Exoplanet atmospheres

Abstract:

Ammonia (NH3) has been detected both on Saturn and Jupiter, and although its concentration and distribution has been well-studied on Jupiter, it has proven more difficult to do so on Saturn due to higher sensitivity requirements resulting from Saturn's lower atmospheric temperatures and the dominance of Saturn's phosphine which masks the ammonia signal. Using far-infrared measurements of Saturn taken by Cassini/CIRS between February 2005 and December 2010, the latitudinal variations of upper tropospheric ammonia on Saturn are studied. Sensitivity to NH3 in the far-infrared is explored to provide estimates of temperature, para-H2 and PH 3, from 2.5 cm-1 spectral resolution measurements alone, 0.5 cm-1 spectral-resolution measurements alone, and 0.5 cm -1 measurements degraded to 2.5 cm-1 spectral resolution. The estimates of NH3 from these three different datasets largely agree, although there are notable differences using the high emission angle 0.5 cm-1 data, which are asserted to result from a reduction in sensitivity at higher emission angles. For low emission angles, the 0.5 cm -1-retrieved values of NH3 can be used to reproduce the 2.5 cm-1 spectra with similar efficacy as those derived directly from the 2.5 cm-1 resolution data itself, and vice versa. Using low emission angle data, NH3 is observed to have broad peak abundances at ±25°latitude, attributed to result from condensation and/or photolytic processes. Lack of data coverage at equatorial latitudes precludes analysis of NH3 abundance at less than about 10°latitude. Noise levels are not sufficient to distinguish fine zonal features, although it seems that NH3 cannot trace the zonal belt/zone structure in the upper troposphere of Saturn. © 2012 Elsevier Ltd. All rights reserved.

Abstract:

HARMONI, the High Angular Resolution Monolithic Optical & Near-infrared Integral field spectrograph is one of two first-light instruments for the European Extremely Large Telescope. Over a 256x128 pixel field-of-view HARMONI will simultaneously measure approximately 32,000 spectra. Each spectrum is about 4000 spectral pixels long, and covers a selectable part of the 0.47-2.45 μm wavelength range at resolving powers of either R=4000, 10000, or 20000. All 32,000 spectra are imaged onto eight HAWAII4RG detectors using a multiplexing scheme that divides the input field into four sub-fields, each imaged onto one image slicer that in turn re-arranges a single sub-field into two long exit slits feeding one spectrograph each. In total we require eight spectrographs, each with one HAWAII4RG detector. A system of articulated and exchangeable fold-mirrors and VPH gratings allows one to select different spectral resolving powers and wavelength ranges of interest while keeping a fixed geometry between the spectrograph collimator and camera avoiding the need for an articulated grating and camera. In this paper we describe both the field splitting and image slicing optics as well as the optics that will be used to select both spectral resolving power and wavelength range. © 2012 SPIE.

Abstract:

KMOS is a multi-object near-infrared integral field spectrograph being built by a consortium of UK and German institutes. We report on the final integration and test phases of KMOS, and its performance verification, prior to commissioning on the ESO VLT later this year. © 2012 SPIE.

Abstract:

HARMONI is a visible and near-IR integral field spectrograph, providing the E-ELT's spectroscopic capability at first light. It obtains simultaneous spectra of 32000 spaxels, at a range of resolving powers from R∼4000 to R∼20000, covering the wavelength range from 0.47 to 2.45 ìm. The 256 ? 128 spaxel field of view has four different plate scales, with the coarsest scale (40 mas) providing a 5? ? 10? FoV, while the finest scale is a factor of 10 finer (4mas). We describe the opto-mechanical design of HARMONI, prior to the start of preliminary design, including the main subsystems - namely the image de-rotator, the scale-changing optics, the splitting and slicing optics, and the spectrographs. We also present the secondary guiding system, the pupil imaging optics, the field and pupil stops, the natural guide star wavefront sensor, and the calibration unit. © 2012 SPIE.

Abstract:

Saturn's moon Titan has a nitrogen atmosphere comparable to Earth's, with a surface pressure of 1.4 bar. Numerical models reproduce the tropospheric conditions very well but have trouble explaining the observed middle-atmosphere temperatures, composition and winds. The top of the middle-atmosphere circulation has been thought to lie at an altitude of 450 to 500 kilometres, where there is a layer of haze that appears to be separated from the main haze deck. This 'detached' haze was previously explained as being due to the co-location of peak haze production and the limit of dynamical transport by the circulation's upper branch. Here we report a build-up of trace gases over the south pole approximately two years after observing the 2009 post-equinox circulation reversal, from which we conclude that middle-atmosphere circulation must extend to an altitude of at least 600 kilometres. The primary drivers of this circulation are summer-hemisphere heating of haze by absorption of solar radiation and winter-hemisphere cooling due to infrared emission by haze and trace gases; our results therefore imply that these effects are important well into the thermosphere (altitudes higher than 500 kilometres). This requires both active upper-atmosphere chemistry, consistent with the detection of high-complexity molecules and ions at altitudes greater than 950 kilometres, and an alternative explanation for the detached haze, such as a transition in haze particle growth from monomers to fractal structures.