Cosmology: supernova element feed found (Introduction)

by David Turell , Friday, November 29, 2019, 02:00 (1823 days ago) @ David Turell

At the bottom of bodies of water:

http://nautil.us/issue/78/atmospheres/the-secret-history-of-the-supernova-at-the-bottom...

"Astronomers have searched the surrounding cosmos for clues, but the most compelling evidence for a nearby supernova comes—somewhat paradoxically—from the bottom of the sea. Here, a dull and asphalt black mineral formation called a ferromanganese crust grows on the bare bedrock of underwater mountains—incomprehensibly slowly. In its thin, laminated layers, it records the history of planet Earth and, according to some, the first direct evidence of a nearby supernova.

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"But to understand just how supernovas affected life, scientists needed to link the timing of their explosions to pivotal events on earth such as mass extinctions or evolutional leaps. The only way to do that is to trace the debris they deposited on Earth by finding elements on our planet that are primarily fused inside supernovas.

"Fields and his colleagues named a few such supernova-forged elements—mainly rare radioactive metals that decay slowly, making their presence a sure sign of an expired star. One of the most promising candidates was Fe-60, a heavy isotope of iron with four more neutrons than the regular isotope and a half-life of 2.6 million years. But finding Fe-60 atoms scattered on the Earth’s surface was no easy task. Fields estimated that only a very small amount of Fe-60 would have actually reached our planet, and on land, it would have been diluted by natural iron, or been eroded and washed away over millions of years.

"So scientists looked instead at the bottom of the sea, where they found Fe-60 atoms in the ferromanganese crusts, which are rocks that form a bit like stalagmites: They precipitate out of liquid, adding successive layers, except they are composed of metals and form extensive blankets instead of individual spires. Composed primarily of iron and manganese oxides, they also contain small amounts of almost every metal in the periodic table, from cobalt to yttrium.

"As iron, manganese, and other metal ions wash into the sea from land or gush from underwater volcanic vents, they react with the oxygen in seawater, forming solid substances that precipitate onto the ocean floor or float around until they adhere to existing crusts. James Hein at the United States Geological Survey, who studied crusts for more than 30 years, says that it remains a mystery exactly how they establish themselves on rocky stretches of seafloor, but once the first layer accumulates, more layers pile on—up to 25 centimeters thick.

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"Unlike the slow-growing crusts, however, supernova explosions happen almost instantly. The most common type of supernova occurs when a star runs out of its hydrogen and helium fuel, causing its core to burn heavier elements until it eventually produces iron. That process can take millions of years, but the star’s final moments take only milliseconds. As heavy elements accumulate in the core, it becomes unstable and implodes, sucking the outer layers inward at a quarter of the speed of light. But the density of particles in the core soon repels the implosion, triggering a massive explosion that shoots a cloud of stellar debris out into space—including Fe-60 isotopes, some of which eventually find their home in ferromanganese crusts.

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" Based on the concentration of Fe-60 in the crust, Knie estimated that the supernova exploded at least 100 light-years from Earth—three times the distance at which it could’ve obliterated the ozone layer—but close enough to potentially alter cloud formation, and thus, climate. While no mass-extinction events happened 2.8 million years ago, some drastic climate changes did take place—and they may have given a boost to human evolution. Around that time, the African climate dried up, causing the forests to shrink and give way to grassy savanna. Scientists think this change may have encouraged our hominid ancestors as they descended from trees and eventually began walking on two legs.

"That idea, as any young theory, is still speculative and has its opponents. Some scientists think Fe-60 may have been brought to Earth by meteorites, and others think these climate changes can be explained by decreasing greenhouse gas concentrations, or the closing of the ocean gateway between North and South America. But Knie’s new tool gives scientists the ability to date other, possibly more ancient, supernovas that may have passed in the vicinity of Earth, and to study their influence on our planet. It is remarkable that we can use these dull, slow-growing rocks to study the luminous, rapid phenomena of stellar explosions, Fields says. And they’ve got more stories to tell."

Comment: No question. Exploding stars brought important elements to the earth. But they had to be far enough away so the radioactive blast did not harm the Earth. Fred Hoyle first showed how stars make carbon.