Lunar Meteorite Source Crater Size: Constraints from Impact Simulations

Hydrocode simulations of lunar impact events show that craters as small as 450 meters in diameter are viable candidates for the source of most lunar meteorites. The lunar cratering flux implies that 6 impact events of this size occurred on the moon in the last 0.1 Ma. This is in good agreement with the number of impacts (7) inferred from the geochemical analysis of the samples in hand. The results from geochemistry and numerical simulations diverge for samples with older CRE ages. This is probably a consequence of the delivery timescales and terrestrial weathering. Introduction: There are at least 12 known lunar meteorites[1]. These samples have been studied in numerous fields, including geochemical analysis (petrology, CRE studies) and celestial mechanics (orbital integration of ejected test particles). It appears that the lunar meteorites are delivered to earth very quickly: approximately 90-95% of the lunar meteorites that reach earth do so in less than 1 Ma[2]. Assigning particular samples to the same source crater (pairing) can be controversial, but it appears that the lunar meteorites represent 11 individual impact events on the moon. Based on geochemical analyses it appears that ten of these impacts probably occurred in the last 1 Ma and 7 in the last 0.1 Ma. It has been estimated from celestial mechanics and CRE data that lunar-meteoriteliberating events occur on a 10-year timescale[1]. These data allow one to estimate the size of the source craters for these meteorites. The approach used here is to simulate impact events numerically. The results can be analyzed to set limits on the minimum required crater. It is a minimum requirement that the various approaches must yield congruent results—for example, agreement on the number of source craters—before the problem can be considered well-understood. Method: I use the SALE 2D hydrocode modified to incorporate multiple materials and fragmentation to simulate impacts onto the lunar surface[3,4]. The impactor and target materials were assumed to be basaltic, using the “gabbroic anorthosite” Tillotson EOS parameters[5]. The impactor diameter studied range from 10 to 100 meters, complementing the 100 to 400 range studied in my work on the martian meteorites. The impactor velocity is 10 km/sec and the implied final crater diameter from π-scaling is 0.45 to 2.71 km. The cell size in the calculation ranged from 0.5 to 2.5 m respectively. The calculation was conducted in two parts: first, an Eulerian calculation with tracer particles generated the input boundary conditions for the subsequent Lagrangian calculation involving fragmentation. Fracture is assumed to occur in tension. This same technique was used to analyze the origin of the martian clan meteorites with good success[6]. The output was analyzed to identify fragments meeting available criteria for the lunar meteorites. The fragment size must be 3-cm or larger, as deduced from the observed size of the meteorites and inferred losses from ablation[7]. The pre-impact depth must be less than 3.2 m (CRE data) and the launch velocity greater than ~2.3 km/sec. Lunar escape velocity is 2.38 km/sec, however, under favorable conditions (full moon at perihelion) material launched at 2.20 km/sec can also escape[7]. The maximum shock pressure allowed ranged from 10 to 40 GPa. The total number of fragments meeting these criteria must exceed a certain value given by the museum efficiency Em, id est, the inverse of the number of particles ejected from a lunar impact required to expect to find one residing in a terrestrial museum. This is defined as Em = EdelEcolltterr/tCRE, where Edel is the total fraction delivered to the Earth (~50%), Ecoll is the fraction of the Earth searched with perfect efficiency (estimated to be ~10), tterr is the characteristic terrestrial age of the sample (~10 ka) and tCRE is the maximum delivery time (~100 ka for the majority of samples). For this problem Em is estimated to be ~10 -10. Once an impact event capable of producing the observed lunar meteorites is identified, the number of such events within the time scale of interest (tCRE again) can be calculated from the estimated lunar cratering flux and the surface area of the moon. This estimate of the number of source craters can then be compared to that derived from geochemical analysis. Results: In earlier work, I determined that martian craters 3 km are larger in diameter produced enough ejecta at 5 km/sec or more to be a candidate martian meteorite source crater[6]. The impactor size in that simulation was 150 meters. Clearly an event of this magnitude is more than sufficient to liberate the lunar meteorites. Indeed, a source crater size of less than 3.6 km was advocated by Warren[7]. I simulated impact events using projectile diameters of 100, 50, 30, and 10 m in diameter (for comparison, the Canyon Diablo meteorite was estimated to be 50 m in diameter) into homogeneous and layered targets. The total number of fragments produced as a function of maximum shock pressure for the two smallest events are shown in Table 1. Even for shock pressures of 10 GPa or less, the Lunar and Planetary Science XXXII (2001) 1768.pdf