Accretion of Moon and Earth and the Emergence of Life
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Chemical Geology 169Ž. 2000 69±82 www.elsevier.comrlocaterchemgeo Accretion of Moon and Earth and the emergence of life G. Arrhenius a,), A. Lepland a,b a Scripps Institution of Oceanography, UniÕersity of California, La Jolla, San Diego, CA 92093-0220, USA b Institute of Geology, Tallinn Technical UniÕersity, EE-0001 Tallinn, Estonia Received 18 March 1999; accepted 9 June 2000 Abstract The discrepancy between the impact records on the Earth and Moon in the time period, 4.0±3.5 Ga calls for a re-evaluation of the cause and localization of the late lunar bombardment. As one possible explanation, we propose that the time coverage in the ancient rock record is sufficiently fragmentary, so that the effects of giant, sterilizing impacts throughout the inner solar system, caused by marauding asteroids, could have escaped detection in terrestrial and Martian records. Alternatively, the lunar impact record may reflect collisions of the receding Moon with a series of small, original satellites of the Earth and their debris in the time period about 4.0±3.5 Ga. The effects on Earth of such encounters could have been comparatively small. The location of these tellurian moonlets has been estimated to have been in the region around 40 Earth radii. Calculations presented here, indicate that this is the region that the Moon would traverse at 4.0±3.5 Ga, when the heavy and declining lunar bombardment took place. The ultimate time limit for the emergence of life on Earth is determined by the effects of planetary accretion Ð existing models offer a variety of scenarios, ranging from low average surface temperature at slow accretion of the mantle, to complete melting of the planet followed by protracted cooling. The choice of accretion model affects the habitability of the planet by dictating the early evolution of the atmosphere and hydrosphere. Further exploration of the sedimentary record on Earth and Mars, and of the chemical composition of impact-generated ejecta on the Moon, may determine the choice between the different interpretations of the late lunar bombardment and cast additional light on the time and conditions for the emergence of life. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Moon; Early Earth; Emergence of life; Lunar bombardment; Earth satellites; Archean sediments; Chemofossils 1. Introduction al.,Ž. 1977 . It was later generally assumed that this protracted event must have affected the Earth vio- The manifestations of a late heavy bombardment lently and rendered our planet uninhabitable well of the Moon were first observed by Papanastassiou past 3.8 GaŽ 1 Ga-109 yr. , with major impacts and WasserburgŽ. 1971 and Tera et al., Ž. 1974 , its extending to 3.45 GaŽ. Fig. 1 . Such a bombardment significance was further discussed by Wasserburg et is thought to have severely affected the possibilities for life to have emerged on Earth in the earliest ) Ž Corresponding author. Archean Stevenson, 1988; Sleep et al., 1989; Maher E-mail addresses: [email protected]Ž. G. Arrhenius , and Stevenson, 1988; Chyba, 1993; Oberbeck and [email protected]Ž. A. Lepland . Fogleman, 1989, 1990. In contrast, the record from 0009-2541r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S0009-2541Ž. 00 00333-8 70 G. Arrhenius, A. LeplandrChemical Geology 169() 2000 69±82 though the original sedimentary features bear an overprint of metamorphism and tectonism. The Isua and Akilia metasediments contain ubiquitous chemo- fossils with carbon isotopic composition suggestive of biochemically advanced microbial life forms Ž.Mojzsis et al., 1996 . Their presence indicates that the planet was not sterilized by impacts during the time periods of deposition of these sediments. This apparent lack of counterparts on Earth, to the late impact features on the Moon, together with new data obtained from the studies of the Martian mete- orites, require a re-evaluation of the events in Earth±Moon space in this critical time interval for the emergence of life. The 3.86 Ga age of the Akilia formation determined by Nutman et al.Ž. 1997, 2000 , has been questioned by Kamber and MoorbathŽ. 1998 and Whitehouse et al.Ž. 1999 , proposing an age of 3.65 Ga for this formation. Regardless of ultimate consensus on the age difference between the Isua and Akilia formations, this difference would, in the present context, be insignificant relative to the spread in time of the late impacts on the Moon, 4.0±3.45 Fig. 1. Radiometric ages of samples from the lunar highlands, GaŽ. Fig. 1 , essential for the comparison with the illustrating the culmination of the late lunar bombardment around record on Earth. 