What are short-lived radioactive isotopes and why do they matter?
They are radioactive isotopes with half-lives of millions of years or less that existed in the early Solar System. They provide precise clocks and heated early asteroids.
Most radioactive isotopes used for dating, such as uranium-238 or potassium-40, have half-lives of billions of years and are still present today. But the early Solar System also contained short-lived radioactive isotopes, with half-lives of up to about 100 million years. They have now decayed completely, but they left fingerprints in meteorites.
Important examples include: - Aluminium-26, half-life about 0.72 million years, decaying to magnesium-26. - Manganese-53, about 3.7 million years, decaying to chromium-53. - Iron-60, about 2.6 million years, decaying eventually to nickel-60. - Hafnium-182, about 9 million years, decaying to tungsten-182. - Iodine-129, about 16 million years, decaying to xenon-129.
Scientists detect them by finding excess daughter isotopes in minerals that originally contained the parent element. For example, CAIs often have extra magnesium-26 in proportion to their aluminium content.
They matter in two ways. First, they are precise clocks for events in the first few million years. Because they decay quickly, small differences in age produce measurable differences. They show, for example, that chondrules formed about 1 to 3 million years after CAIs, and that some iron meteorite cores formed within about a million years.
Second, they were heat sources. Aluminium-26 in particular released enough energy to melt planetesimals that formed early, producing iron cores and basaltic crusts. Where did they come from? Some may have come from a nearby massive star or supernova, while others were produced by radiation from the young Sun.