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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.02.013 Byline: Brad J. Sandor (a), R. Todd Clancy (a)(b), Gerald Moriarty-Schieven (c), Franklin P. Mills (d) Keywords: Venus, Atmosphere; Photochemistry; Atmospheres, Chemistry; Atmospheres, Composition; Abundances, Atmospheres Abstract: First measurements of SO.sub.2 and SO in the Venus mesosphere (70-100km) are reported. This altitude range is distinctly above the [approximately equal to]60-70km range to which nadir-sounding IR and UV investigations are sensitive. Since July 2004, use of ground-based sub-mm spectroscopy has yielded multiple discoveries. Abundance of each molecule varies strongly on many timescales over the entire sub-Earth Venus hemisphere. Diurnal behavior is evident, with more SO.sub.2, and less SO, at night than during the day. Non-diurnal variability is also present, with measured SO.sub.2 and SO abundances each changing by up to 2x or more between observations conducted on different dates, but at fixed phase, hence identical sub-Earth Venus local times. Change as large and rapid as a 5I doubling of SO on a one-week timescale is seen. The sum of SO.sub.2 and SO abundances varies by an order of magnitude or more, indicating at least one additional sulfur reservoir must be present, and that it must function as both a sink and source for these molecules. The ratio SO.sub.2/SO varies by nearly two orders of magnitude, with both diurnal and non-diurnal components. In contrast to the strong time dependence of molecular abundances, their altitude distributions are temporally invariant, with far more SO.sub.2 and SO at 85-100km than at 70-85km. The observed increase of SO.sub.2 mixing ratio with altitude requires that the primary SO.sub.2 source be upper mesospheric photochemistry, contrary to atmospheric models which assert upward transport as the only source of above-cloud SO.sub.2. Abundance of upper mesospheric aerosol, with assumption that it is composed primarily of sulfuric acid, is at least sufficient to provide the maximum gas phase (SO+SO.sub.2) sulfur reported in this study. Sulfate aerosol is thus a plausible source of upper mesospheric SO.sub.2. Author Affiliation: (a) Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, United States (b) Space Science Institute, P.O. Box 3075, Bald Head Island, NC 28461, United States (c) National Research Council of Canada, Joint Astronomy Centre, 660 North Aohoku Place, Hilo, HI 96720, United States (d) RSPE/AMPL, Mills Rd., Australian National University, Canberra, ACT 0200, Australia Article History: Received 6 November 2009; Revised 18 February 2010; Accepted 19 February 2010

Infrared transmission spectra were recorded by the Jet Propulsion Laboratory MkIV interferometer during flights aboard the NASA DC-8 aircraft as part of the Airborne Arctic Stratospheric Expedition II (AASE II) mission in the early months of 1992. In our research, we infer the properties of the stratospheric aerosols from these spectra. The MkIV instrument employs two different detectors, a HgCdTe photoconductor for 650-1850 [cm.sup.-1] and an InSb photodiode for 1850-5650 [cm.sup.-1], to simultaneously record the solar intensity throughout the mid-infrared. These spectra have been used to retrieve the concentrations of a large number of gases, including chlorofluorocarbons, N[O.sub.y] species, [O.sub.3], and ozone-depleting gases. We demonstrate how the residual continua spectra, obtained after accounting for the absorbing gases, can be used to obtain information about the stratospheric aerosols. Infrared extinction spectra are calculated for a range of modeled aerosol size distributions and compositions with Mie theory and fitted to the measured residual spectra. By varying the size distribution parameters and sulfate weight percent, we obtain the microphysical properties of the aerosols that best fit the observations. The effective radius of the aerosols is found to be between 0.4 and 0.6 [micro]m, consistent with that derived from a large number of instruments in this post-Pinatubo period. We demonstrate how different parts of the spectral range can be used to constrain the range of possible values of this size parameter and show how the broad spectral bandpass of the MkIV instrument presents a great advantage for retrieval of both aerosol size and composition over instruments with a more limited spectral range. The aerosol composition that provides the best fit to the measured spectra is a 70-75% sulfuric acid solution, in good agreement with that obtained from thermodynamic considerations.

High-resolution infrared nongas absorption spectra derived from the Atmospheric Trace Molecule Spectroscopy (ATMOS) experiment are analyzed for evidence of the presence of cirrus clouds. Several nonspherical ice extinction models based on realistic size distributions and crystal habits along with a stratospheric sulfate aerosol model are fit to the spectra, and comparisons are made with different model combinations. Nonspherical ice models often fit observed transmission spectra better than a spherical Mie ice model, and some discrimination among nonspherical models is noted. The ATMOS lines of sight for eight occultations are superimposed on coincident geostationary satellite infrared imagery, and brightness temperatures along the lines of sight are compared with retrieved vertical temperature profiles. With these comparisons, studies of two cases of clear sky, three cases of opaque cirrus, and three cases of patchy cirrus are discussed. OCIS codes: 010.1100, 010.1320, 010.3920, 010.7030, 280.1100, 290.1090.

