UChicago scientists have discovered evidence in a meteorite that a rare element, curium, was present during the formation of the solar system. The finding ends a 35-year-old debate on its possible presence in the early solar system, and plays a crucial role in reassessing models of stellar evolution and synthesis of elements in stars. Details of the discovery appear in the March 4 edition of Science Advances.
“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,” said the study’s lead author, François Tissot, PhD’15, now a postdoctoral fellow at the Massachusetts Institute of Technology.
And yet Tissot and his UChicago co-authors, Profs. Nicolas Dauphas and Lawrence Grossman, have found evidence of curium in an unusual ceramic inclusion they called “Curious Marie,” taken from a carbonaceous meteorite. Curium became incorporated into the inclusion when it condensed from the gaseous cloud that formed the sun in the early solar system.
Curious Marie and curium are named after Marie Curie, whose pioneering work laid the foundation of the theory of radioactivity. Curium was only discovered in 1944, by Glenn Seaborg and his collaborators at the University of California, Berkeley, who bombarded atoms of plutonium with alpha particles (atoms of helium) to synthesize a new, very radioactive element.
To chemically identify this new element, Seaborg and his collaborators studied the energy of the particles emitted during its decay at the Metallurgical Laboratory at UChicago (which later became Argonne National Laboratory). The isotope they had synthesized was the very unstable curium-242, which decays in a half-life of 162 days.
On Earth today, curium exists only when manufactured in laboratories or as a byproduct of nuclear explosions. Curium could have been present in the early solar system as a product of massive star explosions that happened before the solar system was born.
“The possible presence of curium in the early solar system has long been exciting to cosmochemists, because they can often use radioactive elements as chronometers to date the relative ages of meteorites and planets,” said Dauphas, the Louis Block Professor in Geophysical Sciences.
The longest-lived isotope of curium (Cm-247) decays over time into an isotope of uranium (U-235). Therefore, a mineral or a rock formed early in the solar system would have incorporated more Cm-247 than a similar mineral or rock that formed later, after Cm-247 had decayed. If scientists were to analyze these two hypothetical minerals today, they would find that the older mineral contains more U-235 (the decay product of Cm-247 than the younger mineral).
“The idea is simple enough, yet, for nearly 35 years, scientists have argued about the presence of Cm-247 in the early solar system,” Tissot said.
long wait to detect curium
Early studies in the 1980s found large excesses of U-235 in any meteoritic inclusions they analyzed, and concluded that curium was very abundant when the solar system formed. More refined experiments conducted by James Chen and Gerald Wasserburg, SB’51, SM’52, PhD’54, at the California Institute of Technology showed that these early results were spurious, and that if curium was present in the early solar system, its abundance was so low that state-of-the-art instrumentation would be unable to detect it.
Scientists had to wait until a new, higher-performance mass spectrometer was developed to successfully identify, in 2010, that tiny excesses of U-235 could be the smoking gun for the presence of Cm-247 in the early solar system.
“That was an important step forward, but the problem is, those excesses were so small that other processes could have produced them,” Tissot noted.
Models predict that curium, if present, was in low abundance in the early solar system. Therefore, the excess U-235 produced by the decay of Cm-247 cannot be seen in minerals or inclusions that contain large or even average amounts of natural uranium. One of the challenges was thus to find a mineral or inclusion likely to have incorporated a lot of curium but containing little uranium.
With the help of Grossman, professor emeritus in geophysical sciences, the team was able to identify and target a specific kind of meteoritic inclusion rich in calcium and aluminum. These calcium, aluminum-rich inclusions are known to have a low abundance of uranium and likely to have high curium abundance. One of these inclusions—Curious Marie— contained an extremely low amount of uranium.
“It is in this very sample that we were able to resolve an unprecedented excess of U-235,” Tissot said. “All natural samples have a similar isotopic composition of uranium, but the uranium in Curious Marie has 6 percent more U-235—a finding that can only be explained by live Cm-247 in the early solar system.”
Thanks to this sample, the research team was able to calculate the amount of curium present in the early solar system and compared it to other heavy radioactive elements such as iodine-129 and plutonium-244. They found that all these isotopes could have been produced together by a single process 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,”Dauphas explained.
The finding of naturally occurring curium in meteorites closes the loop opened 70 years ago by the discovery of manmade curium, and it provides a new constraint, which modelers can now incorporate into complex models of stellar nucleosynthesis and galactic chemical evolution to further understand how elements like gold were made in stars.
Citation: “Origin of uranium isotope variations in early solar nebula condensates” by F.L.H. Tissot, N. Dauphas, and L. Grossman, Science Advances, March 4, 2016. DOI: Vol. 2, No. 3, March 4, 2016, DOI: 10.1126/sciadv.1501400.
Funding: National Aeronautics and Space Administration, National Science Foundation