Astronomy and Astrophysics – Astrophysics
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..598t&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Astronomy and Astrophysics
Astrophysics
Scientific paper
Radioisotope chronologies from both long-lived nuclides (238,235U-206,207Pb, 40K-40Ar [1,2,3]) and shortlived radionuclides (129Xe from 129I; half-live T1/2=16 Myr [4,5], excess 26Mg from 26Al; T1/2=0.73 Myr [6], 53Cr from 53Mn; T1/2=3.7 Myr [7], 182Hf from 182W; T1/2=9 Myr [8,9]) provide a framework for the formation of solids in the early solar system. We present an early solar system chronology based on the calibration of short-lived isotope chronometries to several tie points (CAIs, H chondrites, Acapulco), and planetesimal heating in the early solar system [3,10]. Conditions of formation of the first solids in the solar nebula varied - most probably due to p,T differences imposed by the early sun - with radial distance and/or time, and caused the compositional variety of planetesimals concerning refractory and volatile elements, metals, Mg-rich silicates, and probably also oxygen isotopes [10,11,12]. Radiometric dating and chemical composition suggest that individual planetesimals grew rapidly in the asteroid belt (within < 1 Myr), but different planetesimals formed over a time interval of 4 million years [3,9,10], well within the lifetime of protoplanetary dust disks in extrasolar systems [13,14]. Early planetesimals were heated to varying degrees by decay heat of short-lived nuclides (primarily 26Al) [3]. This caused melting and differentiation in early (within < 2 Ma after CAIs) formed planetesimals and led to the formation of iron cores and basaltic rocks, while planetesimals that accreted later remained undifferentiated [3,9,10]. Chondritic parent bodies experienced severe thermal metamorphism in the case of ordinary chondrites, and aqueous alteration (further modifying the oxygen isotopic composition) in the case of carbonaceous chondrites. As most chondrules were immediately consumed in accreting planetesimals, they were only preserved in unmelted chondritic parent bodies and their age distribution is biased to the formation time interval of chondrites 2-3 Ma after CAIs [10]. The formation of solids in the early solar system (CAIs, chondrules, planetesimals and terrestrial planets) are still insufficiently linked to astrophysically constrained processes like early protostellar activity, disk dissipation, formation and migration of gas planets interacting with young disks [13,14]. Models of Earth and Mars formation based on 182Hf -182W core formation ages estimate the presence of planetary embryos of 60% the size of Mars after 2- 4 Ma [15]. This requires the early presence of Jupiter to effectively prevent the formation of a proto-planet in the asteroid belt. Planetesimal formation in the asteroid belt and the terrestrial planet formation zone at <3 Ma after CAIs was likely accompanied by inner disk clearing permitting solar wind irradiation (and possibly volatile element depletion) of terrestrial - and partly asteroidal - precursor planetesimals [16]. Inner disk gas loss may also have been responsible for preventing the migration of Jupiter into the inner solar system. References [1] Allègre C.J., Manhès G., Göpel C. Geochim. Cosmochim. Acta 59, 1445 (1995) [2] Amelin Y., Krot A. N. et al. Science 297, 1678 (2002) [3] Trieloff M., Jessberger E.K., et al. Nature 422, 502 (2003) [4] Brazzle R.H., Pravdivtseva O.V., Meshik A.P., Hohenberg C.M. Geochim. Cosmochim Acta 63, 739 (1999) [5] Gilmour J.D., Saxton, J.M. Phil. Trans. R. Soc. Lond. A 359, 2037 (2001) [6] Bizzarro, M., Baker, J. A., Haack, H. Nature 431, 275 (2004). [7] Lugmair G.W., Shukolyukov A. Geochim. Cosmochim. Acta 62, 2863 (1998) [8] Kleine, T., Münker, C. et al. Nature 418, 952 (2002) [9] Kleine T., Mezger C. et al. Geochim. Cosmochim. Acta 68, 2935 (2004) [10] Trieloff M., Palme H. (2006) in: Planet Formation - Theory, Observations, and Experiments (Eds. H. Klahr & W. Brandner), Cambridge University Press, pp.64-89 [11] Clayton, R. N. Annu. Rev. Earth Planet. Sci. 21, 115 (1993). [12] Palme, H. Phil. Trans. R. Soc. Lond. A 359, 2061 (2001) [13] Haisch K.E. et al. Astrophys. J. 553, L153 (2001) [14] Briceno, C., Vivas, A. et al. Science 291, 93 (2001) [15] Trieloff M., Kunz J. et al., Science 288, 1036 (2000) [16] Trieloff M., Kunz J. et al., Earth Planet. Sci. Lett. 200, 297 (2002)
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