Landing At Lunar Impact Craters And Impact Basins To Determine The Bombardment Of Ancient Earth
Apollo sample ages indicate impact cratering was particularly severe in the Earth-Moon system during the first billion years of its evolution. A concentration of ages circa 4 billion years ago suggests there may have been a dramatic increase in the impact flux [1,2] that also affected the entire inner Solar System [3,4]. Not only did the bombardment affect the geologic evolution of terrestrial planets, it may have also influenced the origin and evolution of life on the Earth and potentially Mars [e.g., 5]. Because the impact flux to the inner Solar System is both accessible and uniquely preserved on the lunar surface, additional samples to further evaluate the impact flux are among the highest lunar science priorities . To determine that flux and any variations in it, we need to  • Target impact craters and multi-ring basins that are representative of the flux in both time and geographic location on the lunar surface. To provide a temporally broad chronometer, we also need to • Target impact craters that provide surfaces (e.g., crater floors) that can be used to calibrate crater counting chronologies and/or • Target impact craters that provide stratigraphic horizons (e.g., ejecta blankets) that can be used for relative chronologies, even for events that may occur too close in time to be discernable using radiometric techniques. The highest priority targets, the Schrödinger and South Pole-Aitken basins (e.g., [8-10]), are on the farside and presumably not immediately accessible to commercial spacecraft. If one is limited to nearside targets, then the following are suitable. To determine the cadence of pre-Nectarian impacts the Nubium Basin (middle pre-Nectarian) and Smythii Basin (slightly younger) could be targeted. The timing of the latter third of the basin-forming events is better understood because of the availability of Apollo and Luna samples, but links between samples of known ages with specific basins produced during the Nectarian and Early Imbrian remain uncertain. For that reason, better documented samples of impact melt or impact-metamorphosed samples from the nearside Nectaris, Serenitatis, Crisium, and Orientale basins are recommended. Orientale Basin is a particularly attractive target because it is the youngest basin and exquisitely preserved, so that the geological relationships between target rocks and impact lithologies can be mapped. That clarity will dramatically assist with investigations of samples from older basins, including existing samples in the Apollo collection. After the formation of Orientale, the impact flux declined at a still-uncertain rate. We currently have no ages for post-3.8 Ga impact craters. To begin constraining the Late Imbrian, samples are needed from craters that meet target requirements, such as Humboldt and Archimedes. Eratosthenian craters that meet target requirements include Hausen, Pythagoras, Theophilus, and Eratosthenes. The ages of younger impact events during the Copernican Period are also ill-defined. Well-documented impact melt samples from nearside Kepler, Aristarchus, Copernicus, and Tycho craters would refine that chronology and constrain the impact flux during the evolution of complex life on Earth. In addition to solving several chronological problems, these same impact melt samples can be used to determine the source of projectiles and other details of bombardment in the Earth-Moon system. Complex craters and multi-ring basins are also excellent probes of the lunar interior and may provide samples of rock that can be used to assess the lunar magma ocean hypothesis too. References:  Turner G. et al. (1973) Proc. 4th Lunar Sci. Conf., 1889-1914.  Tera F. et al. (1974) Earth & Planet. Sci. Letters, 22, 1-21.  Bogard D. D. (1995) Meteoritics, 30, 244-268.  Kring D.A. and Cohen B.A. (2002) J. Geophys. Res., 107, doi: 10.1029/2001JE001529.  Kring D.A. (2000) GSA Today, 10(8), 1-7.  NRC (2007) The Scientific Context for Exploration of the Moon, 107p.  Kring D. A. (2009) Lunar Reconnaissance Orbiter Science Targeting Meeting, Abstract #6037.  Kring D. A. (2013) European Planetary Science Conference.  Potts J. J. et al. (2015) Advances in Space Research, 55, 1241-1254.  Steenstra E. S. et al. (2016) Advances in Space Research, 58, 1050-1065.