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.
