Enlarge /. The Cassiopeia, a supernova that left this remnant, occurred about 11,000 light years away – far too far to pose a significant threat – and its wavefront likely reached Earth about 300 years ago.
An article published this week by the University of Illinois at Urbana-Champaign, professor of astronomy and physics at Urbana-Champaign, Brian Fields, suggests distant supernovae are the cause of past mass extinctions – particularly the Hangenberg event that marks the boundary between the Devonian – and the carbon period marked. Fields has suggested this before, and both this and his earlier piece are fascinating "what-if" exercises. Each model shows the impact a supernova had on Earth's biosphere and how we can look for evidence that it happened.
It is important to understand, however, that none of these papers should be construed as indicating that there is evidence that the events referred to were caused by a supernova, or representing any general scientific consensus to that effect . They are simply fascinating suggestions and indicate what evidence we should be looking for.
When you say "mass extinction" and "space" in the same sentence, the first thing most people think is an asteroid impact on Earth – even when dinosaur fans think of Chicxulub Crater and pop culture fans think of movies like Deep Impact or Armageddon.
Asteroid impact isn't the only threat Earth faces from space, however – and the Cretaceous Paleogene isn't the only mass extinction Earth has seen. An even larger mass extinction occurred 359 million years ago – it's called the Hangenberg event, which marks the boundary between the Devonian and Carboniferous periods. The Hangenberg event affected both marine and terrestrial biomes and wiped out 97 percent of all vertebrate species.
An asteroid impact has been put forward as the likely cause of the Kellwasser Extinction, which occurred some 10 million years earlier. However, no material impacts dating to the correct time period for the Hangenberg event have been identified. Several other possible mechanisms have been suggested, including sequential effects due to significant changes in plant life and massive atmospheric injections of carbon dioxide and sulfur dioxide due to magmatism. So far, however, there is no "smoking weapon" that directly points to a cause.
What we do know about the Hangenberg extinction is that it has occurred over the course of thousands and maybe hundreds of thousands of years. We also have evidence of ultraviolet damage to pollen and spores over many thousands of years during this event, which in turn suggests possible long-term destruction of the ozone layer.
The length and severity of this period of ultraviolet damage likely precludes most of the local causes of the ozone layer depletion. The ozone depletion is unlikely to occur due to terrestrial causes such as B. increased water vapor in the stratosphere due to higher surface temperatures, is severe enough to lead to large-scale extinctions during this period.
Meanwhile, the ozone layer typically recovers in about a decade from more local catastrophic astrophysical events such as bolide impacts, solar flares, and gamma-ray bursts, which does not take into account the severity or duration of the Hangenberg extinction.
Fields' team suggests that supernovae could possibly explain both the severity and duration of Hangenberg's extinction.
The influence of the biosphere on distant supernovae
Supernova events are popularly imagined as instantaneous – a supermassive star explodes and a radiation wavefront instantly cooks up any nearby objects as they pass by. Within about 25 light years – far closer than any supernova our solar system is exposed to – this is close enough to be precise.
The effects of a supernova event, however, are felt well outside of this relatively narrow "kill radius" (and can potentially cause annihilation events). In 2018, another team, led by Fields, attempted to link periods of decreased biodiversity and increased extinction rates 2.5 million years ago – on the border between the Pliocene and Pleistocene – to a probable supernova event. This article hypothesized that a supernova would occur between 163 and 326 light years away based on globally elevated concentrations of iron-60, a radioactive isotope produced during supernovae. However, extinction coincided with a period of great climate change, so it is not clear that a supernova would be required to explain it.
For this new work, the team used global climate, atmospheric chemistry, and radiation transfer models to study how the flow of cosmic rays from a distant supernova would alter the ozone layer. Thomas told Astrobiology magazine that the effects of a distant supernova don't happen all at once. Instead, the intergalactic medium slows some particles more than others, resulting in a "radioactive iron rain" that can last hundreds of thousands of years.
The key to proving that a supernova occurred in the correct timeframe to explain the older and more devastating Hangenberg event would be the discovery of the radioactive isotopes plutonium-244 and samarium-146 in rocks and fossils deposited during the event. No isotope occurs naturally on Earth, and Fields colorfully describes them as "green bananas".
"If you see green bananas in Illinois," Fields said, "you know they're fresh and you know they weren't grown here." The decay period of Pu-244 and Sm-146 is long enough to be discovered after 360 million years, but short enough to preclude their inclusion in the original Earth formation. Fields goes on to say that the discovery of these isotopes today means "they're fresh and not from here – the green bananas of the isotopic world – and thus the smoking weapons of a nearby supernova."
PNAS, 2020. DOI: 10.1073 / pnas.2013774117