STRUCTURE OF THE VENUS MIDDLE ATMOSPHERE: VENERA 15
FOURIER SPECTROMETRY DATA REVISITED
L. V. Zasova, I. A. Khatountsev, V. I. Moroz and N. I. Ignatiev
Space Research Institute (IKI), Russian Academy of Sciences, Profsoyuznaya 84/32, 117810 Moscow,
Russia
ABSTRACT
The data obtained by Infrared Fourier Spectrometer on board Venera 15 Orbiter are revisited. The new
database of temperature and aerosol profiles is created for the altitude range 55-100 km. The main
improvements concern the involving of the whole spectral range free from absorption by any gases but
CO2 into the temperature retrieval procedure. Besides the CO2 15 m fundamental band, this range also
includes the weak hot and isotopic CO2 bands. HITRAN-96 spectral database was used for calculation of
the gaseous absorption coefficients. The diurnal variations at the isobaric levels are investigated. At low
latitudes at the altitude h > 85 km a minimal temperature is observed in the afternoon, and a maximal one
is on the morning day side. The temperature differences reach 20 K near 0.1 mb level. The temperature
difference changes its sign below 1 mb level: in the afternoon it is warmer by more than 10 K than in the
morning. The density of the clouds at all latitudes is found to be higher in the afternoon than in the
morning. In the coldest parts of the ‘cold collar’ the clouds are found to be composed of the mode 3
particles. The thermal zonal wind field reveals the presence of the midlatitude jet, connected with the
‘cold collar’. The low latitude jet near 85 km, connected with the temperature inversion above this level,
is observed. It is also possible that another low latitude jet exists near the cloud tops at low latitudes.
INTRODUCTION
Infrared Fourier Spectrometer was installed on board Venera 15 Orbiter. The experiment was described in
several papers (see, for example, Moroz et al., 1986, Oertel et al., 1987). About 2000 high quality spectra
in the range 250 – 1650 cm -1 with spectral resolution 4.5 and 6.5 cm-1 were obtained. During an orbit the
nearly meridional measurements were fulfilled, beginning from low latitudes to the North Pole and then
back to low latitudes. As a result, we obtained practically simultaneous measurements for a wide range of
latitudes at the same solar longitude (or local time). This geometry is very convenient for investigation of
three-dimensional fields of temperature, solar related structures and thermal tides in the middle
atmosphere of Venus (55-100 km). The results of previous work may be found in Dubois et al. (1990),
Moroz et al. (1990), Schafer et al. (1987, 1990), Spankuch et al. (1990), , Zasova et al. (1990, 1992, 1993,
1997, 1998a,b, 1999). The new results on water vapor abundance, based on the last version of
temperature-aerosol database one may find in the paper by Ignatiev et al. (1999).
SELFCONSISTENT TEMPERATURE AND AEROSOL RETRIEVAL
In our earlier works we used only the separate channels inside 15 m CO2 band for temperature retrieval.
Now we apply the method of retrieval to the whole spectral range, in which aerosol and carbon dioxide
are the main absorbers. It includes the hot bands near 960 and 1050 cm-1 and isotopic band near 1260cm-1.
This range extends from 540 to 1280 cm-1, excluding the interval 1120-1180 cm-1, where the absorption
of the 1 SO2 band may be essential at high latitudes. Using the additional spectral channels in the weak
bands in other spectral ranges enables to obtain more reliable temperature profiles at lower levels in the
atmosphere below 58 km.
The transfer equation for thermal IR may be written as:
lg p0
I  =  B [T ( p)] K [ p, T ( p)] d lg p
(1)

 t
(2)
lg p
where p0 is the pressure at low boundary (we adopted p0 =1200 mb) and K is the weighting function, which
defines the input of a given level with pressure p and temperature T into the outgoing radiation at
wavenumber , and t is the transmission function:
t  t , g t ,a
(3)
K  -
where t,g relates to gaseous component and t,a to aerosol one.
The Eq.1 was solved by iterative method with relaxation (Chahine, 1970, Twomey et al., 1977, Schofield and
Taylor, 1983, Spankuch et al., 1990). The relationship between vertical temperature profiles at n+1 and n
iterations is defined by the following equation:
I0
T ( n 1) ( Pj )  T ( n ) ( Pj )   TB ( i ) / TB
i 1
(n)
I0
( i )  K ij   i /  K ij   i
,
(4)
i 1
where i is the noise equivalent radiation (NER) for i-th spectral channel, i=1,…,I0, TB (i ) and TB (n) (i ) are
the brightness temperatures measured and calculated at n-th iteration step respectively.
