November 17th, 2012 by James Ayre
Pallasites are a very rare type of meteorite that are composed of brilliant, translucent olivine (peridot) crystals contained in an iron-nickel matrix. Often referred to as ‘space gems’, pallasites have long been of interest to researchers, since they were first identified as being of meteoric origin more than 200 years ago.
And now new research has revealed that the way that they were formed is far more dramatic than first realized. “Using a carbon dioxide laser, a magnetic field, and a sophisticated recording device, a team of geophysicists, led by John Tarduno at the University of Rochester, has shown that the pallasites were likely formed when a smaller asteroid crashed into a planet-like body about 30 times smaller than earth, resulting in a mix of materials 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, who was not involved in the study. “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.”
Pallasites are composed of the translucent, gem-like mineral olivine (peridot), and an iron-nickel matrix. So many researchers have assumed that they were created where these two materials are known to meet, which is “at the boundary of the iron core and rocky mantle in an asteroid or other planetary body.” The researchers discovered “that tiny metal grains in the olivine were magnetized in a common direction, a revelation that led the researchers 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.”
“In order for the metal grains — located in the olivine — to become magnetized, there had to be a churning, molten iron core to create a magnetic field. And temperatures at the core-mantle boundary — which would have been close to 930° C — are simply too hot for magnetization to take place. That means that the pallasites must have formed at relatively shallow depths in the rocky mantle, where it was much cooler.”
By utilizing a carbon dioxide laser, the researchers “were able to heat the metal grains past their individual Curie temperatures — the point at which a metal loses its magnetization. The grains were then cooled in the presence of a magnetic field in order to become re-magnetized, while a highly sensitive measuring instrument called a SQUID (superconducting quantum interference device) was used to record the values. In that way, the research team was able to calculate the strength of the past magnetic field, and then 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. Tarduno said a 200 km radius made the body large enough to be considered a protoplanet — a small celestial object with the potential of developing into planets.”
The new research has also helped to clarify whether small celestial bodies are able to have a rotating, liquid iron core, that is able to generate a magnetic field.
“Our magnetic data join mounting evidence from meteorites that small bodies can, indeed, have dynamo action,” said Tarduno.
The new research was just published on November 17th in the journal Science.
Some background on Pallasites:
“A pallasite is a type of stony–iron meteorite. A common error is to associate their name with the asteroid 2 Pallas but their actual name is after the German naturalist Peter Pallas (1741–1811), who studied in 1772 a specimen found earlier near Krasnoyarsk in the mountains of Siberia that had a mass of 680 kg. The Krasnoyarsk mass described by Pallas in 1776 was one of the examples used by E.F.F. Chladni in the 1790s to demonstrate the reality of meteorite falls on the Earth, which were at his time considered by most scientists as fairytales. This rock mass was dissimilar to all rocks or ores found in this area (and the large piece could not have been accidentally transported to the find site), but its content of native metal was similar to other finds known from completely different areas.”
Image Credits: Arlene Schlazer; Esquel via Wikimedia Commons
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