Monday, July 16, 2012

Study Says Asteroids Delivered Water to Earth

How did the Earth get its oceans? The primordial Earth was a seething ball of magma, so the water that it began with would have evaporated into space. As a result, planetary scientists have long debated which of two types of objects, comets or asteroids, were more responsible for delivering Earth’s water.
A new study, published today in Science, says that asteroids were the source. The authors, led by Conel Alexander of the Carnegie Institution of Washington, in Washington DC, analysed the isotopic abundances of nitrogen and hydrogen in 86 primitive meteorites, and found that they coordinate with Earth’s.
Asteroids had already been the favored source. Studies of solar system dynamics suggest that there was a period of time around 3.9 billion years ago, called the Late Heavy Bombardment, during which the Earth would have been barraged, mostly by asteroids.
Even though comets are ideal sources, with their high percentage content of water, rich with amino acids, there are a few strikes against them. Six studies of comets from the Oort Cloud found that their isotopic ratios of heavy hydrogen (or deuterium) were much higher than Earth’s. When a 2011 Nature paper found isotopic levels of heavy hydrogen in the comet Hartley 2 to be similar to Earth’s, it revived interest in water-from-comets idea. But Alexander and his colleagues suggest that the overall levels of heavy hydrogen in Hartley 2 (and not just the levels in the comet’s ice) would be much higher.
For the first time, researchers have detected water on an asteroid. Two research teams independently determined that the 24 Themis asteroid, which orbits between Mars and Jupiter, is completely covered in a thin layer of frost. The discovery indicates that comets and asteroids may be more similar than previously thought. Comets are traditionally defined as small lumps of rock and ice that form vapor tails when they venture close to the Sun. Asteroids are usually characterized as large dry rocks. Now the distinction between these two types of space rock is not so clear.

The ice on 24 Themis appears to have been present for billions of years. Researchers suggest that an icy reservoir within the asteroid may continually replenish the surface frost. This find is prompting astronomers to hypothesize that an asteroid collision with early Earth might explain how water was delivered to our planet.

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