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 | Subject: Meteorite mineral made in the lab  Tue Aug 11, 2009 4:01 pm | |
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Meteorite mineral made in the lab
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By Victoria Gill
Science reporter, BBC News
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 The team use large projectile launchers to create shock waves
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By slamming materials together, scientists have made a mineral that is found naturally only in meteorites and the deep layers of Earth's mantle.
Their successful "shock" experiment reveals new clues about the formation of our early Solar System.
The team report the production of the mineral wadsleyite in the journal PNAS.
Its creation in the lab shows that objects that collided to form the planets may have been far smaller than previously thought.
This could aid understanding of how the dust and gas that made up our Solar System around 4.6 billion years ago fused into planets.
The team, led by Thomas Ahrens at the California Institute of Technology (Caltech), in Pasadena, recreated early Solar System collisions in the laboratory by launching a bullet down a long gun at two materials - magnesium oxide and silicon dioxide (or quartz).
The two materials were embedded in a steel recovery chamber, bolted to the muzzle of the gun.
"We launched a tantalum bullet that struck the steel chamber and generated a shock wave that travelled through the steel and into the sample," explained Paul Asimow, an author of the study, also a researcher at Caltech.
.jpg) Tenham H6, a well-studied meteorite contains wadsleyite
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"As [the wave] propagated through the material, it generated high pressures and high temperatures for a very short time."
By sawing the steel chamber in half, the scientists were able to recover the sample and examine what they had created using a series of sensitive analytical techniques.
"We confirmed, using scanning electron microscopy, that it was wadsleyite. And that also allowed us to determine the size of the grains," said Dr Asimow.
"That's a key part of our study - we not only made wadsleyite, we made grains of it that were at least a few micrometres (a few thousandths of a millimetre) in size."
Short shock
Previous experiments have succeeded in making wadsleyite, but these efforts involved putting samples under very high pressures for long periods. This is the first time the mineral has been made in a shock experiment.
Dr Asimow explained that, prior to this, most scientists believed that the mineral took a relatively long time to make - and that the meteorites containing it would have to be subjected to high pressures for a few seconds.
Here, the researchers made it in a microsecond - or one millionth of a second.
"These shock waves travel at speeds in the order of 10km per second, so in order to stay at high pressure for a whole second, you would need to run together two things that are 10km in diameter," said Dr Asimow.
Creating the mineral within a microsecond shows that the pressure necessary to create it could be generated by the impact of objects just a few metres in diameter.
"Everything about 4.6 billion years ago was gas and dust, but somehow we ended up with large planets," said Dr Asimow.
"Those dust particles accreted into larger and larger objects. And, it's important for us to understand that process to understand the geochemistry of the Earth."
Douglas Rumble, a geophysicist from The Carnegie Institution of Washington said: "One now has to consider whether shock features in meteorites could have been caused by small or large collisions."
"The next step is to do more experiments and try additional techniques that give [very high] resolution so we can understand the shapes and microstructures... of the wadsleyite crystals." |
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