Jeff Taylor Lunar cataclysm1 Lunar Bombardment History: Was There a Terminal Lunar “Cataclysm”? The concept The importance if it happened The evidence Problems with the evidence: arguments against a cataclysm and counter arguments Testing the idea

Jeff Taylor Lunar cataclysm2 The Concept of a Cataclysm See notes page Graph from Dave Kring (LPI)

Jeff Taylor Lunar cataclysm3 The Concept of a Cataclysm See notes page Graph from Dave Kring (LPI)

Jeff Taylor First Evidence: Numerous samples fall on a line that intercepts concordia at 4.5 Ga and 3.9 Ga. The younger age was taken as evidence for a big event that affected much of the lunar crust. Tera et al (1974)

Jeff Taylor Early Bombardment Rate: A Cataclysm? Lunar cataclysm5 Huge implications for early history of the Solar System

Jeff Taylor Lunar cataclysm6 Importance of the Concept Numerous imaginative ideas to explain early bombardment and cataclysm (if it happened): –Left over debris from formation of terrestrial planets –Late formation of Uranus and Neptune, which scatters nearby planetesimals –Break-up of a large main-belt asteroid –Asteroid scattering by 2-3 planets in the region that is now the asteroid belt –Comet shower caused by close passage of a star –Cataclysm confined to Earth-Moon system Which ever one is correct, there are important implications for how planets formed, and some are wrong if there was or was not a cataclysm.

Jeff Taylor Lunar cataclysm7 Dynamical Evidence for Late Heavy Bombardment Disk of planetesimals beyond 15 AU Saturn and Jupiter migrate slowly, reaching 2:1 resonance This scatters Neptune and Uranus Their extreme migrations scatter planetesimals in disk in a short time interval—a cataclysm Gomes, Levison, Tsganis, Mobidelli (2005)

Jeff Taylor Lunar cataclysm8 Gomes, Levison, Tsganis, Mobidelli (2005) Time when 1:2 is reached can be adjusted Important point is the sudden onset of scattering Note rapid, huge change in orbit of Neptune

Jeff Taylor Lunar cataclysm9 The Evidence Comes from studies of impact melts*: –Identify melt groups –Determine ages –Try to associate them with basins *Ryder claimed that only impact melts provide reliable ages for impact events, but granulites are good, too. Dalrymple and Ryder (1996)

Jeff Taylor Lunar cataclysm10 The Evidence Appears to be a clustering of ages of impact melts around 3.8 to 4 Ga Has led to the idea of a lunar “cataclysm” Age (Gy)

Jeff Taylor Lunar cataclysm11 The Evidence Associated with basins on basis of where Apollo missions and Luna 20 mission landed: –Apollo 14: Imbrium ejecta –Apollo 15: Imbrium ring –Apollo 16: Nectaris ejecta –Apollo 17: Serentatis ring –Luna 20: Crisium ejecta

Jeff Taylor Lunar cataclysm12 Problems with the Evidence

Jeff Taylor Lunar cataclysm13 Problems with the Evidence Everything is from Imbrium—we are dating only one event –Samples all from near side, all sites within reach of Imbrium ejecta –Imbrium area focus of high Th (hence REE etc.), characteristic of most LKFM (which is what is dated) Counter argument: –Melts have different chemical compositions and compositional clusters [But maybe basin-sized impact melts vary in composition more than smaller terrestrial craters that have been studied] –Melts groups have different ages [but maybe trapped Ar in some]

Jeff Taylor Lunar cataclysm14 Problems with the Evidence Stonewall (Hartmann, 1975; 2003): Early, declining bombardment continuously –Resets ages –Comminutes rocks so they are too small to recognize But there are mare basalts 3.85 – 4.23 Gy, yet they survived There are also pristine rocks older than 4 Gy, but Hartmann says there are excavated by events that dig beneath the pulverized zone Hartmann (2003) 14053, 3.95 Ga 0.5 mm

Jeff Taylor Problems with the Evidence: Do we have samples of Serenitatis melt sheet? Lunar cataclysm15 Hartmann (2003) 14053, 3.95 Ga Montes Rook Formation, OrientaleSculptured Hills, Taurus Montes (A-17) Spudis et al. (2011): Similar morphology has led many to postulate similar origins for the two units. Might mean Sculptured Hills are Imbrium ejecta.

Jeff Taylor Serentatis Age Problem Lunar cataclysm16 LROC WAC Digital Terrain Model LROC WAC mosaic Spudis et al. (2011)

Jeff Taylor Serentatis Age Problem Lunar cataclysm17 Spudis et al. (2011)LRO WAC mosaic

Jeff Taylor Serentatis Age Problem Lunar cataclysm18 Spudis et al. (2011)

Jeff Taylor Current Status of Impact Ages Lunar cataclysm19 Fernandes et al. (2013)

Jeff Taylor Lunar cataclysm20 Testing the Cataclysm Hypothesis Date basins that are: –Far from Imbrium –Have compositionally distinct impact melt sheets –Are stratigraphically older Excellent candidate: South Pole – Aitken basin Geological Map of SPA basin. Note superimposed basins

Jeff Taylor LRO LOLA data on LROC WAC context mosaic South Pole – Aitken Basin AitkenCrater South Pole Equator Southern Far Side

Jeff Taylor Lunar cataclysm22 Testing the Cataclysm Hypothesis

Jeff Taylor Lunar cataclysm23 Testing the Cataclysm Hypothesis

Jeff Taylor Lunar cataclysm24 Testing the Cataclysm Hypothesis