A Programme of Research in Planetary Science at Leeds

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.

The proposed research will be of societal and economic benefit in several ways:

1. Educational benefits to the wider public. Planetary exploration - particularly of Mars - is currently of great public interest. The applicants are all aware of the potential of planetary science to excite and enthuse students of all ages, while also providing opportunities to present fundamental aspects of chemistry with clear and interesting applications. The applicants have a strong track record in public outreach e.g. through public lectures and school demonstrations.

2. Space agencies, aerospace companies and satellite operators. Drag affects satellite attitude control, orbit decay and the tracking of potentially hazardous space debris. Large changes in atmospheric density and winds, caused by space weather or gravity waves - can induce significant local variations of drag forces, leading to increased orbit decay and also affecting atmospheric entry - a notorious challenge for spacecraft entering the Martian atmosphere. These issues can be addressed with properly validated whole atmosphere models. The first project in this consolidated grant proposal involves using a global model to study dynamics and transport of metallic ions in the Martian ionosphere. The model simulations will be tested against observations of ion profiles during "deep dip" orbits by NASA's MAVEN spacecraft. Nothing comparable exists for the terrestrial atmosphere (most sub-orbital rocket flights with mass spectrometers only extend to around 110 km). The project will therefore provide a valuable test-bed for informing terrestrial whole atmosphere models, as well as models for Mars.

3. Radio transmission. The metallic ions produced by meteoric ablation are the major constituents of sporadic E layers. These layers have a significant effect on ground-to-satellite and over-the-horizon radio transmissions, which is important to many industrial and governmental organizations. Observations by the MAVEN spacecraft of metallic ions in the Martian thermosphere show that the transport of metallic ions confounds expectation. This probably impacts on our understanding of the processes which control the distribution of metallic ions in the terrestrial thermosphere, and hence predictions of sporadic E occurrence.

4. Numerical weather forecasting organizations (e.g. the UK Met Office) are starting to extend their operational forecast and climate models into the thermosphere. One reason is that a well-constrained mesosphere is now considered to be an important element of climate modelling due to the impact of middle atmospheric chemistry. There is also increasing evidence for an improved accuracy of weather forecasting, particularly if mesospheric data from satellites is assimilated. Another reason for developing high-top models is for space weather prediction, since there is a clear impact in the thermosphere of upward-propagating waves from the lower atmosphere. There is an obvious benefit to comparing whole atmosphere models for Mars and Earth, since these planets have similar lengths of day and rotational axis obliquities.

5. Commercial exploitation of laboratory science. The group has a strong track record of taking fundamental discoveries - of the kind that could emerge from the second proposed project on the nucleation of ice particles - and redirecting them to commercial applications. Two of the applicants have filed three patents recently related to cryopreservation and automobile exhaust catalysis.

6. Kinetic modelling software development. Chemical reactions are sometimes difficult to study experimentally, particularly in temperature and pressure regimes appropriate to certain applications (e.g. combustion or plasma processing). For this reason, a robust theoretical methodology - of the kind which will be developed in the first project - is essential for estimating rate coefficients which can then be used to build chemical networks.