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	Geochemist Bernhard Peucker-Ehrenbrink collects samples of 635-million-year-old rock in Namibia, to study the iridium layer for evidence of snowball earth vs. slushball earth.

In the 1990s, an enigmatic series of rock formations, found all over the world, led scientists to the provocative and controversial Snowball Earth theory: Between 2.2 billion and 635 million years ago, Earth’s climate swung violently between freeze and fry modes. The planet was either covered by ice from pole to pole, including a thick blanket shrouding all its oceans, or it thawed into a hothouse because of a buildup of heat-trapping greenhouse gases.

The theory offers insights into the evolution of our planet, its climate—and also life on Earth. After the last Snowball Earth melted, complex life-forms blossomed on a planet previously populated with nothing but single-celled organisms.

A variation of the theory arose—Slushball Earth—in which most oceans froze but a band of ice-free or thin-ice waters remained around the equator, providing marine organisms with open water and light to survive.

Key evidence for the theory are glacially derived rock formations that lie immediately below rocks that formed in tropical oceans. In the thin inter-layer boundary, unusually high concentrations of iridium, a noble metal, have been detected. Some scientists theorize that it came from space, landed on ice, and was stored there until it was deposited in a gush when Snowball Earth glaciers melted. Alternatively, the iridium may derive from volcanic exhalations, coming from the Earth’s mantle.

In 2005, WHOI geochemist Bernhard Peucker-Ehrenbrink (left) collected samples of 635-million-year-old rocks from the iridium boundary in the Hoanib Valley in Namibia. He will determine whether the iridium layer also contains minute quantities of other noble metals—osmium, rhodium, ruthenium, platinum, and palladium—that should also be found at high concentrations if the iridium reflects an accumulation of extraterrestrial material. With WHOI geochemist Mark Kurz, he will also analyze the rocks’ osmium and helium isotopic compositions, two of the most sensitive indicators of extraterrestrial material on Earth. If the source of the iridium and other noble metals proves to be purely extraterrestrial, Snowball Earth is a more likely scenario than Slushball Earth.

—Lonny Lippsett

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