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Planetary
Atmospheres
is the reaction with chlorine radical O
3
+Cl à O
2
+ClO. This suggests similarity
with terrestrial atmosphere [Montmessin et al., 2011].
Dayside observations Venus by the high spectral resolution channel of VIR‑
TIS/Venus Express have been used to measure the altitude of the cloud tops and
the water vapor abundance at this level. CO
2
and H
2
O bands between 2.48 and
2.60 μm are analyzed to determine the cloud top altitude and water vapor abun‑
dance near this level. At low latitudes (±40°) the mean water vapor abundance
is 3 ± 1 ppm and the cloud top altitude is 69.5 ± 2 km. Poleward from middle
latitudes the cloud top altitude gradually decreases down to 64 km, while the
average H
2
O abundance reaches its maximum of 5 ppm at 80° of latitude with a
large scatter from 1 to 15 ppm. The calculated mass percentage of the sulfuric
acid solution in cloud droplets of mode 2 (~1 μm) particles is in the range 75–
83%. No systematic correlation of the dark UV markings with the cloud top al‑
titude or water vapor has been observed [Cottini et al., 2012].
Sulfur dioxide is a million times more abundant in the atmosphere of Venus
than that of Earth, possibly as a result of volcanism on Venus within the past
billion years. A tenfold decrease in sulfur dioxide column density above Venus’s
clouds measured by the Pioneer Venus spacecraft during the 1970s and 1980s
has been interpreted as decline following an episode of volcanogenic upwelling
from the lower atmosphere. Using the data of SPICAV/Venus Express the sulfur
dioxide column density above Venus’s clouds is found to decrease by an order
of magnitude between 2007 and 2012. This decline is similar to observations
during the 1980s (Fig 2). A strong latitudinal and temporal variability in sulfur
dioxide column density is observed that is consistent with supply fluctuations
from the lower atmosphere. Episodic sulfur dioxide injections to the cloud tops
may be caused either by periods of increased buoyancy of volcanic plumes, or,
in the absence of active volcanism, by long-period oscillations of the general
atmospheric circulation. The 30-year observational record from Pioneer Venus
and Venus Express confirms that episodic injections of sulfur dioxide above the
clouds recur on decadal timescales, suggesting a more variable atmosphere than
expected [Marcq et al., 2011; 2013].
New measurements of sulfur dioxide (SO
2
) and monoxide (SO) in the atmos‑
phere of Venus by SPICAV/SOIR instrument onboard Venus Express orbiter
provide ample statistics to study the behavior of these gases above Venus’ clouds.
Improved calibration of SOIR has been applied [Vandael et al., 2013]. Vertical
profiles result from solar occultations in the absorption ranges of SO
2
(190–230
nm, and at 4 µm) and SO (190–230 nm). The dioxide is detected by the SOIR
spectrometer at the altitudes of 65–80 km in the IR and by the SPICAV spectrom‑
eter at 85–105 km in the UV. The monoxide’s absorption was measured only by
SPICAV at 85–105 km. We analyzed 39 sessions of solar occultation, where
boresights of both spectrometers are oriented identically, to provide complete
204
O. I. Korablev
vertical profiling of SO
2
of the Venus’ mesosphere (65–105 km). Two distinct
SO
2
layers are detected. In the lower layer SO
2
mixing ratio is within 0.02–0.5
ppmv. The upper layer is also conceivable from microwave measurements [Be‑
lyaev et al., 2012].
Fig. 2 More than thirty years of SO2 measurements at Venus’s cloudtop.
Black stands for Pioneer Venus and other previously published measurements.
Red stands for the 8-month moving average of SPICAV/Venus
Express retrievals
[Marcq et al., 2013]
Ground-based observations of the D/H ratios in H
2
O, HCl, and HF on Venus
has been carried out using the CSHELL spectrograph at NASA IRTF. The re‑
trieved D/H ratios are: (D/H)HF is 420 ± 200 times that in the Standard Mean
Ocean Water (SMOW); (D/H)H2O = 95 ± 15 times SMOW; (D/H)HCl = 190 ±
50 times SMOW [Krasnopolsky et al., 2013].
Observations of the 1.10- and 1.18-μm nightside windows by the SPICAV–IR
instrument aboard Venus Express [Korablev et al., 2012] were analyzed to char‑
acterize the various sources of gaseous opacity and determine the H
2
O mole
fraction in the lower atmosphere of Venus. Different approximations of line pro‑
file models are analyzed and an empirical lineshape is suggested. An additional
continuum opacity is required to reproduce the variation of the 1.10- and 1.18-
μm radiances with surface elevation as observed by the VIRTIS-M/Venus Ex‑
press. Also different CO
2
and H
2
O line lists are compared; a lack in CDSD
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Planetary Atmospheres
database, which provide significant opacity in Venus’ deep atmosphere, is found.
A composite line list that best reproduces the observations is suggested. It is
shown that HDO brings significant absorption at 1140–1190 nm. The retrieved
a water vapor mole fraction at 5–25 km is 30–5+10 ppmv. Combined with pre‑
vious measurements in the 1.74- and 2.3-μm windows, this result provides strong
evidence for a uniform H
2
O profile below 40 km, in agreement with chemical
models [Bezard et al., 2012].
3.2. Modeling of Atmospheric Chemistry
A photochemical model of the nighside Venus atmosphere was modified to
account for the recent detection of ozone by SPICAV/Venus Express. The mod‑
el includes more details regarding spectroscopy and chemistry of OH. The com‑
puted
chlorine species, Cl
2
,
ClO and ClNO
3
at 80–90 km is above 1 ppb, and the
possibility of their detection is discussed. The modeled layer of ozone is consist‑
ent with SPICAV observations. Ignoring chlorine chemistry allows to reproduce
the altitude
and the density in the O
3
peak, but results in unrealistic content of
ozone of 4×10
8
cm
-3
at 80 km [Krasnopolsky 2013].
To explain the pronounced difference between Venus Express and ground-
based microwave observations, a photochemical model of Venus atmosphere
covering the attitude range of 47–112 km was updated. The H
2
O profile is com‑
puted by the model. The main feature of the Venus photochemistry is the forming
of H
2
SO
4
in a narrow layer near the upper cloud deck leading to a significant
decrease of SO
2
and H
2
O content above the clouds. The SO
2
and H
2
O
transport
through the bottleneck determines the chemical composition above clouds and
its variations. It is shown that the observed variability of the composition of the
mesosphere can be explained by small changes in atmospheric dynamics around
the cloud layer, and does not require any volcanic activity [Krasnopolsky 2012].
A chemical-kinetic model of the Venus lower atmosphere has been modified to
include the S
4
cycle, and other improvements to be consistent with recent meso‑
sphere [Krasnopolsky 2012] model. The lower atmosphere chemistry is mainly
driven
by sulfur, but also involves chlorine compounds [Krasnopolsky 2013].
Venus Express measurements of the vertical profiles of SO and SO
2
in the
middle atmosphere of Venus provide an opportunity to revisit the sulfur chemis‑
try above the middle cloud tops (~58 km). A 1-D photochemistry-diffusion mod‑
el was used to simulate the behavior of the whole chemical system including
oxygen-, hydrogen-, chlorine-, sulfur-, and nitrogen-bearing species. A sulfur
source is required to explain the SO
2
inversion layer above 80 km. The evapora‑
tion of the aerosols composed of sulfuric acid or polysulfur above 90 km could
provide the sulfur source [Zhang et al., 2012].