Back when the Time Lord and I were still engaged, we went shopping for wedding rings. He only had one criteria: he wanted his ring to be made of platinum or a similar material forged in a supernova. It's not quite as exotic as it sounds: most heavy elements were formed in supernovae, via a process called supernova nucleosynthesis.
Lighter elements form inside stars over the course of billions of years as they slowly burn through their fuel. But when massive stars use up their fuel -- going from hydrogen to helium, to carbon, oxygen and so on -- eventually there's nothing left but iron and nickel. At that point, their cores collapse and they explode into supernovae. When that happens, heavier elements form within seconds -- including gold, silver, lead, and uranium, as well as platinum -- and are ultimately scattered throughout space, seeding the universe, so to speak.
Huge amounts of energy are needed for that to happen, because it requires nuclei to fuse. The energy output from a supernova over the course of two weeks amounts to the equivalent of the total energy of 1 billion sun-like stars over 4 billion years, according to Shawn Bishop, an astrophysicist at the Technical University of Munich, Germany. Bishop was at the American Physical Society's recent April meeting in Denver, reporting on his preliminary evidence that certain fossils of ancient bacteria might contain an iron isotope produced by supernovae around 2.2 million years ago.
Bishop has been investigating this possibility for several years now. Whether we're talking about Type II or Type Ia supernovae, one of the elements that gets scattered throughout space is a neutron-rich isotope of iron, 60Fe. It has a pretty short half-life, however, especially compared to the age of our Solar System, so there shouldn't be any 60Fe on Earth.
Except there is -- a tiny bit, preserved in the ferromanganese crust deep on the ocean floor, discovered several years ago. This indicates that around the same time as that layer formed in the crust, Earth got pelted with some supernovae debris. "That we're here talking about it means it wasn't too close," Bishop joked at a meeting press briefing in Denver.
But if there's 60Fe in the crust on the ocean floor, there might be traces of it elsewhere -- possibly even in microfossils of a certain kind of magnetotactic bacteria found in core sediments taken from the ocean floor. And Bishop has figured out how to search for those traces using accelerator mass spectroscopy (AMS).
The bacteria in question were first described in a 1963 paper by Italian microbiologist Salvatore Bellini, who observed such bacteria orienting themselves toward the North Pole in bog sediment samples when he exampled them under a microscope. He realized they could somehow sense magnetic fields, and used that ability for navigation to find their preferred low-oxygen environments. Richard Blakemore published the first peer-reviewed paper on the critters in Science in 1975; it was Blakemore who first described them as "magnetotactic," and the name has stuck.
Okay, but why does Bishop think these bacteria might be the key to verifying his hypothesis? Their magnetic sense comes from chains of pure magnetite crystals formed by extracting iron from the ocean water and sediments along the sea bed. Bishop thinks it's possible that fine-grained debris from a supernova explosion could pass through Earth's atmosphere, rapidly oxidizing in the process so that they are broken down into tiny nano-oxides.
These would rapidly dissolve in oxygen, form rust, and eventually settle in the sediment along the ocean floor, where the bacteria would suck them up for their crystal chains. When the bacteria eventually die, those chains remain behind in the sediment, and 60Fe would be locked inside. So any traces of 60Fe found in that sediment would constitute a kind of biogenic signature of a supernova event, preserved in the fossil record.
So that's what Bishop set out to find. Over the last few years, he's obtained two sediment cores from the Pacific Ocean near the equator. He developed chemical processes to dissolve the tiny bacteria grains inside those samples, and then ran those samples through an AMS, calibrated using industrial magnetite (found in black printer toner). "We're literally counting individual atoms of 60Fe," he said.
When Bishop analyzed the data, he found strong peaks in 60Fe concentrations corresponding with a possible supernova event around 2.2 million years ago. He also cited a 2002 Physical Review Letters paper analyzing the motion of various star formations relative to earth around the same time period, indicating that there may have been several supernovae in a star cluster called Scorpio Centauri that passed near Earth -- handy "smoking gun" candidates for the source of that 60Fe.
It's worth noting that Bishop emphasized repeatedly that these are merely preliminary results from his analysis, done in January and augmented just two weeks before the Denver meeting. They have not undergone peer review, and are as yet unpublished. He's working on that over the next six months, and will also be moving on to analyzing the second core sample, which has five times more material, thereby improving the statistical quality of his results. He'd also like to extend the AMS analysis to searching for another supernova-produced isotope, aluminum-26, thereby garnering even more evidence for his hypothesis.
So is Bishop right that there is evidence of a supernova locked in the fossils of these ancient bacteria? It's starting to seem plausible, at least. Only time will tell.
Benitez, Narcisco; Maiz-Apellaniz, Jesus; Canelles, Matilde. (2002) "Evidence for Nearby Supernova Explosions," Physical Review Letters 88, letter 081101.
Bishop, Shawn and Egli, R. (2011) "Discovery Prospects for a Supernova Signature of Biogenic Origin," Icarus 212: 960-962.
Blakemore, Richard. (1975) "Magnetotactic Bacteria," Science 190(4212): 377-379.
Knie, K. et al. (2004) "60Fe Anomaly in a Deep-Sea Manganese Crust and Implications for a Nearby Supernova Source," Phys. Rev. Lett. 93, 171103.
Seeger, P. A., Fowler, W.A., Clayton, D. D. (1965) "Nucleosynthesis of heavy elements by neutron capture," The Astrophysical Journal Supplement 11: 121–166.