The evolution of the protosolar disk

这一提议的目的是确定后来构成太阳系行星的固体材料的形成环境。通过对陨石的研究，结合天文观测和建模，我们认为这些行星是由尘埃和气体组成的围绕太阳运行的圆盘(原太阳盘)形成的。这个圆盘的大部分螺旋进入了正在形成的太阳，但也有一些保留了下来，它们可能首先聚集成毫米厘米大小的固体块，然后成为越来越大的物体的一部分，最终进入类地行星。这些毫米大小的物体被保存在一些陨石中，被称为球粒。通过详细观察它们，我们可以了解圆盘是由什么组成的，这些第一批固体是如何形成的，以及圆盘中的条件可能是如何演变的。球粒陨石的化学成分/具体地说，每种元素的丰度可以告诉我们一些关于它们的起源和进化的信息。每种元素都有不同的化学性质，它们的行为将由温度、压力以及存在的氧气和氢气的数量等条件决定。通过测量尽可能多的元素的丰度，我们可以建立球粒形成条件的图景，以及关于球粒前体是由什么组成的想法。有些元素有不止一个同位素--同一元素的核具有不同的质量。单一元素的同位素将具有大致相同的化学性质，但在物理性质上略有不同，例如它们的挥发性。我们计划测量球粒中最常见的四种元素硅、铁、氧和镁的同位素组成，以便更好地了解它们的历史。除了热历史，我们建议测量的几种元素具有一些特殊的性质。太阳系天体的氧同位素组成非常不同，这表明太阳系氧的组成最初发生了变化。这种初始的非均质性可用作固体原始成分的示踪剂。镁同位素也因为另一个原因而特别有趣。同位素26 mg可以是放射性成因，由放射性衰变形成，现已绝迹的同位素26Al。监测26 mg的丰度可以告诉我们一些关于26Al在早期太阳系中的分布情况，26Al是一个潜在的关键热源。这些测量还可以潜在地告诉我们一些时间尺度/早期形成的物体在固化时可能含有更多的26Al，而不是在26Al大部分已经腐烂时形成的固体。测量硅和铁的同位素在某种程度上更具探索性/这两种元素的测量报告还不是很多，因此我们还不清楚我们可以预期的同位素组成的多样性。这些同位素很可能被用作确定球粒陨石的热历史以及它们如何与周围气体(可能以气体形式含有一些铁和硅)相互作用的工具。关于这些物体的化学和同位素组成，特别是关于同一物体的几个参数，缺乏准确的、系统地获得的数据。通过建立这些天体的主、次元素组成数据库，以及主要元素氧、硅、铁、镁的同位素组成，我们可以确定吸积盘中物质成分的多样性。这反过来将使我们能够确定圆盘中的混合和湍流程度，圆盘固体形成的时间尺度和这些物体的热历史。我们可以利用这些信息来与天文圆盘观测进行比较，并评估我们自己的行星系统的演化是否有任何不寻常的地方。

The aim of this proposal is to determine the formation environment of the solid materials that later made up the planets in the Solar System. From studies of meteorites, combined with astronomical observations and modelling, we believe that the planets formed from a disk around the Sun (the protosolar disk) composed of dust and gas. Much of this disk spiralled into the forming Sun, but some remained, that probably accreted firstly into mm-cm sized solid lumps that went on to become part of bigger and bigger objects and eventually into the terrestrial planets. These millimetre-sized objects have been preserved inside some meteorites and are called chondrules. By looking at them in detail we can learn about what the disk was made of, how these first solids formed and how the conditions in the disk may have evolved. The chemistry / specifically, the abundance of each of the elements- of chondrules can tell us something about their origins and evolution. Each element has a different chemical character, and their behaviour will be dictated by conditions such as temperature, pressure and the amount of oxygen and hydrogen present. By measuring the abundance of as many elements as possible, we can build up a picture of the conditions in which the chondrules formed, as well as an idea about what the chondrule precursors were made of. Some elements have more than one isotope- nuclei of the same element that have different mass. Isotopes of a single element will have broadly the same chemical properties, but will slightly differ in their physical properties, for example their volatility. We plan to measure the isotopic composition of the four most common elements silicon, iron, oxygen and magnesium in chondrules, we can build up a better picture of their history. In addition to thermal history, a couple of the elements we propose to measure for isotopes have some special properties. The oxygen isotopic composition of solar system objects is very diverse, and points to an initial variation in the composition of solar system oxygen. This initial heterogeneity can be used as a tracer of the original composition of the solid. Magnesium isotopes are also particularly interesting for another reason. The isotope 26Mg can be radiogenic, formed from the decay of the radioactive, and now extinct, isotope 26Al. Monitoring the abundance of 26Mg can tell us something about the distribution of 26Al, a potentially critical heat source, in the early solar system. These measurements can also potentially tell us something about timescales / early-formed objects are likely to have contained more 26Al when they solidified than solids that formed when most of the 26Al had already decayed away. Measuring silicon and iron isotopes is somewhat more exploratory / there are not very many measurements already reported of these two elements, and so we do not already have a good picture of what diversity in isotopic composition we can expect. It is likely that these isotopes can be used as tools to determine the thermal history of chondrules and how they interacted with the neighbouring gas (that may have contained some Fe and Si in gaseous form). There is a lack of accurate, systematically acquired data about the chemistry and isotopic composition of these objects, and especially of several of these parameters on the same object. By building up a database of the major and minor element composition of these objects, and the isotopic composition of the major elements oxygen, silicon, iron and magnesium, we can determine the diversity of compositions of material in the accretion disk. This in turn will allow us to determine the degree of mixing and turbulence in the disk, the timescales of formation of disk solids and the thermal history of these objects. We can use this information to compare to astronomical disk observations, and assess if there was anything unusual about the evolution of our own planetary system.