Deciphering the Origin and Evolution of the Early Solar System by Isotopes

长期以来，我们太阳系的起源一直是人类好奇心和科学探索的焦点。最近，系外行星的发现提高了人们的兴趣，这表明地球在太阳系外有兄弟姐妹的令人兴奋的可能性，因此，也许有外星生命。关于宜居行星演化的初始条件的宝贵信息可以从我们自己的太阳系中获得，但有几个基本问题必须解决：为了了解太阳系诞生的情况，我们需要找到并检查与太阳系形成过程中存在的原始物质相似的样本。为了理解支配太阳系演化的过程，我们可以比较太阳系中不同的天体和区域，例如，寒冷的外太阳系中的彗星与内太阳系中的小行星和类地行星。我们还需要评估这些地区之间发生的运输和交换机制。例如，彗星可能向地球大气层输送了大部分氮，但只输送了一小部分水。另一方面，在“星尘”任务带回的怀尔德2号彗星的尘埃中，发现了原本预计只在早期太阳系内部高温区域形成的物质。此外，很可能是外太阳系彗星的成分差别很大，因此有必要对更多的彗星进行取样，以更好地了解这一可能向地球输送水和其他挥发性元素的重要来源。但是，取样返回任务费用昂贵，因而受到限制。我从事的一个独特的替代方案是研究在地球上层大气中收集的“行星际尘埃颗粒”（IDPs）。这些尘埃颗粒被认为主要来自彗星。在这里，我建议研究星尘样品、IDPs和来自大量小行星的原始陨石中的稀有气体，主要是氪和氙，以阐明上述基本问题。稀有气体非常适合解决这些问题，因为它们在地球上很罕见，化学活性不强，因此不会失去它们的特征（同位素和元素）组成特征。这使得我们可以使用稀有气体特征来追踪物质的原始储集层。例如，陨石显示出与现在保存在太阳中的原始星云气体不同的特征。然而，探测彗星物质中的重惰性气体是极具挑战性的。国内流离失所者非常小，因此需要一种尖端的高灵敏度仪器。在取样过程中，彗星星尘与收集器的物质紧密混合在一起，需要一种特别有选择性的检测技术来将彗星稀有气体与收集器污染产生的气体分开。这些技术现在可以在曼彻斯特大学使用，我计划首次将这两种最先进的技术结合起来，研究Wild 2彗星尘埃、IDPs和原始陨石的惰性气体含量。通过这些研究，我们将更好地了解原始太阳云物质的原始库存，这些物质是通过吸积形成太阳系天体的，并且从那时起就储存在彗星和原始陨石中。我将限制来自彗星的挥发性物质对类地行星及其大气的贡献，将能够比较来自柯伊伯带采样的不同彗星的物质，并将评估整个早期太阳系中必须发生的运输机制和混合程度。最后，我还可以对这些过程设置时间限制。

The origin of our solar system has long been a focus of both human curiosity and scientific exploration. Interest has recently been heightened by the discovery of extrasolar planets, suggesting the exciting possibility of extrasolar siblings of the Earth and hence, perhaps, extraterrestrial life. Valuable information about the initial conditions from which habitable planets evolve can be obtained from our own solar system but several fundamental issues have to be tackled: To understand the circumstances of the birth of our solar system we need to find and examine samples that resemble the original material that was present during the formation of our solar system. To understand the processes that governed the evolution of the solar system we can compare various bodies and regions in the solar system, e.g., the comets in the cold outer solar system with the asteroids and the terrestrial planets in the inner solar system. We furthermore need to asses the transport and exchange mechanisms that have occurred between these regions. For instance, comets might have delivered most of the nitrogen to the Earth's atmosphere, but only a small fraction of the water. On the other hand, materials that were expected to have formed only in the hot inner regions of the early solar system have been detected in dust from comet Wild 2 that has been returned with the 'Stardust' mission. Also, it might well be that the composition of the comets in the outer solar system largely vary, and it is thus necessary to sample more comets to better understand this important source that may have delivered water and other volatile elements to Earth. However, sample return missions are expensive and thus limited. A unique alternative that I work on is the study of 'interplanetary dust particles' (IDPs) that are collected in the Earth's upper atmosphere. These dust grains are believed to originate mostly from comets. Here I propose to study noble gases, mainly krypton and xenon, in Stardust samples, IDPs and primitive meteorites from a large number of asteroids, to shed light on the above fundamental issues. Noble gases are uniquely suited to tackle these questions as they are rare on Earth, chemically inactive and hence do not lose their characteristic (isotopic and elemental) compositional signatures. This allows us to use noble gas signatures to trace materials back to their original reservoirs. Meteorites, e.g., show different signatures than the original nebula gas that is now in preserved the Sun. However, the detection of the heavy noble gases in cometary materials is extremely challenging. IDPs are very small and therefore a cutting-edge high-sensitivity instrument is needed. Cometary Stardust has been closely intermingled with the collector material during sampling and a particularly selective detection technique is required to separate cometary noble gases from those originating from collector contamination. These techniques are now available at the University of Manchester and I plan to combine, for the first time, these two state-of-the-art techniques to study the noble gas contents of comet Wild 2 dust, IDPs and primitive meteorites. With these studies, we will better understand the original inventory of protosolar cloud materials that accreted to form the solar system bodies and that were stored ever since in comets and primitive meteorites. I will constrain the contribution of volatile materials from comets to the terrestrial planets and their atmospheres, will be able to compare materials from different comets sampling the Kuiper belt and will assess transport mechanisms and the degree of mixing that must have occurred throughout the whole early solar system. Finally I might also be able to set chronological constraints on these processes.