Y. Yung, P. Gao, X. Zhang, D. Crisp, C. G. Bardeen, 2012, Generation of Small-Made Particles
via Nucleation of Meteoric Dust in the Upper Haze of Venus, Bull. Am. Astron. Soc., 508.04
Abstract: Observations by the SPICAV/SOIR instruments aboard Venus Express has revealed
that the Upper Haze of Venus is populated by two particle modes, as reported by Wilquet et al.
(J. Geophys. Res., 114, E00B42, 13pp, 2009). In this work we posit that the large mode is due to
the upwelling of cloud particles, while the smaller mode is generated by the nucleation of
meteoric dust. We test this hypothesis by using version 3.0 of the Community Aerosol and
Radiation Model for Atmospheres, first developed by Turco et al. (J. Atmos. Sci., 36, 699-717,
1979) and upgraded by Bardeen et al. (The CARMA 3.0 microphysics package in CESM, Whole
Atmosphere Working Group Meeting, 2011). Using the meteoric dust production profile of
Kalashnikova et al. (Geophys. Res. Lett., 27, 3293-3296, 2000), the sulfur/sulfate condensation
nuclei production profile of Imamura and Hashimoto (J. Atmos. Sci., 58, 3597-3612, 2001), and
sulfuric acid vapour production profile of Zhang et al. (Icarus, 217, 714-739, 2012), we
numerically simulate a column of the Venus atmosphere from 40 to 100 km above the surface.
Our aerosol number density results agree well with Pioneer Venus data from Knollenberg and
Hunten (J. Geophys. Res., 85, 8039-8058, 1980), while our gas distribution results match that of
Kolodner and Steffes (Icarus, 132, 151-169, 1998). There is a mediocre agreement between our
cloud deck size distribution and Pioneer Venus data. The Upper Haze size distribution shows
two lognormal distributions overlapping each other, possibly indicating the presence of two
modes, though more analysis is required at this time. Finally, we treat the simulated aerosol
particles as Mie scatterers and compute their optical parameters. The results show a minimum in
the optical depth at a wavelength of 300 nm, comparable to the results of Lacis (J. Atmos. Sci.,
32, 1107-1124, 1975).
M. Liang, C. Li, X. Zhang, Y. Yung, 2012, Modeling The Vertical Profile Of Tholin Particles In
The Atmosphere Of Titan, Bull. Am. Astron. Soc., 312.02
Abstract: Detailed vertical profiles of minor species from 100 to 1000 km and of tholins above
~300 km have recently been deduced from Cassini observations. To match the observed profiles
of three major C2-hydrocarbons (C2H2, C2H4, and C2H6), modifications to the existing kinetics
and vertical transport are needed. The latter plays a crucial role. Incorporating these
modifications and a parameterized aerosol formation pathway into a chemistry-diffusion model,
we could explain the observed tholin profile made by the Cassini/UVIS instrument. Processes
that affect the profile are (1) aerosol production, (2) aerosol coagulation, and (3) dynamical
transport. Sensitivity of the resulting tholin profile to the processes is examined and discussed.
J. Kammer, D. E. Shemansky, X. Zhang, Y. L. Yung, 2012, Nitrogen Chemistry in Titan's Upper
Atmosphere, Bull. Am. Astron. Soc., 300.09
Abstract: Titan’s atmosphere has evolved over time into its current state through a complex suite
of photochemical processes involving nitrogen (N2), the dominant molecular species in the
atmosphere, as well as methane (CH4). Since 2004, observations by the Cassini spacecraft have
greatly improved our knowledge of the current composition of Titan’s atmosphere, and
measurements made using the Ultraviolet Imaging Spectrograph (UVIS) in particular are able to
probe the region of interest from roughly 400 km to 1500 km in altitude where much of the
photochemistry on Titan occurs. This photochemistry is responsible for converting nitrogen from
stable N2 molecules into other detectable nitrile compounds such as HCN, HC3N, and heavier
species that are further processed into tholins. Therefore an analysis of nitrogen and nitrile
abundances on Titan is of particular interest, and we examine UVIS solar and stellar occultation
observations in both the EUV and FUV regions of the spectrum in order to directly retrieve the
vertical profiles of N2 in addition to its related hydrocarbon derivatives. Furthermore, global
analysis of these density profiles strongly indicates possible spatial and seasonal variability both
for composition and temperature of the upper atmosphere.
