Far out cosmology: supernova effects on Earth (Introduction)

by David Turell , Saturday, October 29, 2022, 16:08 (754 days ago) @ David Turell

They ae dangerous to life:

https://www.universetoday.com/158316/how-dangerous-are-nearby-supernovae-to-life-on-earth/

Life and supernovae don’t mix.

From a distance, supernovae explosions are fascinating. A star more massive than our Sun runs out of hydrogen and becomes unstable. Eventually, it explodes and releases so much energy it can outshine its host galaxy for months.

But space is vast and largely empty, and supernovae are relatively rare. And most planets don’t support life, so most supernovae probably explode without affecting living things.

Earth is no stranger to supernovae. One hasn’t been close enough to sterilize Earth, but there’s evidence showing supernovae have affected life on Earth.

A 2018 paper presented evidence of a supernova exploding near Earth about 2.6 million years ago. It was about 160 light-years away. The authors of that paper tied the supernova to the Pliocene marine megafauna extinction. In that event, up to a third of Earth’s large marine species were wiped out, but only in shallow coastal waters.

Another paper showed up to 20 supernovae in the last 11 million years in the Scorpius-Centaurus OB association. Some of these were as close as 130 light-years to Earth. The paper’s authors say that about 2 million years ago, one of the supernovae exploded close enough to our planet to damage the ozone layer.

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In a scenario where an SN exploded close to Earth, it can take months or years following the initial explosion for the x-rays to arrive. Interactions with the circumstellar debris cause a delay. The x-rays can deplete Earth’s ozone layer, allowing harmful UV radiation from the Sun to reach the planet’s surface.

After the x-rays arrive, the cosmic rays arrive, similar to other SN. This is a double whammy for Earth’s ozone layer.

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Our Solar System is inside what’s known as the Local Bubble. It’s a cavity carved out of the ISM [interstellar medium] in the Milky Way’s Orion Arm. Multiple supernovae explosions created the bubble in the last 10 to 20 million years. Did those SN affect Earth? (my bold)

Advances in x-ray astronomy will shed more light on the consequences for terrestrial planets, and the authors think there’s lots more to uncover. But their observations show that “… the interacting X-ray phase of an SN’s evolution can entail significant consequences for terrestrial planets. We limit any further speculation until further developments in X-ray astronomy are made; however, the evidence presented here certainly points to this process as capable of imposing lethal consequences for life at formidable distances.”

Scientists know that supernovae have had some effect on Earth. The presence of the radioactive isotope 60Fe has a half-life of 2.6 million years, yet researchers found undecayed 60Fe in ocean samples dating from 2 to 3 Myr ago. It should’ve decayed into nickel long ago. Supernovae can create 60Fe through nucleosynthesis when they explode.

But other things can create 60Fe. Asymptomatic giant branch stars can make it, too, so by itself, it’s not a smoking gun for a nearby supernova.

Researchers also found 53Mn in the same samples of ferromanganese crust that hold the 60Fe. It’s also a radioactive isotope that should’ve decayed by now. Unlike 60Fe, only supernovae can create 53Mn. Its presence is definite proof of nearby supernovae in the recent geological past.

It’s not the presence of these radioactive isotopes that poses a threat to life. It’s the radiation that must’ve struck Earth, and if the supernova that created the isotopes was close enough to spread them to Earth, then the radiation must’ve struck Earth, too.

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SN outbursts have almost certainly struck our planet. The exact consequences are difficult for scientists to untangle. But if the radiation weakened the ozone layer, allowing more UV radiation to reach the Earth’s surface, it would’ve caused mutations. It’s called UV mutagenesis, which may have driven molecular evolution and been critical in the origin of sex. In fact, mutation is evolution’s primary driver.

The fact that supernovae can lead to mutations is the backdrop for the authors’ concluding remarks.

“We thus conclude with the comment that further research into SN X-ray emission has value not just for stellar astrophysics but also for astrobiology, paleontology, and the Earth and planetary sciences as a whole.”

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The researchers urge more long-term study of supernovae for months and years after an outburst and plea for more advancements in x-ray observation to aid the study. “These observations and innovations will shed light on the physical nature of SN X-ray emission and will clarify the danger that these events pose for life in our galaxy and other star-forming regions,” they write.

Comment: note my bold. We are in a very safe spot in the 'bubble'.