Prof. Patrick Irwin

Abstract:

Current observational data of exoplanets are providing increasing detail of their 3D atmospheric structures. As characterisation efforts expand in scope, the need to develop consistent 3D radiative-transfer methods becomes more pertinent as the complex atmospheric properties of exoplanets are required to be modelled together consistently. We aim to compare the transmission and emission spectra results of a 3D Monte Carlo Radiative Transfer (MCRT) model to contemporary radiative-transfer suites. We perform several benchmarking tests of a MCRT code, Cloudy Monte Carlo Radiative Transfer (CMCRT), to transmission and emission spectra model output. We add flexibility to the model through the use of k-distribution tables as input opacities. We present a hybrid MCRT and ray tracing methodology for the calculation of transmission spectra with a multiple scattering component. CMCRT compares well to the transmission spectra benchmarks at the 10s of ppm level. Emission spectra benchmarks are consistent to within 10% of the 1D models. We suggest that differences in the benchmark results are likely caused by geometric effects between plane-parallel and spherical models. In a practical application, we post-process a cloudy 3DHD 189733b GCM model and compare to available observational data. Our results suggest the core methodology and algorithms of CMCRT produce consistent results to contemporary radiative transfer suites. 3D MCRT methods are highly suitable for detailed post-processing of cloudy and non-cloudy 1D and 3D exoplanet atmosphere simulations in instances where atmospheric inhomogeneities, significant limb effects/geometry or multiple scattering components are important considerations.

Abstract:

Observations of Neptune, made in 2018 using the new Narrow Field Adaptive Optics mode of the Multi Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope (VLT) from 0.48 to 0.93 μm, are analysed here to determine the latitudinal and vertical distribution of cloud opacity and methane abundance in Neptune's observable troposphere (0.1–∼ 3bar). Previous observations at these wavelengths in 2003 by HST/STIS (Karkoschka and Tomasko 2011, Icarus 205, 674–694) found that the mole fraction of methane above the cloud tops (at ∼ 2 bar) varied from ∼ 4% at equatorial latitudes to ∼ 2% at southern polar latitudes, by comparing the observed reflectivity at wavelengths near 825 nm controlled primarily by either methane absorption or H2–H2/H2–He collision-induced absorption. We find a similar variation in cloud-top methane abundance in 2018, which suggests that this depletion of methane towards Neptune's pole is potentially a long-lived feature, indicative of long-term upwelling at mid-equatorial latitudes and subsidence near the poles. By analysing these MUSE observations along the central meridian with a retrieval model, we demonstrate that a broad boundary between the nominal and depleted methane abundances occurs at between 20 and 40°S. We also find a small depletion of methane near the equator, perhaps indicating subsidence there, and a local enhancement near 60–70°S, which we suggest may be associated with South Polar Features (SPFs) seen in Neptune's atmosphere at these latitudes. Finally, by the use of both a reflectivity analysis and a principal component analysis, we demonstrate that this depletion of methane towards the pole is apparent at all locations on Neptune's disc, and not just along its central meridian.

Abstract:

Enhanced mid-infrared emission from CH4 and other stratospheric hydrocarbons has been observed coincident with Jupiter’s ultraviolet auroral emission1,2,3. This suggests that auroral processes and the neutral stratosphere of Jupiter are coupled; however, the exact nature of this coupling is unknown. Here we present a time series of Subaru-COMICS images of Jupiter measured at a wavelength of 7.80 μm on 11–14 January, 4–5 February and 17–20 May 2017. These data show that both the morphology and magnitude of the auroral CH4 emission vary on daily timescales in relation to external solar-wind conditions. The southern auroral CH4 emission increased in brightness temperature by about 3.8 K between 15:50 UT, 11 January and 12:57 UT, 12 January, during a predicted solar-wind compression. During the same compression, the northern auroral emission exhibited a duskside brightening, which mimics the morphology observed in the ultraviolet auroral emission during periods of enhanced solar-wind pressure4,5. These results suggest that changes in external solar-wind conditions perturb the Jovian magnetosphere in such a way that energetic particles are accelerated into the planet’s atmosphere, deposit their energy as deep as the neutral stratosphere, and modify the thermal structure, the abundance of CH4 or the population of energy states of CH4. We also find that the northern and southern auroral CH4 emission evolved independently between the January, February and May images, as has been observed at X-ray wavelengths over shorter timescales6 and at mid-infrared wavelengths over longer timescales7.