As suggested in the previous section, solar climate during the first Ga of the Earth was radically different. The earliest relevant factor was excessive solar-flare energetic particle emission, a phenomenon that has been recorded in meteorites (Goswami, 1991). These extraterrestrial samples provide information on events that took place during this early period after the collapse of the solar nebula disk. Gas-rich meteorites have yielded evidence for a more active Sun. A considerable number of young stars with remnants of accretion disks show energetic winds that emerge from the stars themselves. Similar ejections are still currently observed from our Sun. For this reason it is believed that some of the early Solar system material represented by meteorites could have retained the record of such emissions.
Information on the energetic emission of the Sun during this period can be inferred from data on X ray and UV emission (larger than 10 eV) from pre-main-sequence stars. We may conclude that during pre-main-sequence period, solar climate and weather presented an insurmountable barrier for the origin of life anywhere in the Solar system. In the Hadean, conditions may still have been somewhat favourable, especially with the broad set of UV defence mechanisms that are conceivable. The high UV flux of the early Sun would, in principle, cause destruction of prebiotic organic compounds due to the presence of an anoxic atmosphere without the present-day ozone layer (Canuto et al., 1982, 1983). Some possible UV defence mechanisms have been proposed in the past, such as atmospheric absorbers and prebiotic organic compounds (Margulis et al., 1976; Sagan and Chyba, 1997; Cleaves and Miller, 1998).
Inversions of the Earth's geomagnetic dipole represent a well-established geochronological framework. The most recent of these inversions, referred to as the Matuyama-Brunhes (M-B) transition, has been dated to about 780 ka ago.
During a geomagnetic reversal, the dipole field strength is believed to decrease by about an order of magnitude. During this time, galactic cosmic rays can more easily penetrate into the Earth's atmosphere and thus increase the production of cosmogenic isotopes, such as 10Be. Evidence has been presented for enhanced 10Be deposition in the ice at 3,160-3,170 m, interpreted as a result of the low dipole field strength during the Matuyama-Brunhes geomagnetic reversal. If correct, this provides a crucial tie point between ice and marine core records (Raisbeck et al., 2006).
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