ICARUS
119 182–191 (1996) 0010
ARTICLE NO.
Meteoritic, Asteroidal, and Theoretical Constraints on the 500 Ma Disruption of the L Chondrite Parent Body HENNING HAACK Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii 96822, and Fysisk Institut, Odense Universitet, DK-5230 Odense M, Denmark E-mail:
[email protected]
PAOLO FARINELLA Dipartimento di Matematica, Universita´ di Pisa, Pisa, Italy AND
EDWARD R. D. SCOTT 1
AND
KLAUS KEIL1
Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii 96822 Received March 17, 1995; revised August 30, 1995
1. INTRODUCTION The high abundance of heavily shocked and degassed L chondrites with Ar–Ar ages around 500 Myr shows that the L chondrite parent body suffered a major impact 500 Myr ago. We infer from constraints on the thermal evolution of impact heated rocks after the 500-Myr event and the high abundance of shocked L chondrites that the parent body was catastrophically disrupted. The slow cooling rates of some shocked and degassed L chondrites (0.01–18C year21) show that they were derived from kilometer-sized impact-heated fragments or rubble piles that were ejected from near the impact point. We suggest that the catastrophic dispersion of the parent body provided some fragments with sufficiently high velocities to put them into resonances and that this initiated the orbital evolution which resulted in the high flux of L chondrite meteoroids impacting the Earth at present. It appears likely that this was a ‘‘slow-track’’ type of dynamical evolution, with most objects avoiding drastic resonant changes of orbital eccentricity, and undergoing a slow random walk in orbital element space, driven by a sequence of random encounters with Mars and, later on, with the Earth. The n6 secular resonance provides a plausible mechanism to start this evolution, since fragments inserted into it in the main belt frequently have their eccentricities raised to values of about 0.4, sufficient for Mars-crossing but not for Earth-crossing orbits; on the other hand, recent numerical work has shown that the 3 : 1 mean motion resonance leads almost always to a fast-track evolution, ending up with a solar collision or a hyperbolic ejection within a few Myr. 1996 Academic Press, Inc.
1
Also associated with Hawaii Center for Volcanology. 182
0019-1035/96 $12.00 Copyright 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
Present day asteroids are the battered remains of the small planets that formed in the early Solar System. We have been provided with samples of P80 asteroids (e.g., Keil et al. 1994) in the form of meteorites which allow us to reconstruct the early high-temperature history in some detail. The later collisional evolution of asteroids is, however, only sparsely recorded in the meteorites. The collisional evolution of asteroids needs to be established if we are to relate meteorites to present day asteroids. In the absence of direct observations of collisions between asteroids, such events can only be studied experimentally, theoretically, or by studying the fragments in the form of later generations of asteroids or meteorites. Asteroids are converted into meteorites by simple cratering events as well as catastrophic dispersal events. In general, events of both kinds are probably needed to obtain an Earth-crossing meteoroid from a main-belt asteroid. To help decipher the fragmentation history of meteorites a very wide range of meteoritic evidence must be considered. Petrologic studies may be used to constrain shock pressures, peak reheating temperatures, and subsequent cooling rates, while radiometric dating techniques constrain ages of impacts that reheated meteoroids (e.g., Bogard 1995) and exposed them to cosmic rays (e.g., Marti and Graf 1992). Because of the complexity of this evidence and the multiplicity of the events experienced by most meteorites, in no case has the complete fragmentation history of a meteorite been reconstructed.
