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。这些尘埃有两个来源：火星和木星之间的小行星带，以及彗星。彗星是充满尘埃的冰球，在绕太阳运行时蒸发。每24小时大约有3吨尘埃进入火星大气层；金星的输入量约为46吨。灰尘以超高的速度进入，所以一小部分颗粒加热到足以融化和蒸发。这一大气烧蚀过程将各种金属，如铁、镁和钠注入到火星上80公里处和金星上115公里处，形成金属原子和离子层。虽然地球大气中相应的层已经被研究了几十年，但直到最近4年，人们才首次在另一个行星的大气中观察到金属层：美国宇航局的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到达火星之后，一颗来自奥尔特云的彗星险些与火星相撞。“飞掠”的结果是在短短3小时内注入了大约80吨的尘埃，导致MAVEN观测到大量的金属物质。最初，尘埃只被注入地球的一个半球，由此产生的金属在水平和垂直方向上的扩散速度比预期的要快得多。MAVEN也在一个高度椭圆的轨道上，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“深探”。这产生了一个独特的数据集，它带来了几个惊喜。例如，人们认为金属离子会根据质量分离，而较轻的离子在大气中含量相对较高，但事实显然并非如此。与地球相比，这些金属的大气化学也有令人惊讶的差异：它们的日变化似乎是由大气潮汐而不是光化学控制的；而在火星富含二氧化碳的大气中，中性金属原子的丰度也出乎意料地低。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测结果，其中包括大气烧蚀和基于70多种Na， Fe和Mg物种相关反应的实验室研究的金属化学的完整描述。该模型将用于模拟彗星飞掠。第二个项目将研究这些行星大气中的高空云现象。它们在陆地上的对应物，被称为夜光云，是夏季出现在高纬度地区83公里左右的水冰云。在火星上，云在70到100公里之间被观测到，相反，云主要出现在赤道纬度。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，因为温度几乎没有低到足以使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了一种很有希望的可能性，即流星烧蚀的金属会变成金属碳酸盐，这种碳酸盐会吸收H2O形成“脏冰”颗粒，这些颗粒是二氧化碳冰的有效种子。现在将在实验室进行测试。以金星为例，我们有第一个初步的报告，一层云周期性地出现在大约85 - 90公里处，即远高于覆盖整个行星约70公里的厚硫酸云。在这个项目中，我们将通过实验来验证我们的假设，即这些分离的云是由硫酸液滴吸收H2O并被稀释到足以冻结的结果。在海拔120千米左右的地方，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，我们将模拟这些云的可能可见性，而我们的项目合作伙伴则使用金星快车（Venus Express）宇宙飞船搜索这些云。

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.