Friday, November 13, 2015

Earth's water as old as the planet itself

by Rogelio Estrada:

Recent theoretical studies have found that a few water molecules could have clung tightly to the coalescing dust particles even in the hot conditions of Earth's formation, but Hallis's study is the first to provide firm factual evidence. He adds, "And it wasn't added by comets". As Earth assembled, this waterlogged dust created a wet planet right from the start, Hallis and colleagues propose. So water enriched in deuterium is a sign that the H2O formed in the solar system's chillier regions, such as the outer areas where comets originate. But the water covering Earth did not have the same hydrogen composition, a sort of water fingerprint, as comets that have been tested.

But they were missing one key piece of information.

Earlier studies of Earth's surface water, however, found ratios that favored deuterium.

During extensive geological surveys, the team managed to uncover this volcanic rocks in Padloping Island and in Iceland.

The Baffin Island rocks are believed to have originated in the Icelandic deep mantle plume hotspot. "It's a time capsule to back in the early Earth", says Hallis. The lavas formed with small bits of glass encased inside a mineral called olivine.

When such material rises to Earth's surface and spills out as lava, it can quickly harden and trap water and other compounds from the deep mantle in crystallized bubbles known as melt inclusions.

Water was first sealed in tiny glass pockets.

"Measurements of the water inside Martian meteorites suggests that Mars has a similar hydrogen composition inside its mantle to the hydrogen composition that we've measured in the deep Earth mantle, suggesting that both Earth and Mars got their water from the same source in the solar system".

The D/H ratio also is an indication of chemical processes over time. Delivery of Earth's water by these solar system outliers could explain a higher ratio of deuterium.

The ratio of deuterium to hydrogen in the universe was fixed shortly after the Big Bang. By using an ion microprobe, they went on to measure the ratio of deuterium to hydrogen.

Analysis of water samples from Earth's deep mantle suggest that the planet's water has been present since Earth's formation, a finding that has implications for other rocky bodies in the solar system and planets beyond as well, a study released on Thursday shows.

As the hydrogen composition of those water molecules differed significantly from those seen on icy comets, the rocks disputed the popular comet theory.

Carl Sagan famously dubbed Earth the "pale blue dot" for our planet's abundant water. Scientists thought that Earth would have been too hot and any water would have boiled away, explains Hallis. 

'Even though a good deal of water would have been lost at the surface through evaporation in the heat of the formation process, enough survived to form the world's water. Understanding the origins of our own water could help scientists hone in on planets that might also have ingredients for life. Hallis says that her results could mean that water-rich planets like Earth are not so rare after all.

Meech, an astronomer specializing in comets, isn't so sure comets can be counted out of the equation yet.

"I don't think that we got 100 percent of our water from this accretion method", says Hallis. "That would make habitable worlds much more likely", says Marschall.

She said the Baffin Island rocks had been analysed extensively since their collection in 1985 and scientists "know they contain a component from Earth's deep mantle". As the surface would cool down, the particles descended, thus forming water.

But, "We're in the beginnings of understanding this and we're very far from saying this is definitely what happens", Hallis says. In the prevailing model of an initially dry Earth, hydrating the planet seemed like "more of a one-off event", Hallis says. "There's lots of water down there".
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