New York | Scientists have discovered evidence in a meteorite that a rare radioactive element, curium, was present during the formation of our solar system, ending a 35-year-old debate. The finding plays a crucial role in reassessing stellar evolution models and synthesis of elements in stars, researchers said. The discovery ends a 35-year-old debate on the possible presence of curium in the early solar system, they said. 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 Francois Tissot, a former student at the University of Chicago, now at the Massachusetts Institute of Technology in US. Curium became incorporated into the inclusion when it condensed from the gaseous cloud that formed the Sun early in the history of the solar system. On Earth, curium exists only when manufactured in laboratories or as a byproduct of nuclear explosions. It could have been present, however, early in the history of the 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 study co-author Nicolas Dauphas, a professor at University of Chicago. The longest-lived isotope of curium (247Cm) decays over time into an isotope of uranium (235U). Therefore, a mineral or a rock formed early in the solar system, when 247Cm existed, would have incorporated more 247 Cm than a similar mineral or rock that formed later, after 247Cm had decayed.
If scientists were to analyse these two hypothetical minerals today, they would find that the older mineral contains more 235U (the decay product of 247Cm) than the younger mineral. Models predict that curium, if present, was in low abundance in the early solar system. Therefore, the excess 235U produced by the decay of 247Cm cannot be seen in minerals or inclusions that contain large or even average amounts of natural uranium. The team was able to identify a specific kind of meteoritic inclusion rich in calcium and aluminium.
These CAIs (calcium, aluminium-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. The finding of naturally occurring curium in meteorites by scientists closes the loop opened by the discovery of man-made Curium and it provides a new constraint, which modellers can now incorporate into complex models of stellar nucleosynthesis and galactic chemical evolution to further understand how elements like gold were made in stars.
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