500 MA DISRUPTION OF THE L CHONDRITE PARENT BODY
To maximize our understanding of the meteorite record of Solar System evolution, we need to relate meteorites to specific asteroids or asteroid types and understand the complex chains of collisional events which convert asteroids to meteorites. To achieve these goals, progress is needed in many related fields: (i) We need to improve our understanding of the relationship between asteroids and meteorite classes, in particular the asteroidal sources of ordinary chondrites. (ii) We need a better understanding of the evolution of asteroid families as possible outcomes of catastrophic disruption events. (iii) We need cratering models for asteroids which can provide mass and velocity distributions of ejecta for simple cratering events through super-catastrophic impacts. (iv) We need quantitative dynamic models for the delivery of ejecta from main-belt asteroids to Earth-crossing orbits. (v) We need a better understanding of the petrographic, chronological, and cosmic-ray exposure information in meteorites that relates to their impact histories. Evidence from Asteroids and Large Craters Fragments from catastrophic disruption events may be observed directly in the form of asteroids. Asteroid families such as Themis, Eos, and Koronis are believed to be fragments from very large disruption events (Chapman et al. 1989). These families can therefore be used to study the fate of fragments from catastrophic collisions (Fujiwara 1982, Zappala` et al. 1984). The Koronis asteroid 243 Ida and its moon, which were observed from the Galileo spacecraft, could be rubble piles which formed from a cluster of fragments generated during the break-up of the Koronis parent asteroid. Clusters of fragments with small internal velocity variations and, therefore, a potential for reaccretion, have been observed in experiments (Martelli et al. 1993). Earth- and space-based observations of contact or nearly contact binary asteroids appear to provide additional evidence for the existence of rubble piles (Hudson and Ostro 1994). Indirect evidence for the existence of binary asteroids can also be found in the relatively high abundance of doublet craters on the Earth and Venus (Melosh and Stansberry 1991, Cook et al. 1995). Constraints on the size of individual fragments produced in large impacts can be obtained by studying secondary impact craters on the Moon, Mercury, and Mars (Vickery 1986, 1987). These studies suggest that fragments of kilometer size (or tightly clustered fragment swarms) can be ejected in large impacts. Kilometer-sized fragments have also been reported in the Ries crater (Ho¨rz et al. 1983, Sto¨ffler and Ostertag 1983). Binzel and Xu (1993) observed 20 small (diameters P 10 km) asteroids with spectra similar to that of Vesta and inferred that they had been spalled off Vesta in a large cratering impact.
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Thermal models of the fragments produced in large impacts require constraints on the amount of impact melt produced. Model calculations of melt volume to crater diameter ratios in terrestrial craters (Grieve and Cintala 1992) may be applied to asteroids using the scaling factors for crater dimensions derived by Schmidt and Housen (1987). The calculated volumes are consistently higher than the observed impact melt volumes in terrestrial craters (Grieve and Cintala 1992) and, therefore, represent upper limits on the actual melt volumes. The models may then be used to constrain the thermal evolution of impact-heated material for different sized craters. Transfer of Meteoroids into Earth-Crossing Orbits It has been proposed that the meteorites are being brought into Earth-crossing orbits through mean motion and secular resonances (Farinella et al. 1993, 1994a, Morbidelli 1993, Morbidelli et al. 1994). However, very different time scales may apply to the different transfer routes. ‘‘Fast-track’’ resonant routes have lifetimes less than a few Myr, which is enough for the resonances to pump up the eccentricities to values close to unity, after which the bodies end up hitting the Sun or being ejected into hyperbolic orbits (Farinella et al. 1994b, Froeschle´ et al. 1995). Other orbits get out of the ‘‘resonant channels’’ and evolve in a slower, random-walk fashion, mainly as a consequence of close encounters with terrestrial planets, and this implies dynamical lifetimes on the order of 10–100 Myr (for Earthapproaching objects) or 100–1000 Myr (when only encounters with Mars can occur) (Milani et al. 1989, Jopek et al. 1995, Michel et al. 1995). Thus, a constraint on the time from breakup to Earth encounter can be used to discriminate between different transfer mechanisms. Evidence from Meteorites Although many meteorites are probably derived from cratering events that left most of the asteroid undisturbed, in some instances meteorite parent bodies were catastrophically disrupted (Keil et al. 1994). Studies of these groups of meteorites can provide estimates of post-impact cooling rates (related to fragment sizes) (Bogard et al. 1995), peak pressures and temperatures (Sto¨ffler et al. 1992), and radiometric ages of the impact events. These data may be used to constrain the mechanics of large impacts and the journey time and transfer mechanism for delivering meteorites to the Earth. In this paper, we discuss the evidence for catastrophic disruption of the L chondrite parent body 500 Myr ago and the constraints on models of catastrophic disruptions which can be derived from studies of this group of meteorites. 2. L CHONDRITES
