It is possible that the ancient Earth was "attacked" by exploding stars

It is possible that the ancient Earth was "attacked" by exploding stars
It is possible that the ancient Earth was "attacked" by exploding stars

Astronomers believe that several supernovae explode in the Milky Way every hundred years. During the existence of the planet, some of them should have exploded very close to it and, possibly, with catastrophic consequences. What impact did this have on the life of the Earth in general and on human evolution in particular?

For our ancestors Australopithecus, who roamed the vastness of Africa 2.5 million years ago, a bright new star in the sky would undoubtedly spark curiosity. As bright as the full moon, it would cast shadows at night and be visible during the day. As the supernova disappeared over the next months, it probably disappeared from memory as well. But she left other traces that are being discovered today.

Over the past two decades, researchers have discovered hundreds of radioactive atoms "walled up" in mineral deposits on the seabed, formed by an ancient explosion that marked the death of a nearby star. After burning nuclear fuel, the star collapsed, the core collapsed, and the resulting shock wave threw out its outer layers to the sides, forming an expanding ball of gas and dust, the temperature of which was so high that for a moment the dying star flashed brightly, like a galaxy. As a result, these atoms fell on the Earth like hail, which are eloquent evidence of what happened.

The bursts of X-rays and gamma rays emanating from hundreds of light-years away probably did not harm the Earth. But the expanding fireball accelerated cosmic rays - mostly hydrogen and helium nuclei - to speeds close to the speed of light. These "shells" flew in unnoticed, decades later, turning into an invisible barrage, like shelling, which could last for thousands of years and could affect the atmosphere - and life.

As a result of much research and speculation, astronomers have outlined their possible implications. It is quite possible that under the influence of a squall of cosmic rays, the mutation frequency increased due to the destruction of the protective ozone layer of the Earth and the creation of secondary flows of particles penetrating into tissues. Apparently, breaking through the atmosphere, these particles also created channels for the passage of lightning, possibly causing an outbreak of forest fires. At the same time, atmospheric reactions caused by radiation could cause rain from nitrogen compounds, which served as fertilizer for plants that absorb carbon dioxide. Thus, this astronomical event could cause a cooling of the climate and contribute to the beginning of the ice age 2.5 million years ago, at the beginning of the Pleistocene epoch. These consequences, even taken together, "are not like the dinosaur extinction event - it is not so visible and local," says astronomer Brian Thomas (Brian Thomas) of Washburn University, who has been studying the consequences of cosmic catastrophes on Earth for almost 20 years.

Few astronomers suggest that supernovae caused any significant extinction at the time, and even fewer paleontologists are willing to believe them. “Death that came from space always looks very cool,” says paleontologist Pincelli Hull of Yale University. "The factual material is interesting, but it has not quite reached the level that is necessary to fit in my head."

Nevertheless, scientists looking for supernovae believe that other explosions, more distant in time, occurred closer to Earth.And, in their opinion, these supernovae may be the cause of some extinction events that lack the usual triggers like volcanic eruptions or asteroid collisions. Adrian Melott, an astronomer at the University of Kansas at Lawrence who is investigating how cosmic cataclysms in the vicinity of Earth might not have affected it, says it's time to start a more thorough study of Earth's history for ancient supernovae. This will not only help astrophysicists understand how the explosions formed the planets in the vicinity of the solar system and “seeded” it with heavy elements, but it could also give paleontologists the opportunity to take a fresh look at recurring global changes. “This is something new and unfamiliar,” says Melotte. "It takes time to accept it, to believe it."

Astronomers believe that several supernovae explode in the Milky Way every hundred years. According to the law of averages, during the entire existence of the Earth over 4.5 billion years, some of them should have exploded very close to the Earth - at a distance of within 30 light years - and possibly with catastrophic consequences. Even explosions that occur up to 300 light-years away should leave trails in the form of dust particles ejected with a shell of debris known as supernova remnants. When physicist Luis Alvarez and his son, geologist Walter Alvarez went to study the upper layers of sediments associated with the extinction of dinosaurs 65 million years ago in the 1970s, they expected to find supernova dust there. Instead, they discovered iridium, an element rarely found on the Earth's surface but found in abundance on asteroids.

In any case, the Alvarez did not have the tools to search for supernova dust. Since the Earth already is already largely composed of elements that arose in supernovae billions of years ago, before the Sun, most traces of later explosions cannot be detected. But some can be found. In the 1990s, astrophysicists realized that supernova dust could also precipitate radioactive isotopes with a half-life of millions of years, too short to have existed since Earth's origins. Those found must have been scattered by recent (from a geological point of view) explosions. One of the key indicators is iron-60, an isotope of iron formed in the cores of large stars with a half-life of 2.6 million years and not naturally occurring on Earth.

