Chris Parkinson, Stephen Bougher, Yuk Yung, Amanda Brecht. Photochemical Distribution of
Venusian Sulfur and Halogen Species, Bull. Am. Astron. Soc., 45, 9, 118.05 (2013).
Recent observations of enhanced amounts of SO2 at 100 km by Venus Express (Bertaux et al,
2009; Marcq et al, 2012) suggest that there is a hitherto unknown source of gaseous sulfur
species in the upper atmosphere of Venus. The observations of Sandor and Clancy (2010) also
show short and longterm variations in mesopause-level (90-100 km) SO and SO2. Zhang et al
(2010) argue that the photolysis of H2SO4 vapor derived from evaporation of H2SO4 aerosols
provides a source of SO3, which upon photolysis yields SO2. In this study, the photochemistry
and dynamics of Venus’ atmosphere from the cloud tops to 110 km has been modeled using an
updated/expanded chemical scheme, with the view to improving our understanding of the
vertical and global distributions of sulfur and halogen species by application of the 1-D
Caltech/JPL KINETICS chemistry tracer species profiles to theVenus Thermospheric General
Circulation Model (VTGCM) (Bougher et al, 1997) for comparison to VEx and ground-based
datasets. Specifically, we compare our model results with the SO2 observations of Bertaux et al
(2009), Sandor and Clancy (2010), and Marcq et al (2012) in our analysis. We mainly follow
Yung and DeMore (1982), Mills (1998), Pernice et al. (2004), Krasnopolsky (2010), and Zhang
et al. (2010) in our choice of chemical reactions, chemical rate constants, and boundary
conditions for 38 species. We will examine a model with an HCl mixing ratio of 1E-7
corresponding to Venus Express observations made at high northern latitudes. Our modeling
agrees satisfactorily with stratospheric observations of key species such as CO, O2 and SO2, and
we better quantify the implications of the different HCl mixing ratios observed. As well, we also
include other tunable parameters in our study such as considering a range of eddy diffusion
profiles that vary by a factor of 10 and other sensitivity studies such as wave drag using Rayleigh
friction parameters.
Peter Gao, Xi Zhang, David Crisp, Charles G. Bardeen, Yuk L. Yung. Bimodal Distribution of
Sulfuric Acid Aerosols in the Atmosphere of Venus. Bull. Am. Astron. Soc., 45, 9, 118.07
(2013).
Observations by the SPICAV/SOIR instruments aboard Venus Express have revealed that the
upper haze of Venus, between 70 and 90 km, is variable on the order of days and that it is
populated by two particle modes. In this work, we posit that the observed phenomena are caused
by the transient mixing of the clouds and the haze, as well as another source of sulfuric acid
aerosols in the upper haze that nucleate on meteoric dust. We test this hypothesis by simulating a
column of the Venus atmosphere from 40 to 100 km above the surface using a model based upon
the Community Aerosol and Radiation Model for Atmospheres and consider the effects of
meteoric dust and polysulfur acting as condensation nuclei in the upper haze and upper cloud,
respectively, as well as transient winds at the cloud tops caused by subsolar convection. Our
aerosol number density results are consistent with Pioneer Venus data from Knollenberg and
Hunten (1980), while our gas distribution results match the Magellan radio occultation data as
analyzed by Kolodner and Steffes (1998) below 55 km. The size distribution of cloud particles
shows two distinct modes in the upper clouds region and three distinct modes in the middle and
lower clouds regions, qualitatively matching the observations of Pioneer Venus. The UH size
distribution shows one distinct mode that is likely an upwelled cloud particle population with
which an in situ meteoric dust condensation particle population has coagulated. The results of the
transient wind simulations yield a variability timescale that is consistent with Venus Express
observations, as well as a clear bimodal size distribution in the UH.
Michael R. Line, Yuk L. Yung. A Novel Diagnosis of Chemical Disequilibrium in Extrasolar
Planet and Substellar Object Atmospheres. Bull. Am. Astron. Soc., 45, 9, 200.05 (2013).
With recent advancements in spectroscopic capabilities, characterization of the temperatures and
compositions of extra-solar planets is becoming more prevalent. One outstanding issue one can
address with temperature and abundance determinations is the role that disequilibrium plays in
sculpting the atmospheric compositions. A variety of disequilibrium models now exist in the
literature to explore various planets and the effects that photochemistry and vertical mixing have
on the molecular abundances. In this investigation we introduce a novel approach to diagnosing
disequilibrium without the need of sophisticated photochemical/kinetics models. In short, we
define an “equilibrium” line as a function of a representative planet temperature. If we can
determine the abundances of H2O, CH4, CO, and H2, we can evaluate a simple analytic formula
to quickly assess whether or not the atmospheric composition is in disequilibrium. We verify the
usefulness of this approach with state-of-the-art chemistry-transport models and find that, as
expected, cooler planets tend to show the strongest signs of disequilibrium.
