The Earth-Moon system during the Late Heavy Bombardment period

📝 Abstract

The Late Heavy Bombardment (LHB) period is the narrow time interval between 3.8 and 3.9 Gyr ago, where the bulk of the craters we see on the Moon formed. Even more craters formed on the Earth. During a field expedition to the 3.8 Gyr old Isua greenstone belt in Greenland, we sampled three types of metasedimentary rocks, that contain direct traces of the LHB impactors by a seven times enrichment (150 ppt) in iridium compared to present day ocean crust (20 ppt). We show that this enrichment is in agreement with the lunar cratering rate, providing the impactors were comets, but not if they were asteroids. Our study is a first direct indication of the nature of the LHB impactors, and the first to find an agreement between the LHB lunar cratering rate and the Earth’s early geochemical record (and the corresponding lunar record). The LHB comets that delivered the iridium we see at Isua will at the same time have delivered the equivalent of a km deep ocean, and we explain why one should expect a cometary ocean to become roughly the size of the Earth’s present-day ocean, not only in terms of depth but also in terms of the surface area it covers.

💡 Analysis

The Late Heavy Bombardment (LHB) period is the narrow time interval between 3.8 and 3.9 Gyr ago, where the bulk of the craters we see on the Moon formed. Even more craters formed on the Earth. During a field expedition to the 3.8 Gyr old Isua greenstone belt in Greenland, we sampled three types of metasedimentary rocks, that contain direct traces of the LHB impactors by a seven times enrichment (150 ppt) in iridium compared to present day ocean crust (20 ppt). We show that this enrichment is in agreement with the lunar cratering rate, providing the impactors were comets, but not if they were asteroids. Our study is a first direct indication of the nature of the LHB impactors, and the first to find an agreement between the LHB lunar cratering rate and the Earth’s early geochemical record (and the corresponding lunar record). The LHB comets that delivered the iridium we see at Isua will at the same time have delivered the equivalent of a km deep ocean, and we explain why one should expect a cometary ocean to become roughly the size of the Earth’s present-day ocean, not only in terms of depth but also in terms of the surface area it covers.

📄 Content

arXiv:0907.4104v1 [astro-ph.EP] 23 Jul 2009 The Earth-Moon system during the Late Heavy Bombardment period – geochemical support for impacts dominated by comets. Uffe Gr˚ae Jørgensen1,∗, Peter W.U. Appel2, Yuichi Hatsukawa3, Robert Frei4, Ma- sumi Oshima3, Yosuke Toh3, Atsushi Kimura3 1Niels Bohr Institute and Centre for Star and Planet Formation, Juliane Maries Vej 30, 2100 Copen- hagen, Denmark, ∗Corresponding author email address: uffegj@nbi.dk 2Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 Copenhagen, Denmark, 3Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan, 4Institute of Geography and Geology, and Nordic Center for Earth Evolution, Øster Voldgade 10, 1350 Copenhagen, Denmark. 1 Abstract. The solid planets assembled 4.57 Gyr ago during a period of less than 100 Myr, but the bulk of the impact craters we see on the inner planets formed much later, in a narrow time interval between 3.8 and 3.9 Gyr ago, during the so-called Late Heavy Bombardment (LHB). It is not certain what caused the LHB, and it has not been well known whether the impactors were comets or asteroids, but our present study lend support to the idea that it was comets. Due to the Earth’s higher gravity, the impactors will have hit the Earth with ∼twice the energy density that they hit the Moon, and the bombardment will have continued on Earth longer than on the Moon. All solid surface of the Earth will have been completely covered with craters by the end of the LHB. However, almost nothing of the Earth’s crust from even the end of this epoch, is preserved today. One of the very few remnants, though, is exposed as the Isua greenstone belt (IGB) and nearby areas in Western Greenland. During a field expedition to Isua, we sampled three types of metasedimentary rocks, deposited ∼3.8 billion years ago, that contain information about the sedimentary river load from larger areas of surrounding land surfaces (mica-schist and turbidites) and of the contemporaneous seawater (BIF). Our samples show evidence of the LHB impacts that took place on Earth, by an average of a seven times enrichment (150 ppt) in iridium compared to present day ocean crust (20 ppt). The clastic sediments show slightly higher enrichment than the chemical sediments, which may be due to contamination from admixtures of mafic (proto-crustal) sources. We show that this enrichment is in agreement with the lunar cratering rate and a corre- sponding extraterrestrial LHB contribution to the Earth’s Hadean-Eoarchean crust, provided the bulk of the influx was cometary (i.e., of high velocity and low in CI abundance), but not if the impactors were meteorites (i.e. had velocities and abundances similar to present day Earth crossing asteroids). Our study is a first direct indication of the nature of the LHB impactors, and the first to find an agreement between the LHB lunar cratering rate and the Earth’s early geochemical record (and the corresponding lunar record). The LHB comets that delivered the iridium we see at Isua will at the same time have delivered the equivalent of a ∼1 km deep ocean, and we explain why one should expect a cometary ocean to become roughly the size of the Earth’s present-day ocean, not only in terms of depth but also in terms of the surface area it covers. The total impacting mass on the Earth during the LHB will have been ∼1000 t/m2. Keywords: comets; meteorites; geological processes; ices 2 Introduction. While the lunar craters could in principle represent the end of the planetary accretion, most evidence point to the planetary accretion epoch and the lunar crater formation as being two different events in the history of the solar system, separated in time by several hundred million years. Comparison of the relative age and size distribution of craters throughout the solar system, indicates that the event that created the ancient lunar craters during the LHB period, (i.e. the crater-rich highlands and the basins that later became the Mare regions) also formed the craters on Mars, asteroids, Mercury, and elsewhere in the solar system; in other words that the LHB was unique, heliocentric, and the most violent independent event that has happened during the whole history of our solar system since the formation of the planets (e.g., Gomes et al. 2005, Martin et al. 2006, Hartmann et al. 2000, Kring & Cohen 2002, Ryder 2002, Pater & Lissauer 2001). Very little, however, is known about its effect on the Earth. Several studies have searched for signs of the LHB on the Earth, throughout many years, without conclusive evidence and often with seemingly contradictory results. It is often assumed that the traces of the LHB on Earth has been erased due to Earth’s dynamic geology. However, the LHB was so intense, in particular on the Earth, that it has radically affected the bulk composition of the atmosphere and hydrosphere, and probably the crust and mantle, too. Judging from the size of the lunar craters, the largest LHB impacts on Earth will have

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