Spatial and temporal scales of aqueous alteration in icy planetesimals

这项研究计划的目的是更好地了解位于太阳系外层的小型岩石和冰体的演化。这些天体统称为“行星体”，行星科学家对它们非常感兴趣，因为在太阳系45亿年的历史中，它们大部分时间都保持不变。因此，他们可以告诉我们许多关于它是如何形成和发展的。这项研究项目是关于含有液态水的行星体，可能是在它们形成后不久的一段短时间内。这些小行星的碎片以陨石的形式落到了地球上，这些陨石被称为碳质球粒陨石，科学家们对它们非常重视，因为它们的化学成分表明它们是可供研究的蚀变最少的岩石。然而，尽管碳质球粒陨石的化学成分非常原始，但它们主要是由水与母星小行星反应形成的矿物组成，这种水蚀变过程预计也会改变岩石的化学成分。这种原始化学和次生矿物学之间的矛盾只能在小行星内的水是静止的情况下才能解释。然而，最近对小行星演化的计算机模拟一直预测，只有最小的天体才可能含有静水，而且在大多数情况下，静水肯定是流经岩石，改变了岩石沿途的化学成分。在这个研究计划中，我们将使用一组被称为CMS的碳质球粒陨石，通过获得关于行星体内水的行为和历史的新信息，来测试这些模型的假设和预测。这些陨石含有从水中结晶出来的称为碳酸盐的矿物的小晶体。通过使用一系列基于显微镜的技术研究碳酸盐晶体的组成、内部结构和分布，我们将解决以下问题：水是静止的，还是像热水在地球上的岩石中流动一样流动？水是在一个小天体中只存在了很短的一段时间，还是在一个较大的小行星中存在了数百万年？小行星的内部是否含有水，或者它是否只存在于靠近其表面的地方？这项研究的结果将增加我们对行星体如何形成和演化的理解，并将使我们和其他科学家能够评估和潜在地修改小行星内部的计算机模型。归根结底，这项工作对于我们了解太阳系的早期历史，以及对当今彗星和小行星的组成和内部结构都具有重要意义。这些机构目前是大量国际研究活动的重点，最近几个空间探测器访问了这些机构，也是未来无人和可能有载人探索的目标。

This aim of this research programme is to understand better the evolution of the small bodies of rock and ice that lie in the outer parts of the solar system. These bodies are collectively called 'planetesimals', and are of great interest to planetary scientists because they have remained unchanged for most of the 4,500 million year history of the solar system. Thus, they can tell us much about how it formed and developed. This research project is concerned with planetesimals that contained liquid water, probably for a brief period soon after their formation. Pieces of these planetesimals have fallen to Earth as meteorites called 'carbonaceous chondrites', which are highly valued by scientists because their chemical compositions indicate that they are least altered rocks available for study. However, despite their very primitive chemistry, the carbonaceous chondrites are made mainly of minerals that were formed by water reacting with their parent planetesimal, and this process of aqueous alteration would be expected to have also modified the chemical composition of the rock. This contradiction between a primitive chemistry and secondary mineralogy can only be explained if water within the planetesimal was static. However, recent computer simulations of planetesimal evolution consistently predict that only the smallest bodies could have contained static water and in most it must have flowed through the rocks, modifying their chemical compositions along its path. In this research programme we will test the assumptions and predictions of these models by obtaining new information on the behavior and history of water within planetesimals using one group of carbonaceous chondrites called the CMs. These meteorites contain small crystals of minerals called carbonates that crystallized from the water. By examining the compositions, internal structures and distributions of carbonate crystals using a range of microscope-based techniques, we will address the following questions: Was the water stationary or did it flow in the same way that hot water moves through rocks on Earth? Did the water exist for only a brief period in a small body or was it present for millions of years within a larger planetesimal? Did planetesimal interiors contain water or was it present only close to their surface? Results of this research will increase our understanding of how planetesimals formed and evolved and will enable us and other scientists to assess and potentially modify the computer models of planetesimal interiors. Ultimately this work is significant for our understanding of the early history of the solar system but also of the present-day composition and internal structure of comets and asteroids. These bodies are currently the focus of a great deal of international research activity, having been visited recently by several space probes, and are targets for future unmanned and possibly manned exploration.