L chondrites are the most common group of meteorites with a relative fall frequency of 38% (Sears and Dodd 1988,
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Keil et al. 1994). The average shock level is higher for the L chondrites than for any other type of chondrites. Sto¨ffler et al. (1992) studied shock levels of 50 L chondrites and found that 54% have been shocked to more than 15 GPa. Rubin (1994) found that 40% of 35 L chondrites have been shocked to more than 15 GPa. The abundance of shocked L chondrites is considerably higher than the abundances of shocked material found in ejecta from terrestrial craters. Ho¨rz et al. (1983) found that 90% of the ejecta from the Ries crater in Germany were shocked to less than 5 GPa. 40 Ar– 39Ar and 87Rb– 87Sr ages of the shocked and degassed L chondrites cluster around 500 Myr, which suggests that the L chondrite parent body suffered a single major impact at that time (Heymann 1967, Turner 1969, 1988, Bogard and Hirsch 1980, Bogard et al. 1976, 1995; McConville et al. 1988, Nakamura et al. 1990, Fujiwara and Nakamura 1992, Keil et al. 1994). The recorded shock levels (Sto¨ffler et al. 1992) are therefore most likely the result of a single major impact rather than an accumulated effect acquired through numerous impacts over time. Metallographic cooling rates of lightly shocked or unshocked L chondrites are in the range 1–108C Myr21 (Taylor et al. 1987). The slow cooling rates seem to require that the radius of the original parent body was more than 50 km even if the surface was covered with a layer of insulating regolith (Wood 1979, Haack et al. 1990). The cooling rate can only constrain the maximum size of the body involved in the 500-Myr event, since later impacts may have reduced the size of the original parent body significantly. The 500Myr event may thus only have involved the eroded remnants or just a fragment of the original asteroid. Studies of L chondrites have provided constraints on their thermal evolution during and after the impact on the parent body 500 Myr ago. The heavily shocked 105-kg L6 chondrite Chico contains approximately 60% impact melt (Bogard et al. 1995). The large volume of impact melt and in particular the heavily shocked host rock suggest that this rock was close to the center of the impact site of the 500-Myr event. Cooling rates of L chondrites which were strongly reheated in the 500 Myr event have been determined for seven L chondrites (Smith and Goldstein 1977, Bogard et al. 1995). These cooling rates at P6008C range from 0.01 to 18C year21 for Chico and Wickenburg (Smith and Goldstein 1977, Bogard et al. 1995) to 1008C day21 for Ramsdorf (Smith and Goldstein 1977). We will use these cooling rates of L chondrites as constraints on the catastrophic and non-catastrophic models of the 500-Myr event discussed below. The low abundance of solar gas-bearing L chondrites also provides evidence of the catastrophic nature of the 500-Myr event (Crabb and Schultz 1981, Rubin et al. 1983). ˚ of an asteroid surface is subject to The upper 1000 A implantation of solar wind gases. Solar gas-bearing meteorites are always regolith breccias which contain a large frac-
tion of material which, at some point in time, has been exposed to the solar wind. Fragmental breccias without solar gases may not come from the surface since it is possible that the L chondrite parent body has experienced a breakup and reassembly event prior to the 500-Myr event (Taylor et al. 1987, Keil et al. 1994). Although meteoroids en route to the Earth are exposed to the solar wind, space erosion and atmospheric ablation removes any solar wind implanted material. Solar gas-bearing meteorites can therefore only be derived from the surface of a parent body with a gravity field strong enough to allow regolith processing. Only 3% of the L chondrites are solar gasbearing, in contrast to 15% of the H chondrites (Crabb and Schultz 1981). The low abundance of solar gas-bearing L chondrites suggests that a large fraction of the meteorites has been derived from the interior of the parent body, suggesting breakup by a major impact. Alternatively, the low abundance of solar gas-bearing L chondrites can be a result of their high shock level. Sto¨ffler et al. (1992) found that the solar gas concentration decreases