Wednesday, October 07, 2015

Meteorite Markings Offer Clues to Their Past

Most iron meteorites are thought to be the remnants of planetesimals that grew large enough to differentiate very early during the formation of the solar system. Later destroyed by violent collisions, the parent body broke into pieces, some of them fragments of the nickel-iron core at the center, and others parts of the silicate crust and mantle. Some of these fragments were perturbed in their orbits enough to careen into the inner solar system, and a lucky few have ended up on Earth.

Nickel-iron phase diagram.
Image courtesy wiki user Tobias1984, CC BY-SA.
Slice one open, polish the surface, and coat it with a corrosive acid, and you might just reveal the secrets of its birth. Widmanst├Ątten patterns, or Thomson structures (as they’re sometimes known), are common among some types of iron meteorites, and — with a little knowledge of the nickel-iron phase diagram — they can tell us about the cooling history and size of the original parent body that was broken up long ago. In this episode, we catch up with geochemist and meteorite expert Dr. Melanie Channon to find out how.

Like Earth’s core, iron meteorites are primarily made of iron and nickel, and these two constituents can be arranged into different phases depending on the temperature, pressure, and proportion of nickel to iron you have. Newly-formed planetesimals started out hot and cooled down over time. At first, all of the nickel-iron solid solution would have crystallized as a phase called taenite, but as the temperature continued to drop, the nickel atoms diffused away to form thin bands, or lamellae, of kamacite, a phase with a slightly different crystal structure. These lamellae grew over millions of years to form the beautiful interlocking pattern we can see today. By measuring the nickel content and width of the kamacite lamellae in a given sample, it’s possible to back-calculate the cooling rate of the original planetesimal, and that information in turn can shed light on the size of the parent body through the physics of heat conduction. That slice of meteorite in your hand has a lot to say about its past, as long as you know how to listen!

Podcast and post by Meg Rosenburg

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