For aerosol retrieval we chose 10 spectral intervals (they are marked by vertical lines at the top frame of
Figure 2a) where gaseous absorption is negligible. The aerosol vertical profile we present as the number
density of equivalent particles. We adopted the 75% H2SO4 particles of mode 2 having log-normal sizes
distribution with r0 =1.05 m and 0 =1.21 (Pollack et al., 1980). Actually, what we retrieve by this method is
the vertical profile of the optical depth. This parameter is wavelength dependent, so taking the hypothesis
about the equivalent particles, we accept the wavelength dependence of aerosol extinction cross section for
them as the first approximation, which then may be corrected.
The relaxation equation for number density of equivalent particles is
I0
I0
i 1
i 1
N ( n 1) ( Pj )  N ( n ) ( Pj )   I ( n ) ( i ) / I ( i )  K ij   i /  K ij   i
(5)
where I (i ) and I (n) (i ) are the brightness measured and obtained at n-th iteration respectively. At some level
j in the atmosphere the aerosol profile is corrected taking into account the weighted input of all spectral
channels, for which the weighting functions at this level are non zero. The weighting functions are
recalculated at each iteration step.
The absorption coefficients of CO2 are temperature dependent, so it is necessary to recalculate transmission
functions during temperature retrieval procedure. For this purpose we use the fast interpolative algorithm,
which allows to recalculate transmission functions convoluted with instrumental profile (Spankuch et al.,
1990, Zasova et al., 1998b, Zasova et al.,1999). Transmission of the atmosphere, tj, from the upper boundary
to the level j for the temperature profile Tj (j=1,...,J0) is determined by
ln t j  ln t j 1  (ln t  ln t
0
j
0
j 1
 Tj0 
) 
T 
 j
 j ( )

Tj  Tj0 


exp  j ( )
0 

Tj Tj 

where tj0 is the transmission function for nominal temperature profile Тj0 , and
Tj  Tj 1
Tj0  Tj01
0
,
.
Tj 
Tj 
2
2
For the upper boundary of the atmosphere t1 = 1.
(6)
(7)
To define the coefficients j() and j() the transmission functions for three temperature profiles (nominal,
minimal and maximal) were calculated line by line (using HITRAN 96 database) and convoluted with
instrumental function. The coefficients j() and j() were calculated for each j-th level in the atmosphere
in the range 0.001 – 1200 mb and for each i-th wavenumber. The nominal profile was adopted as the average
over all temperature profiles, taken from the earlier Venera 15 database. The minimal and maximal
temperature profiles were chosen in a way to be sure that all temperature profiles to be retrieved are inside
the interval (see Figure 4 in Zasova et al., 1998b). The error of transmission, obtained for temperature
profiles, which are more or less parallel to model ones, does not exceed 0.02%, but for more complicated
temperature profiles it may reach 2% in the Q-branches.
As we interpolate the convoluted transmission functions, the angular dependence of transmission functions
should also be investigated. It was found that the angular dependence of the convoluted transmission function
may by presented as (Spankuch et al., 1990):
0.64
(8)
t ( )  t (0) (1 /  )
where =cos() and  is the zenith angle of observation. This experimentally obtained relationship gives
accurate presentation of angular dependence for   600. At higher zenith angles the error may exceed 1%.
To obtain the temperature and aerosol profiles we alternate the iterations for aerosol and temperature, starting
with some initial temperature and aerosol profiles. The convergence is considered to be achieved, when the
difference between calculated and measured spectrum becomes less than NER and when the convergent
process stopped. If convergence stopped, but the difference between measured and calculated spectra still
exceeds NER, then we restore the spectral dependence of aerosol extinction cross section, using the simple
equation
0
(9)
Qi  Qi  I ( i ) / I ( n) ( i )
where Qi is the extinction cross section in the aerosol channels. Note, that actually we retrieve the aerosol
optical depth, which is
(10)
 a ( i , Pj )    Qi  r 2  N ( Pj )  H a
where the angular brackets show the extinction cross section, averaged over particle sizes distribution, r is
the particle radius, Ha is the aerosol scale height.
RESULTS
In Figure 1 one can see the normalized extinction efficiencies Q(i )/Q(1218) vs. wavenumber for different
modes of particles, observed in the Venus clouds (Pollack et al., 1980). The spectral channel 1218 cm-1 is the
most opaque aerosol channel. There is no crucial difference in spectral dependence of normalized extinction
between mode 1 (r0 =0.15 m), mode 2 (r0 =1.05 m) and mode 2’ (r0 =1.4 m). The particles of these modes
are small comparing to the wavelength in our spectral range, so the normalized efficiency of extinction falls
down steeply, when the wavelength increases. Its value reaches 0.1 near 300 cm-1.