with increasing shock level among ordinary chondrites. Exposure ages of L chondrites can also be used to constrain the fragmentation history of L chondrite material between the 500-Myr impact event and atmospheric entry. The exposure age of a meteorite is a measure of the time it has spent as a meter-sized object exposed to cosmic rays (Voshage 1967, Voshage and Feldmann 1979, Marti and Graf 1992). Several groups of meteorites such as the IIIAB iron meteorites show clusters in exposure ages consistent with a common launch time as meter-sized meteoroids from their parent asteroid. There are, however, only a few stony meteorites with cosmic ray exposure ages of more than P50 Myr (Crabb and Schultz 1981). This is usually attributed to shorter lifetimes for meter-sized stony meteoroids exposed to space erosion than similarly sized iron meteoroids. The 500-Myr event is therefore not recorded in the cosmic ray exposure ages of L chondrites. The L chondrites show peaks at 5, 28, and 40 Myr in their cosmic ray exposure age spectra (Marti and Graf 1992). The presence of these peaks implies that most of the L chondrites with cosmic ray exposure ages around 5, 28, or 40 Myr resided in either one object that was bombarded three times, or several (up to three) that formed after the 500Myr event. 3. DISCUSSION
There seems to be little doubt that the L chondrite parent body experienced a major collision P500 Myr ago. Although there is no conclusive evidence that the L chondrites are derived from a single parent body, the high abundance of shocked and degassed L chondrites with similar Ar–Ar ages makes it likely. If the lightly shocked or unshocked L chondrites were derived from a second
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body, our estimates of the energy of the 500-Myr event would only be increased. The question we wish to address below is whether the 500-Myr event was a simple cratering event, or whether it was of sufficient magnitude to catastrophically fragment and possibly disperse the asteroid. Most of the projectile kinetic energy is converted to heat (e.g., Davis et al. 1994). Since the kinetic energy of the projectile must be smaller in the simple cratering case constraints on the heat pulse generated by the impact can be used to discriminate between simple cratering and dispersion. Simple Cratering Event Here we investigate the hypothesis that the 500-Myr event was a simple cratering event, i.e., that it did not fracture the entire parent body. In this case, the L chondrites could either be ejecta produced by the crater-forming event, material ejected by later impacts, or fragments from a later breakup. The much higher fraction of shocked L chondrites in comparison with shock levels observed in ejecta of terrestrial craters (Ho¨rz et al. 1983) makes it unlikely that the L chondrites are ejecta samples from the 500-Myr event. If the L chondrites were derived from the parent body by later impacts or a breakup event, the high fraction of shocked L chondrites suggests that we preferentially sampled the 500-Myr old crater. In this case, the L chondrites were probably all derived from a single later impact event which fortuitously excavated only highly shocked material from within the old crater. The three peaks in the cosmic ray exposure ages (e.g., Marti and Graf 1992) are therefore most likely due to later events involving material derived from the 500-Myr old crater. The range of thermal histories experienced by different L chondrites during and after the 500-Myr event appears to be inconsistent with excavation from a limited volume inside the inferred crater. The low cooling rate of Chico and Wickenburg indicates that they cooled inside a volume of heated material with dimensions of hundreds of meters. It is therefore enigmatic that Chico and Wickenburg seem to be unique if this entire volume and surrounding rocks, which were not heated during the 500-Myr event, were excavated in a later impact. The large dimensions also indicate that the excavation impact would have to be very large—yet no L chondrites have been thoroughly reset after the 500-Myr event (Bogard 1995). We can constrain the dimensions of the inferred crater on the basis of the cooling rates of the fragments. The most likely origin of a heavily shocked sample with abundant impact melt such as Chico is from a dike complex in the crater floor directly beneath the impact site (Melosh 1984, 1989, Sto¨ffler et al. 1991, Bogard et al. 1995). We assume that the melt breccia was evenly distributed over
FIG. 1. Cooling rate at 6008C against depth for a 300-m-thick sheet of melt breccia. The initial temperature of the melt breccia is assumed to be 10008C and heat is conducted away at the bottom of the sheet and radiated away at the top. Cooling rates within a melt breccia with a thickness of 3000 m (e.g., one order of magnitude more) exhibit the same pattern but are two orders of magnitude lower.
