Curium, an element heavier than any that exist naturally on Earth today, played a part in the formation of the Solar System, traces left behind in a meteorite suggest. The discovery provides us with a better understanding of how the Sun and planets formed, and how giant stars die.
Elements heavier than uranium, known as transuranics, exist only in the laboratory on Earth. Curium, jointly named after Marie and Pierre Curie, has an atomic number of 96, four places beyond uranium. "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 Dr. François Tissot of the Massachusetts Institute of Technology in a statement.
Like all transuranics, curium's isotopes have half-lives short enough that any formed in supernovae, and incorporated into planets at the birth of the Solar System, decayed long ago. Most curium isotopes have half-lives of a few thousand years or less. The longest lived isotope, Cm-247, has a half-life of 15.6 million years, tiny compared to the age of the Solar System (4.6 billion years).
However, when Tissot examined the Allende meteorite, he found a portion of it was a ceramic. He dubbed it “Curious Marie,” suspecting it might once have contained curium. In Science Advances, he reveals evidence for this theory.
Cm-247 decays via plutonium 243 to eventually become uranium 235. Any material that formed with Cm-247 in it should have more U-235, relative to other isotopes of uranium, than the same material formed in the absence of Cm-247. On Earth, geological mixing obscures such variations, but meteorites preserve a record of the Solar System's formation.
"The idea is simple enough, yet, for nearly 35 years, scientists have argued about the presence of Cm-247 in the early Solar System," said Tissot. Some studies found excess U-235 in meteorites, but other explanations have been made. Finding traces of curium is hard because it is estimated that even in the early Solar System there was almost 10,000 times less Cm-247 than U-235.
False color close up of the "Curious Marie" inclusion. Calcium is in red, aluminum is blue, green for magnesium; field of view is 0.5 millimeters (0.01 inches). François L.H. Tissot
Tissot's approach was to study a portion, known as an inclusion, rich in calcium and aluminum, rather than the whole meteorite. The chemistry of these inclusions excludes most uranium, in this case 99.9 percent, but should incorporate curium. "We were able to resolve an unprecedented excess of U-235," Tissot said. "A finding that can only be explained by live Cm-247 in the early Solar System."
"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 coauthor Professor Nicolas Dauphas of the University of Chicago.
Dauphas concluded that the quantity of Cm-247 produced indicated it was formed in the same process as iodine 129 and plutonium 244, two other long-decayed isotopes whose legacy we detect. The discovery will help us understand how supernovae form heavy elements.