A Programme of Research in Planetary Science at Leeds

这里提出的两个项目将调查火星和金星的高层大气，重点是宇宙尘埃粒子的影响。尘埃来自两个来源：火星和木星之间的小行星带和彗星，彗星是充满尘埃的冰球，它们在绕太阳运行时蒸发。每24小时约有3吨尘埃进入火星大气层；进入金星的尘埃约为46吨。尘埃以超热的速度进入，因此一部分颗粒的热量足以融化和蒸发。在火星上约80公里处和金星上115公里处，这种陨石烧蚀过程注入了铁、镁和钠等多种金属，形成了金属原子层和离子层。尽管对地球大气层中相应的金属层的研究已经有几十年了，但直到过去4年才首次在另一个行星大气中观察到金属层：NASA的MAVEN航天器于2014年底抵达火星，并测量了一系列金属离子和原子。此外，就在MAVEN到达火星后，奥尔特云的一颗彗星险些与该行星相撞。“飞越”的结果是在短短3个小时内注入了大约80吨尘埃，导致MAVEN观测到了大量的金属物种。起初，尘埃只注入了地球的一个半球，由此产生的金属在水平和垂直方向上的扩散速度都比预期的要快得多。MAVEN也处于一个高度椭圆形的轨道上，当它接近火星时，它可以在距离火星表面仅120公里的地方进行一次“深度俯冲”。这产生了一个独特的数据集，它带来了几个惊喜。例如，预计金属离子将根据质量分离，较轻的离子在大气中的较高位置相对丰富--但显然情况并非如此。与地球相比，这些金属的大气化学也有惊人的不同：它们的日变化似乎由大气潮汐而不是光化学控制；中性金属原子在火星富含二氧化碳的大气中以出人意料的低丰度出现。在第一个项目中，我们将通过火星的全球大气模型来探索这些观察结果，其中包括流星烧蚀和基于对Na、Fe和Mg物种70多个相关反应的实验室研究得出的金属化学的完整描述。该模型随后将用于模拟彗星掠过。第二个项目将研究这些行星大气中的高空云现象。它们的地面对应云被称为夜光云，是夏季出现在高纬度约83公里处的H2O冰云。在火星上，观测到的云层在70到100公里之间，相比之下，大部分出现在赤道纬度。此外，它们是二氧化碳冰云。一个主要的挑战是解释这些云是如何形成的，因为温度几乎从来没有低到足以让二氧化碳凝结在金属硅酸盐颗粒上。然而，申请者在最近的一篇论文中提出的一个有希望的候选方案是，陨石侵蚀的金属转化为金属碳酸盐，吸收H2O形成“脏冰”颗粒，这些是二氧化碳冰的有效种子。这将在实验室进行测试。在金星的例子中，我们首次初步报告了在85-90公里处周期性地出现一层云层，即远远高于笼罩整个地球约70公里的厚厚的硫酸云层。在这个项目中，我们将从实验上检验我们的假设，即这些分离的云是由于硫酸液滴吸收H2O并变得足够稀薄而冻结所致。再高一些，大约120公里，温度下降到足以形成二氧化碳冰云的程度，就像在火星上一样。因此，我们将模拟这些云层的可能可见度，同时我们的项目合作伙伴将使用金星快车航天器搜索这些云层。

The two projects proposed here will investigate the upper atmospheres of Mars and Venus, with a focus on the impacts of cosmic dust particles. The dust originates from two sources: the asteroid belt between Mars and Jupiter, and comets which are dust-laden balls of ice that evaporate as they orbit towards the sun. Around 3 tonnes of dust enters Mars' atmosphere every 24 hrs; the input for Venus is around 46 tonnes. The dust enters at hyperthermal speeds, and so a fraction of the particles heat sufficiently to melt and evaporate. This process of meteoric ablation injects a variety of metals such as Fe, Mg and Na at around 80 km on Mars and 115 km on Venus, giving rise to layers of metal atoms and ions.Although the corresponding layers in the Earth's atmosphere have been studied for decades, it is only in the past 4 years that metals layers were first observed in another planetary atmosphere: NASA's MAVEN spacecraft arrived at Mars in late 2014 and has measured a range of metallic ions and atoms. Moreover, just after MAVEN reached Mars, a comet from the Oort Cloud narrowly avoided colliding with the planet. The result of the "fly-by" was the injection of around 80 tonnes of dust in only 3 hours, leading to huge quantities of metallic species being observed by MAVEN. Initially the dust was injected in only one hemisphere of the planet, and the resulting metals spread much more rapidly - both horizontally and vertically - than expected.MAVEN is also in a highly elliptical orbit, and as it approaches Mars it can make a "deep dip" down as far as only 120 km above the surface. This has produced a unique dataset which throws up several surprises. For example, the metallic ions would be expected to separate according to mass with the lighter ions being relatively more abundant higher in the atmosphere - but this is clearly not the case. There are also surprising differences in the atmospheric chemistry of these metals compared with the Earth: their diurnal variation seems to be controlled by atmospheric tides rather than photochemistry; and neutral metal atoms occur at unexpectedly low abundances in the CO2-rich atmosphere of Mars.In the first project we will explore these observations through a global atmospheric model of Mars which includes meteoric ablation and full descriptions of the metal chemistry based on laboratory studies of over 70 relevant reactions of Na, Fe and Mg species. The model will then be used to simulate the cometary flyby.The second project will examine the phenomenon of high-lying clouds in the atmospheres of these planets. Their terrestrial counterparts, known as noctilucent clouds, are H2O-ice clouds which occur around 83 km at high latitudes in summer. On Mars, clouds have been observed between 70 and 100 km, and in contrast mostly occur at equatorial latitudes. Furthermore, they are CO2-ice clouds. A major challenge has been to explain how these clouds can form, since the temperatures are hardly ever low enough for CO2 to condense on metal silicate particles. However, a promising candidate - proposed in a recent paper by the applicants - is that the meteor-ablated metals turn into metal carbonates, which absorb H2O to form "dirty ice" particles and these are effective seeds for CO2 ice. This will now be tested in the laboratory.In the case of Venus, we have the first tentative reports of a layer of clouds periodically appearing around 85 - 90 km i.e. well above the thick sulphuric acid clouds which envelop the entire planet up to around 70 km. In this project we will test experimentally our hypothesis that these detached clouds result from sulphuric acid droplets absorbing H2O and becoming dilute enough to freeze. Higher still around 120 km, the temperature falls far enough for CO2 ice clouds to form in the same way as on Mars. We will therefore model the likely visibility of these clouds, while our project partners search for the clouds using the Venus Express spacecraft.