Formation of the earliest solids: Clues from 26Al

我们知道太阳系是在45亿年前形成的，当时一团尘埃和气体云坍塌形成了太阳和行星。当太阳形成时，它周围形成了一个旋转的尘埃盘，称为吸积盘。这个圆盘上剩余的物质最终形成了行星。我们可以通过研究这个时代的一些陨石，球粒陨石，来了解太阳系在这个阶段的情况。如果你观察球粒陨石，你会发现它们含有直径约0.1到1毫米的小圆球，称为球粒。它们形成于新生太阳周围的吸积盘。从构成吸积盘的星际纳米级颗粒到直径数千公里的行星，球粒的形成是颗粒生长层次结构的重要一步。球粒在高达2000摄氏度的高温下熔化了几分钟后就形成了圆形。尽管单个球粒的形成时间很短，但有一些初步证据表明，导致这种“闪热”的过程持续了1- 200万年。这项研究的目的之一是限制时间间隔期间，球粒形成是一个活跃的过程，在吸积盘。也有一些迹象表明，在不同的时间形成了具有不同化学成分的球粒。这将是一个令人兴奋的发现，因为它将记录下吸积盘随时间的化学演变，记录在球粒中。这将帮助我们了解在行星形成过程中吸积盘是如何随时间变化的。然而，目前可用的数据仍然不明确。因此，研究的重点是确定单个球粒的化学成分和年龄，并验证它们之间是否存在相关性。我们可以通过放射性同位素26Al来了解这一切。铝-26存在于早期的太阳系中，但由于它是放射性的，半衰期很短，所以它会衰变，所以在太阳系形成后的1000万年左右，铝-26的数量基本上减少到零。然而，如果在这1000万年的早期形成了一种矿物，并含有26Al，那么它的衰变产物26Mg今天可以测量，我们可以重新计算球粒中最初有多少26Al。这可能会让我们知道它的年龄。有些球粒可能含有旧物质的碎片，这可能会影响26Al的数据，这是一个复杂的问题。我们将通过视觉研究球粒并对其进行化学测量来寻找这一点。

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.