The early Earth, in other words, may have been an interrupted Eden - a planet where life repeatedly evolved and diversified, only to be sent back to square one by asteroids 10 or 20 times wider than the one that hastened the dinosaurs` demise. When the surface of the Earth finally became inhabitable again, thousands of years after each asteroid impact, the survivors would have emerged from their hiding places and spread across the planet - until another asteroid struck and the whole cycle was repeated.

``We know that large asteroid impacts can sterilize or partially sterilize planets,`` says Norman Sleep, a professor of geophysics at Stanford who will present the theory at the fall meeting of the American Geophysical Union in San Francisco on Friday, Dec. 14.

``An asteroid a few hundred kilometers in diameter will boil off much of the ocean and leave the rest of the ocean very hot, so all that will survive will be high-temperature organisms living deep in the subsurface,`` he says. Rock vapor and water would fill the atmosphere, killing off any life on the surface with temperatures upwards of 1,000 C (1,800 F).

The only organisms that could survive such an impact are thermophiles - heat-loving microbes - buried half a mile or more below the Earth`s surface, where the effects of the burning atmosphere would have been muted to a survivable 100 C (212 F). Those organisms may have given rise to much of the life on today`s Earth.

Sleep calls the region where those organisms would have lived the ``Goldilocks Zone`` - deep enough for microbes to avoid the heat of the burning atmosphere, but not so deep that they ran afoul of the Earth`s internal he
'"/>

Contact: Mark Shwartz
mshwartz@stanford.edu
650-723-9296
Stanford University
14-Dec-2001