1.4
4
1.2
Q(V) / Q(1200)
1.0
0.8
3
0.6
2
0.4
1
0.2
0.0
100
300
500
700
900
1100
1300
1500
1700
Wavenumber, cm-1
Fig. 1. Normalized extinction efficiency for log-normal particle sizes distribution: 1 –
mode 1, r0 = 0.15 m,  = 1.91; 2 – mode 2, r0 = 1.05 m,  = 1.21; 3 – mode 2’, r0 = 1.4
m,  = 1.23; 4 – mode 3, r0 = 3.85 m,  = 1.30.
200
280
400
600
800
1000
1200
1400
1600
280
3
260
260
5
1
Tb, K
240
240
6
2
220
220
4
200
200
180
180
160
200
400
600
800
1000
1200
1400
160
1600
Wavenumber, cm-1
Fig. 2a. The spectra, averaged over typical regions: 1 -  < 35, LS = 20 - 90; 2 -  < 35,
LS = 270-310; 3 - 10 <  <+10, LS = 75; 4 – ‘cold collar’,  = 60 - 80; 5 – N-pole,  >
85; 6 – ‘hot dipole’,  = 75 - 85, 7 - warm areas,  = 60 - 80. The vertical lines at the
top show the position of the spectral channels, which were chosen for aerosol retrieval.
5
4
2
- Log P, b
3
3
1
2
6
4
1
7
5
0
150
170
190
210
230
250
270
290
310
330
350
Tempetrature, K
Fig. 2b. Temperature profiles corresponding to the spectra in Fig. 2a.
The wavelength dependence of normalized extinction of mode 3 particles is quite different (curve 4 in Figure
1). It does not fall down so drastically in the long wavelength part of spectrum. Hence, mode 3 particles may
be distinguished from mode 2 particles using our observations.
In general, the temperature and aerosol vertical profiles have been retrieved using about 2000 spectra for
latitude range from 20 to 87 N, for local time 4 - 10:30 AM and 4 - 10:30 PM. For one single orbit the
latitude interval was from 20N to 65S near 7 AM local time.
The spectra, averaged over typical regions are presented in Figure 2a. Three spectra for low latitudes are
shown, all for the day side: in the morning (1), in the afternoon (2), and at 7 AM (3). Spectrum 3 corresponds
to the nadir observations; for the cases (1) and (2) the averaged angle  is about 60. A minimal temperature
in the center of 15 m CO2 band (which corresponds to the temperature at the altitude 85-95 km) was
obtained for the afternoon observation. Note, that the temperature inversion at these altitudes was obtained
for the morning hours. At high latitudes the temperature variations in this altitude range are not too
pronounced, and the temperature is systematically higher than at low latitudes. Outside of the 15 m CO2
band the main radiation comes from the clouds. The most transparent range is near 365 cm-1, where radiation
comes mainly from the altitudes of 55-60 km at all latitudes. The highest temperature was observed in the
equatorial spectrum, which agrees with the well known fact that the temperature decreases towards high
latitudes at these levels (see for example Seiff et al., 1985). In a short wavelength part of the spectrum the
radiation comes from the levels near the upper boundary of the clouds. The shape of the spectra reveals that
the aerosol profile should have significant scale height at low latitudes, and also in the ‘warm’ areas at the
latitudes of the ‘cold collar’ (curve 7). For high latitudes we usually have a sharp upper boundary of the
clouds, observed in the thermal IR. The hot and isotopic bands of CO2, which are clearly seen in the ‘hot
dipole’ spectrum in absorption and in the ’cold collar’ spectrum in emission indicate the low altitude of the
clouds. In the short wavelength part of the spectra the highest brightness temperatures correspond to the ‘hot
dipole’, and the lowest ones relate to the ‘cold collar’.
