Formation of the earliest solids: Clues from 26Al

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We know that the solar system formed about four and a half billion years ago when a cloud of dust and gas collapsed to form the Sun and planets. As the Sun formed, a swirling dusty disk evolved around it, called the accretion disk. Left over material from this disk eventually became the planets. We can learn about the solar system when it was at this stage by studying some meteorites that date from this era, the chondrites. If you look at chondritic meteorites you can see that they contain little round balls with a diameter of about 0.1 to 1 mm, called chondrules. These formed in the accretion disk around the nascent sun. Chondrule formation is an important step in the hierarchy of grain growth from interstellar, nanometer sized grains that made up the accretion disk to planets, which are thousands of kilometres in diameter. Chondrules obtained their round shape when they were molten for only a few minutes at very high temperatures of up to >2000 degree C. Although the formation of an individual chondrule was brief, there is some first evidence that the process causing this 'flash' heating was active for 1-2 Million years. One aim of this study is to constrain the time interval during which chondrule formation was an active process in the accretion disk. There is also some indication that chondrules with a different chemical compositions formed at different times. This would be an exciting finding, as it would document a chemical evolution of the accretion disk with time, recorded in chondrules. This would help us understand how the accretion disk changed with time as the planet forming process progressed. However, the data available so far are still ambiguous. A focus of the study is therefore to determine both the chemical composition and the age of a single chondrule and verify whether there is a correlation between them. We can learn about all this using a radioactive isotope, 26Al. Aluminium-26 was present in the early solar system, but since it is radioactive with a very short half-life it decayed away and so the quantity lessened to essentially zero about 10 Million years after the beginning of the solar system. However, if a mineral formed early within this 10 Million years and incorporated 26Al, its decay product 26Mg can be measured today and we can re-calculate how much 26Al was initially in the chondrule. This may then give us an idea of its age. There is a complication in that some chondrules may contain fragments of older materials and addition of this may affect the 26Al data. We will look for this by studying the chondrule visually and making chemical measurements on it.