Prof. Patrick Irwin

Abstract:

We report interferometric observations of carbon monoxide (CO) and its isotopologues in Titan's atmosphere using the Atacama Large Millimeter/submillimeter Array (ALMA). The following transitions were detected: CO (J = 1-0, 2-1, 3-2, 6-5), 13CO (J = 2-1, 3-2, 6-5), C18O (J = 2-1, 3-2), and C17O (J = 3-2). Molecular abundances and the vertical atmospheric temperature profile were derived by modeling the observed emission line profiles using NEMESIS, a line-by-line radiative transfer code. We present the first spectroscopic detection of 17O in the outer solar system with C17O detected at >8σ confidence. The abundance of CO was determined to be 49.6 ± 1.8 ppm, assumed to be constant with altitude, with isotopic ratios 12C/13C = 89.9 ± 3.4, 16O/18O = 486 ± 22, and 16O/17O = 2917 ± 359. The measurements of 12C/13C and 16O/18O ratios are the most precise values obtained in Titan's atmospheric CO to date. Our results are in good agreement with previous studies and suggest no significant deviations from standard terrestrial isotopic ratios.

Abstract:

The climate on Earth is generally determined by the amount and distribution of incoming solar radiation, which must be balanced in equilibrium by the emission of thermal radiation from the surface and atmosphere. The precise routes by which incoming energy is transferred from the surface and within the atmosphere and back out to space, however, are important features that characterize the current climate. This has been analysed in the past by several groups over the years,based on combinations of numerical model simulations and direct observations of theEarths climate system. The results are often presented in schematic form to show the main routes for the transfer of energy into, out of and within the climate system. Although relatively simple in concept, such diagrams convey a great deal of information about the climate system in a compact form. Such an approach has not so far been widely adopted in any systematic way for other planets of the Solar System, let alone beyond, although quite detailed climate models of several planets are now available, constrained bymany new observations and measurements. Here we present an analysis of the global transfers of energy within the climate systems of a range of planets within the Solar System,including Mars, Titan, Venus a nd Jupit er, a s mo delled by rela t ively co mprehens iveradiative transfer and (in some cases) numerical circulation models. These results are presented in schematic form for comparison with the classical global energy budget analyses (e.g.Trenberth et al. 2009; Stephenset al.2012; Wildet al.2013; IPCC 2013)for the Earth, highlighting important similarities and differences. We also take the first steps towards extending this approach to other Solar System and extra-solar planets,including Mars, Venus, Titan, Jupiter and the ‘hot Jupiter’ exoplanet HD189733b, presenting a synthesis of `both previously published and new calculations for all of these planets.

Abstract:

In its decade of operation the Cassini mission has allowed us to look deep into Saturn’s atmosphere and investigate the processes occurring below its enshrouding haze. We use Visual and Infrared Mapping Spectrometer (VIMS) 4.6—5.2 µm data from early in the mission to investigate the location and properties of Saturn’s cloud structure between 0.6 and 5 bars. We average nightside spectra from 2006 over latitude circles and model the spectral limb darkening using the NEMESIS radiative transfer and retrieval tool. We present our best-fit deep cloud model for latitudes −40◦ < λ < 50◦ , along with retrieved abundances for NH3, PH3 and AsH3. We find an increase in NH3 abundance at the equator, a cloud base at ∼2.3 bar and no evidence for cloud particles with strong absorption features in the 4.6—5.2 µm wavelength range, all of which are consistent with previous work. Non-scattering cloud models assuming a composition of either NH3 or NH4SH, with a scattering haze overlying, fit limb darkening curves and spectra at all latitudes well; the retrieved optical depth for the tropospheric haze is decreased in the northern (winter) hemisphere, implying that the haze has a photochemical origin. Our ability to test this hypothesis by examining spectra at different seasons is restricted by the varying geometry of VIMS observations over the life of the mission, and the appearance of the Saturn storm towards the end of 2010.