![]() ![]() Astronomers estimate that the stars that dump the iron-60 each contained about 10 times the mass of the Sun, and should live only a relatively short 30 million years. The stars that make the powerful supernovae have fairly short lives, so a group that is born together with the same starting mass will tend to explode more or less together. The Orion Nebula (M42), a favorite of amateur astronomers, is a large example of this. The massive stars that make type II supernovae are often born in associations, and therefore clump together. It may sound unreasonable to have so many supernovae all going off in the same area at nearly the same time, but it isn’t. A chain of supernova explosions may have formed it, perhaps the same ones that deposited the iron-60. The Local Bubble is an irregularly shaped region of hot (million-degree) but tenuous gas (plasma) in which our solar system and many other stars reside. ![]() This concept would explain the extended deposits on Earth and also sounds reasonable, considering the energetics needed to form the Local Bubble. Another idea is that a lot of supernovae occurred at various distances, as many as a dozen or more. One interpretation suggests that the dust grains containing iron-60 were caught up in interstellar clouds, which confined them or modified their trajectory, keeping them in our neighborhood so they could fall to Earth more than once. In space, the iron would pass Earth as part of the blast wave and be deposited for only a short time. There are indications of another event 7 million or 8 million years ago, also near enough to deposit iron-60 on Earth. All recent publications agree that something happened 2.5 million to 2.6 million years ago at a distance between 150 and 300 light-years from Earth.īeyond that, there is no consensus. Researchers can date the age of the event from the age of the sediments in which iron-60 is found. Iron-60 is created in supernovae, so identifying times of great increase in the isotope is a good sign that supernovae have gone off not too far from Earth. Because our planet is 4.5 billion years old, no original iron-60 should be left on Earth, unless it came from space. It is radioactive and decays with a half-life of 2.6 million years. ![]() The dominant, stable form of iron is iron-56, whose nucleus contains 26 protons and 30 neutrons. That picture is based on detection of iron-60, an isotope. That work was followed by others who reported data from the remnants of fossilized bacteria, from the Moon, even from cosmic rays, all finally creating a consistent picture. These were data from the ocean bottoms in a variety of locations around the world. There had previously been some claimed detections, but many doubted them because the work is literally at the level of counting atoms! I wrote about more detections in Nature in 2016. Fields at the University of Illinois at Urbana-Champaign and his colleagues have already predicted that a supernova near enough to affect Earth would dump some radioactive residue here that we might detect in ocean sediments. In the past year, everything has changed. Somewhat more distant supernovae go off more often, but scientists assume the effects on Earth would be similar, though weaker. It has probably happened a few times, based only on the rate of supernovae, but we don’t have any direct evidence. A really nearby event - 30 light-years away or closer - would induce a mass extinction from radiation destroying the ozone layer, allowing lots of ultraviolet radiation through to damage life on the surface. Supernovae, the explosions of stars, have been the main focus. But because these events left few clues behind, for a time we could only speculate about any connection between events on Earth and those in space. We know there have been mass extinctions and sudden changes in Earth’s climate. We expect serious, life-threatening events every couple of hundred million years or so, on average. From this rate, we can infer the likelihood of such an event close enough and powerful enough to affect life on Earth. From astronomical observations, we can infer the average rate of supernovae, gamma-ray bursts, and solar outbursts. Scientists know there must have been radiation events, just based on the odds. Nearly all the work in this field has been a “what if” game. I’ve written about the generalized threat from astrophysical radiation events. ![]()
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