibit some range of albedoes (Chapman and Salisbury i i 1973) and the relationship between the albedoes of meteor- 161 H Chondrite 14 ites and those of the surface of meteoroid bodies, of prime 12 importance in this study, is not known.
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