Some of the materials essential for life as we know it—iron, potassium, and iodine, for example—are created by supernovae and then distributed throughout space. But these stellar explosions also create lead, silver, and gold, as well as heavier radioactive elements like uranium and plutonium. “Small amounts of debris from these distant explosions fall on the Earth as it travels through the galaxy,” says Australian National University’s Anton Wallner.
To determine the amount of heavy elements in what they thought was supernova dust in the deep sea, Wallner and colleagues analyzed a 10-centimeter-thick sample of the Earth’s crust (pictured above) that represents two dozen million years of accretion, as well as sediment collected from a stable area at the bottom of the Pacific Ocean. They focused on plutonium-244, which acts like a clock thanks to the nature of its radioactive decay: It has a half-life of 81 million years. They were surprised.
“We’ve analyzed galactic dust from the last 25 million years that has settled on the ocean and found there is much less of the heavy elements such as plutonium and uranium than we expected,” Wallner says in a news release. “We found 100 times less plutonium-244 than we expected.”
Any plutonium-244 that existed when our planet formed from intergalactic gas and dust over four billion years ago has long since decayed. Wallner adds: “So any plutonium-244 that we find on earth must have been created in explosive events that have occurred more recently, in the last few hundred million years.”
The findings, published in Nature Communications this week, suggest that the heaviest elements weren’t formed in standard supernovae. Rather, this discrepancy indicates that the heavy, radioactive members of the periodic table were created in much rarer, more explosive events—such as the merging of two neutron stars (pictured to the right), each of which may contain the mass of the sun in a sphere the size of a city.
Images: A. Wallner et al., Nature Communications 2015 (top), NASA/Goddard Space Flight Center (bottom)
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