January 20, 2015 | David F. Coppedge

Blow to Supernova Nucleogenesis Theory

There’s 100 times less of a radioactive element on the ocean floor than expected.

According to commonly accepted theory, heavy elements are cooked inside stars and distributed via supernova explosions. One particular unstable isotope, Plutonium-244 with a half-life of 81 million years, should be a good tracer of supernova explosions in the earth’s vicinity. Since the earth is believed to have formed 4.5 billion years ago, any primordial Pu-244 should be long gone. New Pu-244 should have come from supernovae since earth’s birth. But there’s a problem. PhysOrg starts an article with the rumble of a paradigm shaking:

Scientists plumbing the depths of the ocean have made a surprise finding that could change the way we understand supernovae, exploding stars way beyond our solar system.

What happened? Dr. Anton Wallner of Australian National University and 12 colleagues went looking for Pu-244 on the ocean floor, thinking there should be some specks from supernova explosions during the past 100 million years. Earth should receive a sprinkling of this isotope because, according to theory, abundances of heavy elements should reach a steady state in interstellar dust.

“We’ve analysed 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.

The findings are at odds with current theories of supernovae, in which some of the materials essential for human life, such as iron, potassium and iodine are created and distributed throughout space….

“We found 100 times less plutonium-244 than we expected,” Dr Wallner said.

Scratching his head, Wallner wondered if theories of nucleosynthesis of heavy elements are wrong.  Maybe it takes the collision of neutron stars to form this isotope.  Where do large radioisotopes come from?

The fact that these heavy elements like plutonium were present, and uranium and thorium are still present on earth suggests that such an explosive event must have happened close to the earth around the time it formed, said Dr Wallner.

“Radioactive elements in our planet such as uranium and thorium provide much of the heat that drives continental movement, perhaps other planets don’t have the same heat engine inside them,” he said.

His findings not only question nucleosynthesis, therefore, but add an additional constraint on habitability. Without the radiogenic heat to drive plate tectonics, it is unlikely a planet would be suitable for complex life. Wallner et al.‘s study is published in Nature Communications.

We offer this astro-geo-physical tidbit to individuals who may wish to explore the implications. One possibility Wallner did not think to consider was whether the earth is younger than he assumes.




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