The Onset Of The Cataclysm: In Situ Dating Of A Nearside Basin Impact Melt Sheet

Barbara Cohen
Abstract Title: 
The Onset Of The Cataclysm: In Situ Dating Of A Nearside Basin Impact Melt Sheet
Presentation PDF: 
Abstract Type: 
Abstract Body: 
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 [5]. 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 [6], 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 [7]; 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 [11]. 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 [10]. 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. [1] Ryder, G. (1990) EOS 71, 313, 322-323. [2] Gomes, R., et al. (2005) Nature 435, 466-9. [3] Liu, D., et al. (2012) Earth Planet. Sci. Lett. 319-320, 277-286. [4] Snape, J.F., et al. (2016) Geochim. Cosmochim. Acta 174, 13-29. [5] Stöffler, D. and G. Ryder (2001) Space Sci. Rev. 96, 9-54. [6] Swindle, T.D., et al. (1991) Proc. Lunar Planet. Sci. Conf. 21, 167-181. [7] Dalrymple, G.B. and G. Ryder (1996) J. Geophys. Res. 101, 26,069-26,084. [8] Schmitt, H.H., et al. (2017) Icarus 298, 2-33. [9] Fassett, C.I., et al. (2012) J. Geophys. Res. 117. [10] Spudis, P.D., et al. (2011) J. Geophys. Res. Planets 116. [11] Fernandes, V.A., et al. (2013) Met. Planet. Sci. 48, 241-269. [12] Spudis, P.D. and M.U. Sliz (2017) Geophys. Res. Lett. 44, 1260-1265. [13] Spudis, P.D. (2013) European Planetary Science Congress, EPSC2013–758. [14] Cohen, B.A., et al. (2014) Geost Geoanal Res 38, 421-439. [15] Devismes, D., et al. (2016) Geost Geoanal Res. DOI: 10.1111/ggr.12118. [16] Cho, Y., et al. (2016) Planet Space Sci 128, 14-29.
N. E. Petro, S. J. Lawrence