D. E. Shemansky, X. Zhang, M. Liang, Y. L. Yung, 2012, Properties of the Scattered FUV Solar
Spectrum at Titan, Bull. Am. Astron. Soc., 312.14
Abstract: The strong solar reflection spectrum in the FUV in Cassini UVIS observations with
line of sight below the solid limb allow the examination of spectral properties of the scattering
medium. At wavelengths longward of 1525 A the spectrum is characterized mainly by solar
emission structure. Absorption features in acetylene , the main known absorber in this region, are
weak. The spectrum therefore shows the properties of a Rayleigh scattering continuum in a
wavelength range of 1920. - 1525.1 A, below which the solar continuum and line emissions are
cut-off by absorption in the medium. The observed emission is evidently multiply scattered. At a
line of sight altitude of 1040 km, analysis assuming Mie scattering gives an estimated particle
radius of 76 A. The evident strong absorber at lower altitude giving the sharp extinction of the
emission spectrum below 1525 A needs investigation. The PAH naphthalene, with an ionization
potential of 1526.9 A is the nearest candidate identified to date. The spectral properties of the
scattering medium in the 1920. - 1525.1 A region will be described.
X. Zhang, C. A. Nixon, R. Shia, R. A. West, R. Morales-Juberias, R. Cosentino, P. G. Irwin, T.
Dowling, M. A. Allen, Y. L. Yung, 2012, Radiative Forcing of the Stratosphere of Jupiter from
Cassini Observations, Bull. Am. Astron. Soc., 205.05
Abstract: In this study we present a detailed analysis of the instantaneous zonally averaged
radiative forcing of the Jovian stratosphere during the Cassini flyby in Dec. 2000. It provides the
necessary data for an accurate simulation of the state of the stratosphere of Jupiter. First, we
retrieved the global map of stratospheric temperature field and hydrocarbon species based on the
measurements from the Cassini Composite Infrared Spectrometer (CIRS). The associated
uncertainties are carefully characterized. From the simultaneous observations by the multiple
filters of Cassini Imaging Science Subsystem (ISS), a global map of stratospheric aerosol is
obtained via a nonlinear optimization approach. Large difference between the scattering
properties of aerosols and clouds in the equatorial region and polar region indicate different types
of aerosol and clouds in the two regions. Secondly, based on the retrieved data, a line-by-line
radiative transfer model including the Non-LTE effect is introduced to calculate the solar heating
and mid-infrared cooling rates for each latitude and altitude. The fractal aggregated multisphere
particles with monomer radius of tens of nanometers contribute to the heat source in the upper
stratosphere of the polar region. Finally, preliminary results of a stratospheric circulation model
driven by the radiative forcing and a two-dimensional chemical transport model forced by the
circulation will be presented.
C. Li, X. Zhang, J. A. Kammer, M. C. Liang, Y. L. Yung, 2012, Revisiting the Photochemical
Processes on Titan: Insight from Cassini Observations, Bull. Am. Astron. Soc., 300.06
Abstract: The recent measurements from the nadir-view and limb-view soundings of
Cassini/CIRS (Vinatier et al., 2010) and the stellar occultations from Cassini/UVIS (Kammer et
al., 2010, Koskinen et al., 2011) revealed the complete vertical profiles of minor species (mainly
C2H2 and C2H4) from 100 to 1000 km in the atmosphere of Titan. In this study, we introduce a
new eddy diffusivity profile and revise the rate coefficients for the chemistry of hydrocarbons,
especially for C2H4. Our new results are in better agreement with the new Cassini measurements
than the previous 1D chemistry-diffusion models (e.g., Yung et al., 1984; Lavvas et al., 2008 a,
b; Krasnopolsky, 2010). An inversion technique is developed to retrieve the vertical profile of
eddy diffusivity. We use the abundance of C2H2 as a proxy to retrieve the eddy diffusivity
profile and verify it though the chemistry-diffusion modeling of other tracers. We found that the
new eddy profile features a turnover near the altitude of detached haze layer (~550 km), which
might lead to the formation of the detached haze layer through a potential positive feedback
process. Secondly, the same retrieval method is applied to the chemistry of C2H4. The result
suggests a revision of rate coefficients for the three body reactions of C2H4 so as to keep the
simulated profile close to the observation. The revised rate coefficients will also be used and
tested for the hydrocarbon chemistry on giant planets.
M. R. Line, E. Ellison, X. Zhang, Y. Yung, 2012, Secondary Eclipse Spectral Retrievals: Trends
in Chemistry, Bull. Am. Astron. Soc., 103.03
Secondary eclipse spectra of exoplanets probe the thermal emission of the atmosphere and can
therefore tell us about their composition and temperature. Here we will apply the optimal
estimation retrieval approach (Lee et al. 2012, Line et al. 2012) to all available secondary eclipse
spectra (in some cases multiple data sets per planet) in order to identify compositional trends
amongst different atmospheres, such as the CH4/CO ratio as a function of effective temperature
etc. We will compare our retrieved results to what is expected from thermal equilibrium
chemistry in order to assess how strongly out of equilibrium the planets as a whole agree with or
deviate from thermal equilibrium.