the crater floor and therefore had lateral dimensions much larger than the vertical dimension. The slowest cooling rates found for L chondrites reheated in the 500-Myr event are 0.01–18C year21 (at 500–7008C) for Chico and Wickenburg (Bogard et al. 1995, Smith and Goldstein 1977). Since the higher cooling rates of other L chondrites can be attributed to the proximity to a cold region and/or the surface, we will only consider the lowest cooling rate. The initial temperature of the melt breccia and the dike complex must have been between the initial temperature of the pure melt (P13008C, Bogard et al. 1995) and the temperature of relatively cold clasts. We assume an initial temperature of 10008C for the mixture of melt and cold clasts (corresponding to 33 vol% cold clasts), a fixed surface temperature of 2708C, and a thermal conductivity of 2 W m21 K21. Since we do not have data to justify a distinction in thermal properties between the melt breccia and the dike complex, we will use the term melt breccia for both in the following. Figure 1 shows the cooling rate at 6008C as a function of depth through a 300-m-deep melt breccia. The cooling rate profile through a 3000-m-deep melt breccia exhibits the same pattern as that of a 300-m-deep melt breccia but is two orders of magnitude lower (i.e., on the order of 0.018C year21). We find that the melt breccia must have been at least 300 m deep if parts of its interior cooled at no more than 18C year21. If the cooling rate was closer
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to 0.018C year21, a depth of 3000 m seems more likely. The estimates of the depth of the melt breccia may then be related to the diameter of the crater using the model of Grieve and Cintala (1992), which relates crater diameter to melt volume. On asteroids with lower gravities than the Earth, impacts can excavate a much larger transient cavity and therefore eject a higher fraction of the melt. Since the remaining melt volume will be distributed in an enlarged crater, it becomes very difficult to create a thick melt breccia on a low-gravity object like an asteroid. We cannot constrain the increase in the amount of ejected melt but it is possible, using the scaling factors of Schmidt and Housen (1987), to scale the diameters of the craters to asteroid gravities (0.1 m sec22 for an P100 km radius asteroid) and impact velocities of 5 km sec21 Vm P 1025 D3.85
(1)
where Vm is the melt volume in km3 and D the crater diameter in km. Since a substantial fraction of this melt may have been ejected from the asteroid, Eq. (1) gives an upper limit on the amount of melt produced in a crater with diameter D. Assuming for simplicity that the melt breccia is evenly distributed throughout the crater and allowing for 33 vol% cold clasts in the melt breccia, the depth of the melt breccia, d, measured in meters, may be expressed as d P 2 3 1022 D1.85
(2)
Using this expression, a depth of between 300 m and 3 km would require a crater diameter of 200 to 600 km (Fig. 2). Even if we assume that we have underestimated the melt volume by an order of magnitude and, furthermore, use the minimum melt breccia depth of 300 m, a crater diameter of 60 km is required. A larger crater is probably required because Eq. (1) is based on the Grieve and Cintala (1992) model, which tends to overestimate the melt volume by up to an order of magnitude by comparison with melt volume estimates in terrestrial craters, and because we have not considered ejection of melt from the crater. The required minimum diameter of the crater is therefore approximately equal to the estimated diameter of the entire original parent body which was in all likelihood much larger than the diameter of the eroded remains of the parent body that suffered the impact 500 Myr ago. If the target was Vesta-sized and survived the 500-Myr impact we should also have expected it to survive to the present. Since there are no Vesta-sized ordinary chondrite parent body candidates, we infer that the L chondrite parent body suffered a catastrophic disruption 500 Myr ago. Craters on a 100-km-radius asteroid could have diameters of up to P70 km and, therefore, depths of the melt breccia of P50
FIG. 2. Melt breccia depth versus crater diameter on a 100 km radius asteroid based on Eq. (2) with an estimated one order of magnitude uncertainty (dotted area). The two horizontal lines show the range of melt breccia depths inferred from the cooling rates of Chico and Wickenburg. For the melt sheet thicknesses derived from the cooling rates, the suggested crater diameter is 2–6 times larger than the diameter of the original asteroid. Therefore, a non-catastrophic crater forming impact seems inconsistent with the cooling rates of impact in L chondrites. On a smaller asteroid with less gravity, the diameter of the crater for a given melt volume would be larger, thus making the depth of the melt breccia even more shallow (Schmidt and Housen 1987).
