Prof. Patrick Irwin

Abstract:

Acetylene (C2H2) and ethane (C2H6) are by-products of complex photochemistry in the stratosphere of Saturn. Both hydrocarbons are important to the thermal balance of Saturn's stratosphere and serve as tracers of vertical motion in the lower stratosphere. Earlier studies of Saturn's hydrocarbons using Cassini-CIRS observations have provided only a snapshot of their behaviour. Following the vernal equinox in August 2009, Saturn's northern and southern hemispheres have entered spring and autumn, respectively, however the response of Saturn's hydrocarbons to this seasonal shift remains to be determined. In this paper, we investigate how the thermal structure and concentrations of acetylene and ethane have evolved with the changing season on Saturn. We retrieve the vertical temperature profiles and acetylene and ethane volume mixing ratios from δν̃=15.5cm-1 Cassini-CIRS observations. In comparing 2005 (solar longitude, Ls~308°), 2009 (Ls~3°) and 2010 (Ls~15°) results, we observe the disappearance of Saturn's warm southern polar hood with cooling of up to 17.1K±0.8K at 1.1mbar at high-southern latitudes. Comparison of the derived temperature trend in this region with a radiative climate model (Section 4 of Fletcher et al., 2010 and Greathouse et al. (2013, in preparation)) indicates that this cooling is radiative although dynamical changes in this region cannot be ruled out. We observe a21±12% enrichment of acetylene and a 29±11% enrichment of ethane at 25°N from 2005 to 2009, suggesting downwelling at this latitude. At 15°S, both acetylene and ethane exhibit a decrease in concentration of 6±11% and 17±9% from 2005 to 2010, respectively, which suggests upwelling at this latitude (though a statistically significant change is only exhibited by ethane). These implied vertical motions at 15°S and 25°N are consistent with a recently-developed global circulation model of Saturn's tropopause and stratosphere(Friedson and Moses, 2012), which predicts this pattern of upwelling and downwelling as a result of a seasonally-reversing Hadley circulation. Ethane exhibits a general enrichment at mid-northern latitudes from 2005 to 2009. As the northern hemisphere approaches summer solstice in 2017, this feature might indicate an onset of a meridional enrichment of ethane, as has been observed in the southern hemisphere during/after southern summer solstice. © 2013 Elsevier Inc.

Abstract:

We have searched for the presence of simple P and S-bearing molecules in Titan's atmosphere, by looking for the characteristic signatures of phosphine and hydrogen sulfide in infrared spectra obtained by Cassini CIRS. As a result we have placed the first upper limits on the stratospheric abundances, which are 1ppb (PH3) and 330ppb (H2S), at the 2-σ significance level. © 2013.

Abstract:

The Uranian atmosphere was observed in August 2009 from 0.8 to 1.8. μm using the near-infrared spectrometer, SpeX, at NASA's Infrared Telescope Facility. The observations had a spectral resolution of R=. 1200 and an average seeing of between 0.5' in the H-Band (1.4-1.8. μm) and 0.6' in the I-Band (0.8-0.9. μm). The reduced data were analyzed with a multiple-scattering retrieval code. We were able to reproduce observations when using a vertically-compact cloud in the upper troposphere and a vertically-extended, optically-thin haze above the 1-bar level. The existence of these two clouds is consistent with previous studies.The sub-micron portion of the data are most sensitive to very small scattering particles, allowing more insight into particle size than other portions of the infrared spectrum. This portion of the spectrum was therefore of particular interest and was not available in most previous studies of the planet. We assumed the particles in both clouds to be relatively strong forward scatterers (with a Henyey-Greenstein asymmetry factor of g=. 0.7). Given this assumption, we found single-scattering albedos in the tropospheric cloud particles to be ω̄=0.7 at wavelengths above 1.4. μm and to gradually increase to ω̄=1.0 at wavelengths shortward of 1.0. μm. In the upper haze, we found single-scattering albedos to be ω̄=1.0 with the exception of a narrow drop at 1.0. μm to ω̄=0.6. We found a preference for upper haze particle radii at r=. 0.10. μm. Retrievals of base pressure, fractional scale height, and optical depth in both cloud layers showed the best agreement with data when the base pressure of the upper haze was fixed just above the tropospheric clouds, rather than at or above the tropopausal cold trap. We found that these same retrievals strongly preferred tropospheric cloud particles of 1.35-μm radii, and observed cloud top height to increase away from the equator in the case of latitudinally invariant methane abundance.Latitudinal methane variability was also considered, both through a reflectivity study at the 825-nm collision-induced hydrogen absorption feature, as well as through radiative transfer analysis, using forward modeling and retrievals of cloud properties and methane abundance. The data suggested that methane abundance above the tropospheric clouds increased when moving from the midlatitudes towards the equator by at least 9%. The peak of this equatorial methane enrichment was determined to be at 4. ±. 2° S latitude, having moved nearly 15° northward since a reflectance study of 2002 data (Karkoschka and Tomasko, 2009). © 2013 Elsevier Inc.