Prof. Patrick Irwin

Abstract:

The Voyager 1 flyby of Titan in 1980 gave a first glimpse of the chemical complexity of Titan's atmosphere, detecting many new molecules with the infrared interferometer spectrometer (IRIS). These included propane(C3H 8) and propyne (CH3C2H), while the intermediate-sized C3Hx hydrocarbon (C3H 6) was curiously absent. Using spectra from the Composite Infrared Spectrometer on Cassini, we show the first positive detection of propene (C 3H6) in Titan's stratosphere (5σ significance), finally filling the three-decade gap in the chemical sequence. We retrieve a vertical abundance profile from 100-250 km, that varies slowly with altitude from 2.0 ± 0.8 ppbv at 125 km, to 4.6 ± 1.5 ppbv at 200 km. The abundance of C3H6 is less than both C3H 8 and CH3C2H, and we remark on an emerging paradigm in Titan's hydrocarbon abundances whereby alkanes > alkynes > alkenes within the C2Hx and C3Hx chemical families in the lower stratosphere. More generally, there appears to be much greater ubiquity and relative abundance of triple-bonded species than double-bonded, likely due to the greater resistance of triple bonds to photolysis and chemical attack. © 2013. The American Astronomical Society. All rights reserved.

Abstract:

Previous studies have demonstrated an external source of CO on Jupiter, Saturn, and Neptune. However, it has not been possible to demonstrate this on Uranus because of its low CO abundance, low upper-tropospheric temperatures, and low stratospheric thermal gradient, which make detection very challenging. Here we use 17 Herschel/SPIRE observation sequences spanning 3 yr (2009-2012), which cover 14.6-51.8 cm-1 with a combined integration time of 5 hr. These spectra were originally taken for routine calibration purposes, so were corrected for continuum offsets prior to analysis. The final stacked spectra had an extremely low noise level of 10-50 pW cm-2 sr-1/ cm-1. Despite this, CO was not observed, but we were able to obtain stringent 3σ upper limits at the 0.1-0.2 bar level of 2.1 ppb for a uniform profile, and 9.4 ppb for a stratosphere-only profile - an order of magnitude improvement over previous studies. Comparison with observed CO fluorescence by Encrenaz et al. suggests the majority of Uranus' stratospheric CO has an external origin. It thus appears that external supply of oxygen species - via comets, micrometeorites, or dust - is an important process on all giant planets in our solar system. © 2013. The American Astronomical Society. All rights reserved.