Rick Robinson

May 18th 2022

Plutonium on the Ocean Floor Provides Clues to Stellar Explosions


Generations of physicists have puzzled over the origins and very existence of heavy elements in the universe, and now they are turning toward the ocean floor. Many feel confident that lighter elements such as hydrogen and helium were created in the original Big Bang some 14 billion years ago, but other elements have proven more difficult to pinpoint. One theory for heavier elements, such as iron or gold, is that they were produced in massive stars, then spread about by powerful stellar winds as the stars aged.

But even these relatively steady processes cannot account for the heaviest elements, let alone even more massive or unstable atoms of radioactive elements such as plutonium.

New evidence from the ocean floor may finally explain the origin of the most massive elements. Or the evidence may end up showing that no single mechanism could produce all the heavy elements we see, indicating that multiple processes were needed to produce them.

What’s most interesting of all, perhaps, is gaining new insight into what happens when scientists disagree.

When Supernovae Aren’t Enough

The challenge of producing heavy elements is that, for atomic nuclei heavier than iron, the process of nuclear fusion that builds lighter elements has to work “uphill.” Instead of releasing energy, as the fusion of light elements (such as hydrogen to helium) typically does, more energy must be pumped into the nucleus.

Building more massive elements thus requires two things not easy to come by at the same time: a lot of energy, plus a lot of neutrons. According to Quanta magazine, by the 1990s, astrophysicists were pretty confident that they had a culprit: supernova explosions. These are among the most violent events in the universe — explosions that tear apart massive stars.

These produce plenty of energy, and since some supernovae produce neutron stars, neutrons are also plentiful.

But there was one problem. As the astrophysicists ran more sophisticated mathematical models of supernova processes on more powerful computers, they ran into more and more trouble. The models did not produce enough gold and other massive elements to account for the quantity of these elements that we observe — here on earth or elsewhere in the universe.

Enter Neutron Stars

The challenge of explaining heavy elements has led some astrophysicists to look at variations of the stellar-explosions theme. For example, supernovae can produce neutron stars, and in some cases these neutron stars end up orbiting each other. These orbits gradually decay, culminating in a collision of neutron stars — which may be another possible mechanism for the “more explosive environment” needed for heavy-element production, as Livescience describes.

Neutron-star collisions are rare events — rare even compared with supernovae. If these collisions are the main source of heavy elements in the universe, their distribution should be clumpy. At Quanta, astrophysicist Enrico Ramirez-Ruiz compares the pattern with chocolate — specifically the chocolate in a chocolate chip cookie versus a smoother chocolate glaze. The “glaze” would be the result if most heavy elements were produced in ordinary supernovae, while neutron star events would likely create something more along the lines of chocolate chips.

So, is it chocolate chip or chocolate glaze? As Livescience reports, to determine the answer to this mystery of astrophysics, researchers have been looking in a not-so-obvious place: a mile below the Pacific Ocean, in sediments that formed so gradually that it took nearly half a million years to lay down one millimeter of crust.

Searching for Clues on the Ocean Floor

There, the research team went looking for traces of two rare, radioactive isotopes: plutonium-244 and a form of iron (iron-60). Because these isotopes are highly radioactive and prone to decay, atoms of either one left over from the initial formation of Earth have long since vanished. Any atoms we find must have reached Earth much later.

What the researchers found was evidence for at least two supernovae that blew up relatively near Earth in the last few million years — one close enough that it would have been as bright in the sky as the full moon, easily visible in broad daylight. And the timing for these explosions fits the decay rates for both iron-60 and plutonium-244.

So, does this settle the matter? Further investigation is underway, using a sediment sample ten times larger. But in the meanwhile, the answer depends somewhat on which scientists you ask. As Science Daily reports, another recent study suggests that yet another type of stellar explosion might be the key to heavy-element production.

Called a collapsar, this type of eruption is produced by the collapse of an aged star that then forms a black hole. Under the right circumstances, it may release enormous amounts of heavy elements.

Is it enough to explain what we see, and resolve the chocolate chip versus chocolate glaze debate? Not quite yet. Sooner or later, the astrophysicists will probable settle the question, and it will be neatly wrapped up and tied with a bow in physics textbooks.

But for now, researchers are still asking the questions and the science is still being done — the answers are still uncertain. Indeed, in a Smithsonian magazine article, nuclear physicist Anton Wallner explains that the “data actually suggests that it might be that both scenarios are necessary.”

With careful observation, you can even detect the occasional particle of snark. In the Quanta article, physicist Selma de Mink says that if advocates of the neutron-star collision model “still have not found something, there will be a moment in which [they] should wonder, and get back to the board.”

A useful reminder that astrophysics and indeed all scientific inquiry remains very much a work in progress.

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