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Saturday, November 19th, 2005, 11:16 PM
New View of Early Earth: A Habitable Place

Robert Roy Britt (http://us.rd.yahoo.com/dailynews/space/sc_space/byline/newviewofearlyearthahabitableplace/17138922/SIG=11mbivi1v/*http://www.space.com/php/contactus/feedback.php?r=rb)
LiveScience Managing Editor

LiveScience.com (http://us.rd.yahoo.com/dailynews/space/sc_space/byline/newviewofearlyearthahabitableplace/17138922/SIG=10sog4vj6/*http://www.livescience.com)

There are a trio of longstanding views of what Earth might have looked like in its formative years: a moon-like desert, a fiery volcanic hell, or a waterworld with no firm footing.
All three may be wrong.

A new study concludes Earth had continents and oceans 4.3 billion years ago, which is just a geological eyeblink after the planet is thought to have formed, in the wake of the Sun's birth 4.6 billion years ago.
A separate study reported in May came to a similar conclusion, also suggesting that notions of a fiery, hellish planet back then have been overblown.

Here's why it matters: A world with water and land and somewhat moderate temperatures and volcanic conditions would have been habitable. That does not mean there was life, but the conditions were in place.

The new study was reported this week in the online version of the journal Science. University of Colorado researcher Stephen Mojzsis explained what our world might have looked like back then:
"Before 4 billion years ago, the Earth would not be recognizable for the Pale Blue World that we are familiar with today. Indeed, although we now understand that there were significant landmasses already present by that time, the denser carbon dioxide-rich atmosphere would have given the sky a reddish-tinge," Mojzsis told LiveScience.

"The oceans, with a much higher concentration of iron than our contemporary oceans, would look a dark greenish-blue and these oceans would have bathed hundreds of small continents akin to New Zealand or the Japan arc," he said.
The conclusion is based on an analysis of hafnium, a rare element in ancient minerals from the Jack Hills in Western Australia. The rocks are thought to be among the oldest on Earth, dated to 4.4 billion years ago.
"The evidence indicates that there was substantial continental crust on Earth within its first 100 million years of existence," Mojzsis said.

The research, led by Mark Harrison of the Australian National University, builds on work Mojzsis and colleagues reported in 2001 that showed evidence for water on Earth's surface roughly 4.3 billion years ago.
"The view we are taking now is that Earth's crust, oceans and atmosphere were in place very early on, and that a habitable planet was established rapidly," Mojzsis said.

The air would have been an unbreathable mix of carbon dioxide, water vapor, sulfur gases and methane. Yet for many microbes, "this is the preferred environment," Mojzsis points out.

Scientists do not know exactly when life began or how it got started. If it did begin 4.3 billion years ago, it may have been wiped out by space rock impacts, only to start up again, other theorists say. At any rate, Earth was a treacherous place for the first billion years or so, until it had helped scoop up many of the asteroids and comets that filled the early solar system.

Gorm the Old
Sunday, November 20th, 2005, 01:13 AM
This agrees well with modern biochemical theories of the origin of the banded ironstone and the sedimentary hematite event of about 2.2 billion years B.P.Earth would have had a reducing atmosphere and oceans saturated with ferrous iron for over 2 billion years. During this time, only anaerobic bacteria could have survived. Chemoautotropes which produced oxygen as a waste product of their metabolism could survive only where there was sufficient ferrous ion to act as an oxygen acceptor. Being anaerobes, they would be poisoned by their own waste product, oxygen, unless it were removed from their immediate environment as by oxidation of ferrous iron. It had been proposed that these anaerobes survived in any one place until they had exhausted the ferrous iron in the water and then died out from oxygen poisoning. While this was going on, hematite (Fe2O3) would be precipitated along with inorganically precipitated silica, producing an iron-rich layer of siliceous sedimentary rock. After the anaerobic bacteria died out, except for the few which still could use the small amount of remaining ferrous iron as an oxygen acceptor. The silica would be deposited uninterruptedly, but there would be no hematite being produced. The next layer deposited would be iron-poor. This went on for at least one billion years, depositing the laminated chert deposits with alternately iron-rich and iron-poor layers which we now call taconite. About 2.2 billion years ago, something new happened, bacteria devloped which, even if they did not use oxygen in their metabolism, could at least tolerate it. Aerobic bacteria had appeared. Not being dependent on an oxygen acceptor in their environment to survive, they underwent a population explosion, excreting enormous quantities of oxygen which oxidized all the ferrous iron in the seas, precipitating thick deposits of hematite , the iron ores of , e.g., the Great Lakes area, and ending the deposition of taconite. Prior to this event, any free oxygen in the atmosphere would react with the ferrous iron in the seas. Until the appearence of aerobic bacteria in the seas, there could be no free oxygen in the Earth's atmosphere. After the sedimentary hematite event of 2.2 billion years ago, oxygen could slowly leak out of the seas into the atosphere, paving the way for life on land. The earliest land organisms, scorpions, appeared about 350 million years ago, so the process of enriching the atmosphere in oxygen to a breathable level was a very slow one, requiring about 1,850,000,000 years.