BEIJING, Jan. 9 (Xinhua) — In February 1987, Neil Gehrels, a young researcher at NASA’s Goddard Space Flight Center, boarded a military plane bound for the Australian interior. Grylls carried some special cargo: a polyethylene space balloon and a set of radiation detectors he had just built in the lab. His destination was Alice Springs, a remote town in the Northern Territory. There, Grylls will use the devices to get a glimpse of one of the most exciting events in the universe above the Earth’s atmosphere: a supernova explosion in a satellite galaxy near the Milky Way.
Rocks under the sea may tell us more about stellar eruptions.
Like many supernovae, SN 1987A announced the dramatic collapse of a large-mass star. What makes this explosion unusual is that it is so close to Earth that it is the latest supernova explosion since The Kepler Supernova (SN 1604) in 1604. SN 1604 is by far the last visible supernova in the Milky Way, and German astronomer Kepler recorded the eruption, as has it in China’s Ming History. Since SN 1604, scientists have asked many questions, and to answer them, a closer look at new supernova events is required. One question is this: How close can supernovae be to destroy life on Earth?
As early as the 1970s, researchers hypothesized that radiation from neighboring supernovae would destroy the Earth’s ozone layer, expose plants and animals to harmful ultraviolet rays, and further lead to mass extinction. With new data from SN 1987A, Grylls can now calculate the theoretical “radius of destruction”, where supernovae can have a serious impact, and he can calculate the probability that a dying star will appear within that radius.
Most importantly, there may be a supernova that is close enough to Earth, which will have a huge impact on the Earth’s ozone layer about every billion years. However, this does not happen very often, and no stars have been found that threaten the solar system. But the Earth has been around for 4.6 billion years, and life has been about half the time it has been, meaning that at some point in the past there was a good chance of a supernova explosion. The question is, when exactly did this outbreak happen? Moreover, since supernovae primarily affect the atmosphere, it is difficult to find conclusive evidence.
Astronomers are looking for clues in the universe around the Milky Way, but the most compelling evidence of supernovae comes from the ocean floor. This sounds a bit contradictory. On the bare bedrock of the underwater mountains, there is a black mineral called the ferromanganese crust that is growing slowly – at an incredibly slow rate. In the thin layer structure of this mineral, the history of the Earth is recorded, from which we can obtain the first direct evidence of adjacent supernovae.
James Hein collects manganese iron crusts near Hawaii. Despite their common appearance, these rocks are scientifically important.
These clues to the ancient supernova explosion are valuable to scientists, who speculate that supernovae may have played a little-known role in the evolution of life on Earth, which may well be part of the story of life on Earth. To understand how supernovae affect the continuation of life on Earth, scientists need to link the timing of their eruptions to key events on Earth, such as mass extinctions or evolutionary jumps. The only way to do this is to trace the debris deposited on Earth by supernova eruptions, i.e. to find elements on our planet that fuse primarily within supernovae.
The decay of rare radioactive metals is slow, so its existence is conclusive evidence of the death of a star. One of the most promising candidates is Fe-60, an iron isotope with four neutrons more than conventional isotopes, with a half-life of about 2.6 million years. However, finding Fe-60 atoms scattered on the Earth’s surface is not easy, and only a very small number of Fe-60s will reach our planet. On land, the Fe-60 is diluted by natural iron, or eroded over millions of years, and eventually washed away by water.
So the scientists looked at the bottom of the ocean and found that the manganese iron crust contained Fe-60 atoms. The formation of these rocks is a bit like stalagmites: they are precipitated from the liquid, accumulated layer by layer, but the manganese iron crust is made of metal, forming a wider shell, unlike stalagmites, which are not separate pointed cones. Manganese iron crusts are mainly composed of iron and manganese oxides and also contain trace elements of almost all metals on the periodic table of elements, from cobalt to niobium.
