Microscopic crystals reveal violent aftermath of ancient meteor strike

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Aerial view of Meteor Crater in northern Arizona(Credit: NASA Earth Observatory)

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An international team of researchers has calculated the cataclysmic conditions created when an asteroid struck northern Arizona roughly 49,000 years ago by analyzing the structure of microscopic crystals known as zircons. The research could be used as a basis to piece together the violent pasts of other solar system bodies including that of Earth's Moon.

Under a powerful microscope, granular zircons appear as a collection of thousands of BB gun-like pellets. Zircons are thought to be created in the minutes directly following a devastatingly-powerful meteor strike, and capable of maintaining their structure for billions of years.

The impact of the Arizona meteor as it struck the Earth's surface created temperatures hot enough to melt the quartz grains embedded in the sandstone present in the surrounding crust. Under immense pressure and heat, the quartz formed a glass-like "shock-melted silica," that enveloped the crystals formed in the event. Samples from the region extracted by prospectors over a century ago were used as the basis for the new study.

The team analyzed the crystals by directing a beam of electrons at 14 zircons, which, upon bouncing off the samples and striking a detector, allowed the researchers to piece together the structure of the unusual crystals. It was found that the orientation of the grains accorded to the level of violent shock that heralded their creation.

A moderate shock would result in what is known as planar cleavage. A more violent impact would cause parts of the crystal structure to orientate in a different direction, and the most cataclysmic of the impacts would transform the zircon into the rare mineral reidite.

The analysis of the crystal structure led the team to estimate that when the Arizona meteor struck the surface, it created a pressure the equivalent of 300,000 atmospheres, and temperatures exceeding 2,000º C (3,632º F). These conditions were sufficient to cause the zircons to reform their structure into the granular formation observed in the samples.

It is hoped that the research will help scientists unravel the history of meteor fragments, and the journey that they took through space prior to striking our planet, and may even lead to a prediction regarding the frequency of cataclysmic meteor strikes capable of causing an extinction event. Moving forward, team members are applying the technique of examining the structural qualities of the crystals to Moon rock samples obtained during the Apollo era.

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