tomorrow (tuesday) night the history channel will broadcast a show called "how the earth was made." this episode's description is that asteroid impacts are responsible for a large nickel deposit in canada, oil deposits in the gulf of mexico, and the gold in south africa. check your local listings.

There's a theory kicking about that has the early Earth colliding with a Mars sized object which contributed most of its iron to the Earth's core, with the remaining fragments forming our moon. It's one way of explaining the high iron content of Earth relative to Venus and Mars.

This has interesting consequences. For one, the iron core generates a magnetic field which shields the Earth from solar radiation that would otherwise be lethal to life on land. Also, the addition of the Moon led to much higher tides than the sun can generate, which probably accelerated the movement of life from sea to land.

This event may also be responsible for the earth's rotation, and for it's relatively circular orbit. Important factors in keeping water in a liquid state over the eons, another essential factor for supporting life.

What it all hints at is that planets like ours may be few and far between, and that life in the universe may be very rare indeed, at least at the state of evolution we have reached.

All the more reason to take better care of the place, I'd say.

ebear