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Planet Earth is home to the ocean depths a radioactive plutonium deposit that could only be formed in space, during a violent cosmic cataclysm. Although there are reserves of this radioactive dust at great depths, it has been proven that it continues to rain down on us today. That would lead one to think that it was a recent cataclysm in astronomical terms. However, according to a recently published study by German scientistsit was hundreds of millions of years ago. Two isotopes to understand everything. Plutonium-244 does not exist naturally on Earth. In fact, the only isotope of this element that can be produced naturally in some geological processes is plutonium-239. and it does so mostly in the form of traces. Plutonium-244 is the heaviest isotope of this element. That is, the one with the most neutrons. It is known that it is usually formed by cosmic phenomena during something known as the r process, where lighter atoms quickly absorb neutrons into their nuclei. Generally, the event that usually gives rise to this phenomenon is the kilonova, an explosion resulting from the merger of two neutron stars. In the process, curium-247 is also formed, which is why these scientists have also analyzed its levels. Taking this data into account, they have discovered that the explosion in question must have occurred more than 100 million years ago, but less than one billion years ago. And, also, that the radioactive fallout has not stopped since then. The key is in the ferromanganese crust. Ferromanganese bark It is a layer of the ocean floor which is formed when metals dissolved in sea water, such as iron and manganese, are deposited and solidify. This occurs at a fairly slow rate, with growth of between 1 and 10 millimeters per million years. The deposits do not only have iron and manganese. Mixed with them are other substances that have fallen into the sea at that time. Therefore, this crust is a perfect chemical photograph of the history of our planet. A section with surprise. The authors of this study analyzed a section of this crust extracted at a depth of 4,830 meters in 1976. This had already been analyzed previously and had pointed out something surprising. And, in addition to plutonium, iron-60 was also found, another radioisotope associated with supernova explosions, which has a fairly short half-life of 2.6 million years. This figure means that, every 2.6 million years, half of the initial atoms of this isotope will have decayed. In another 2.6 million years half of what remained and so on. Since it is a fairly short half-life, it was concluded at the time that the kilonova that caused the fall of radioactive dust took place about 3 million years ago. However, the authors of the study just published debunked that hypothesis. Half-life of the study isotopes Curio to the rescue. The formation of plutonium-244 when neutron stars merge is always accompanied by the formation of curium-247. The plutonium isotope has a half-life of 81 million years, while that of curium “only” has a half-life of 15.6 million years. When analyzing the ferromanganese bark sample, these researchers found no curium. Therefore, it must have completely disintegrated. That places the explosion more than 100 million years ago. Be careful, remember that the half-life is the time it takes for half of the radioactive material to decay. Every 15.6 million years, half of it disintegrates, so in 100 million years there should be no curium left, but a lot of plutonium, which only lost half of it 19 million years ago. For plutonium to completely disappear, it would take 1 billion years. What about iron? The reason why there is iron-60 in the sample, despite having a lower half-life than that of curium-247, is that they originated in different events. In fact, the level changes of iron do not coincide with those of plutonium. On the other hand, it has been seen that plutonium continues to appear uniformly in the upper layers, hence it has been concluded that the radioactive fallout has not ended. At least it hadn’t ended in 1976 and that in astronomical terms was before yesterday. And now what? These scientists think that the cataclysm that released this long radioactive fallout must have been immense. Possibly even affected life on Earth. But at the moment it is something that cannot be known. We will have to continue investigating to have the answer. Image | University of Warwick/Mark Garlick | B. Schröder/HZDR/NASA, ESA, J. Hester, A. Loll/ASU In Xataka | Gravitational waves work their magic: we are closer to revealing the enigmas of neutron stars