The wavelength-dependent aerosol extinction in the 800-1250-cm-1 region has been derived from ATMOS (atmospheric trace molecule spectroscopy) high-spectral-resolution IR transmission measurements. Using models of aerosol and cloud extinction, we have performed weighted nonlinear least-squares fitting to determine the aerosol-volume columns and vertical profiles of stratospheric sulfate aerosol and cirrus cloud volume. Modeled extinction by use of cold-temperature aerosol optical constants for a 70-80% sulfuric-acid-water solution shows good agreement with the measurements, and the derived aerosol volumes for a 1992 occultation are consistent with data from other experiments after the eruption of Mt. Pinatubo. The retrieved sulfuric acid aerosol-volume profiles are insensitive to the aerosol-size distribution and somewhat sensitive to the set of optical constants used. Data from the nonspherical cirrus extinction model agree well with a 1994 mid-latitude measurement indicating the presence of cirrus clouds at the tropopause.

This paper deals with the composition and chemical processes occurring in the neutral atmosphere of Venus. Since the last synthesis, observers as well as modellers have emphasised the spatial and temporal variability of minor species, going beyond a static and uniform picture that may have prevailed in the past. The outline of this paper acknowledges this situation and follows closely the different dimensions along which variability in composition can be observed: vertical, latitudinal, longitudinal, temporal. The strong differences between the atmosphere below and above the cloud layers also dictate the structure of this paper. Observational constraints, obtained from both Earth and Venus Express, as well as 1D, 2D and 3D models results obtained since 1997 are also extensively referred and commented by the authors. An non-exhaustive list of topics included follows: modelled and observed latitudinal and vertical profiles of CO and OCS below the clouds of Venus; vertical profiles of CO and SO2 above the clouds as observed by solar occultation and modelled; temporal and spatial variability of sulphur oxides above the clouds. As a conclusion, open questions and topics of interest for further studies are discussed. [ABSTRACT FROM AUTHOR]

Thermal inertia and albedo provide information on the distribution of surface materials on Mars. These parameters have been mapped globally on Mars by the Thermal Emission Spectrometer (TES) onboard the Mars Global Surveyor. Two-dimensional clusters of thermal inertia and albedo reflect the thermophysical attributes of the dominant materials on the surface. In this paper three automated, non-deterministic, algorithmic classification methods are employed for defining thermophysical units: Expectation Maximisation of a Gaussian Mixture Model; Iterative Self-Organizing Data Analysis Technique (ISODATA); and Maximum Likelihood. We analyse the behaviour of the thermophysical classes resulting from the three classifiers, operating on the 2007 TES thermal inertia and albedo datasets. Producing a rigorous mapping of thermophysical classes at ∼3 km/pixel resolution remains important for constraining the geologic processes that have shaped the Martian surface on a regional scale, and for choosing appropriate landing sites. The results from applying these algorithms are compared to geologic maps, surface data from lander missions, features derived from imaging, and previous classifications of thermophysical units which utilized manual (and potentially more time consuming) classification methods. These comparisons comprise data suitable for validation of our classifications. Our work shows that a combination of the algorithms–ISODATA and Maximum Likelihood–optimises the sensitivity to the underlying dataspace, and that new information on Martian surface materials can be obtained by using these methods. We demonstrate that the algorithms used here can be applied to define a finer partitioning of albedo and thermal inertia for a more detailed mapping of surface materials, grain sizes and thermal behaviour of the Martian surface and shallow subsurface, at the ∼3 km scale. [ABSTRACT FROM AUTHOR]

The hydroxyl radical (OH) plays an important role in middle atmospheric photochemistry, particularly in ozone (O3) chemistry. Because it is mainly produced through photolysis and has a short chemical lifetime, OH is expected to show rapid responses to solar forcing [e.g., the 11-y solar cycle (SC)], resulting in variabilities in related middle atmospheric O3 chemistry. Here, we present an effort to investigate such OH variability using long-term observations (from space and the surface) and model simulations. Groundbased measurements and data from the Microwave Limb Sounder on the National Aeronautics and Space Administration's Aura satellite suggest an ∼7-10% decrease in OH column abundance from solar maximum to solar minimum that is highly correlated with changes in total solar irradiance, solar Mg-II index, and Lyman-α index during SC 23. However, model simulations using a commonly accepted solar UV variability parameterization give much smaller OH variability (∼3%). Although this discrepancy could result partially from the limitations in our current understanding of middle atmospheric chemistry, recently published solar spectral irradiance data from the Solar Radiation and Climate Experiment suggest a solar UV variability that is much larger than previously believed. With a solar forcing derived from the Solar Radiation and Climate Experiment data, modeled OH variability (∼6-7%) agrees much better with observations. Model simulations reveal the detailed chemical mechanisms, suggesting that such OH variability and the corresponding catalytic chemistry may dominate the O3 SC signal in the upper stratosphere. Continuing measurements through SC 24 are required to understand this OH variability and its impacts on O3 further. [ABSTRACT FROM AUTHOR]