Astrophysics and cosmology represent science on the grandest of scales and study the most powerful events in nature, including the big bang and the explosive deaths of stars. Often these phenomena are far removed from Earthly concerns, but my group brings them close to home--even dangerously close. We study the astrophysics and astrobiology of near-Earth supernova explosions.
[cr][lf]Credit: NASA/CXC/SAO
The most massive stars are the celebrities of the cosmos: they are very rare, but they live extravagantly and die in spectacular and violent supernova explosions. While these events are awesome to observe, they can take a sinister shade when they occur closer to home, because an explosion inside a certain “minimum safe distance” would pose a grave threat to Earthlings. Amazingly, there is evidence this threat is real. Recent data points to a global, catastrophic loss of ozone at the end of the Devonian period 359 million years ago--in association with a large biological extinction event. In a paper by a team that includes current and former Illinois students, as well as professors from the US and Europe specializing in particle physics, astrobiology, and paleontology, we offer a new interpretation of the ozone loss.
In the Proceedings of the National Academy of Sciences, we point out that a nearby supernova can explain the ozone loss and thus may be the trigger for one or more late Devonian biological extinctions. We present tests for this scenario, including the search for trace amounts of radioactivity possibly created by the supernova(e) and delivered to Earth. The presence of live - that is, undecayed - radioactivity would be a telltale signature of the supernova, since there is no natural background for the isotopes we consider, plutonium-224 and samarium-146.
This work draws on our group's systematic study of the rich astrophysics of near-Earth supernovae, which both uses and sheds new light on fundamental questions. Detection of supernova radioactivity brings debris from the explosion into the laboratory, opening new windows into the conditions at the hearts of massive stars before, during, and after their deaths. This offers a new laboratory window into stellar nucleosynthesis, similar in sprit to the analysis of meteorites. But meteorites sample and average the output of many stars whose deaths enriched the composition of the protosolar nebula; nearby supernova debris probes individual events without any averaging.
The propagation of supernova material from the explosion to the Earth encodes a wealth of information about the cycling of matter through interstellar space. Moreover the supernova matter arriving at Earth comes in the form of microscopic dust particles. Thus the radioisotope signal we see offers new information about the nature of supernova dust production, transport, and evolution. Indeed, our group has modeled supernova dust and discovered that magnetic fields likely played a pivotal role in its evolution.
Finally, the collision between the supernova blast and the solar wind sheds new light on the nature of the heliosphere in which the Earth resides. The blast also acts as an accelerator of high-energy cosmic rays, which bombard the Earth's atmosphere and lead not only to ozone loss but also to intense, high-energy muon radiation that bathes the Earth's surface and reaches far below the surface of the land and oceans, damaging most of the biosphere.
Fortunately, today there are no supernova candidates close enough to be menacing. But if the Devonian extinctions were caused by supernovae, then we literally can count the ways these explosions have affected us: the extinctions saw the deaths of many land-dwelling tetrapods, but the ones that survived happened to have five toes–as do their descendants, including us! Thus, if our proposal is correct, we humans have five fingers (and do math in base ten) due to an ancient supernova.
Brian Fields is a Professor of Astronomy and Physics at the University of Illinois.
The paper "Supernova triggers for end-Devonian extinctions" was published in Proceedings of the National Academy of Sciences in July 2020 and is available online.