Mathematics – Logic
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..409n&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 409
Mathematics
Logic
Lunar Crust, Palaomagnetism, Radiogenic Ages, Solar Matter, Volatile Elements
Scientific paper
To explain the origin of the natural remanent magnetism (NRM) of lunar rocks [1,2] and lunar asymmetries [3-5], many models suppose an asymmetric arrangement of matter within the early Moon [6,7]. Presented here is a model of lunar evolution that explains both the appearance of the lunar asymmetries and the presence of a high lunar palaeomagnetic field in the vicinity of 3.9 Ga starting from a symmetrically differentiated Moon. According to this model, less than 0.2 Ga after its formation as a completely molten planetary body, the Moon differentiated in concentric layers to form an anorthositic crust, a mantle, and an iron core [8]. Lunar asymmetries appeared at about 3.9 Ga ago when the Sun underwent a very violent explosive phase, generating the second great thermal event (SGTE) from the history of the solar system. Solar plasma into expansion interacted with the solar system bodies, erasing previous records on a large part of their surfaces and generating thermal effects of various kinds. Figure 1 presents the manner in which the solar plasma "excavated" within the lunar crust. The result was the displacement of the lunar center-of-figure (CF) and center-of mass (CM) from O in their present positions [3-5]. A similar "excavation" within the primordial crust of the Earth generated the appearance of a protocontinental land (Proto-Pangaea). Eccentric placement of the lunar core immediately after SGTE occurrence and its movement toward the new position of the lunar CM, along with change of the lunar spin into a synchronous rotation, generated lunar internal motions vigorously enough to drive lunar dynamo. Figure 2 presents the lunar palaeomagnetic field intensity as a function of time according to this scenario. Immediately after SGTE occurrence, the lunar crust was subjected to intensive bombardment with impacting masses belonging to Population II impactors [9]. The larger impacts generated zones of weakness and fissures within the lunar crust, but the isostatic answer of the lunar interior and its asymmetric structure permitted mare volcanism to occur only on the nearside [10]. This model is strongly supported by isotopic, petrologic, and magnetic analyses of lunar nearside rocks that give abundant evidence for a widespread thermal metamorphism at about 3.9 Ga. It is reflected especially in partial to complete equilibration of the isotopic systems [8], in the redistribution of volatiles on lunar surface [11-13] and in the thermoremanent magnetization of lunar breccias [14]. This model also explains the asymmetric distribution of the thorium concentrations in the lunar surface [15] and the presence, near the lunar surface, of rocks of deep-seated origin [16]. References: [1] Nagata T. et al. (1970) Proc. Apollo 11 LSC, 2325. [2] Strangway D. W. (1971) EPSL, 13, 43. [3] Kaula W. M. et al. (1972) Proc. LSC 3rd, 2189. [4] Kaula W. M. et al. (1974) Proc. LSC 5th, 3049. [5] Haines E. L. and Metzger A. E. (1980) Proc. LPSC 11th, 689. [6] Stevenson D. J. (1980) Nature, 287, 520. [7] Ransford G. and Sjogren W. (1972) Nature, 238, 260. [8] Wasserburg G. J. et al. (1977) Phil. Trans. R. Soc. Lond., A285, 7. [9] Neagu A. (1993) 18th EGS Meeting, Wiesbaden. [10] Neagu A. (1992) Meteoritics, 27, 267. [11] Tera F. and Wasserburg G. J. (1972) EPSL, 14, 281. [12] Nunes P. D. and Tatsumoto M. (1973) Science, 182, 916. [13] Cirlin E. H. and Housley R. M. (1980) Proc. LPSC 11th, 349. [14] Gose W. A. et al. (1978) EPSL, 38, 373. [15] Metzger A. E. et al. (1977) Proc. LSC 8th, 949. [16] James O. B. (1980) Proc. LPSC 11th, 365. Fig. 1, which appears here in the hard copy, shows a schematic equatorial cross section of the Moon showing the lunar internal structure before and after SGTE occurred. Fig. 2 appears here in the hard copy.
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