Another meaty scientific lecture for members of Wycombe Astronomical Society, this time from Professor Patrick Irwin from Oxford University. He spoke authoritatively about the exploration of other planets’ atmospheres since, amongst other things, he was directly involved with sounding out Saturn’s atmosphere by the Cassini Mission.
Prof Irwin described the process of “atmospheric sounding” using infra-red detectors. Atmospheric components (for example carbon monoxide, methane, nitrous oxide, ozone, carbon dioxide, water) absorb infrared light differentially, giving rise to peaks in a spectrum that permit not only the identification of the components of the atmosphere, but also their temperatures.
He was directly involved with the design and construction of the radiative cooler of the
Composite Infrared Spectrometer (CIRS) on board the Cassini spacecraft. He presented results from this element of Cassini’s mission showing extraordinary detail in Saturn’s and Titan’s atmospheres. But actually working out the atmospheric properties from thousands of spectra obtained by CIRS was not straightforward – the so-called “Retrieval Problem”. Prof Irwin briefly described the use of NEMESIS (Non-linear Optimal Estimator for MultivariatE Spectral AnalySIS), which is a general purpose combined radiative transfer and retrieval tool. It has two main components: firstly, a forward model which, given an assumed atmospheric structure, calculates a synthetic spectrum for comparison with that measured, and secondly an inversion retrieval model, which compares the measured and modelled spectra and then adjusts the atmospheric parameters in such a way as to minimise any discrepancy.
We also learned about ground observations of the atmospheres of Jupiter, Saturn, Uranus and Neptune. Carbon dioxide and water in the Earth’s atmosphere complicate matters here, and reasons for placing telescopes in high places like Chile and Hawaii to minimise this were explained. The use of adaptive optics on the instruments at these locations further improves images, producing yet clearer images for study. Even the use of Lucky Imaging, popular with amateurs for lunar and planetary imaging, is now also being used by the professionals. By way of illustration, it was widely believed that the atmosphere of Uranus was featureless, following the fly-by of Voyager 2 in 1986. But it’s not true! It was just quiet then when Voyager 2 flew by! Now from land-based imaging we can see many features of Uranus’s atmosphere – for example it contains hydrogen sulphide.
Finally, Prof Irwin touched on the advances being made in the search for exoplanets and, more relevan
tly to this lecture, the detection of the presence and composition of their atmospheres. This work involves studying images in the visible part of the spectrum (red, green and blue) which is beginning to reveal information about these distant worlds. The launch of the James Webb Space Telescope later this year is eagerly anticipated and will revolutionise exoplanetary spectroscopy.
Sandy