The Imbrian Crater Record and the Lunar Cataclysm
Multiple impact basins on the Moon have been dated to about 3.8 Gyr ago. This unexpected outcome of the Apollo missions has since been referred to as the lunar cataclysm or late heavy bombardment. While controversial, many authors interpret this bombardment as an impact spike (Tera et al. 1974) common to the whole inner Solar System and primarily caused by destabilized main-belt asteroids (Strom et al. 2005). A popular hypothesis for the cause of the cataclysm is a delayed planetary migration, commonly known as the Nice model (Gomes et al. 2005). We show that the absolute spatial density of impact craters on the Orientale ejecta blanket (deposited about 3.8 Gyr ago) implies that young (class 1) craters on the lunar highlands were mostly formed in the immediate aftermath of the cataclysm rather than over the last 3.5 Gyr. The formation of class 1 craters during the cataclysm is supported by the size-frequency distribution of craters assigned to th e Imbrian system by Wilhelms et al. (1978). We then argue that the projectiles bombarding the Moon at the time of the cataclysm had a size-frequency distribution that is not similar to the current asteroid belt. Thus, the lunar cataclysm was not caused by a population of main-belt asteroids destabilized by purely gravitational means.
Manicouagan and the Moon: Reassessing Impact Melt - Crater Affiliations
When Apollo samples were first returned to Earth, comparisons were made with several terrestrial impact melt sheets to aid in the interpretation of the samples. Manicouagan was considered representative of a 60 to 100 km size complex crater with a supposedly undifferentiated, chemically homogeneous, although somewhat texturally heterogeneous, impact-melt sheet. Based on the belief that craters in the size range of Manicouagan produced chemically homogeneous melt sheets, Simonds et al. (1976) identified four distinct compositions of lunar melt in Apollo 16 breccia samples, attributing each to four different impact-melt sheets formed during discrete cratering events. However, recent drilling activities at Manicouagan, combined with surface sampling and geochemical analysis, have revealed that its impact-melt sheet is not of uniform composition as suggested by past field work. This calls into question previously held assumptions regarding the identification and interpretation of lunar impact melts. Drilling has revealed an unexpectedly varied topography to the melt sheet-basement contact in the centre of the structure at Manicouagan. An elongate, impact-melt filled, N-S trough extending at least 8 km from the southern flanks of the uplifted Mont de Babel anorthosite has been identified. The trough varies in depth from 600 m at the northern and southern extremes, to 1430 m in the middle, resulting in substantially thicker melt sections than previously identified by field work, which estimated current impact-melt sheet thickness to be 200 to 300 m. Our geochemical analysis of 88 core and field impact melt samples reveal that the more typical 300 m thick sections and the newly discovered 600 m thick sections intersected within the central trough in drill holes MAN0501 and 0511, exhibit a homogeneous, quartz monzodiorite composition comparable with previous average impact melt compositions. In contrast, the 1100 m clast-free melt sequence encountered in the centre of the graben in hole MAN0608 is segregated into two compositionally distinct layers, separated by a transition zone: a 450 m thick lower monzodiorite; a 180 m thick transition zone of quartz monzodiorite (the same as the average composition of the impact-melt sheet intersected in the other drill holes), and a 450 m thick upper quartz monzonite. The identification of a thicker, fractionated impact-melt sheet section at a crater the size of Manicouagan (90 km) has implications for the interpretation of lunar samples. It is apparent that samples previously assigned to separate impact events on the Moon may be differentiates of a common impact-melt sheet. Critically, this may occur at smaller diameters than previously considered. Simonds, C.H., Warner, J.L. and Phinney, W.C. 1976. Thermal regimes in cratered terrains with emphasis on the role of impact melt. Am. Mineral. 61, 569-577.
