This ends a 35-year-old debate!
Scientists from the University of Chicago have discovered evidence in a meteorite that a rare element, curium, was present during the formation of the solar system. The findings finally end a 35-year-long debate and will likely change our current theories on how stars produce elements.
“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,” the study's lead author, François Tissot, currently a W.O. Crosby Postdoctoral Fellow at the Massachusetts Institute of Technology, said in a press release.
Despite this, Tissot and colleagues, Nicolas Dauphas and Lawrence Grossman, found evidence of curium in an unusual ceramic inclusion they called Curious Marie, taken from a carbonaceous meteorite. Curium was incorporated into the inclusion when it condensed from a gaseous cloud that formed the sun early in the history of the solar system.
Curious Marie and curium are both named after Marie Curie — a pioneer in the theory of radioactivity. Curium was discovered in 1944 by Glenn Seaborg, who bombarding atoms of plutonium with alpha particles (atoms of helium) and synthesized a new, very radioactive element.
On Earth, curium only exists when it is manufactured in laboratories or as a byproduct of nuclear explosions. However, curium could have been present in the early history of the solar system as a product of star explosions that happened before our 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, Professor in Geophysical Sciences.
The longest-lived isotope of curium (247Cm) decays over time into an isotope of uranium (235U). Therefore, a mineral or rock formed earlier in the solar system, when more 247Cm existed, would contain more 235U than a younger mineral.
“The idea is simple enough, yet, for nearly 35 years, scientists have argued about the presence of 247Cm in the early solar system,” Tissot said.
Previous studies conducted in the 1980s found large excesses of 235U in meteoritic inclusions they analyzed, and concluded that curium was very abundant when the solar system formed. However, more refined experiments conducted later showed that those results were misleading.
So, scientists waited until 2010, when higher-performance mass spectrometers were developed in order to identify any small excesses of 235U.
“That was an important step forward but the problem is, those excesses were so small that other processes could have produced them,” Tissot noted.
However, models predicted that curium, if present, was in low abundance in the early solar system, so the excess 235U produced by the decay of 247Cm would not be seen in minerals that contain average amounts of natural uranium. One of the challenges was to find a mineral likely to have incorporated a lot of curium but containing little natural uranium.
With the help of study co-author Grossman, Professor in Geophysical Sciences, the team was able to identify a specific kind of meteoritic inclusion rich in calcium and aluminum, which are known to have low amounts of uranium and likely high curium abundance. Curious Marie, was one of these inclusions.
Luckily, the team was able to calculate the amount of curium present in the early solar system and compare it to the amount of other heavy radioactive elements such as iodine-129 and plutonium-244, which are all isotopes that 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 findings finally end the 35-year-old debate. Now, modelers can incorporate this information to better understand how other elements, like gold, were made in stars.