Modern landscapes offer glimpses of the way our planet may have looked
billions of years ago.
By Tim Appenzeller Photographs by Frans Lanting
The early Earth was a vision of hell, all scalding rock and choking fumes. Since
then, its surface has cooled, continents have drifted, mountains have risen and
eroded, and life has emerged, benign and green. Nearly all traces of the planet
as it was have been wiped away. But from clues in the oldest rocks, deepest
magmas, and even the cratered face of the moon, scientists have traced the
planet's beginnings. As those early days have come into focus, so have the rare
scenes, found today in some of Earth's harshest places, that recall its ancient
self.
Its birth pangs began some 4.6 billion years ago as rock and ice particles swirling
around the young sun collided and merged, snowballing to produce ever larger
planetary building blocks. In violent pileups, they smashed together to create
planets, including the infant Earth. In the turmoil, another body, as big as Mars,
struck our planet with the energy of trillions of atomic bombs, enough to melt it all
the way through. Most of the impactor was swallowed up in the bottomless
magma ocean it created. But the collision also flung a small world's worth of
vaporized rock into orbit. Debris quickly gathered itself into a ball, and since then
Earth history has unfolded beneath the blank stare of the moon.
After the moon's fiery birth, the Earth's surface cooled. Even so, our planet
remained an alien world for the next 700 million years; scientists call this time the
Hadean, after the Greek underworld. Rafts of solid rock drifted in the magma like
dark ice floes. Gases hissed from the cooling rock—carbon dioxide, nitrogen,
water vapor, and others—enveloping the planet in a scalding atmosphere devoid
of oxygen. As the temperature dropped further, the steam condensed into rain
that fell in primordial monsoons and filled the ocean basins.
These first oceans may have been short-lived. Space rubble left over from the
birth of the planets—chunks of rock tens to hundreds of miles across—
bombarded Earth throughout the Hadean. The greatest impacts might have
boiled the oceans away, forcing the process of cooling and condensation to
begin again.
By 3.8 billion years ago the impacts relented. Liquid water could persist. About
that time, perhaps in the oceans, lifeless chemical reactions crossed a threshold,
producing molecules complex enough to reproduce themselves and evolve
toward greater complexity. Life was on a road that led, as early as 3.5 billion
years ago, to single-celled, blue-green cyanobacteria that flourished in the sunlit
parts of the oceans. By the trillions, these microscopic organisms transformed
the planet. They captured the energy of the sun to make food, releasing oxygen
as a waste product. Little by little they turned the atmosphere into breathable air,
opening the way to the diversity of life that followed.
Those days are long gone, but the processes that turned our planet from a hell to
a habitable world are still on view today, as the images on these pages show.
Primordial heat left over from the planet's formation still bursts out in volcanic
eruptions, spilling lava that exudes gases like the young, cooling Earth.
In the planet's harshest environments today, cyanobacteria reign as they have for
billions of years. And each time a plant gains a toehold on newly cooled lava, the
victory of life over lifeless rock—won so long ago on the young Earth—is affirmed
again.
Photos and article at http://www7.nationalgeographic.com/ngm/0612/feature2/index.html