Lunar Granulitic Breccias: Geochemistry, 40Ar-39Ar Geochronology and Conditions of Metamorphism
Lunar granulitic breccias are ancient (>3.9 Ga) metamorphosed impact breccias from the early lunar highlands. This suite of rocks was formed when impact breccias containing a mixture of igneous rock clasts were heated to form their diagnostic granoblastic to poikiloblastic matrix texture. They were subsequently excavated during later impact(s) and brought to surface. They lack KREEP-contamination but contain moderately high amounts of meteoritic siderophile elements, indicative of a history dominated by multiple impact events. The lunar granulitic breccias were metamorphosed at ~1,000°C but their conditions of metamorphism have remained enigmatic. Here, we argue for a contact metamorphism origin. We believe that the parental lithologies of the granulitic breccias were buried beneath thick (>1 km), superheated (>1900°C) impact-melt sheets or associated ejecta blankets, which provided the source of heat for metamorphism. 40Ar/39Ar data from this study reveal that three samples (60035, 77017, and 78155) have peak metamorphic ages of 4.1 Ga. Sample 79215 has a peak metamorphic age of 3.9 Ga, which may be related to Serenitatis basin formation. Post-peak metamorphism, low-temperature (<300°C) events caused the partial resetting of argon in the finer-grained granulites, but did not alter the texture or mineralogy of the rocks. The granulitic breccias have been described as "homogeneous on a mm-scale" throughout the literature; however, this study shows that large mineral clasts embedded in their metamorphic matrices preserve primary (igneous) trace element zoning, while their major element contents have been homogenized. The presence of this zoning indicates that the rocks were metamorphosed for a shorter period of time than was previously thought. Linescans and X-ray maps have been performed on these rocks in order to examine the extent of the zoning. Here we constrain conditions of metamorphism for the granulitic breccias using pyroxene thermometry, X-ray mapping, diffusion calculations, and modeling. The ubiquitous but uncommon nature of these rocks, their high temperatures of metamorphism, their moderately high siderophile contents, and their time scales of metamorphism all suggest that these rocks were metamorphosed in relatively near- surface settings. Our results increase the amount of high precision data available for the granulitic breccias and lunar highlands material. The results demonstrate the survival of pre-Nectarian material on the surface of the Moon and document the effects of contact metamorphic and impact processes during the pre-Nectarian Epoch.
Lunar Crustal Magnetism: What can we Learn From the Highs and Lows?
On the Earth, crustal magnetization has provided a window on the physical processes active below the surface. At the Moon, the highly variable distribution of lunar crustal magnetic fields may provide a similar Rosetta stone, if we can only learn how to interpret it. We are still far from being able to read the whole story of lunar magnetism, but we have made substantial progress since we first opened the book. We see a magnetic Moon that has been dominated by impact processes, with large concentrations of strong magnetic fields antipodal to young large impact basins. Meanwhile, impact sites tend to be demagnetized by the combination of heating and high shock pressures, but some larger basins show a secondary signature of remagnetization processes. The key question remains: Do the magnetic signatures that we observe today imply the presence of an early lunar dynamo? We will discuss measurements from the Lunar Prospector Magnetometer and Electron Reflectometer instruments. By focusing on the fields of impact basins and craters, and comparing measurements from two different altitudes, we present constraints on the properties of lunar crustal magnetization. Lunar crustal magnetization appears to be more incoherent, with less spatial correlation, than the terrestrial analogue. This may suggest that local processes (for instance, impacts) have dominated the creation and evolution of the current distribution of lunar crustal magnetization.
Could transient fields be responsible for the large-scale magnetization of the lunar crust?
A re-evaluation of the reliability of paleointensities inferred from Apollo samples has shown that the remanence they hold cannot be definitively identified as primary thermoremanent magnetization (TRM), thus undercutting the principal piece of evidence for a lunar "magnetic era" from 3.9-3.6 Ga. We evaluate global magnetic field data sets based on satellite observations in order to elucidate the roles of transient, impact-generated and persistent, internally-generated fields. We find that almost all Nectarian and Pre-Nectarian basins enclose significant magnetic anomalies; those from Hertzsprung (late Nectarian) onward do not. Most Nectarian basins (and at least one Pre-Nectarian) show central and rim-clustered anomalies, suggestive of TRM of the melt sheet and shock remanent magnetization (SRM) of the surroundings, respectively. Modeling of the shock and heating from a basin-forming impact shows that the crust beneath the basin floor is raised above the Curie point to 1/3-1/2 of the radius of the basin during the impact process; SRM may occur from this point out to about 2 radii. This result implies that 1) central anomalies cannot be produced by SRM during the impact, as rocks in the central region of a basin would be too hot to retain a stable remanence and 2) rim-clustered anomalies may be produced by SRM during the impact. The strongest anomalies on the Moon occur in an area on the northwestern rim of South Pole-Aitken basin that is at least 3 radii from any but the oldest Pre-Nectarian basins and encloses the antipode of the Imbrium basin. We present evidence that these anomalies are older than the Imbrium event and suggest that strong magnetic anomalies that are expected to have formed at Imbrium's antipode as a result of the impact are more localized than previously thought. An analysis of the global magnetic field distribution and its relationship to the locations of large impact basins shows that several basin- enclosed magnetic anomalies lie far enough from any younger or coeval basin or antipode that they cannot have been produced by SRM acquired in a transient field. Thus, magnetization due to transient fields alone cannot account for the large-scale magnetic field of the lunar crust. We conclude that an internally-generated field is the most probable scenario, likely ending prior to the Hertzsprung impact.