m, which is one to two orders of magnitude less than implied by the cooling rates. A high porosity of the target could increase the melt volume for a given projectile energy over that found by Grieve and Cintala (1992). Because of the low pressures of asteroidal interiors it is unlikely that porosity caused by previous large impacts could be removed by creep. Seismic studies of the Moon suggest that the lunar surface is brecciated, and thus porous, down to a depth of 20 km (e.g., Carr 1979). The pressure at 20 km depth on the Moon is approximately 100 MPa or approximately an order of magnitude more than the pressure inside a 50- to 100km radius asteroid. It is therefore possible that the entire asteroid had some porosity prior to the 500-Myr event. Schaal et al. (1979) found that the melt volume produced in a fine-grained particulate basalt (35–40 vol% porosity) could be up to an order of magnitude more than the melt volume produced in similar non-porous target for shock pressures .35 GPa. This is an upper limit, since the porosity in the interior of an asteroid is not likely to exceed that of fine-grained basalt. Two other factors control the difference between melt
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production in porous and non-porous targets. One is that a rubble pile target can sustain a more energetic impact without being dispersed. Therefore, the upper limit of the energy available to melt material is increased. The other factor is that the shock wave dissipates faster in a porous target, e.g., the volume of heavily shocked and heated material is more confined (e.g., a deeper melt breccia). However, since the depth of the melt breccia would have to be increased by approximately an order of magnitude for the largest possible crater even on the largest possible L chondrite parent body, assuming that all of the melt stayed in the crater, we find that it is very unlikely that a melt breccia in a crater could be sufficiently deep. Another cause of concern is that displaced regolith due to an adjacent impact after the 500-Myr event could insulate the melt breccia and thus lead to an overestimation of the thickness of the melt breccia. Since the cooling rate estimates from Chico and Wickenburg indicate that the melt breccia cooled from 1000 to 6008C at 0.01 to 18C year21, the adjacent impact had to occur within 400 to 40,000 years after the 500-Myr event. It is, however, highly unlikely that such an impact would take place so shortly after the 500-Myr event. The low cooling rates of Chico and Wickenburg and the high fraction of heavily shocked L chondrites therefore appear to be inconsistent with a non-disruptive crater-forming impact event 500 Myr ago. Catastrophic Fragmentation Catastrophic fragmentation of the L chondrite parent body 500 Myr ago is consistent with the high abundance of heavily shocked and outgassed L chondrites, with Ar–Ar and Rb–Sr ages around 500-Myr, and with the scarcity of solar gas-bearing L chondrites. The cooling rates of the impact melt in L chondrites such as Chico constrain the thermal evolution after the 500-Myr event. In order to relate the measured post-impact cooling rates to fragment sizes, we need to know whether the impact melt cooled in reaccreted rubble piles or in individual fragments produced in the 500-Myr event. If the asteroid was catastrophically fragmented, the L chondrites could have been derived from three different sources: (a) from a rubble pile generated in a breakup and reassembly event; (b) from multiple rubble piles accreted from clusters of fragments ejected during a catastrophic dispersion event (e.g., Martelli et al. 1993); (c) from large coherent fragments generated in a catastrophic dispersion event. (a) Breakup and reassembly event. Formation of a rubble pile after a catastrophic impact is more likely for a large parent body with a large gravitational binding energy than for a smaller body. For small bodies, the energy range between the energy required to break up the body and the energy required to disperse the fragments is very narrow. It therefore seems unlikely that a significant number of
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fragments would reaccrete after a catastrophic impact, unless the diameter of the possibly eroded remains of the parent body 500 Myr ago was still more than 10–100 km (Davis et al. 1979, 1983, Scott et al. 1995). Even in this case, a sufficiently energetic event would disperse the fragments. The large dimensions of the (transient) crater calculated above, based on the constraints on the volume of heated material, seem to indicate that the kinetic energy of the projectile required to heat a large enough volume in the target was also sufficient to disperse the fragments. It therefore seems likely that the 500-Myr event injected the L chondrite meteoroids into transfer orbits. (b) Clusters of fragments. The fragment jets and clusters of fragments observed in the explosive experiments of Martelli et al. (1993) suggest that it is possible to generate rubble piles even in a situation where the energy is sufficiently high to disperse the fragments. If Ida and its moon are indeed rubble piles formed during the possible breakup of the