Planetary atmosphere observation analysis

Abstract:

© 2014 Elsevier Ltd. One of the difficulties in modeling Venus' nightside atmospheric windows is the need to apply CO2 continuum opacity due to collision-induced CO2 bands and/or extreme far wings of strong allowed CO2 bands. Characterizing the CO2 continuum absorption at near-IR wavelengths as well as searching for a possible vertical gradient of minor species near the surface require observations over different surface elevations. The largest change in altitude occurs during a passage above Maxwell Montes at high northern latitudes. In 2011, 2012 and 2013 the SPICAV instrument aboard the Venus Express satellite performed three sets of observations over Maxwell Montes with variation of surface altitude from -2 to 9km in the 1.10, 1.18 and 1.28-μm windows. The retrieved CO2 continuum absorption for the 1.10- and 1.18-μm windows varies from 0.29 to 0.66×10-9 cm-1 amagat-2 and from 0.30 to 0.78×10-9 cm-1 amagat-2, respectively, depending on the assumed input parameters. The retrieval is sensitive to possible variations of the surface emissivity. Our values fall between the results of Bézard et al., (2009, 2011) based on VIRTIS-M observations and laboratory measurements by Snels et al. (2014). We can also conclude that the continuum absorption at 1.28μm can be constrained below 2.0×10-9 cm-1 amagat-2. Based on the 1.18μm window the constant H2O mixing ratio varying from 25.7+1.4 -1.2 ppm to 29.4+1.6 -1.4 ppm has been retrieved assuming the surface emissivity of 0.95 and 0.6, respectively. No firm conclusion from SPICAV data about the vertical gradient of water vapor content at 10-20km altitude could be drawn because of low signal-to-noise ratio and uncertainties in the surface emissivity.

Abstract:

Imaging and spectroscopy of Neptune's thermal infrared emission from Keck/LWS (2003), Gemini-N/MICHELLE (2005); VLT/VISIR (2006) and Gemini-S/TReCS (2007) is used to assess seasonal changes in Neptune's zonal mean temperatures between Voyager-2 observations (1989, heliocentric longitude Ls=236°) and southern summer solstice (2005, Ls=270°). Our aim was to analyse imaging and spectroscopy from multiple different sources using a single self-consistent radiative-transfer model to assess the magnitude of seasonal variability. Globally-averaged stratospheric temperatures measured from methane emission tend towards a quasi-isothermal structure (158-164K) above the 0.1-mbar level, and are found to be consistent with spacecraft observations of AKARI. This remarkable consistency, despite very different observing conditions, suggests that stratospheric temporal variability, if present, is <±5K at 1mbar and <±3K at 0.1mbar during this solstice period. Conversely, ethane emission is highly variable, with abundance determinations varying by more than a factor of two (from 500 to 1200ppb at 1mbar). The retrieved C2H6 abundances are extremely sensitive to the details of the T(p) derivation, although the underlying cause of the variable ethane emission remains unidentified. Stratospheric temperatures and ethane are found to be latitudinally uniform away from the south pole (assuming a latitudinally-uniform distribution of stratospheric methane), with no large seasonal hemispheric asymmetries evident at solstice. At low and mid-latitudes, comparisons of synthetic Voyager-era images with solstice-era observations suggest that tropospheric zonal temperatures are unchanged since the Voyager 2 encounter, with cool mid-latitudes and a warm equator and pole. A re-analysis of Voyager/IRIS 25-50μm mapping of tropospheric temperatures and para-hydrogen disequilibrium (a tracer for vertical motions) suggests a symmetric meridional circulation with cold air rising at mid-latitudes (sub-equilibrium para-H2 conditions) and warm air sinking at the equator and poles (super-equilibrium para-H2 conditions). The most significant atmospheric changes have occurred at high southern latitudes, where zonal temperatures retrieved from 2003 images suggest a polar enhancement of 7-8K above the tropopause, and an increase of 5-6K throughout the 70-90°S region between 0.1 and 200mbar. Such a large perturbation, if present in 1989, would have been detectable by Voyager/IRIS in a single scan despite its long-wavelength sensitivity, and we conclude that Neptune's south polar cyclonic vortex increased in strength significantly from Voyager to solstice. © 2013 Elsevier Inc.