New studies extend period of asteroid bombardment of Earth

Nature : From the evidence of impact craters, it is well known that the Moon and Earth were battered by asteroids during their early history. Computer simulations had indicated that the late heavy bombardment (LHB), which followed the planets’ initial formation through accretion, lasted from 4.1 billion to 3.8 billion years ago. Two recent studies published in Nature suggest, however, that the battering lasted much longer. In the first study , William Bottke at the Southwest Research Institute in Boulder, Colorado, and colleagues propose that during the LHB, the inner boundary of the asteroid belt lay closer to the Sun than it does now; asteroids dislodged from that part of the belt would have been even more likely to end up in Earth-crossing orbits and prolonged Earth’s LHB. In the second study , Brandon Johnson and Jay Melosh at Purdue University in West Lafayette, Indiana, looked at the rock record from as recently as 2 billion to 3.5 billion years ago and found evidence of impact spherules—tiny droplets of molten rock flung into the atmosphere by asteroid impacts—that scattered, solidified, and settled back to Earth. Although the consequences of asteroid impacts are not fully known, the fact that the impacts may have continued for nearly the first half of Earth’s history could have profoundly affected the planet’s biosphere and evolution of life.