Formation and evolution of small planets and moons

我们的研究旨在帮助理解较小的类地行星核(如水星、月球、火星和木卫三)以及外太阳系的冰冻卫星和较小天体的形成和演化。我们将通过以下方式完成这项工作：(I)通过研究核心结晶过程(项目1)，(Ii)通过确定在相关压力和温度条件下形成核心的物质的物理性质(项目2)，以及(Iii)通过研究高压和极低温度下结冰物质的物理性质(包括其振动光谱)(项目3)。近期和未来飞行任务的主要观测对象是表面形貌、光谱观测、磁场、重力数据，以及月球和火星的地震观测(通过目前的洞察任务)。磁场的产生依赖于能量来源，包括重力能和结晶潜热的释放。对这些小行星核心中的能量来源的详细了解目前还不确定，因为甚至不知道它们是自下而上结晶还是自上而下结晶，这对发电机驱动机制具有一级影响；这些是项目1要解决的主要问题。该项目的主要成果将是对与水星、木卫三、火星和月球核心相关的整个压力-温度范围的从头计算预测：i)铁合金的绝热温度梯度和熔化曲线，这将使我们能够确定核心结晶的模式；ii)液体的密度和液固密度对比，这有助于重力能量的释放；以及iii)铁合金系统中的熔化潜热。项目2主要是实验性的，并解决了我们在较小行星核心内发现的条件下对铁合金和相关材料的基本物理性质的了解方面的主要空白。如果我们要了解行星的演化，行星内部的地球物理模型--其结果可以对照航天器和地面观测的数据进行检验--是必不可少的，但只有在相关的压力和温度范围内准确地知道组成核心的材料(主要是铁和铁合金)的物理性质，这些地球物理模型才是可靠的。目前，月球、水星、火星和木卫三内部预期的情况往往并非如此。令人惊讶的是，即使是最简单的近似--一个纯铁的核心--目前可用的数据也是不充分的。该项目的主要成果将是确定铁合金的相图和物理性质，特别是三元系，如Fe-Ni-Si和Fe-S-Si，压力高达~40 Gpa。项目3涉及一类不同的材料--水冰和水合硫酸盐--它们构成了外太阳系冰冷天体的地幔。对这些物质施加压力很容易引起晶体结构的变化，对于水合硫酸盐来说，可能会导致水被排出并转变为较低的水合状态。这些结构变化的结果是在冰体内产生层状结构。此外，伴随着脱水的大体积变化可能导致表面的全球扩张和裂解和/或全球收缩和皱缩。该项目的主要成果包括相图、状态方程以及这些结冰材料在相关压力和温度条件下的导热系数和电导率。除此之外，我们还将确定这些不同相的振动光谱，以便与飞行任务数据进行直接比较。

Our research aims to aid understanding of the formation and evolution of smaller terrestrial planetary cores (e.g., Mercury, the Moon, Mars and Ganymede) and the icy moons and smaller bodies of the outer solar system. We shall do this in the following ways: (i) by investigating core crystallisation processes (Project 1), (ii) by determining the physical properties of core-forming materials under the relevant conditions of pressure and temperature (Project 2) and (iii) by investigating the physical properties of icy materials (including their vibrational spectra) at high pressures and very low temperatures (Project 3).Key observables from recent and future missions are surface morphologies, spectroscopic observations, magnetic fields, gravity data and, for the Moon and for Mars (via the current InSight mission) seismic observations. The generation of magnetic fields relies on energy sources which include the release of gravitational energy and latent heat of crystallisation. Detailed understanding of these energy sources in small planetary cores is currently uncertain, as it is not even known whether they crystallise from the bottom up or from the top down, and this has first order implications for dynamo driving mechanisms; these are the main issues to be addressed by Project 1. Key deliverables of this project will be ab initio predictions over the entire pressure-temperature range relevant to the cores of Mercury, Ganymede, Mars and the Moon of: i) the adiabatic temperature gradients and melting curves of iron-alloys, which will allow us to determine the mode of core crystallisation, ii) the densities of the liquids and the liquid-solid density contrast, which contributes to gravitational energy release, and iii) the latent heat of fusion in iron alloy systems.Project 2 is predominantly experimental, and addresses major gaps in our knowledge of the fundamental physical properties of iron alloys and related materials at the conditions found within the smaller planetary cores. If we are to understand planetary evolution, geophysical modelling of planetary interiors - the outputs of which can be tested against data from spacecraft and ground-based observations - is essential, but these geophysical models will be reliable only if the physical properties of the constituent core-forming materials (essentially iron and iron alloys) are accurately known in the relevant pressure and temperature ranges. At present, this is often not the case for the conditions expected in the interiors of the Moon, Mercury, Mars and Ganymede. Surprisingly, even for the simplest approximation - a core of pure iron - the data currently available are inadequate. Key deliverables of the project will be the determination of the phase diagrams and physical properties of iron alloys, especially ternary systems such as Fe-Ni-Si and Fe-S-Si, at pressures up to ~40 GPa. Project 3 addresses a different class of materials - water ice and hydrated sulphate salts - which form the mantles of the icy bodies of the outer solar system. Application of pressure to these readily induces changes in crystal structure and, in the case of the hydrated sulphate salts, can lead to expulsion of water and transformation to a lower hydration state. The consequence of these structural changes is to produce a layered structure within the icy body. In addition, the large volume changes accompanying dehydration may result in global expansion and rifting and/or global contraction and crumpling of the surface. Key deliverables of this project include the phase diagrams, equations of state and thermal and electrical conductivities of these icy materials under the relevant conditions of pressure and temperature. In addition to this we shall determine the vibrational spectra of these various phases which will allow direct comparison with mission data.