August 11, 2009

Lab-Made Meteorite Mineral Hints At Solar System Formation

Scientists have found that a collision of materials can form a mineral only found naturally in meteorites and the deep layers of Earth's mantle.

The success of their "shock" experiment gives new clues about how our Solar System was formed.

The journal Proceedings of the National Academy of Sciences (PNAS) published the team's report on the production of the mineral wadsleyite.

The team found by its creation in the lab that the objects believed to have collided to form the planets could have been much smaller than previously thought.

The new information could give greater understanding to how dust and gas fused into planets to ultimately make our Solar System approximately 4.6 billion years ago. 

Thomas Ahrens at the California Institute of Technology (Caltech), in Pasadena, headed the team in recreating early Solar System collisions in the laboratory by firing a bullet down a long gun at two materials, magnesium oxide and silicon dioxide (or quartz).

They then observed that 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 traveled through the steel and into the sample," explained Paul Asimow, an author of the study and a researcher at Caltech.

"As [the wave] propagated through the material, it generated high pressures and high temperatures for a very short time."

The scientists recovered the same by sawing the steel chamber in half and they were then able to examine what they had made with the use of 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 micrometers (a few thousandths of a millimeter) in size."

Although experiments performed before have successfully made wadsleyite, these efforts involved putting the samples under very high pressures for long periods of time. This is the first time the mineral has been created in a shock experiment.

Prior to this, Dr. Asimow explained, the majority of scientists believed that the mineral took a relatively long time to make and that the meteorites containing it would have to be subjected to a few seconds of extreme high pressures.

However, the researchers were able to create the mineral in a millionth of a second in this case.
"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.

The ability to create the mineral within a microsecond proves that the pressure it takes could be generated by the impact of objects only a few yards 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."

"One now has to consider whether shock features in meteorites could have been caused by small or large collisions," said Douglas Rumble, geophysicist from The Carnegie Institution of Washington.

"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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