Thursday 04 Jun 2015: Seminar: NEMESIS - the retrieval of atmospheric properties from remotely-sensed planetary, exoplanetary and brown dwarf spectra. (THU 4pm)
Prof Patrick Irwin - Oxford University
Physics, 4th floor 16:00-15:00
Telescope and spacecraft observations of the visible-infrared spectra of solar system planets, exoplanets and brown dwarfs can be used to determine the temperature and composition of their atmospheres. However, the problem is fundamentally ill-posed in that since temperature and composition profiles are continuous functions they cannot be uniquely constrained by a single spectrum measured with a finite number of wavelengths. In fact, it is found that there are a range of possible profiles that can fit an observed spectrum (and error estimates) equally well. Early analyses of solar system targets compared observed spectra with a library of model spectra calculated with different temperatures/abundance assumptions and the closet matching case taken to be the solution. However, as computing power has increased it has become possible to use 'retrieval' models that compare the measured spectrum with one calculated from model atmosphere and then iteratively adjust the model to minimise the difference between the measured and modelled spectra. NEMESIS is just such a retrieval model that was initially developed by the Oxford Planetary Group to model Cassini/CIRS observations of Saturn and Titan, but has since been applied to observations of every planet in our solar system with an atmosphere. With increasing interest (and better measurements) of exoplanets and brown dwarfs, NEMESIS and the retrieval models of several other international groups have been extended to model exoplanetary transit spectra and brown dwarf spectra to better characterise their atmospheres.
In this talk I will describe the NEMESIS model and its application to several solar system planets. I will then go on to describe its extension to brown dwarfs and exoplanets and discuss the development of this area and the unique challenges it poses.