In the late 1990s, astro-particle physicist Gunther Korschinek of the Technical University of Munich (TUM) decided to look for it, in part because the university had a powerful accelerated mass spectrometer (AMS) suitable for this purpose. After ionizing the sample, the mass spectrometer accelerates the charged particles to high energies and passes them through a magnetic field. Under the influence of a magnetic field, the direction of their movement changes, and they rush to the detectors. The direction of movement of the heaviest atoms changes the least because they have more inertia.

It is especially difficult to separate iron-60 atoms from the equally massive, but differently charged atoms of nickel-60, but the mass spectrometer from the University of Munich, built in 1970, was one of the few in the world that had enough power to separate them from each other. from friend.

In addition, Korshinek needed a "correct" sample - a fragment of geological deposit formed over millions of years, in which traces of iron can be distinguished. Cores taken in the ice of Antarctica were not suitable for this, since their age is only about a couple of million years. Most ocean sediments build up so quickly that any trace of iron-60 dissolves to unnoticeable levels.Korshinek ended up using a fragment of the surface layer of ferromanganese deposits recovered from a seamount in the North Pacific by the German research vessel Valdivia in 1976. This surface layer forms in areas of the seabed where sediment cannot settle due to slope or currents. At a suitable pH of water, metal atoms selectively precipitate from the water at a rate of several millimeters per million years, slowly forming a mineral crust.

Korshinek and his colleagues cut their sample into layers of different ages, chemically isolated the iron, and passed the atoms through their mass spectrometer. In 1999, in the scientific journal Physical Review Letters, published by the American Society of Physicists, scientists wrote that among the thousands of trillion atoms of ordinary iron, they found 23 iron-60 atoms dating back to a period less than three million years ago. The era of supernova geochemistry has begun. “We were the first to start experimental research,” says Korshinek.

Others followed. Iron-60 has been found in the surface layers of ocean sediments taken from other parts of the world and even in the microfossils of ocean sediments, microscopic fossil remains of living things that, to the delight of supernova hunters, "absorbed" and accumulated iron in their bodies. Most of the results pointed to a local supernova explosion between two and three million years ago - with some traces pointing to a second supernova explosion several million years earlier.

Although the remnants of these explosions have long swept past the Earth, the rain from the atoms ejected by them continues. In 2019, Korshinek's team obtained iron by running half a ton of fresh Antarctic snow through their mass spectrometer and discovered several iron-60 atoms that he estimates have fallen to Earth over the past 20 years. Another group of scientists found a small number of atoms in cosmic rays detected by the advanced Explorer spacecraft, launched by NASA to study the composition of celestial bodies, halfway between the Sun and Earth. Researchers have found iron-60 even in the lunar soil brought back by the Apollo missions. "The moon has confirmed that it was not just some earthly phenomenon," says astronomer Adrienne Ertel of the University of Illinois at Urbana-Champaign (UIUC).

Astronomer Dieter Breitschwerdt of the Technical University of Berlin is trying to trace the path of iron to its source in the sky. When he became aware of the results of Korshinek's research, he studied the local "bubble", an area of ​​space around the solar system, "cleared" of most of the gas and dust. The likely broomsticks were supernovae, so he began tracking groups of stars in the vicinity of the solar system to see if they were close enough to the sun to throw iron-60 onto Earth when some of these stars exploded.

Using data from the Hipparcos satellite, launched by the European Space Agency to map the starry sky, Breitschwerdt looked for clusters of stars on common trajectories and tried to turn back time to see where they were millions of years ago. It seemed that about 2.5 million years ago in an ideal location - at a distance of 300 light-years from Earth - there were two clusters that are part of the Scorpio-Centauri OB Association (Sco OB2) today. “It felt like a miracle,” he says. The likelihood that the explosion occurred at the "right" time was high. Supernova core collapse occurs in high-mass stars. Based on age and mass data for the 79 stars remaining in the clusters, Breitschwerdt suggests that over the past 13 million years, about a dozen of the former stars that made up these clusters have exploded as supernovae.

