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. 2008 Nov 18;105(46):17654-8.
doi: 10.1073/pnas.0806596105. Epub 2008 Nov 10.

Toward Understanding Early Earth Evolution: Prescription for Approach From Terrestrial Noble Gas and Light Element Records in Lunar Soils

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Toward Understanding Early Earth Evolution: Prescription for Approach From Terrestrial Noble Gas and Light Element Records in Lunar Soils

Minoru Ozima et al. Proc Natl Acad Sci U S A. .
Free PMC article

Abstract

Because of the almost total lack of geological record on the Earth's surface before 4 billion years ago, the history of the Earth during this period is still enigmatic. Here we describe a practical approach to tackle the formidable problems caused by this lack. We propose that examinations of lunar soils for light elements such as He, N, O, Ne, and Ar would shed a new light on this dark age in the Earth's history and resolve three of the most fundamental questions in earth science: the onset time of the geomagnetic field, the appearance of an oxygen atmosphere, and the secular variation of an Earth-Moon dynamical system.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Mixing diagrams of Earth Wind (EW) and Solar Wind (SW) components. All of the curves suggest the mixing of two components, SW and terrestrial components, which are attributable to EW transported from the Earth's upper atmosphere. Figures are redrawn from Ozima et al. (4).
Fig. 2.
Fig. 2.
Schematic view of interaction of the SW with Earth's upper atmosphere. (Upper) A magnetic Earth with the present GMF and with biotic (oxygenic) atmosphere. The SW is stopped at about 10 Earth radii (RE) by the GMF. Because of efficient ionization of oxygen, O+ becomes dominant above 100 km and picked up by the SW from the magnetosphere (light blue) to escape from the Earth as EW. The Moon spends ≈10% of its revolving time under the shadow of the EW tail, that is, ≈10% of the EW hits the lunar surface. Here, we assumed on the basis of Geotail observation that the EW has a circular flow with 2 RE (6). (Lower) The SW can approach very close (a few hundred kilometers) to the Earth and pick up ions from the ionopause (darker blue). The probability of the EW tail shadowing the Moon is calculated to be ≈0.3% (for assumed Earth–Moon distance of 40 RE) of the whole revolving time of the Moon (4), that is ≈0.3% of the EW flux reaches the Moon surface. Although interacting volume of the upper atmosphere with the SW is much smaller than in a magnetic Earth, much thicker atmospheric pressure because of the shorter distance to the Earth largely counterbalances to give rise to a substantial amount of pick-up ions from the ionopause.
Fig. 3.
Fig. 3.
Schematic representation of algorithm in estimating surface exposure time T of a lunar ilmenite grain (also see text).
Fig. 4.
Fig. 4.
Depth profile of O+ (16 keV) implanted in FeTiO3 target. Oxygen content is normalized to the maximum concentration. Calculation was made with SRIM-code −2003 (34). Implantation energy of 16 keV is assumed in accordance with the mean SW energy of 1 keV per nucleon, because picked-up ions in EW are carried away with the same speed as the SW due to electromagnetic coupling. Note that implanted O+ becomes maximum at ≈30 nm with a rough Gaussian distribution, being highly concentrated in an extremely thin skin layer of <10 nm (also see text). The calculated profile approximately mimics oxygen depth profile in metal particles observed by Ireland et al. (24) (not shown) apart from the very surface layer less than a few nanometers, which was disturbed by surface contamination (24).
Fig. 5.
Fig. 5.
Schematic representation of isotopic analyses of light elements (see text for details).

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