The Onset Of The Cataclysm: In Situ Dating Of A Nearside Basin Impact Melt Sheet
The impact history of the Moon has significant implications beyond simply excavating the surface of our nearest neighbor. The age distribution of lunar impact breccias inspired the idea of a catastrophic influx of asteroids and comets about 4 billion years ago and motivated new models of planetary dynamics [1, 2]. An epoch of heavy bombardment after planets had atmospheres and continents would have influenced the course of biologic evolution. The story of a cataclysmic bombardment, written in the rocks of the Moon, has far-reaching consequences. Linking lunar samples to specific basins underpins the lunar cataclysm. The inferred age of Imbrium, being the stratigraphically penultimate basin with a distinctive KREEP signature, has a U-Pb age of 3.92 Ga [3, 4] and slightly younger Ar-Ar ages around 3.85 Ga . Until recently, we thought we also had definitive dates of Nectaris, Serenitatis, and Crisium. Luna 20 samples also yielded ages of 3.85–3.89 Ga , but whether they are Crisium impact melt is debated.. Apollo 17 samples contain impact-melt rocks with ages of 3.89 Ga argued to be ejecta from the Serenitatis basin ; however, debate continues about the geological relationships at the site and the relative age of Serenitatis [8-10]. The age of materials thought to originate in Nectaris varies wildly, from 3.9 to 4.2 Ga, with little agreement on what samples in our collection represent Nectaris, if any . The lack of definitive ages for major basins throws the key arguments supporting a lunar cataclysm into doubt. To directly constrain the onset of the cataclysm requires targeted dating of a stratigraphically critical lunar basin. The Decadal Survey twice recognized the importance of understanding the cataclysm by recommending sample return from the South Pole-Aitken basin, which would enable high-precision measurements by complementary laboratory methods to resolve sample petrology and ages. However, it may be possible to understand the formation age of individual craters using in situ dating in a Discovery-class package. The Nectaris and Crisium basins define stratigraphic horizon based on relationships between ejecta units . Although both basins experienced basaltic infill and impact erosion, small areas have been identified as remnants of the basin impact melt sheets (Fig 1: Nectaris, and Fig. 2: Crisium) [12, 13]. For such key basins, outcrops of recognizable, datable impact-melt rocks are a significant find and an important potential landing site. A stationary lander could retrieve small (1-3 cm) sized rocks by scooping and sieving the regolith at a single location. Nectaris or Crisium impact-melt rocks should be aluminous and possibly slightly iron-rich – readily distinguished from KREEPy Imbrium and basaltic mare materials. Sieved samples could be dated using an ~30 kg in situ geochronology package such as the LIBS-MS technique for planetary missions, which multiple laboratories have proven to TRL 4 (Fig. 3) [14-16]. Assessing the onset of the cataclysm using the age of Nectaris or Crisium requires only modest precision: if either were 3.9 Ga, we would infer a robust cataclysm; if 4.1 Ga (as suggested by some older samples), a more expansive epoch of bombardment would be allowable; if even older, there may have been no unusual spike in flux but rather a declining rate. These intervals can be recognized with ages ±100 Myr (or less), currently achievable with in situ techniques. An in situ mission to such a landing site would constrain the onset of the cataclysm by determining the age of samples directly sourced from the impact melt sheet of a major lunar basin.  Ryder, G. (1990) EOS 71, 313, 322-323.  Gomes, R., et al. (2005) Nature 435, 466-9.  Liu, D., et al. (2012) Earth Planet. Sci. Lett. 319-320, 277-286.  Snape, J.F., et al. (2016) Geochim. Cosmochim. Acta 174, 13-29.  Stöffler, D. and G. Ryder (2001) Space Sci. Rev. 96, 9-54.  Swindle, T.D., et al. (1991) Proc. Lunar Planet. Sci. Conf. 21, 167-181.  Dalrymple, G.B. and G. Ryder (1996) J. Geophys. Res. 101, 26,069-26,084.  Schmitt, H.H., et al. (2017) Icarus 298, 2-33.  Fassett, C.I., et al. (2012) J. Geophys. Res. 117.  Spudis, P.D., et al. (2011) J. Geophys. Res. Planets 116.  Fernandes, V.A., et al. (2013) Met. Planet. Sci. 48, 241-269.  Spudis, P.D. and M.U. Sliz (2017) Geophys. Res. Lett. 44, 1260-1265.  Spudis, P.D. (2013) European Planetary Science Congress, EPSC2013–758.  Cohen, B.A., et al. (2014) Geost Geoanal Res 38, 421-439.  Devismes, D., et al. (2016) Geost Geoanal Res. DOI: 10.1111/ggr.12118.  Cho, Y., et al. (2016) Planet Space Sci 128, 14-29.