Structure and dynamics of small planets and moons

这项拟议的研究旨在了解较小行星的内部结构和演化，包括冰行星(例如木星卫星)和岩石行星(例如水星)。同样的卫星：围绕气态巨行星运行的是许多冰冷的卫星，它们的大小从10公里到2500公里不等，比水星还大。三颗最大的冰冻卫星是木卫三和木卫四(围绕木星运行)，以及土卫六(围绕土星运行)；它们的半径和体积密度相似，但经历了截然不同的地质历史。卡利斯托似乎根本没有进化，它的内部是近乎均匀的岩石和冰的混合物，地表地质由撞击坑主导。木卫三的金属、岩石和冰的成分已经分离出来，形成了一个铁核、一个岩石地幔和一个厚厚的冰壳，它撕裂了地壳，导致了液态水的喷发--相当于地球上的火山岩浆。土卫六也经历了内部分离，形成了一个被厚厚的冰壳覆盖的致密核心。与木卫三不同，土卫六可能仍然活跃。最引人注目的发现是，所有这些大型冰体在10-200公里厚的冰壳下都有全球性的液态水海洋。这些海洋可能是我们太阳系外部的外星生命的栖息地。为了理解为什么大小和组成相似的冰体会过着如此不同的生活，我们必须构建内部结构和热流的数学模型。这个模型依赖于冰层如何将热量从核心输送到表面的知识。在冰体内部的高压下，水-冰以几种不同的结晶形式存在，每一种都具有非常不同的热物理性质。此外，可能存在丰富的富含水的各种分子的水合物，如氨，以及许多可溶的硫酸盐。这些化合物的导热系数通常比水冰小；就像厚厚的冬季被子可以让你在床上取暖一样，低导热系数的行星壳将使地球内部比其他情况下温暖得多，从而使地下海洋在整个地质历史中保持液态。对于大多数冰和水合物，在相应的压力和温度下，我们构建准确模型所需的物理性质是未知的。在这个项目中，我们将测量热膨胀、导热系数和比热容等性质，并用计算机模拟来支持我们的测量。然后我们将把结果合并到我们的行星模型中，从而研究冰冷卫星的内部结构和演化。水星是两个轨道任务的目标，信使号(Current)和贝皮哥伦布(Bepi-Columbo)(2019)，这两个任务都有仪器来研究它的内部结构、组成和磁场。水星的密度只比地球小一点，但比地球小得多，因此水星内部的物质没有地球那么强的压缩。要使水星具有如此高的密度，其核心必须很大(体积占40%，质量占70%)，并且富含铁(约70%铁，约30%硅酸盐)。水星的小体积也表明它一定比地球冷却得更快，因此会有不同的化学和进化史。磁场的存在表明水星有一个熔融区，尽管快速冷却意味着这可能被限制在一个相当薄的壳层中。由于这些差异，支持水星磁场的发电机可能与地球上的发电机有很大的不同。了解水星的内部需要我们构建其内部结构和演化的地球物理模型。要做到这一点，我们必须知道组成其内部的材料的物理性质；这些可以通过基于固态和液态铁合金在高压和高温下的量子力学计算来获得。

The proposed research aims to understand the interior structure and evolution of smaller planetary bodies, both icy (e.g., the Jovian moons) and rocky (e.g., the planet Mercury).Icy moons:Orbiting the gas-giant planets are many icy moons, which vary in size from 10s of kilometres across to >2500 km, larger than the planet Mercury. The three largest icy moons are Ganymede and Callisto (orbiting Jupiter), and Titan (orbiting Saturn); they have similar radii and bulk densities, but have experienced radically different geological histories. Callisto appears not to have evolved at all, its interior is a near uniform mixture of rock and ice, and the surface geology is dominated by impact craters. Ganymede's metal, rock, and ice components have separated out to form an iron core, a rocky mantle, and a thick icy shell, which has rifted the crust and caused the eruption of liquid water - an icy equivalent to Earth's volcanic magma. Titan has also undergone internal segregation to form a dense core coated by a thick icy shell. Unlike Ganymede, Titan may still be active. The most remarkable discovery is that all of these large icy bodies have global oceans of liquid water beneath icy crusts 10-200 km thick. These oceans are possible niches for extraterrestrial life in the outer reaches of our solar system.To understand why icy bodies of otherwise similar size and composition have led such different lives, we must construct mathematical models of the internal structure and heat flow. This modelling relies upon knowledge of how the icy layer transports heat from the core to the surface. Under the high pressures in the interior of an icy body, water-ice exists in several different crystalline forms, each with very different thermo-physical properties. In addition, there are likely to be abundant water-rich hydrates of various molecules, such as ammonia, and many soluble sulfates. These compounds often have a smaller thermal conductivity than water ice; just as a thick winter quilt will keep you warm in bed, a low-thermal-conductivity planetary crust will keep the interior much warmer than it would be otherwise, allowing subsurface oceans to stay liquid throughout geological history. For most of ices and hydrates, the physical properties we need to construct accurate models are not known at relevant pressures and temperatures. In this project we will measure properties such as the thermal expansion, thermal conductivity and specific heat capacity, supporting our measurements with computer simulation. We shall then incorporate the results into our planetary models and thus investigate the internal structure and evolution of the icy moons.MercuryMercury is the target of two orbital missions, MESSENGER (current) and Bepi-Columbo (2019) both of which have instruments on board to study its internal structure, composition and magnetic field. Mercury is only slightly less dense than the Earth but is much smaller and therefore the material within its interior is not as strongly compressed. For Mercury to have such a high density, its core must be large (>40% by volume, <70% by mass) and iron-rich (~70% Fe, ~30% silicate). Mercury's small size also suggests it must have cooled more rapidly than the Earth and therefore will have a distinct chemistry and evolutionary history. The presence of a magnetic field suggests that Mercury has a molten region, although fast cooling means that this may be confined to a rather thin shell. As a result of these differences, it is possible that the dynamo that supports the magnetic field of Mercury differs substantially from the Earth's dynamo. Understanding Mercury's interior requires us to construct geophysical models of its internal structure and evolution. To do this we must know the physical properties of the materials that make up its interior; these can be obtained through calculations based on quantum mechanics for both solid and liquid iron alloys at high pressures and temperatures.