Koronis parent asteroid, it seems likely that they formed from clusters of fragments. Therefore, in a catastrophic dispersion event, a number of rubble piles and a large number of individual fragments could be generated. The slow cooling of Chico and Wickenburg could therefore be due to insulated cooling in a rubble pile which accreted from a cluster of fragments. The thermal conductivity of megaregolith is approximately an order of magnitude lower than the thermal conductivity of equivalent coherent rock. Therefore, the minimum sizes inferred from the cooling rate of Chico are smaller for rubble piles (by a factor of 3) relative to coherent rock fragments. (c) A large fragment. If the impact melt in Chico cooled within a single hot fragment, the low cooling rate (0.01–18C year21) (Bogard et al. 1995) indicates that the fragment was of at least kilometer-size (Fig. 3). This is inconsistent with numerical models of catastrophic fragmentation which predict that the fragment size approaches zero near the impact site (Paolicchi et al. 1989). However, studies of secondary impact craters on the Moon, Mars, and Mercury and the observations of asteroids which appear to have been ejected from Vesta suggest that 5- to 10-km fragments can be ejected from large impact craters (Vickery 1986, 1987, Binzel and Xu 1993). The thermal constraints inferred from the cooling rates of Chico and Wickenburg can be satisfied with a single large fragment since the much higher energy involved in a dispersive collision increases the amount of melt and therefore makes it possible to heat kilometer-sized fragments to P10008C. Kilometer-sized, fractured but coherent fragments have also been observed around the Ries crater in Germany (Sto¨ffler and Ostertag 1983). These fragments were probably derived from the crater floor since the stratigraphy shows that they were uplifted by P500–1000 m. The Ries fragments (megablocks) are, however, only weakly
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FIG. 3. Central cooling rate at 6008C of a spherical chondritic fragment. The initial temperature of the fragment is assumed to be 10008C and the surface is kept at 200 K.
shocked and do not contain impact melt. Numerous dikes in the megablocks are filled with shocked fragments rather than impact melt. The megablocks are therefore not equivalent to the rock type sampled by Chico, but their dimensions show that large fragments can survive the impactgenerated stress fields in the vicinity of large impacts. If Chico and Wickenburg cooled slowly within large volumes of melt breccia, then it is surprising that we have received so few L chondrites with slow cooling rates and a high fraction of impact melt. Chico and Wickenburg do, however, have unusual exposure ages which makes it possible that they were derived from very poorly sampled material. Garrison et al. (1992) found that Chico has an unusually long exposure history different from that of other L chondrites, and Heymann (1967) found an unusually short exposure age for Wickenburg (2.6 Myr). The low cooling rates of Wickenburg and Chico may therefore equally well be explained in terms of one or more rubble pile(s) or one or more large fragment(s). In either case, the L chondrite meteoroids were probably ejected from the parent body 500 Myr ago, initiating the orbital evolution which resulted in the high flux of L chondrite meteoroids impacting the Earth at present. 4. TRANSFER OF FRAGMENTS TO EARTH-CROSSING ORBIT
If the 500-Myr L chondrite impact event catastrophically disrupted the parent body, we have a unique opportunity to study transfer times of fragments from the asteroid belt
to the surface of the Earth. Possible transfer mechanisms are the mean motion resonances (e.g., Froeschle´ and Greenberg 1989) and secular resonances (e.g., Scholl et al. 1989). The peaks in the cosmic ray exposure ages imply that the L chondrites were involved in several collisions after the 500-Myr event. None of these collisions have, however, reset the Ar–Ar ages, which suggests that the later collisional events were minor. Two types of chaotic evolution are possible for fragment orbits injected into resonances from main-belt parent bodies. Both mean motion and secular resonances can cause drastic increases of the orbital eccentricity over time scales of a few Myr, and in this case the object is bound to fall into the Sun or to leave the Solar System after a jovian encounter within such a short time scale. According to numerical integrations, such ‘‘fast-track’’ evolution patterns are typical of a significant fraction (on the order of 20%) of both near-Earth asteroids (Farinella et al. 1994a) and meteoroids (Jopek et al. 1995). The observed bodies on such orbits must be relatively young, i.e., their ejection from the main belt must have occurred less than, say, 10 Myr ago, implying relatively short exposure ages. On the other hand, many near-Earth asteroids/meteoroids appear to be currently parked into ‘‘slow-track’’ non-resonant orbits, which evolve slowly as a consequence of chance encounters with Mars and/or the Earth, and have dynamical lifetimes ranging from tens (for Earth-crossing orbits) to many hundreds of Myr (for orbits reaching Mars