Exoplanet atmospheres

Abstract:

Observations of Uranus were made in September 2009 with the Gemini-North telescope in Hawaii, using both the NIFS and NIRI instruments. Observations were acquired in Adaptive Optics mode and have a spatial resolution of approximately 0.1″ NIRI images were recorded with three spectral filters to constrain the overall appearance of the planet: J, H-continuum and CH4(long), and long slit spectroscopy measurements were also made (1.49-1.79μm) with the entrance slit aligned on Uranus' central meridian. To acquire spectra from other points on the planet, the NIFS instrument was used and its 3″×3″ field of view stepped across Uranus' disc. These observations were combined to yield complete images of Uranus at 2040 wavelengths between 1.476 and 1.803μm. The observed spectra along Uranus central meridian were analysed with the NEMESIS retrieval tool and used to infer the vertical/latitudinal variation in cloud optical depth. We find that the 2009 Gemini data perfectly complement our observations/conclusions from UKIRT/UIST observations made in 2006-2008 and show that the north polar zone at 45°N has continued to steadily brighten while that at 45°S has continued to fade. The improved spatial resolution of the Gemini observations compared with the non-AO UKIRT/UIST data removes some of the earlier ambiguities with our previous analyses and shows that the opacity of clouds deeper than the 2-bar level does indeed diminish towards the poles and also reveals a darkening of the deeper cloud deck near the equator, perhaps coinciding with a region of subduction. We find that the clouds at 45°N,S lie at slightly lower pressures than the clouds at more equatorial latitudes, which suggests that they might possibly be composed of a different condensate, presumably CH4 ice, rather than H2S or NH3 ice, which is assumed for the deeper cloud. In addition, analysis of the centre-to-limb curves of both the Gemini/NIFS and earlier UKIRT/UIST IFU observations shows that the main cloud deck has a well-defined top, and also allows us to better constrain the particle scattering properties. Overall, Uranus appeared to be less convectively active in 2009 than in the previous 3years, which suggests that now the northern spring equinox (which occurred in 2007) is passed the atmosphere is settling back into the quiescent state seen by Voyager 2 in 1986. However, a number of discrete clouds were still observed, with one at 15°N found to lie near the 500 mb level, while another at 30°N, was seen to be much higher at near the 200 mb level. Such high clouds are assumed to be composed of CH4 ice. © 2011 Elsevier Inc.

Abstract:

The book quantifies the outcomes of different stabilization targets for greenhouse gas concentrations using analyses and information drawn from the scientific literature.

Abstract:

We demonstrate experimentally that slicer based integral field spectrographs are an attractive choice for the next generation of exoplanet direct detection instruments. By propagating a single simulated speckle though a slicer based integral field spectrograph (IFS) and performing the post processing technique of spectral deconvolution we are able to achieve a speckle rejection factor of ∼600 in broadband images (and ∼100 in individual wavelength channels) with contrasts only appearing to be limited by calibration errors in the IFS datacube. This is over an order of magnitude improvement on the current state-of-the-art and well within the requirements of EPICS (Exo Planet Imaging Camera and Spectrograph for the E-ELT) for post coronagraphic speckle rejection thus proving that slicers will not impose a limit on the achievable contrast. When using prior knowledge of the diffraction-limited size of real objects we further improve the speckle rejection factor such that it exceeds 103.