In a discovery that will end a more than three decade-long debate, a team of scientists from the University of Chicago discovered evidence in a meteorite indicating that the rare element curium was present during the formation of the solar system.
For 35 years, researchers have debated whether or not curium, an element first discovered by a team of Berkeley researchers in 1944, played a role in stellar evolution and the synthesis of elements in stars, UChicago alum François Tissot, currently a postdoctoral fellow at MIT, and his colleagues reported in the March 4 edition of Science Advances.
Credit: University of Chicago
While studying a carbonaceous meteorite, Tissot’s team found evidence of the element in an unusual ceramic inclusion they called “Curious Marie,” which like curium itself, was named in honor of Marie Curie, one of the pioneers of radioactivity. Not only did they find evidence that the element existed in the early solar system, they were able to determine how much of it there was in relation to plutonium-244 and other heavy radioactive elements.
Each of these radioactive isotopes, they explained, could have been produced at the same time by a single process in stars, and the curium could have become part of the process after it condensed from the same gas cloud from which the sun formed during the early days of the solar system.
The ‘smoking gun’ for curium’s existence in the early solar system
The Allende carbonaceous meteorite was analyzed and found to contain traces of the rare element. Credit: François L.H. TissotClose
As Tissot explained in a statement, “Curium is an elusive element. It is one of the heaviest-known elements, yet it does not occur naturally because all of its isotopes are radioactive and decay rapidly on a geological time scale.” For 35 years, scientists debated whether or not the element would have been present when the solar system originally formed.
When curium was first discovered, it was because the UC Berkeley scientists bombarded atoms of plutonium with alpha particles (atoms of helium), and studied the new radioactive element as it decayed so that they could chemically identify it. What they had synthesized turned out to be a highly unstable isotope, curium-242, that had a half-life of just 162 days.
Curium can only exist on Earth when manufactured in laboratories, or when created as the result of a nuclear explosion, the study authors explained. However, scientists hypothesized that it may have been produced by massive stellar explosions predating the solar system. Curium-247, which is the longest-lived isotope of the element, eventually decays into uranium, they noted, meaning that minerals rich in uranium may have at one time contained curium-247.
“The idea is simple enough, yet, for nearly 35 years, scientists have argued about the presence of 247Cm in the early solar system,” said Tissot. Previous studies were divided on the topic until, in 2010, a new, high-performance mass spectrometer enabled scientists to detect the telltale isotope of uranium that could have resulted from the decomposition of curium-247. It was, as university officials put it, the “smoking gun” for the existence of curium in the early solar system.
Discovery could explain how gold, other elements came to be
Tissot called that research “an important step forward,” but added that the amounts were so small that they could have been the result of other processes. So as part of their research, his team set out to find a mineral or inclusion that likely incorporated a significant amount of curium, but that contained a comparatively low amount of uranium, which could have occurred naturally.
They identified a type of meteoritic inclusion that was abundant in calcium and aluminum, better known as calcium aluminum-rich inclusions of CAIs. CAIs, they explained, have low amounts of uranium and an abundant amount of curium. One such inclusion, Curious Marie, contained very little uranium, and using this sample, Tissot said that he and his colleagues were able to “resolve an unprecedented excess of 235U.”
“All natural samples have a similar isotopic composition of uranium,” he said, “but the uranium in Curious Marie has six percent more 235U, a finding that can only be explained by live 247Cm in the early solar system.” Using this sample, the study authors were able to calculate the amount of curium present in the early solar system, and found that it was likely produced alongside other radioactive elements as part of a single process that took place in stars.
“This is particularly important because it indicates that as successive generations of stars die and eject the elements they produced into the galaxy, the heaviest elements are produced together, while previous work had suggested that this was not the case,” said co-author Nicolas Dauphas, also from the University of Chicago. Their work means that curium can now be added to models of stellar nucleosynthesis and chemical evolution, which should help explain how elements such as gold were originally produced.
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Image credit: University of Chicago
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