3.8±3.9 Ga and tailing off toward 3.5 GaŽ. after Dalrymple, 1991 . The dashed line shows the minimum age of the Isua metasedi- ments and delineates the overlap of the decaying late lunar bombardment with the early sedimentation record of Earth. Com- 2. The picket fence model and the sedimentary pare interpretations in Fig. 3. record We are considering two possible explanations for the oldest known Isua and Akilia metasedimentary the apparent incongruence between the lunar and rocks from southern West Greenland, extending back terrestrial records. If the bombardment was caused in time beyond 3.75 GaŽ Fig. 2; Moorbath et al., by invading translunar objects, it is likely to have 1973; Rosing et al., 1996; Nutman et al., 1997. show been episodic. The effects could then have been sequences of banded ironstones without any clearly overlooked in the discontinuous geologic record in- identifiable disturbances that could have formed at vestigated from the early Archean. This concept is asteroidal impacts. Such disturbances, in the form of embodied in the ``picket fence'' modelŽ. Fig. 3 surge deposits, are exemplified by the Ordovician proposed by Mojzsis and ZahnleŽ Mojzsis et al., Lockne impact structure in SwedenŽ. Sturkell, 1998 1998. Life could, in each such major occult impact and consist of coarse-grained rock fragments, trans- event, have been extinguished, only to arise again in ported and sorted by the strong tidal flow generated the intervening quiescent time intervals of a few ten by the impact. Crushing-, flow- and transport-effects or hundred million years. Or it could have found a at impacts of the size indicated by the lunar maria, niche for survival in the deep ocean or crust, and and scaled up for Earth's gravitation, would have have spread again from there, between each pair of been correspondingly violent. They would have left major assaultsŽ. Sleep et al., 1989 . records that are likely to be distinguishable from the Sequences of metasedimentary cherts and banded layer structure of the banded iron formation, even iron formations in the Isua Supracrustal BeltŽ. ISB , G. Arrhenius, A. LeplandrChemical Geology 169() 2000 69±82 71 Fig. 2. Time scale of events related to accretion of Moon and Earth and the emergence of life. The 3.8 Ga age line indicates the culmination of the late bombardment observed on the Moon. referred to above, are exposed in outcrops and in a pel, 1991. The metasedimentary units are commonly series of several-hundred meter-long drill coresŽ Ap- intersected by intrusions of mafic rocks. Although individual 10±100 m sections of coherent sequences, represent comparatively long accumulation periods, no certain evidence has so far been found for ca- tastrophic surge deposits. Reliable stratigraphic interpretation is, however, hindered by the tectonic overprinting; the formation is isoclinally folded. Therefore, it cannot be excluded that some of the sequences represent repetitions of a single unitŽ Ros- ing, 1998. In the northeastern part of the ISB, the sequence of finely-laminated banded iron formation, contains strongly deformed 0.1±1 m thick layers, sandwiched between undeformed strata. These de- formed packages probably mark tectonic shear zones Fig. 3. Three generalized interpretations of the late lunar bom- bardment data shown in Fig. 2. The concurrent record on Earth is that resulted from small-scale translations parallel to represented by the Isua sediments that have a minimum age of the bedding. Some of the thicker deformed packages 3.75 Ga. Retrieved segments of these deposits do not bear evi- Ž.Fig. 4 , display grading of the component rock dence of contemporaneous bombardment of Earth at the scale fragments, opening the possibility that they could be observed on the Moon. This suggests, either that the correspond- surge deposits formed by violent resuspension and ing time±rock units on Earth are missing or have not been found Ž.cf. Fig. 4 , or that the late lunar bombardment was restricted to sorting during resettling. If meteorite impact is as- lunar phase space.Ž Zahnle and Mojzsis, unpubl., reproduced in sumed as a cause of such implied surges, an en- Mojzsis et al., 1998. hancement should be evident of the platinum group 72 G. Arrhenius, A. LeplandrChemical Geology 169() 2000 69±82 process, but was largely limited to the lunar orbit. This possibility may be evaluated against the back- ground of several current theories for the origin of the Moon, that can all be taken to provide more or less compelling support for such a conjecture. One currently popular scenario for the formation of the Moon assumes that Earth, in a relatively advanced state of accretion, including or followed by core formation, would have collided with a hypo- thetical planet moving in an Earth-crossing orbit and with a mass larger than that of MarsŽ Hartmann and Davis, 1975; Cameron and Ward, 1976; WankeÈ et al., 1984; Cameron and Benz, 1991; Cameron, 1985, 1997; Canup and Esposito, 1996; Ida et al., 1997. Ejecta from the impact would have been placed in prograde equatorial orbit around Earth and coalesced to form the Moon.