There is no visible evidence of the existence of these supernovae in Sco OB2 for a long time: after about 30 thousand years, supernova remnants scatter, and black holes or neutron stars that they leave behind are difficult to detect. But the direction in which the iron dust hit the Earth could theoretically indicate its source. Samples from the seabed do not provide any directional information, as the dust moves as it settles under the influence of wind and ocean currents. However, the moon "has no atmosphere, so it stays where it settled," says Brian Fields, an astronomer at the University of Illinois at Urbana-Champaign. Since the Moon rotates, the longitudinal direction is not appropriate, but if more iron-60 was found at one of the poles than, for example, at the equator, this could serve as confirmation of Breitschwerdt's hypothesis that the source is Sco OB2. Fields and several of his colleagues want to test this idea, and they have asked NASA to provide them with samples of lunar soil that will be collected and delivered to Earth during future launches of moon rovers or flights to the moon with a man on board.

Korsinek's team now has a rival in the search for iron from supernovaeThis is a group of scientists led by Anton Wallner, who worked after his doctoral dissertation in Korshinek's team. He is analyzing several samples of the surface layer of ferromanganese deposits recovered from the Pacific Ocean by a Japanese mining company at the Australian National University (ANU) using an upgraded accelerated mass spectrometer. “Now we have pushed aside Munich,” says Wollner.

This year, Wollner's team more accurately studied the time of the appearance of recent supernovae, cutting a sample of the surface layer of sediments into 24 layers of one millimeter thick, each of which corresponds to 400 thousand years, about which they wrote in an article published in the American scientific journal Science Advances. “This has never been done with such a temporal resolution before,” says Wallner of the Helmholtz Center Dresden-Rossendorf. The 435 iron-60 atoms that they extracted made it possible to associate their appearance with the explosion of the most recent supernova 2.5 million years ago and confirm the facts indicating another supernova explosion that occurred earlier, according to their calculations, 6, 3 million years ago. By comparing the concentration of iron-60 in the surface layer of sediments with models indicating the amount of jelly-60 formed in a supernova, scientists have calculated the approximate distance from these supernovae to Earth - 160-320 light years.

In addition, Wallner's team discovered 181 plutonium-244 atoms, another radioactive isotope that may have been formed from the explosion not of a precursor star (like iron-60), but of the supernova itself. True, there is a fierce debate about the source of plutonium-244: some researchers believe that the process of formation of plutonium-244 in large quantities in supernovae is difficult. They believe it is the result of collisions between neutron stars, and is supposedly the ash left behind by supernovae.

These collisions, called kilons, occur 100 times less often than supernovae, but they produce much more of the heaviest elements. “When neutron stars merge, plutonium is easily formed,” says Rebecca Surman, an astrophysicist at the University of Notre Dame. "And in supernovae it is much more difficult."

But Surman still believes that the role of supernovae is great. She believes that plutonium-244, found on the seabed, is a sign that kilonova in the distant past "covered" our interstellar space with heavy elements. She suggests that when the last two supernovae exploded, their expanding remnants may have captured some of this interstellar plutonium-244 and delivered, along with their own iron-60, to Earth.However, Korshinek said that more data on the plutonium wave and its timing would be needed to convince him that several rare events happened so close and so recently.

What impact could nearby supernovae have, besides the fact that they bombard the Earth with nuclei of rare elements? In 2016, a team of scientists led by Melott and Thomas estimated the flux of various forms of light and cosmic rays that would likely reach Earth in an explosion 300 light-years away. In an article published in the American scientific journal Astrophysical Journal Letters, they concluded that the most active, potentially destructive photons - X-rays or gamma rays - would have minimal impact. “This produces so much high-energy radiation,” says Thomas. They speculated that a few weeks of bright light would have no more effect than disturbing sleep patterns.

It is quite another matter - cosmic rays, particles accelerated to a speed close to the speed of light, by shock waves in an expanding supernova fireball. Because they are charged, they can be tilted away from the Earth by galactic magnetic fields. But it is believed that the local "bubble" is largely devoid of fields, so cosmic rays, located only 300 light-years away from us, will hardly encounter obstacles in their path.

Melotte and Thomas found that the atmosphere would be bombarded. “Reactivation is a slow process lasting at least several decades,” Thomas says, explaining that the peak is reached about 500 years after a supernova explosion, and there is a 10-fold increase in atmospheric gas ionization that will persist for five thousand years. … Using a chemical model of the atmosphere developed by NASA, they calculated that chemical changes caused by ionization in some places would deplete the ozone layer by about 7% or more and accelerate the formation of nitrogen oxide compounds that serve as fertilizers by 30%. Perhaps the associated sharp increase in the number of plants was enough to make the climate colder, and conditions for the beginning of the Pleistocene arose.

