Zircon Crystals Could Reveal Earth’s Path Among the Stars

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By Tom Metcalfe, Eos/AGU. 

Excerpt: Tiny crystals in Earth’s crust may have recorded meteorite and comet impacts as our planet traveled through the spiral arms of the Milky Way over more than 4 billion years, according to new research. ...The key to the latest research was in the ratios of isotopes—forms of the same chemical element that have different numbers of neutrons—in the oxygen atoms of zircon’s silicate group. The relative levels of oxygen isotopes in samples of zircon crystals can tell geologists whether the crystals formed high in the crust, perhaps while interacting with water and sediments, or deeper within Earth’s mantle. ...The scientists evaluated the data’s “kurtosis,” or the measure of how flat or peaked a distribution is. ...The researchers determined that periods of high oxygen isotope kurtosis corresponded to times when our solar system was crossing the dense spiral arms of the Milky Way galaxy. Such crossings occurred roughly every 187 million years on average during our solar system’s 748-million-year orbit around the galactic center at a speed of about 240 kilometers per second. The study is one of the first to suggest that galactic-scale processes can affect Earth’s geology. ...Some other experts suggest the new study is notable for outlining the concept that galactic processes could have left geological traces, but it is not yet conclusive proof.... 

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