November 16, 2012
Pallasites Created By Dramatic Collision Of Asteroid And Protoplanet
April Flowers for redOrbit.com - Your Universe Online
Some meteorites found on Earth contain strikingly beautiful, translucent, olive-green crystals embedded in an iron-nickel matrix. These "space gems," called pallasites, are only found in a tiny fraction of the total number of meteorites, but they have fascinated scientists since they were first identified as originating in outer space more than 200 years ago.A new study, published in the journal Science and led by John Tarduno at the University of Rochester, reveals the origins of these space gems are more dramatic than originally thought. A team of geophysicists used a carbon dioxide laser, a magnetic field, and a sophisticated recording device to show that the pallasites were likely formed when a smaller asteroid crashed into a planet-like body approximately 30 times smaller than Earth. This impact resulted in a mix of material that make up the distinctive meteorites.
"The findings by John Tarduno and his team turn the original pallasite formation model on its head," said Joshua Feinberg, assistant professor of earth sciences at the University of Minnesota. "Their analysis of the pallasites has helped to significantly redefine our understanding of how these objects formed during the early history of our solar system."
The composition of pallasites — iron-nickel and the translucent, gem-like mineral olivine — leads many scientists to assume they were formed where those two materials typically come together - at the boundary of the iron core and rocky mantle in an asteroid or other planetary body. The team has discovered that tiny metal grains in the olivine were magnetized in a common direction. This revelation led the scientists to conclude that the pallasites must have been formed much farther from the core.
"We think the iron-nickel in the pallasites came from a collision with an asteroid," said research team member Francis Nimmo, professor of earth and planetary sciences at the University of California Santa Cruz. "Molten iron from the core of the smaller asteroid was injected into the mantle of the larger body, creating the textures we see in the pallasites."
"Previous thinking had been that iron was squeezed up from the core into olivine in the mantle," said Tarduno. "The magnetic grains in the olivine showed that was not the case."
For the metal grains in the olivine to become magnetized a churning, molten iron core is required to create a magnetic field. Temperatures at the core-mantle boundary — which reach approximately 930 Celsius — are too hot for that field to form, meaning that the pallasites must have formed at relatively shallow depths in the rocky mantle where temperatures were much cooler.
The scientists were able to heat the metal grains past their individual Curie temperatures–the point at which a metal loses its magnetization by using a carbon dioxide laser. A highly sensitive measuring instrument called a SQUID (superconducting quantum interference device) was used to record the values as the grains were cooled in the presence of a magnetic field in order to become re-magnetized. The team was able to calculate the strength of the original magnetic field and determine the rate of cooling using prior published work on metal microstructures.
"The larger the parent body was, the longer it would have taken for the samples to cool," said Nimmo. "Our measurements, combined with a computer model we developed, told us that the parent body had a radius of about 200 km–some 30 times smaller than earth."
The measurements helped the scientists to classify the parent body of the pallasites as a protoplanet — a small celestial object with the potential of developing into a planet.
The study also clears up questions about the possibility of protoplanets having dynamo activity - a rotating, liquid iron core that can create a magnetic field.
"Our magnetic data join mounting evidence from meteorites that small bodies can, indeed, have dynamo action," said Tarduno.