robert karl stonjek

A Cool Early Earth?
The textbook view that the earth spent its first half a billion years
drenched in magma could be wrong. The surface may have cooled quickly--with
oceans, nascent continents and the opportunity for life to form much earlier
By John W. Valley

In its infancy, beginning about 4.5 billion years ago, the earth glowed like
a faint star. Incandescent yellow-orange oceans of magma roiled the surface
following repeated collisions with immense boulders, some the size of small
planets, orbiting the newly formed sun. Averaging 75 times the speed of
sound, each impactor scorched the surface--shattering, melting and even
vaporizing on contact.

Early on, dense iron sank out of the magma oceans to form the metallic core,
liberating enough gravitational energy to melt the entire planet. Massive
meteorite strikes continued for hundreds of millions of years, some blasting
craters more than 1,000 kilometers in diameter. At the same time, deep
underground, the decay of radioactive elements produced heat at rates more
than six times greater than they do today.

These fiery conditions had to subside before molten rock could harden into a
crust, before continents could form, before the dense, steamy atmosphere
could pool as liquid water, and before the earth's first primitive life
could evolve and survive. But just how quickly did the surface of the earth
cool after its luminous birth? Most scientists have assumed that the hellish
environment lasted for as long as 500 million years, an era thus named the
Hadean. Major support for this view comes from the apparent absence of any
intact rocks older than four billion years--and from the first fossilized
signs of life, which are much younger still.

In the past five years, however, geologists--including my group at the
University of Wisconsin-Madison--have discovered dozens of ancient crystals
of the mineral zircon with chemical compositions that are changing our
thinking about the earth's beginnings. The unusual properties of these
durable minerals--each the size of the period in this sentence--enable the
crystals to preserve surprisingly robust clues about what the environment
was like when they formed. These tiny time capsules bear evidence that
oceans habitable to primitive life and perhaps continents could have
appeared 400 million years earlier than generally thought.

Full Text at Scientific American
http://www.sciam.com/article.cfm?cha...7C83414B7F0000

Posted by
Robert Karl Stonjek