When iron, manganese, and other metal ions are washed from land into the sea, or ejected from the craters of the seafloor, they react with the oxygen in the sea, forming solid sons, precipitating to the sea floor or floating around, until they are attached to the existing hard crust, and the exact process by which the manganese iron crust initially formed in the rocky areas of the seafloor remains a mystery. Once the first layer accumulates, more rock formations will accumulate, eventually up to 25 cm thick.
Thus, manganese iron crusts can be used as “cosmic historians” who record changes in the chemical composition of seawater, including elements that can indicate dying stars. In the 1980s, geologists salvaged one of the oldest manganese iron crusts in southwestern Hawaii, dating back more than 70 million years. Dinosaurs were still roaming the earth at the time, and the Indian subcontinent was just an island between Antarctica and Asia.
The growth of manganese iron crusts is one of the slowest scientifically known processes, increasing by only about 5 mm per million years. By contrast, human nails grow about 7 million times faster. The reason is simple: there is less than one iron or manganese atom per billion water molecules in the ocean, and they must resist the pull of passing ocean currents and other chemical interactions to be fixed in a new crust.
Klaus Kearney used manganese iron crusts collected from a depth of 4,830 meters in the Pacific Ocean to track the iron isotope Fe-60, which can be up to 25 cm thick
Unlike slow-growing manganese iron crusts, supernova eruptions occur almost instantaneously. In the most common supernova type, a star depletes hydrogen and helium fuel, and its core begins to burn heavier elements until it eventually produces iron. This process can take millions of years, but the star’s final moments take only a few milliseconds. As heavy elements accumulate in the core of a star, the core becomes unstable and implosion occurs, sucking outer matter into the core at a quarter of the speed of light. But the density of particles in the core quickly stopped implosion, triggering a big explosion that sent a large cluster of stellar fragments into space, including the Fe-60 isotope, some of which ended up in the manganese iron crust.
Klaus Knie was one of the first people to look for the Fe-60 in a manganese iron crust when he was an experimental physicist at the Technical University of Munich, Germany. However, his team did not study supernovae or manganese iron crusts, but was developing methods for measuring rare isotopes of various elements, including the Fe-60. At the time, another scientist measured an isotope of radon, which could be used to determine the age of the manganese iron crust. So Klaus Kearney decided to test the Fe-60 for the same sample. By this time he had known that the Fe-60 was produced in a supernova. “We’re part of the universe, and if we find the right place, we have a chance to hold this ‘astrophysical’ matter in our hands,” said Kearney, who now works at the Helmholtz Heavy Ion Research Center. “
The manganese iron crust used in the study was also obtained from the seabed not far from Hawaii. The test results showed that the location was indeed the right one. Klaus Kearney and his colleagues found a Fe-60 spike in a crust that dates back about 2.8 million years, marking the death of a neighboring star. This discovery is of great significance. This is the first evidence that the remnants of supernovae can be found on Earth, pinpointing the approximate time of the last supernova explosion in the nearby universe (if there are more recent events, researchers may find closer Fe-60 spikes). However, the findings also suggest an interesting evolutionary theory for Kney.
Based on the content of Fe-60 in the manganese iron crust, Kney estimates that the supernova bursts at least 100 light-years from Earth. That’s three times the distance the ozone layer could be destroyed, but it’s enough to potentially change cloud formation and thus climate. Although there were no mass extinctions 2.8 million years ago, some dramatic climate change did occur, and these changes may have contributed to human evolution. Around that time, Africa’s climate became dry, causing forests to shrink and replace them with large grasslands. Scientists believe the change may have helped our primitive human ancestors come down from trees and eventually start walking on two legs.
The idea, like any young theory, is speculative, and some scholars disagree. Some scientists believe that the Fe-60 may have been brought to Earth by meteorites, while others believe that these climate changes millions of years ago could be explained by a drop in greenhouse gas concentrations or the closure of ocean channels between North and South America. However, Kney et al.’s research does provide scientists with new tools to determine the age of other potentially older supernovae passing near Earth and to study their effects on the planet. Fields says it’s remarkable that we can use these dimly colored, slow-growing rocks to study the rapid glow of stellar bursts, and they will tell us more stories in the future. (Any day)