And cosmic rays have not disappeared anywhere. When high-energy particles enter the upper atmosphere, they create cascades of secondary particles. Most of them disappear on further collisions, but muons - heavy, rapidly decaying relatives of electrons - continue to exist. Creatures on the Earth's surface would receive three times the normal dose of radiation - the equivalent of one or two computed tomography scans per year. "It's an increased risk [of cancer], but not radiation damage," Thomas says. In general, according to these scientists, the consequences were “not catastrophic,” but could be found in the fossil record if, for example, some vulnerable species disappeared and others survived.

In an article published in the scientific journal Astrobiology in 2019, Melotte and two colleagues concluded that if the supernova had exploded not 300 light-years away, but only 150 light-years away, muon radiation would be surprisingly severe. a blow to marine animals. Water blocks most of the particles that fall from the sky, but muons can penetrate as deep as one kilometer. Marine creatures, usually protected from almost all types of radiation, would be the victims of the maximum relative dose increase and would be the most affected. This compares with the extinction of the marine megafauna in the early Pleistocene, only recently discovered in the fossil record.

Then, last year, supporters of the supernova hypothesis suggested that a similar scenario could be the cause of a major extinction event 359 million years ago, at the end of the Devonian period.A team of scientists led by John Marshall of the University of Southampton found that the spores of fern plants of the time suddenly warped and darkened, attributable to exposure to ultraviolet radiation. At the same time, the scientists did not name the astronomical reason. But in an article published in the journal Proceedings of the National Academy of Sciences, astronomers saw this as a possible result of a nearby supernova explosion. They speculated that an explosion, perhaps only 60 light-years away, could flood the Earth with ultraviolet light, destroying the ozone layer. “This is purely hypothetical,” admits one of the authors of the article, John Ellis, a theoretical scientist at King's College London, since it is currently impossible to identify the radioactive traces of a supernova explosion of this age.

In an article published in The Journal of Geology in 2020, Melotte and Thomas look further back in their hypotheses. They noted that secondary cosmic rays, stripping electrons from air molecules, could create channels for lightning, which increased the likelihood of thunderstorms, as a result of which not only more nitrogen compounds were formed, but also forest fires. It is curious that at the beginning of the Pleistocene, in some parts of the world, a layer of soot was found on rock paintings. Melotte and Thomas theorized that these wildfires, triggered by a supernova explosion, may have caused primitive humans to climb out of trees and move to the savannah, leading to upright posture, increased brain size, and everything that followed. "It is curious to assume that 2.5 million years ago, a supernova had such a significance [on evolution] that we are now talking on Skype," says Korshinek.

Such scenarios are not very popular with paleontologists. “Timing is the trivial answer to everything,” Hull says. "Whenever something dies out, something happens." Moreover, she said, the transition to the Pleistocene "needs no explanation." She says that other events around the same time, such as the closure of the Isthmus of Panama, which seriously altered ocean circulation, may have had a greater impact on the global climate.

To prove that they are right, she says, astronomers need to more accurately determine when ancient supernovae appeared. They "need to study more surface sediment samples." But finding supernova trails is not getting any easier. In 2019, the Technical University of Munich stopped work on its mass spectrometer with an accelerator, and now only the Australian National University has a powerful enough accelerator on which iron-60 can be separated.

As for the rarer isotopes, such as plutonium-244, they could allow researchers to look even further back in time, but they require accelerated mass spectrometers, in which the main thing is not just power, but sensitivity. There are very few mass spectrometers in the world that meet these requirements, Wallner said. He has secured funding for the construction in the German city of Dresden of a new scientific laboratory equipped with an accelerator mass spectrometer, specializing in the study of the heaviest elements, it should be opened by 2023. In order to resume the search for iron-60, his research team also applied for funding from the state budget to create a new powerful mass spectrometer with an accelerator, which could be operational in seven years.

For astronomers, a sudden flash of light in the sky today would be the best opportunity to see how a supernova is affecting Earth. But the chances of us seeing a light show like the one that may have blinded our distant ancestors are slim.Betelgeuse, a restless red giant star that is likely to explode sometime in the next 100,000 years, has calmed down in recent months, and is more than 500 light-years away anyway. Sco OB2 is now moving away from the Sun. And using data from the Gaia space telescope, the successor to Hipparcos launched by the European Space Agency, Dieter Breitschwerdt tracked 10 more star clusters. “None of them come close to us,” he says. "This is a bright future for the Earth, not a supernova."

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