Day 4/5, Session 2: Dust, UV, Reflection Spectra, Polarisation

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Christiane Helling (University of St Andrews) gave us a talk about the challenges involved in modelling the formation and destruction of solid airborne particles in planetary atmospheres. Her approach is to consider the process from the bottom up, with tiny solid ‘seed’ particles made of materials like TiO2 nucleating larger dust particles that grow through solid-gas molecule collisions.  From what I could gather, our understanding of these microphysical processes is almost totally reliant upon numerical simulations, which are tough to do. Nonetheless, it sounds like some good progress is being made. These airborne particles are significant sinks of various chemical species and they also have a big effect on the wavelength-dependent opacities of the atmosphere.

Jeffrey Linsky (University of Colorado) reminded us just how important UV radiation from a stellar host is in driving photochemistry and mass loss in the upper layers of planetary atmospheres. Jeffrey highlighted the fact that most of the UV radiation a planetary atmosphere receives is concentrated in the Lyman-alpha emission line.

Ruth Murray-Clay  (Harvard CfA) described in more detail a way in which UV radiation a hot Jupiter atmosphere can result in a hydrodynamical escape of hydrogen atoms in the atmosphere. I think the basic idea is that gas in the heated layers accelerates outwards as a result of thermal expansion into regions of the atmosphere with lower density, whereupon it can be lost to space if its speed exceeds a supersonic threshold. This is essentially analogous to the Parker wind mechanism for the solar wind.

Yuka Fujii (University of Tokyo) then gave a fascinating talk on what we could conceivably learn one day by monitoring the reflection spectrum of a nearby (~10pc) terrestrial planet over periods of days, months and even years. By carefully accounting for geometric effects, it would in theory be possible to reconstruct a broad brush map of the surface of the planet in different wavelengths. This might allow us to map out features such as clouds, oceans and continents, and even things like desert versus vegetation coverage on the latter. For instance, Yuka showed us the method applied to some remote-sensing Earth data, and by taking the difference between the brightness maps in two wavelengths bracketing the famous red edge at ~700nm, the densely forested regions of South America and sub-Saharan Africa were clearly identifiable. Using this technique to monitor cloud cover,  seasonal variations of ice and vegetation cover etc could reveal lots of information about the planetary atmosphere and its dynamics. I find ambitious concepts like this really exciting, although I suspect we’ve got quite a few years to wait before it becomes a reality…

In the final talk of the day, Enric Palle (Instituto de Astrofisica de Canarias) guided us through some increasingly information-rich measurements of exoplanet atmospheres that can be made, starting at single band-pass photometry and ending at spectropolarimetry. There were lots of interesting things in this talk, not least of which was how Enric and his team had managed to successfully distinguish between the North American and Asian continents and the relatively dark waters of the Atlantic and Pacific Oceans using Earth shine observations.

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About Author

I'm a PhD student at the University of Oxford. My work focuses on transiting exoplanets and, in particular, what we can learn about the atmospheres of these systems. A large part of this involves getting a better handle on the various instrumental systematics that contaminate the small signals we're trying to measure, and devising methods to remove them from the data. I'm also investigating ways of correcting for the effect of star spots on planetary transmission and emission spectroscopy measurements. My supervisor is Suzanne Aigrain.