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The Impact Environment of the Hadean Earth

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The Impact Environment of the Hadean Earth Chemie der Erde 73 (2013) 227–248 Contents lists available at ScienceDirect Chemie der Erde jou rnal homepage: www.elsevier.de/chemer Invited review The impact environment of the Hadean Earth a b c,d,e,∗ Oleg Abramov , David A. Kring , Stephen J. Mojzsis a United States Geological Survey, Astrogeology Science Center, 2255 North Gemini Drive, Flagstaff, AZ 86001, USA b USRA – Lunar and Planetary Institute, Center for Lunar Science & Exploration, 3600 Bay Area Boulevard, Houston, TX 77058-1113, USA c University of Colorado, Department of Geological Sciences, NASA Lunar Science Institute, Center for Lunar Origin and Evolution (CLOE), 2200 Colorado Avenue, UCB 399, Boulder, CO 80309-0399, USA d Ecole Normale Supérieure de Lyon and Université Claude Bernard Lyon 1, Laboratoire de Géologie de Lyon, CNRS UMR 5276, 2 rue Raphael Dubois, Villeurbanne 69622, France e Hungarian Academy of Sciences, Research Center for Astronomy and Earth Sciences, Institute for Geological and Geochemical Research, 45 Budaörsi ut, H-1112 Budapest, Hungary a r t i c l e i n f o a b s t r a c t Article history: Impact bombardment in the first billion years of solar system history determined in large part the initial Received 1 July 2013 physical and chemical states of the inner planets and their potential to host biospheres. The range of Accepted 13 August 2013 physical states and thermal consequences of the impact epoch, however, are not well quantified. Here, we assess these effects on the young Earth’s crust as well as the likelihood that a record of such effects could be Keywords: preserved in the oldest terrestrial minerals and rocks. We place special emphasis on modeling the thermal Hadean effects of the late heavy bombardment (LHB) – a putative spike in the number of impacts at about 3.9 Gyr Zircon ago – using several different numerical modeling and analytical techniques. A comprehensive array of Late heavy bombardment impact-produced heat sources was evaluated which includes shock heating, impact melt generation, Origin of life uplift, and ejecta heating. Results indicate that ∼1.5–2.5 vol.% of the upper 20 km of Earth’s crust was Thermal modeling ∼ Cratering processes melted in the LHB, with only 0.3–1.5 vol.% in a molten state at any given time. The model predicts that approximately 5–10% of the planet’s surface area was covered by >1 km deep impact melt sheets. A global average of ∼600–800 m of ejecta and ∼800–1000 m of condensed rock vapor is predicted to have been deposited in the LHB, with most of the condensed rock vapor produced by the largest (>100-km) projectiles. To explore for a record of such catastrophic events, we created two- and three-dimensional models of post-impact cooling of ejecta and craters, coupled to diffusion models of radiogenic Pb*-loss in zircons. We used this to estimate what the cumulative effects of putative LHB-induced age resetting would be of Hadean zircons on a global scale. Zircons entrained in ejecta are projected to have the following average global distribution after the end of the LHB: ∼59% with no impact-induced Pb*-loss, ∼26% with partial Pb*-loss and ∼15% with complete Pb*-loss or destruction of the grain. In addition to the relatively high erodibility of ejecta, our results show that if discordant ca. 3.9 Gyr old zones in the Jack Hills zircons are a signature of the LHB, they were most likely sourced from impact ejecta. © 2013 Elsevier GmbH. All rights reserved. Contents 1. Introduction . 228 1.1. The late heavy bombardment hypothesis. 228 1.2. Effects of the late heavy bombardment on Earth . 228 2. Thermal models for global bombardments . 230 2.1. Model construction . 230 2.2. Crater cooling . 234 2.3. Ejecta cooling . 235 2.4. Global bombardment models . 236 2.5. Diffusion models for zircons . 236 ∗ Corresponding author at: University of Colorado, Department of Geological Sciences, Center for Lunar Origin and Evolution (CLOE), 2200 Colorado Avenue, UCB 399, Boulder, CO 80309-0399, USA. Tel.: +1 303 492 5014; fax: +1 303 492 2606. E-mail address: [email protected] (S.J. Mojzsis). 0009-2819/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.chemer.2013.08.004 228 O. Abramov et al. / Chemie der Erde 73 (2013) 227–248 3. Results and analysis . 238 3.1. Global bombardment models . 238 3.2. Thermal fields within individual impact craters . 238 3.3. Thermal fields from globally emplaced impact ejecta . 240 4. Discussion . 242 4.1. LHB-induced ages to undamaged vs. damaged zircons in the Hadean crust . 242 4.2. Effects of zircon grain size. 242 4.3. Effects of target lithotype . 242 4.4. Predictions for the Hadean terrestrial zircon record. 242 5. Conclusions . 244 Acknowledgements . 245 References . 245 1. Introduction extend for long after (e.g., Bogard, 1995; Ash et al., 1996; Turner et al., 1997; Kring and Cohen, 2002; Bogard, 2011; Cohen, 2013; see Impacts fundamentally contribute to several key physical and Bottke et al., 2012). The most intense epoch of this solar-system- chemical aspects of the evolution of the terrestrial (a.k.a. silicate wide bombardment is now commonly referred to as the “late heavy or “rocky”) planets, the largest of the asteroids, as well as icy bombardment” (LHB), a term we use in this review. Thermal events objects of the outer solar system. The planetary-scale effects of recorded in pre-4.0 Ga terrestrial zircons which cluster at ca. 3.9 Ga bombardments range from profound to subtle, and include (i) mod- may also be suggestive of this bombardment (Trail et al., 2007; ifications in surface morphology expressed as cratered terrains; (ii) Abbott et al., 2012; Bell and Harrison, 2013), but aside from this compositional changes via delivery of exogenous materials to the evidence there is little reliable confirmation of the LHB from the ter- crust or deep interior, melt mixing and differentiation of meteoritic restrial rock record (e.g., Anbar et al., 2001; Schoenberg et al., 2002; and crustal components; (iii) alterations to primordial atmosphere cf. Frei and Rosing, 2005). We discuss the nature of the Hadean zir- compositions and atmospheric densities, and thus strong forcing con evidence for an LHB in more detail later. Finally, the population on paleoclimate; and (iv) defining the initial conditions that helped structure of the main asteroid belt appears to preserve a record of to determine the overall thermal structures of the affected worlds. giant planet migration that has been implicated in triggering the Impacts on Earth have also had important biological consequences LHB (Minton and Malhotra, 2009). A self-consistent mechanism for over geologic time. its origin has been proposed (Gomes et al., 2005; Morbidelli, 2010; The effect of major bolide impacts on the habitability of rocky Morbidelli et al., 2012) which involved a rapid migration of giant worlds is a two-edged sword. Impacts may be conducive to ancient planets which strongly perturbed both the asteroid belt and the icy biospheres via the formation of new habitats such as hydrothermal planetessimal disk outside their initial orbits. Much remains to be systems (e.g., Kring, 2000, 2003). Studies of preserved remains of done, however, to further constrain the timing, duration and inten- post-impact hydrothermal systems (e.g., Versh et al., 2006) lend sity of the LHB, and to understand its physical effects on planetary credence to the idea that the consequences of an impact could be surfaces. benign for microbial life, or even advantageous to it (Cockell and It is important to note that although the LHB hypothesis pro- Lee, 2002). They may.
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