Prof. Patrick Irwin

Abstract:

Inspection of near-infrared images from Cassini's Imaging Science Subsystem and Visual and Infrared Mapping Spectrometer have revealed a new feature in Titan's haze structure: a narrow band of increased scattering by haze south of the equator. The band seems to indicate a region of very limited mixing in the lower stratosphere, which causes haze particles to be trapped there. This could explain the sharp separation between the two hemispheres, known as the north-south asymmetry, seen in images. The separation of the two hemispheres can also be seen in the stratosphere above 150 km using infrared spectra measured by Cassini's Composite Infrared Spectrometer. Titan's behaviour in the lower tropical stratosphere is remarkably similar to that of the Earth's tropical stratosphere, which hints at possible common dynamical processes. © 2009 Elsevier Inc. All rights reserved.

Abstract:

Five years of Cassini CIRS infrared spectra have been used to determine the tilt of Titan's stratospheric symmetry axis with respect to the solid body rotation axis. Measurements of HCN abundance centred around 5 mbar (125 km altitude) at equatorial latitudes show the symmetry axis is tilted by 4.0 ± 1 . 5{ring operator} in a direction 70 ± 40{ring operator} W of the sub-solar point. This value is consistent with tilts determined from temperature and haze measurements by Achterberg et al. (2008a) and Roman et al. (2009). The consistency of results from three independent methods suggests that Titan's entire stratosphere is tilted and provides a powerful constraint on the underlying atmospheric dynamics. © 2010 Elsevier Ltd. All rights reserved.

Abstract:

The Rosetta spacecraft accomplished a flyby of Mars on its way to 67P/Churyumov-Gerasimenko on 25 February 2007. In this paper we describe the measurements obtained by the M channel of the Visual and Infrared Thermal Imaging Spectrometer (VIRTIS-M) and the first scientific results derived from their analysis. The broad spectral coverage of the VIRTIS-M in the IR permitted the study of various phenomena occurring in the Martian atmosphere; observations were further exploited to achieve accurate absolute radiometric calibration. Nighttime data from the VIRTIS-M constrain the air temperature profile in the lower atmosphere (5-30 km), using variations in CO2 opacity at 4.3 mm. A comparison of this data with the global circulation model (GCM) by Forget et al. (1999) shows a trend of slightly higher air temperature in the VIRTIS-M retrievals; this is accompanied by the presence of moderate decreases (∼5 K) in large sections of the equatorial region. This is potentially related to the occurrence of water ice cl uds. Daytime data from the VIRTIS-M reveal CO 2 non-local thermodynamic equilibrium emission in the high atmosphere. A mapping of emission intensity confirms its strict dependence on solar zenith angle. Additionally, devoted limb observations allowed the retrieval of vertical emission intensity profiles, indicating a peak around 105 km in southern tropical regions. Ozone content can be effectively monitored by the emission of O2 (a1Δg) at 1.27 μm. Retrieved emission intensity shows that polar regions are particularly rich in ozone. Aerosol scattering was observed in the 1-2.5 μm region above the night region above the night disk, suggesting the occurrence of very high noctilucent clouds. Copyright 2010 by the American Geophysical Union.

Abstract:

Long-term (2004-2009) on-orbit observations by Cassini Composite Infrared Spectrometer are analyzed to precisely measure Saturn's emitted power and its meridional distribution. Our evaluations suggest that the average global emitted power is 4.952 ± 0.035 W m-2 during the period of 2004-2009. The corresponding effective temperature is 96.67 ± 0.17 K. The emitted power is 16.6% higher in the Southern Hemisphere than in the Northern Hemisphere. From 2005 to 2009, the global mean emitted power and effective temperature decreased by ∼2% and ∼0.5%, respectively. Our study further reveals the interannual variability of emitted power and effective temperature between the epoch of Voyager (∼1 Saturn year ago) and the current epoch of Cassini, suggesting changes in the cloud opacity from year to year on Saturn. The seasonal and interannual variability of emitted power implies that the energy balance and internal heat are also varying. Copyright © 2010 by the American Geophysical Union.