[meteorite-list] Earth's Water Likely Came from Very Early Asteroid Strikes

[Ron Baalke]

http://www.space.com/23336-earth-water-origin-asteroid-impacts.html 

Earth's Water Likely Came from Very Early Asteroid Strikes
by Mike Wall
space.com
Oct 25, 2013

Earth got most of its water from asteroid impacts nearly 4.6 billion years 
ago, shortly after the solar system first took shape, a new study suggests.

Researchers studying a meteorite that fell to Earth in 2000 found evidence 
that the water in its parent asteroid disappeared soon after the space 
rock formed, when its insides were still warm. Asteroids that slammed 
into Earth several hundred million years after the solar system's birth 
were thus probably relatively dry, scientists said.

"So, our results suggest that the water [was] supplied to Earth in the 
period when planets formed rather than the period of late heavy bombardment 
from 4.1 billion years to 3.8 billion years ago," study lead author Yuki 
Kimura, of Tohoku University in Japan, told LiveScience via email. 

Kimura and his colleagues analyzed the Tagish Lake meteorite, which landed 
in Canada's Yukon Territory in January 2000. Scientists think this rock 
- a type of meteorite known as a carbonaceous chondrite - is a piece of 
an asteroid that originated in the main belt between Mars and Jupiter.

The scientists used a transmission electron microscrope to observe tiny 
particles of magnetite, which arranged themselves within the meteorite 
into three-dimensional "colloidal crystals."

These crystals can be formed during the sublimation of water - the transition 
of the material directly from ice to vapor - but not during freezing, 
Kimura said. This implies that the parent asteroid's bulk water disappeared 
in the early stages of the solar system's formation, before the space 
rock's innards had a chance to cool down, he added.

Other studies have also found support for very early water delivery to 
Earth. For example, a paper published this May in the journal Science 
found that water on the moon and Earth come from the same source.

The simplest explanation for this latter observation, researchers say, 
is that Earth was already wet by about 4.5 billion years ago, when a planet-size 
body is believed to have smashed into our planet, ejecting a huge amount 
of debris that eventually coalesced into the moon.

In addition to water, impacts likely delivered to the young Earth organic 
molecules - the carbon-containing building blocks of life as we know it. 
Indeed, the colloidal crystals in the Tagish Lake meteorite have an organic 
layer on their surface, Kimura said.

"Further analysis might give us some information about evolution of organic 
molecules in the early solar system," he said.