[meteorite-list] Giant Moon-Forming Impact On Early Earth May Have Spawned Magma Ocean

[Ron Baalke]

http://www.space.com/23514-moon-crash-earth-magma-ocean.html

Giant Moon-Forming Impact On Early Earth May Have Spawned Magma Ocean
by Katia Moskvitch
space.com
November 8, 2013

LONDON - Billions of years ago, the Earth's atmosphere was opaque and 
the planet's surface was a vast magma ocean devoid of life.

This scenario, says Stanford University professor of geophysics Norman 
Sleep, was what the early Earth looked like just after a cataclysmic impact 
by a planet-size object that smashed into the infant Earth 4.5 billion 
years ago and formed the moon. The moon, once fully formed, which would 
have appeared much larger in the sky at the time, since it was closer 
to Earth

Hundreds of millions of years later, he added, the first forms of life 
appeared, possibly having hitched a ride on a rock from Mars. The scenario 
is one presented by Sleep at a recent Royal Society conference here called 
Origin of the Moon. A paper detailing Sleep's study was submitted to the 
symposium volume.

Although many elements of the theory have been around for some time, Sleep's 
synthesis is "like putting together a jigsaw puzzle with some pieces already 
known and some that are speculative and have new aspects," said Dave Stevenson, 
a Caltech professor of planetary science who was not involved with Sleep's 
study.

One of these new aspects is how Earth cooled down to the temperatures 
necessary for life to evolve, following the - presumed - giant impact 
that formed the moon.

The processes Sleep discussed took place in the period called Hadean, 
about 4 billion to 4.5 billion years ago - before the first organisms 
came into being, and well before more complex life-forms, including dinosaurs, 
started roaming the Earth.

Back then, the Earth was nothing like the blue Earth we know today.

Scorching world

Instead, the entire Earth was hot and molten all the way to its inner 
core, a mixture of molten rock and liquid.

No life would have been able to survive these brutally high temperatures, 
which reached 2,000degrees Celsius (more than 3,600 degrees Fahrenheit). 
Liquid water had no chance to form.

The Earth's atmosphere at this time was also much heavier. Its mass was 
similar to that of today's oceans, and it pushed down on Earth's surface 
with a pressure of hundreds of bars. (For comparison, the average pressure 
at the Earth's surface today is 1 bar).  It was also opaque - "you would 
not have been able to see much, just clouds covering everything," Stevenson 
said.

Beneath the clouds, a magma ocean swayed, with partially molten rock pushed 
around by tides, Sleep thinks.

These tides were due to the mutual attraction of the Earth and the moon, 
and were much stronger than those in today's watery oceans, as the moon 
was sitting much, much closer to the Earth back then.

The tides constantly stirred the ocean, causing the mantle to lose heat, 
similar to stirring and blowing on a bowl of soup. But once released from 
the Earth's depths, the heat was trapped at the surface, held back by 
the thick, opaque primordial atmosphere.

The heat could only escape the planet (and cool it down) at so-called 
cloud-top temperature levels - where it would be as cold as on a modern 
high mountain summit. But for the first 10 million years, the temperatures 
were much, much higher, Sleep said.

The energy loss caused by the mutual attraction of the Earth and the moon 
was also making the moon gradually pull away. This made the tides progressively 
weaker, so the molten rock was being stirred less and less, and the Earth's 
mantle began to solidify in stages.

"While at the top of the Earth there was still partially molten slurry 
with a bit of liquid left, in the middle there was a mushy layer, but 
the deep mantle was becoming solid,” Sleep said. "Lava was probably still 
coming up and erupting and freezing at the top, and then falling back 
in large, kilometer-size pieces that were sinking into the Earth."

Slowly, the internal heat flow ceased to dominate the climate, and the 
temperatures at the surface began to drop, with the heat being able to 
escape the atmosphere at last.

Life from Mars?

The sweltering temperatures and trapped heat were not the only obstacles 
for life to appear, Sleep said.

Another issue was overabundance of carbon dioxide in the primordial atmosphere. 
Carbon dioxide doesn't dissolve in molten rock, so it was bound to bubble 
up from the magma ocean, creating a so-called runaway greenhouse effect, 
Sleep said.

For the Earth to become habitable, most of this carbon dioxide had to 
vanish.

Sleep said this happened when the tectonic plates began to move in the 
late Hadean, some 4.4 billion years ago. With the plates moving, the carbon 
dioxide began to enter the mantle in a process called subduction, when 
one tectonic plate moves under another and sinks into the mantle.

Liquid water oceans had already begun to condense around that time, and 
once the Earth cooled sufficiently and most of the carbon dioxide was 
safely tucked away in the mantle, life did finally appear, Sleep said, 
adding that chances are that this life arrived on Earth from Mars.

"We know life was present on Earth about 3.9 billion years ago, but Mars 
was probably habitable for a long time before the Earth," Sleep said. 
"So you had hundreds of millions of years when Mars was not a particularly 
unpleasant place, with liquid water. If life evolved on Mars, rocks get 
knocked off by asteroids all the time - so at some point, a rock from 
Mars could've come in, trying to hit us on the head." 

And if conditions on Earth were just right for life to start, this Martian 
rock could've been the beginning of everything we know today.

But it's still only an idea, yet a testable one. Unlike Earth, though, 
the ancient geological record exists on Mars, Sleep said. It is just hard 
to examine.