A Programme of Research in Planetary Science at Leeds

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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.