Prof. Patrick Irwin

Abstract:

In this paper we describe the first quantitative search for several molecules in Titan's stratosphere in Cassini CIRS infrared spectra. These are: ammonia (NH3), methanol (CH3OH), formaldehyde (H2CO), and acetonitrile (CH3CN), all of which are predicted by photochemical models but only the last of which has been observed, and not in the infrared. We find non-detections in all cases, but derive upper limits on the abundances from low-noise observations at 25 degrees S and 75 degrees N. Comparing these constraints to model predictions, we conclude that CIRS is highly unlikely to see NH3 or CH3OH emissions. However, CH3CN and H2CO are closer to CIRS detectability, and we suggest ways in which the sensitivity threshold may be lowered towards this goal.

Abstract:

Fine scale layering of haze and composition in Titan's stratosphere and mesosphere was investigated using visible/UV images from Cassini's Imaging Science Sub-system (ISS) and IR spectra from Cassini's Composite Infra-Red Spectrometer (CIRS). Both ISS and CIRS independently show fine layered structures in haze and composition, respectively, in the 150-450 km altitude range with a preferred vertical wavelength of around 50 km. Layers are most pronounced around the north polar winter vortex, although some weaker layers do exist at more southerly latitudes. The amplitude of composition layers in each trace gas profile is proportional to the relative enrichment of that species in the winter polar vortex compared to equatorial latitudes. As enrichment is caused by polar subsidence, this suggests a dynamical origin. We propose that the polar layers are caused by cross-latitude advection across the vortex boundary. This is analogous to processes that lead to ozone laminae formation around Earth's polar vortices. © 2009 Elsevier Inc. All rights reserved.

Abstract:

Although propane gas (C3 H8) was first detected in the stratosphere of Titan by the Voyager IRIS infrared spectrometer in 1980, obtaining an accurate measurement of its abundance has proved difficult. All existing measurements have been made by modeling the ν26 band at 748 cm- 1: however, different analyzes over time have yielded quite different results, and it also suffers from confusion with the strong nearby ν5 band of acetylene. In this paper we select large spectral averages of data from the Cassini Composite Infrared Spectrometer (CIRS) obtained in limb-viewing mode at low latitudes (30{ring operator}S-30{ring operator}N), greatly increasing the path length and hence signal-to-noise ratio for optically thin trace species such as propane. By modeling and subtracting the emissions of other gas species, we demonstrate that at least six infrared bands of propane are detected by CIRS, including two not previously identified in Titan spectra. Using a new linelist for the range 1300-1400cm- 1, along with an existing GEISA list, we retrieve propane abundances from two bands at 748 and 1376cm- 1 . At 748cm- 1 we retrieve 4.2 ± 0.5 × 10- 7 (1 - σ error) at 2 mbar, in good agreement with previous studies, although lack of hotbands in the present spectral atlas remains a problem. We also determine 5.7 ± 0.8 × 10- 7 at 2 mbar from the 1376cm- 1 band - a value that is probably affected by systematic errors including continuum gradients due to haze and also an imperfect model of the ν6 band of ethane. This study clearly shows for the first time the ubiquity of propane's emission bands across the thermal infrared spectrum of Titan, and points to an urgent need for further laboratory spectroscopy work, both to provide the line positions and intensities needed to model these bands, and also to further characterize haze spectral opacity. The present lack of accurate modeling capability for propane is an impediment not only for the measurement of propane itself, but also for the search for the emissions of new molecules in many spectral regions. © 2009 Elsevier Ltd.