Sunscreen for the Young Earth

摘要

What were the conditions on the young Earth that allowed life to form? The obvious one, the presence of water, has driven the search for extra-solar planets whose orbits lie in the so-called habitable zone. A less well known requirement, however, is the need for Earth to generate a large-scale magnetic field to shield it from the high-energy radiation and wind from the Sun. Without such a magnetic field, the atmosphere would have been exposed to ionization and erosion by the charged particles in the solar wind, and any form of early life would have been irradiated by the intense x-ray and high-energy ultraviolet emission from the young Sun. On page 1238 of this issue Tarduno et al. (1) show that an appreciable magnetic field had developed as early as 3.45 billion years ago (Ga). The timing-not long after the Late Heavy Bombardment (3.90 Ga) but before the development of an oxygen-rich atmosphere (2.30 Ga)-suggests that the magnetic field may predate the establishment of life. The measured field strengths, while less than that of the present-day Earth, may have been sufficient to shroud the Earth in a protective shield.