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Heavy Bombardment of the Earth at -3.85 Ga: the Search for Petrographic and Geochemical Evidence

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Heavy Bombardment of the Earth at -3.85 Ga: the Search for Petrographic and Geochemical Evidence Heavy Bombardment of the Earth at -3.85 Ga: The Search for Petrographic and Geochemical Evidence Graham Ryder Lunar and Planetary Institute Christian Koeberl U"iversity 0/Vienna Stephen J. Mojzsis University o/California, Los Angeles The Moon experienced an interval of intense bombardment peaking at -3.85 ± 0.05 Ga; subsequent mare plains as old as 3.7 or 3.8 Ga are preserved. It can be assumed that the early Earth must have been subjected to an even more intense impact flux resulting from its larger size and because of its proximity to the Moon. Siderophile-element analyses (e.g., Ir abun­ dance) of the oldest sediments on Earth could be used to indicate past escalated influxes of extraterrestrial material. In addition, shocked minerals may also be present in the oldest extant rocks of sedimentary origin as detrital minerals. and remnants of impact ejecta might exist in early Archean formations. Searches for impact signatures have been initiated in the oldest sedi­ ments on the Earth, from the early Archean (>3.7 Ga) terrane of West Greenland; some of these rocks have been interpreted to be at least 3.8 Ga in age. So far, unequivocal evidence of a late heavy bombardment on the early Earth remains elusive. We conclude that either the sedimen­ tation rate of the studied sediments was too fast and therefore too diluting to record an obvi­ ous signal, or the ancient bolide flux has been overestimated, or the bombardment declined so rapidly that the Greenland sediments, some even at -3.85 Ga in age, do not overlap in time with it. 1. INTRODUCTION Earth appears to have been completed about 50-100 m.y. after the initial collapse of the solar nebula (Lee and Halli­ Collisions between planetary bodies have been funda­ day, 1995, 1996; Halliday et al.. 1996; Podosek and Ozima, mental in the evolution of the solar system. Studies under­ 2000), in a timescale apparently more protracted than that taken over the last few decades have convinced most for smaller planetesimals and Mars. As a result of later workers that the planets formed by collision and hierarchi­ geological activity, no record of any primary accretion bom­ cal growth starting from small objects, i.e., from dust to bardment history remains on the surface of the Earth. planetesimals to planets (e.g., Wetherill. 1994; Taylor, Thc period on Earth between the end of accretion and I992a,b; see also chapters in section II of this volume), and the production of the oldest known crustal rocks is com­ not from condensation downward. Late during the accre­ monly referred to as the Hadean Eon (Cloud, 1976. 1988; tion of the Earth (some time after -4.5 Ga), when it had Harland et aI., 1989; Taylor, 2000), which is a chronostratic reached about 70% of its eventual mass, it was most prob­ division (Fig. I). Its terminal boundary is actually not de­ ably impacted by a Mars-sized or larger body (see Cameron, fined on the Earth; Harland et af. (1989) equate it with the 2000). The consequences of such an impact event for the Orientale impact on the Moon. Others do not even use the proto-Earth would have been severe and seminal, ranging teml Hadean, either distinguishing the chronometric divi­ from almost complete melting and formation of a magma sions of Archean Eon and (older) Priscoan time (Harland ocean, thermal loss of preexisting atmosphere, changes in et al., 1989), provisionally at 4.0 Ga. Here we use the term spin rate and spin-axis orientation, to accretion of material Hadean to represent the time period between the formation from the impactor directly, or through rapid fall-out from of the Moon at -4.5 Ga and the beginning of the continu­ orbital debris below the Roche limit. Much of the material ous terrestrial rock record at 3.8 Ga. blasted off in the impact eventually reimpacted the Earth; In contrast with the youthful age for the crust of the some of the ensuing Earth-orbiting debris would have rap­ Earth, the surface of the Moon displays abundant evidence idly coalesced to form most of the Moon and probably some of an intense bombardment at some time between its origi­ smaller moonlets. Some of this geocentrically orbiting nal crustal formation and the outpourings of lava that form material would have continued to impact both bodies for the dark mare plains. Even prior to the Apollo missions, perhaps tens of millions of years after the lunar forming these plains were calculated to be about 3.6 Ga in age based impact. The essential accretion and core formation of the on crater counts and realistic flux estimates. Hence, the 475 Origin ofthe Earth and Moon Eds.: Robin Canup and Kevin Righter University of Arizona Press, Tucson (2000) Fig. L. Comparative