The production of heavy metals in the cosmos is largely delegated to rapid neutron capture processes by some types of supernovae and neutron star mergers. By analyzing the radioactive isotope content of iron and plutonium in the oceanic crust, a group of researchers investigated the relative contribution of these two phenomena to the enrichment of the Solar System over time and, with it, of the Earth. The study is published in Science
The Sun comes from a cloud of gas and dust secondhand, “recycled” after the explosion of a supernova , and about 5-10 million years going through a region called the Local Bubble ( local bubble ), what remains of another supernova explosion. This is why the Earth is rich in chemical elements, including metals such as iron and even heavier ones, which have allowed the richness of shapes, colors and living organisms that we see. It does not end there, because other elements have probably been deposited in the course of history by other similar stellar catastrophes that occurred in the vicinity. But what is the contribution of all these contaminations? A study published today in Scienceregarding the analysis of the chemical abundances of some heavy radioactive isotopes present in the oceanic crust, it would seem to suggest a rather complex picture.
We are chemically children of the stars, this is now well established. Stars are large natural furnaces that produce chemical elements heavier than hydrogen. We are children of the stars, but not of all in the same way. Those similar to the Sun, however – or a little smaller, but also a little larger – are unable to produce heavy metals during their life. Not because they don’t live long enough, but because the production of these elements doesn’t happen by means of nuclear fusion or, in astronomical jargon, stellar nucleosynthesis.. Iron itself is at the limit: it is produced by fusion but, unlike the lighter elements – such as helium, nitrogen, carbon, oxygen – it does not return the favor by providing useful energy to the star to counteract the gravitational force and not fall under its own. same weight.
To form the heavier elements, then, real catastrophic events are needed, such as the explosion of a supernova or the merger of two neutron stars . Half of the elements heavier than iron in the cosmos are produced by rapid neutron capture ( r processes ). Supernovae, specifically, create many of the building blocks of human life, such as iron, potassium and iodine. Even heavier elements, such as gold, uranium and plutonium, are instead attributed to rarer events, such as the merger of two neutron stars. Together with the heavy elements, however, radioactive isotopes are also generated, which decay over time as they are unstable.
Star explosions, supernovae, neutron stars: all catastrophic events far away in time and space, all events that have not made their extreme violence felt so far. Not directly, at least. Instead, they threw their products into space that reached the Earth several times, creating deposits that remained almost unchanged – subject only to natural decay – in the most remote places.
The authors of this study went looking for them in the oceanic crust . They were looking for two in particular: iron-60 – produced mainly in massive stars and supernovae, and with a half-life of 2.6 million years – and plutonium-244 – produced exclusively by r- processes and with a much longer half-life, 80.6 million years. Two isotopes that directly testify violent cosmic events that occurred in the vicinity of the Earth millions of years ago, and whose relationship can tell us something more about the astronomical catastrophes that have given metals to the solar system.
What events are we talking about, exactly, and when did they happen?
“The story is complicated,” explains first author Anton Wallner , a professor at the Australian National University . “Maybe this plutonium-244 was produced in supernova explosions or it could have remained from a much older event. But it could have been produced in an even more spectacular way, as in the detonation of a neutron star “
Any isotope of plutonium-244 and iron-60 that was present when the Earth formed from interstellar gas and dust more than four billion years ago has long since completely decayed. The traces found now, therefore, must have originated from more recent events . In particular, the dating of the sample allows us to trace at least two supernova explosions that occurred near the Earth, in the last 10 million years. In both, the relationship between the isotope of iron and that of plutonium is similar. The plutonium abundance, however, is lower than expected if the supernovae were the only ones responsible for the r processes . We need to think of something new.
“Our data could be the first evidence that supernovae actually produce plutonium-244, ” Wallner continues. “Or maybe it was already in the interstellar medium before the supernova exploded, and it was pushed through the Solar System along with the material ejected from the supernova.”
The data collected in this study, therefore, on the one hand are compatible with the scenario according to which the passage of the Solar System through the local bubble enriches interstellar space and our planet, more frequently than the radioactive half-life of the deposited elements themselves. On the other hand, contaminations from rarer astrophysical sources – such as neutron star mergers – are less frequent, but indispensable to explain the isotope ratios found. The hypothesis of a near rare event that occurred before the formation of the solar system remains open. In short, where does that family gold chain come from, that bracelet you gave away or that ring from which you never separate, scientists still cannot say exactly. In the meantime, you can appreciate them even more, thinking that deep down,
Featured image: Artist’s impression of the merger of two neutron stars. Credits: Robin Dienel – Carnegie Institution for Science
To know more:
- Read in Science the article ” 60 Fe and 244 Pu deposited on Earth constrain the r-process yields of recent nearby supernovae ” by Wallner, MB Froehlich, MAC Hotchkis, N. Kinoshita, M. Paul, M. Martschini, S. Pavetich , SG Tims, N. Kivel, D. Schumann, M. Honda, H. Matsuzaki, T. Yamagata
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