Rare meteorites created in violent celestial collision

November 15, 2012
The Esquel meteorite, consisting of iron-nickel and olivine, was discovered in central Argentina. Credit: Photo by Arlene Schlazer

A tiny fraction of meteorites on earth contain strikingly beautiful, translucent, olive-green crystals embedded in an iron-nickel matrix. Called pallasites, these "space gems" have fascinated scientists since they were first identified as originating from outer space more than 200 years ago.

Now a new study published this week in Science (Nov. 16) shows that their origins were more dramatic than first thought. Using a , a , and a sophisticated recording device, a team of , 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 made of iron-nickel and the translucent, gem-like mineral olivine, leading many scientists to assume they were formed where those two materials typically come together—at the boundary of the and in an asteroid or other . Tarduno 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 using a carbon dioxide laser at the University of Rochester, the scientists 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.

Their work also helped clear up questions about whether small celestial bodies were capable of 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.

Explore further: Dynamo theory: How small planets can have self-sustaining magnetic fields

More information: "Evidence for a Dynamo in the Main Group Pallasite Parent Body," by J.A. Tarduno et al., Science, 2012.

Related Stories

Earth's inner core is melting... and freezing

May 18, 2011

The inner core of the Earth is simultaneously melting and freezing due to circulation of heat in the overlying rocky mantle, according to new research from the University of Leeds, UC San Diego and the Indian Institute of ...

Ancient lunar dynamo may explain magnetized moon rocks

November 9, 2011

The presence of magnetized rocks on the surface of the moon, which has no global magnetic field, has been a mystery since the days of the Apollo program. Now a team of scientists has proposed a novel mechanism that could ...

Cosmic voyager has a layover in St. Louis

November 10, 2011

Last January two amateur meteorite hunters dropped by Randy Korotev's office at Washington University in St. Louis to show him their latest purchase, a 17-kilogram pallasite meteorite found in 2006 near Conception Junction ...

A new kind of metal in the deep Earth

December 19, 2011

(PhysOrg.com) -- The crushing pressures and intense temperatures in Earth's deep interior squeeze atoms and electrons so closely together that they interact very differently. With depth materials change. New experiments and ...

First evidence of dynamo generation on an asteroid found

October 11, 2012

About 4.6 billion years ago, the solar system was little more than a tenuous disk of gas and dust. In the span of merely 10 million years, this soup evolved to form today's massive, complex planets. In the intervening period, ...

Recommended for you

Distant planet's interior chemistry may differ from our own

September 1, 2015

As astronomers continue finding new rocky planets around distant stars, high-pressure physicists are considering what the interiors of those planets might be like and how their chemistry could differ from that found on Earth. ...

New Horizons team selects potential Kuiper Belt flyby target

August 29, 2015

NASA has selected the potential next destination for the New Horizons mission to visit after its historic July 14 flyby of the Pluto system. The destination is a small Kuiper Belt object (KBO) known as 2014 MU69 that orbits ...

4 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

cantdrive85
1.2 / 5 (9) Nov 15, 2012
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.

Electric currents can do the same, they can also produce the field aligned phenomenon of the metallic grains that was observed.
Silverhill
3.7 / 5 (3) Nov 15, 2012
Moving, ionized matter constitutes an electric current. The liquid part of a planetary core is hot enough to be ionized, therefore conductive. If it is rotating, you have a current and therefore a magnetic field.
cantdrive85, your counter-proposal is not actually 'counter' at all.
ValeriaT
1 / 5 (1) Nov 15, 2012
A nifty piece of pallasite found near Fukang in China.
Torbjorn_Larsson_OM
not rated yet Nov 17, 2012
@ cantdrive85:

When you hear hoof beats, think horses not zebras. Dynamo fields are known from many planets. Local currents would have to be large and are haphazard in direction, note that the magnetization was global as would be predicted by an external field.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.