200
O. I. Korablev
comparative studies of atmospheres of planets and their satellites. New methods and
instruments for the studies of planetary atmospheres are also included.
2. Mars
2.1. Hydrological Cycle
The measurements of water vapor in the Martian atmosphere, which are car‑
ried out regularly from the spacecraft, are important for understanding the hy‑
drological cycle of the planet: Formation of clouds, transfer of water, status of
the icy polar caps. Water vapor, a small condensable specie, is a good indication
of the global thermal regime of the planet. The longest series of homogeneous
observations is published after the measurements with SPICAM instrument, op‑
erating onboard Mars Express [Trokhimovsky et al., 2015]. Long-term monitor‑
ing of water vapor on Mars and comparison of different observations, including
Viking and Mars Express orbiters, in the same water vapor absorption band and
processed in the same manner demonstrate the stability of water vapor on the
timescale of several tens of years. SPICAM/Mars Express measuring the waver
vapor is the acousto-optic (AOTF) spectrometer, which has been criticized for
putative spectral leakage, or stray light, to be responsible for underestimation of
water amount measured by SPICAM. Laboratory calibrations of the spare SPI‑
CAM unit have demonstrated high fidelity of the AOTF measurements [Korablev
et al., 2013].
Only a few measurements of the vertical distribution of water vapor have been
available until recently. Limb radiometer at Mars Reconnaissance Orbiter (MRO),
specifically dedicated for profiling, has only completed its mission for the aerosol
and atmosphere density profiles: The channel dedicated to water vapor is not
operational. Solar occultation’s with SPICAM/Mars Express allowed to fill the
gap, providing extensive dataset of water profiles for different seasons and geo‑
graphic locations [Maltagliati et al., 2013]. Maltagliati et al. [2011] has provided
evidence that the H
2
O at 30–40 km is supersaturated. The observed supersatura‑
tion is significantly higher than that known in the terrestrial atmosphere. The su‑
persaturated state allows water to travel higher, than the hygropause (saturation
level), where it becomes involved in active meridional transport.
2.2. Aerosol
Bimodal aerosol distribution in the atmosphere of Mars has been determined
and characterized. A small fraction of the Martian dust has been suspected since
long time, explaining well the UV and visible measurements from orbiters and
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Planetary Atmospheres
astronomy observations. In turn they are basically incompatible with surface
observations. Solar occultation observed by SPICAM/Mars Express simultane-
ously in 200–300 nm and 1–1.7 µm ranges allowed to detect the bimodal distri-
bution unambiguously. The particle radii are 0.04–0.07 µm for the small mode
and 0.7–0.8 µm for the large mode. This is the fi rst direct evidence of presence
of two different particle modes in the atmosphere of Mars [Fedora et al., 2014].
This study has employer a numerical model of aerosol microphysics [Burlakov
and Rodin, 2012].
Fig 1. Bimodal distribution of Martian dust. A typical evolution with the altitude
from solar occultation by SPICAM/Mars Express [Fedorova et al., 2014]
2.3. Dayglow and Nightglow
Dayglow and Nightglow of O
2
(a
1
Δ
g
) at 1.27 µm was studied using ground-
based astronomy and spacecraft observations.
The Mars O
2
(a
1
Δ
g
) nightglow ahs been predicted long ago, but fi rst detected
in polar regions of Mars by OMEGA/Mars Express. The oxygen atoms are
formed at the dayside, transported to the nightside by meridional Hadley circu-
lation, and recombine at 50–60 km. The nightglow has been also observed by
SPICAM/Mars Express in 2010. Seven profi les were retrieved in the Southern
polar region. The profi les peak at 42–50 km, and their line intensity is 0.24–0.45
MR. After the discovery the corresponding OMEGA channel stopped working,
while SPICAM has acquired a large number of observations, awaiting analysis
[Fedorova et al., 2012]. An attempt to detect the nightglow from ground-based
observations has resulted in upper limit of 40 kern A prolonged ground-based
202
O. I. Korablev
monitoring of the O
2
dayglow during three Mars years allowed to detect interan‑
nual variations in the season of L
S
≈ 15, 65, и 110°. The variations of ~20% are
detected during Northern spring and summer [Krasnopolsky 2013].
O
2
(a
1
Δ
g
) dayglow and its connection to water distribution was studied using
limb observations by SPICAM/Mars Express [Guslyakova et al. 2014]. Gravity
waves were detected in the О
2
1.27-µm patterns and compared to meteorological
model. The most likely reason is the surface topography [Altieri et al., 2012].
Also, variations of temperature and CO mixing ratio at 50 km were studied
from ground-based observations of the CO dayglow at 4.7 μm on Mars [Kras‑
nopolsky 2014].
2.4. Other Minor Constituents
A search for methane and other minor constituents was performed in ground-
based observations performed with CSHELL spectrometer at NASA IRTF in
2006 and 2009. The 2006 data (L
S
= 10°) revealed ~10 ppb of methane in the
latitude range 45°S …7°N (Mariners Vallis), and ~3 ppb elsewhere. In 2009 the
upper limit for methane was 8 ppb. The results are consistent with detections by
Mumma et al. [2009] in 2006, and are smaller than PFS/Mars Express mapping.
The search for ethane in the 2977 cm
‑1
band resulted in an upper limit of 0.2 ppb.
This new upper limit does not contribute to solving the problem of methane or‑
igin on Mars. A repeated analysis of TEXES data to search for SO
2
has confirmed
the previous upper limit of 0.3 ppb. The very low limit on SO
2
suggests the
volcanic source of methane on Mars is insignificant given the similarity of vol‑
canic degassing on terrestrial planets [Krasnopolsky, 2012].
3. Venus
3.1. Composition of the Atmosphere
Ozone was detected in the Venus atmosphere by SPICAV/Venus Express.
Occulations of stars at the night side of the planet allowed detection of ozone in
the Hartley band at 260 nm by UV spectrometer SPICAV. The ozone layer is
located at ~100 km. Retrieved vertical profiles show volume concentrations of
10
7
–10
8
molec.cm
-3
. The ozone content is consistent with photochemical model
predictions, and with recent detection of OH by VIRTIS/Venus Express, which
can be formed as O
3
+H à O
2
+OH*. The spatial distribution of ozone revealed no
significant features but the absence of ozone in the antisolar point. The same is
observed for OH, that suggests prevailing chemistry over dynamics in forming
its distribution. A possible mechanism of ozone destruction in the antisolar point
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