chronostratigraphies of the Earth and Moon, based on Harland el al. (1989) and Wilhelms (1987). The times of interest in this paper are the Isuan and Hadean Eras for the Earth and the Pre-Nectarian, Nectarian, and Imbrian Periods for the Moon. The Imbrian is divided into the two Epochs of Early Imbrian and Late Imbrian, which have greatly differing styles of geological activity (rock stratigraphic units, i.e, systems, are not used in this paper). Although the chronostratic divisions into these two Epochs (the Nectarian and the pre-Nectarian) are perfectly clear, the cotTeiation with absolute time is less established, although the age of the Fra Mauro Formation (Imbrium ejecta morphology) that defines the division of Early Imbtian and Nectarian is fairly well established at 3.84 or 3.85 Ga (e.g., Dalrymple and Ryder, 1993). Ryder et al.: Heavy Bombardmenl olthe Earth 477 heavy bombardment was inferred to be ancient (Hartmann, according to the recognition oflunar ferroan anorthosite of 1966). Lunar highland sample data show isotopic resetting that age. The present morphology of the highlands of the from thermal heating, for which there is abundant evidence Moon reflects, almost exclusively, a history of numerous for impact sources dominated by ages of around 3.8-3.9 Ga. subsequent impacts that occurred prior to the extrusion of The most ancient volcanic rocks from mare plains have ages the volcanic flows that form the visible mare plains (e.g., of about 3.8 Ga (see, e.g., Taylor, 1982; Wilhelms, 1987). Wilhelms, 1984, 1987). These ancient impact structures in­ The highland ages have been interpreted to either represent clude giant multiring basins and their debris (Spudis, 1993), a short and intense heavy bombardment period at 3.85 ± as well as a size-seriate range of smaller craters. Hartmann 0.05 Ga or so (e.g., Tera et al., 1974; Ryder, 1990), or the (1965, 1966) recognized that most ofthis cratering occurred tail end of a prolonged postaccretionaly bombardment (e.g., early in lunar history according to an estimate of the aver­ Baldwin, 1974; Hartmann, 1975), as discussed in Hartmann age age of mare plains of 3.6 Ga, which was calculated et af. (2000). In any case, the bulk of this bombardment, based on present-day cratering rates. He inferred a cratering which produced size-seriate scars up to multiring basins rate averaging roughly 200x higher for the first one-seventh many hundreds of kilometers across, preceded 3.8 Ga. We of lunar history than for the remainder. The general correct­ will use the term late heavy bombardment to refer specifi­ ness of Hartmann's conclusion was demonstrated by the cally to that bombardment of the Moon and the Earth from return ofApollo samples, and the dating ofthe oldest mare -3.90 to 3.80 Ga. plains at close to 3.8 Ga (Wilhelms, 1987). In any given time-span, the Earth must have been sub­ Geochronological studies of impact-brecciated highland jected to a significantly greater bombardment than was the samples show thermal events, most of them of impact ori­ Moon, as it has a larger diameter and a much larger gravi­ gin, concentrated at -3.8-3.9 Ga. These ages have been tational cross section, thus making it an easier target to hit taken to represent the tail end of a heavy but declining (e.g., Maher and Stevenson, 1988; Oberbeck and Fogleman, bombardment dating back to the accretion of the Moon 1989; Zahnle and Sleep, 1997). If a late heavy bombard­ (e.g., Shoemaker, 1972, 1977; Hartmann, 1975. 1980; ment occurred on the Moon, the Earth was subject to a flux Neukum et af., 1975; Baldwin, 1971, 1974. 1981, 1987; scaling because of the ratio of the impact cross sections Taylor, 1982; Wilhelms, 1987); alternatively, they may record (Sleep et al., 1989), which may have resulted in an impact a sharp or cataclysmic increase in bombardment for that rate ~20x greater than the lunar one, containing both more short interval (e.g., Tera et aI., 1974; Ryde/; 1990; Dalrymple and larger impact events. The consequences for the hydro­ and Ryder, 1993, 1996). There exists a sharp drop-off in sphere, atmosphere, and even the lithosphere of Earth at that estimates for the cratering rate from the youngest high­ time must have been devastating (Zahnle and Sleep, 1997; land surfaces, the Orientale and Imbrium ejecta blankets, Grieve, 1980; Frey. 1980). There is evidence that the Earth's to the oldest mare surfaces. This is according to crater upper mantle had already undergone some differentiation counts of those surfaces, which differ by a factor of -3-4 at the time of formation of the oldest igneous rocks, sug­ (e.g., Wetherill. 1977. 1981; B VSp, 1981). As a result of the gesting the prior existence of chemically evolved crust (e.g., difference in cratering rates, a flux at least lOOx higher can Harper and Jacobsen, 1992; McCulloch and Bennett, 1993; be calculated for this transition period, even if those young­ Bowring and Housh, 1995).
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