The Earth–Moon system during the late heavy bombardment period – Geochemical support for impacts dominated by comets

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 corresponding 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/m 2 .

Atmospheric erosion and replenishment induced by impacts upon the Earth and Mars during a heavy bombardment

“…Even the total impactor mass that hit Earth and Mars, respectively, during a (late) heavy bombardment era is a matter of controversial discussions (Levison et al, 2001;Ryder, 2002;Dauphas, 2003;Koeberl, 2003;Marty and Meibom, 2007;Bottke et al, 2007;Frey, 2008;Jørgensen et al, 2009). It can be derived by impact basin counting and the use of scaling laws but this provides only rough estimates because of the unknown ages.…”

Re-examination of the formation ages of the Apollo 16 regolith breccias

“…When coupled to the temporal record of their delivery (e.g., time of impact), these samples help to provide better geochemical and chronological constraints for models of Solar System dynamics (e.g., Gomes et al, 2005;Morbidelli et al, 2005;Tsiganis et al, 2005;Bottke et al, 2007;Levison et al, 2009;Jørgensen et al, 2009;Malhotra, 2009, 2010), and causes of impact spikes to the Earth-Moon system (Turner et al, 1973;Tera et al, 1974;Ryder, 1993, 1996;Cohen et al, 2000;Kring and Cohen, 2002;Strom et al, 2005;Norman et al, 2006;Cuk et al, 2010). The integration of geochemical and chronological evidence from lunar samples, therefore, informs our understanding of the evolution of the Earth-Moon system, and the wider inner Solar System.…”

“…Chemical analyses of other Apollo samples indicate the impactors were dominated by asteroids rather than comets (e.g., Kring and Cohen, 2002) and an analysis of the size distribution of craters on the Moon points specifically to the main belt of asteroids and allows for no more than 15% comets (Strom et al, 2005). Nonetheless, it has been suggested that the impactor population was dominated by comets and/or asteroids from the outer Solar System (Jørgensen et al, 2009) or a mixture of outer Solar System comets and main belt asteroids in near-equal abundances , if not dominated by comets. The uncertainties among model assumptions (e.g., Levison et al, 2009) and the discrepancy between many models and the observed lunar data are perplexing.…”

Cometary impact rates on the Moon and planets during the late heavy bombardment

“…Gråe Jørgensen et al (2009) argued for cometary bombardment based on Ir abundances in the Isua Greenstone belt (Greenland) rocks and lunar Apollo samples, while Strom et al (2005) found the size distribution of lunar craters to be consistent with asteroidal projectiles and different from the likely cometary crater size distributions on the giant planet satellites. Moreover, estimated that practically all the mare basins could have an E-belt origin, leaving nearly nothing for the comets.…”