only). Among these objects, we may be seeing fragments that have been wandering in the planet-crossing region since they were ejected in collisions which took place in the main belt hundreds of Myr ago. When they become Earth-crossing, both types of orbits have collisional lifetimes versus Earth impact of the order of 100 Myr (Milani et al. 1990, Bottke et al. 1995). This means that only a small fraction of the fast-track objects (but a significant fraction of the slow-track ones) end up hitting a terrestrial planet instead of colliding with the Sun or undergoing hyperbolic ejection. Also, due to the different dynamical lifetimes, the ratio of slow-track to fast-track objects in the observed, quasi-steady state population of near-Earth bodies (and in the flux of Earth-impacting objects) must be much higher than the ratio between the original fluxes into the corresponding orbital routes. As for L chondrites, the evidence summarized above (500-Myr old disruption of the parent body, subsequent minor collisions 5, 28, and 40 Myr ago) appears to favor the slow-track transport mechanism from the site of the 500-Myr event to the Earth. A fast-track route might be viable only if the 500-Myr event parked a number of fragments close to one of the resonant channels in the main belt; later and smaller impacts on these fragments might have then injected material in the resonance. But it appears
500 MA DISRUPTION OF THE L CHONDRITE PARENT BODY
unlikely that this two-stage process could have provided us with the high observed flux of L chondrites: there are certainly many other asteroids close to the resonances in the main belt, and all of them should be capable of delivering ejecta to Earth-crossing orbits (see Farinella et al. 1993), so why would we see such a high percentage of L chondrite meteorites among falls? Also, the minor collisions recorded in the exposure ages appear too old for impacts that injected meteoroids into the fast-track orbits, because most bodies in such orbits are eliminated by solar collisions or hyperbolic ejections within a few Myr. Thus it appears likely that the L chondrite fragments from the 500-Myr event ended up in slow-track orbits, and probably remained in Mars-crossing but not Earth-crossing (i.e., Amor-like) orbits for most of the time. Mars’ smaller mass and cross section make it much less efficient than the Earth (and Venus) in modifying the orbits through close encounters, and for the same reason collisions with Mars provide a much slower sink mechanism to remove material. Recent dynamical work (Morbidelli et al. 1994) has shown that the n6 secular resonance can often pump up the orbital eccentricity just to Mars-crossing values (about 0.4); whence, Mars encounters are required to change the orbit in such a way to allow further increases in eccentricity. This would provide the fragments with a long enough interval of time between the original break-up event in the main asteroid belt and their eventual transformation into Earthcrossing meteoroids. On the other hand, this appears more difficult for the 3 : 1 route. Both analytical work and numerical experiments (Moons and Morbidelli 1995a, 1995b, Farinella et al. 1994b) have shown that the 3 : 1 jovian mean motion resonance is normally associated with fast-track dynamical evolution routes, leading to very high orbital eccentricities and therefore to solar collisions or hyperbolic ejections within a typical time span of a few Myr. Thus, the 3 : 1 resonance could provide a source of slow-track bodies only provided encounters would extract the orbits from it at an early stage of their evolution (i.e., at moderate values of the eccentricity), so as to leave them ‘‘parked’’ for a comparatively long time in a non-resonant zone of the phase space. This scenario appears quite unlikely, though not impossible; only further long-term numerical experiments will provide statistical information on the corresponding probabilities and evolution time scales. Independently of their original source and detailed dynamical evolution mechanisms, the slow-track scenario described above makes the high abundance of L chondrites among the observed falls consistent with the fact that the majority of near-Earth asteroids, and in particular the largest ones, such as 1036 Ganymede and 433 Eros, are currently on slow-track orbits driven by Mars encounters, and probably had a dynamical history similar to that outlined above for the L chondrites. Another clue in this direction has been provided by Jopek et al. (1995), who
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found this kind of slow-track evolution for the numerically integrated pre-fall orbits of some meteoroids associated with meteorite finds. ACKNOWLEDGMENTS We thank Stan Love, Dieter Sto¨ffler, and Jeff Bell for valuable discussions. Mike Nolan and Alan Rubin are thanked for helpful reviews. This work was supported in part by NASA Grant NAGW 3281 (K. Keil, PI). This is Hawaii Institute of Geophysics and Planetology publication No. 855, and School of Ocean and Earth Sciences and Technology publication No. 3975. H.H. acknowledges support from the Danish National Science Research Council.
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