Yuk Yung, Cheng Li, Xi Zhang, Joshua A. Kammer, Mao-Chang Liang, Run-Lie Shia.
Modeling Photochemical Processes on Titan: Comparison to Cassini Observations. Bull. Am.
Astron. Soc., 45, 9, 207.03 (2013).
The recent data from the Cassini/CIRS soundings (Vinatier et al., 2010), the Cassini/UVIS stellar
occultations (Koskinen et al., 2011, Kammer et al. 2013) and the Cassini/INMS in situ
measurements (Westlake et al., 2011) revealed vertical profiles of minor species from 100 to
1500 km in the atmosphere of Titan. In this study, we introduce a new optimization method of
deriving the best-fit eddy diffusivity profile. We also make systematic revisions of the rate
coefficients for the chemistry of hydrocarbons within the uncertainties of laboratory
measurements. After a careful search, we found that reproducing the abundance profile of C2H2
is the best strategy for retrieving the eddy diffusivity profile. We verify the validity of the eddy
diffusivity profile though its application to the modeling of other tracers. We found that our
objectively derived new eddy profile is consistent with that from Lavvas et al. (2008), but differs
significantly from those used by other groups. We re-examined the chemistry of C2H4. The
results suggest a revision of rate coefficients for three-body reactions related to C2H4 so as to
keep the simulated profile close to the observation. The rate coefficients for reactions that control
the abundance of C2H6, C3H8, CH3C2H and C4H2 are also modified in the model to provide
better fit to Cassini observations. We identified the rate coefficients that need improved
laboratory data appropriate for the conditions of Titan’s atmosphere.
Mao-Chang Liang, J. Kammer, X. Zhang, D. Shemansky, Y. Yung. Revised tholin profile in the
atmosphere of Titan. Bull. Am. Astron. Soc., 45, 9, 305.02 (2013).
We present a recent effort for analyzing the stellar occultation data from Cassini/UVIS
instrument. The mean optical depth as a function of line of sight impact parameter is derived for
the spectral range between 1700 and 1900 A from four stellar occultations. Vertical profiles of
tholin particles above ~300 km are then retrieved. The profiles show two depletion regions: 750800 km and 400-500 km ranges. A photochemistry-transport is developed and used to interpret
the production, loss, and transport in the atmosphere. Processes that affect the profile are (1)
aerosol production via ion-neutral and neutral-neutral chemical reactions, (2) aggregation to form
fractal aggregates, (3) aerosol coagulation, and (4) diffusion and dynamical transport via
gravitational settling. Sensitivity of the resulting tholin profile to the above processes is
examined and discussed.
Michael Wong, Xi Zhang, Cheng Li, Renyu Hu, Run-Lie Shia, Claire Newman, Ingo MüllerWodarg, Yuk Yung. Titan 2D: Understanding Titan’s Seasonal Atmospheric Cycles. Bull. Am.
Astron. Soc., 45, 9, 309.10 (2013).
In this study, we present results from a novel two-dimensional (2D) model that simulates the
physics and chemistry of Titan’s atmosphere. Despite being an icy moon of Saturn, Titan is the
only Solar System object aside from Earth that is sheathed by a thick nitrogen-dominated
atmosphere. This vulnerable gaseous envelope—an embodiment of a delicate coupling between
photochemistry, radiation, and dynamics—is Nature’s laboratory for the synthesis of complex
organic molecules. Titan’s large obliquity generates pronounced seasonal cycles in its
atmosphere, and the Cassini spacecraft has been observing these variations since 2004. In
particular, Cassini measurements show that the latitudinal distribution of Titan’s rich mélange of
hydrocarbon species follows seasonal patterns. The mixing ratios of hydrocarbons increase with
latitude towards the winter pole, suggesting a pole-to-pole circulation that reverses after equinox.
Using a one-dimensional photochemical model of Titan’s atmosphere, we show that
photochemistry alone cannot produce the observed meridional hydrocarbon distribution. This
necessitates the employment of a 2D chemistry-transport model that includes meridional
circulation as well as diffusive processes and photochemistry. Of additional concern, no previous
2D model of Titan extends beyond 500 km altitude—a critical limitation since the peak of
methane photolysis is at 800 km. Our 2D model is the first to include Titan’s stratosphere,
mesosphere, and thermosphere. The meridional circulation in our 2D model is derived from the
outputs of two general circulation models (GCMs): the TitanWRF GCM (Newman et al. 2011)
covering the troposphere, stratosphere, and lower mesosphere, and a thermosphere general
circulation model (TGCM) covering the remainder of the atmosphere through the thermosphere
(Müller-Wodarg et al. 2003; 2008). This presentation will focus on the utilization of these
advances applied to the 2D Caltech/JPL KINETICS model to understand the seasonal and
meridional distribution of hydrocarbons as well as other atmospheric cycles on Titan.