A small piece of rock pulled from the bottom of the Pacific Ocean in 1976 is giving scientists new clues about an ancient cosmic event. More than a hundred million years ago, two neutron stars collided. The resulting energetic kilonovas sent a rain of long-lived elements, such as plutonium isotopes, across space. Eventually, this stellar “debris” settled on Earth. Some of it sank to the bottom of the ocean and became embedded in a piece of iron manganese rock. Hidden inside were a few hundred atoms of radioactive plutonium isotopes. They provide the strongest evidence as to why and when the merger occurred.
Plutonium exists in the form of Pu-244, which has a half-life of 81.3 million years. This helped a team of scientists from the Helmholtz-Zentrum Dresden-Rossendorf Foundation in Germany and researchers at the Australian Nuclear Science and Technology Organization (ANSTO) determine the age of the explosion at approximately 100 million years ago. They also found that the sample lacked another element related to the collision: curium-247. It has a half-life of 16 million years.
“The absence of the radioactive isotope curium Cm-247, which was also produced in the explosion, tells us this happened a very long time ago,” said Dr Michael Hotchkiss of ANSTO. “But no more than a billion years ago, otherwise Pu-244 would also have been undetectable.”
Research team member Dominic Cole holds a sample of rock crust extracted from the Pacific Ocean. Courtesy Ansto.
Drill cores reveal kilonova elements
To reach the hidden PU-244 and learn the age of the neutron star merger debris, the science team drilled three cores into the rock. Then they began a careful chemical analysis. The cores were dated using the beryllium isotope Be-10, which has a half-life of 1.5 million years. They also found traces of the iron isotope Fe-60 in one core. The Earth’s crust grows very slowly, with each core measuring up to 3 cm in length, and extends over ten million years.
The remaining cortex was imaged using X-ray computed tomography and encased in resin. This allowed scientists to cut thin layers, each corresponding to about a million years of growth. Each sample was then divided and processed to extract the plutonium. During this analysis, the team also found traces of material from known supernova events that occurred between 2 and 7 million years ago. They also found some curium, but not the specific isotopes created in the neutron star collision, according to Hotchkiss. “The only possible explanation is that the cosmic explosion responsible for the plutonium occurred so long ago that curium had already decayed into practically nothing,” he said.
Making items
We all know that elements like helium, carbon, nitrogen and oxygen – all the way to iron – are made inside stars, a process called stellar nucleosynthesis. The Sun, for example, fuses hydrogen in its core to form helium. Within a few billion years, it will begin fusing helium to make carbon, and then continue to produce carbon and oxygen. When the Sun begins to become a white dwarf, it will release all the elements into space. In stars more massive than the Sun, the process is more complex, but essentially continues until iron is formed. Since it takes more energy to make iron and anything heavier, the process stops, the core collapses and the star explodes with all its elements into space. Elements such as gold, platinum, uranium, nickel and zinc are created in such events.
About half of the heaviest elements are formed in massive events such as neutron star collisions Which leads to kilonova events. This process is called the “r process” and includes elements such as thorium and uranium, and transuranic elements, such as plutonium and curium. Theories of r-process nucleosynthesis suggest that both Cm-247 and Pu-244 are produced simultaneously, in approximately equal proportions, in such an event. Because curium decays more quickly than plutonium, this sets a lower limit on the neutron star merger age, while Pu-244 helps set an upper limit.
* The periodic table of the elements, specifying the origin of each element. Elements heavier than iron are created in supernovae, while some are created only in neutron star mergers. Courtesy Camgli. CC BY-SA 3.0*
Exploring R process dust on Earth and beyond
Detailed study of these isotopes, as well as other isotopes found in a rock sample on the ocean floor, shows that debris from cosmic events can reach Earth in pulses. Some of them are associated with nearby supernova explosions. However, the small sample of Pu-244 was present in all layers of the rock chips. This means that the plutonium very likely came from a neutron star/kilonova merger. It has appeared on Earth as a continuous flow for the 100 million years since the event.
The research team is looking for other samples to enhance the discovery of neutron star mergers using radioactive isotope samples. There must be more pieces of ancient crust on Earth that contain the products of the r process that occurred. It is possible that the dust resulting from this long-ago event settled on the Moon and other worlds. The Apollo rocks could be a suitable place to study, and future missions could provide another way to access dust from the ancient past.
Space missions such as the Chandra X-ray Observatory, the James Webb Space Telescope, and others have witnessed neutron star mergers at different wavelengths. So scientists knew it happened. However, this “chemical analysis” of the debris resulting from such events is a major step forward in dating the events and monitoring the results of the nucleosynthesis process.
An artist’s view of a neutron star merger, accompanied by two views taken by the Chandra X-ray Observatory. This type of event results in extremely high-energy conditions that help form some heavier elements such as plutonium.
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