Temperature profiles, retrieved from these averaged spectra, are presented in Figure 2b. The
corresponding equivalent number densities are shown in Figure 2c. Aerosol optical depth in the
4
3
3
7
2
5
6
1
1
2
4
0
10
-1.0
10
0.0
10
1.0
10
2.0
10
3.0
10
4.0
10
5.0
Nubmer density if equivalent particle,
Fig. 2c. Aerosol vertical profiles corresponding to the spectra in Fig.2a. Curve 4,
aerosol vertical profile for the ‘cold collar’ is number density of mode 3 particles.
most opaque spectral regions are given in Figures 2d. By comparing the morning and the afternoon
temperature profiles, one can see that the temperature is lower in the afternoon at the altitudes above 85
km, but is higher between 75 and 85 km, and becomes lower again below 75 km. A comparison of the
aerosol vertical profiles for the morning and afternoon sides reveals that the clouds upper boundary is
higher in the afternoon. The major temperature variations take place above 1 mb (80 km) level at low
latitudes, and below 10 mb level at high latitudes. The most pronounced temperature inversion below 100
mb is observed in the ‘cold collar’ (curve 4, Figure 2a-d). The lowest position of the clouds was observed
there. The obtained spectral dependence of aerosol extinction coincides with the curve 3 in Figure 1
(mode 2) for ‘warm’ areas and for mode 3 (curve 4) for the coldest parts of the ‘cold collar’, where
position of the upper boundary of clouds is the lowest one. In the ‘cold collar’ a simple rule fulfils: the
lower is the temperature, the lower is the upper boundary of the clouds. Retrieved values of normalized
extinction coefficient vs. wavelength for the latitudes 60-80 N fills the space between curve 3 for mode 2
and curve 4 for mode 3 particles in Figure 1. Hence, the clouds are composed of mixture of small and
mode 3 particles in such a way, that the content of mode 3 particles increases when the upper boundary of
clouds lowers. Another IR feature, the ‘hot dipole’ (curve 6), differs from the surrounding polar region
(curve 5) by lower position of clouds and by the absence of inversion in the temperature profile near 58
km, which usually presents in the polar region. This inversion probably dissipates in the downward flux.
-log p, b
75
2.000
73
71
1.575
Altitude, km
69
67
2
65
7
5
1
1.150
3
63
4
61
0.725
6
59
57
55
0.300
0
1
2
3
4
5
6
Aerosol optical depth at 1218 cm-1
Fig. 2d. Vertical profile of aerosol optical depth (curves are labeled as in Fig. 2a)
The hypothesis about the composition of the clouds of mode 2 particles does not contradict to the
observations of the other regions of the planet. However, one can see in Figure 2c that at low latitudes
below 57 km the number densities of mode 2 particles reach the value of 1000 cm -3. In this altitude range
our measurements are sensitive to the aerosol profile, because the optical depth in the spectral range near
365 cm-1 is about 1 there. The ratio between absorption cross section for mode 2 and 3 particles is about
100. Hence, the density 1000 cm-3 of mode 2 particles in the spectral range 365 cm-1 is equivalent to 10
cm-3 for mode 3 particles. It is in a good agreement with the measurements of Pioneer Venus
(Knollernberg and Hunten, 1980).
It is quite natural to accept that the mode 2 particles prevail in the upper clouds in the ‘hot dipole’: large
particles may be evaporated in the downward flux there. In the polar region the upper boundary of clouds
is sharp, so our spectra are not so sensitive to the changes of the spectral dependence of absorption.
0
60
180
240
300
u()=-98.44+5.78*cos(+89.64)+9.36*cos(2*+88.65)+
+0.19*cos(3*+32.99)+3.52*cos(4*+34.69)
-120
Zonal wind speed, m/s
120
360
-120
-110
-110
-100
-100
-90
-90
0
60
120
180
240
300
360
Solar longitude, deg
Fig 3. Zonal wind speed in the midlatitude jet vs. solar longitude. “+” relate to the data
averaged over 200 of solar longitude, “x” relate to the individual sessions.
The thermal zonal wind field was obtained without global averaging, for each individual session. This
approach enables to restore the meridional structure of zonal wind. The results of determination of tidal
components in the midlatitude jet are presented in Figure 3. The solar related periods of 1, 1/2, 1/3, 1/4
days were found with maximal amplitude for 1/2 day period. It was also discovered that parallel with
variations of wind speed, the jet changes its position, both by latitude and altitude, in such a way that the
correlation between wind speed and its latitude and altitude corresponds to the laws of the conservation of
momentum and flux (Khatountsev and Zasova, 1997, Zasova et. al., 1999). The global averaged
temperature and zonal wind fields are presented in Figure 4. The midlatitude jet, connected with the ‘cold
collar’, reveals itself near latitude 50 and altitude 70 km, close to the upper boundary of the clouds. The
low latitude jet at the latitude near 20 N and altitude about 85 km evidently also exists, being connected
with the temperature inversion, observed at the altitude about 90 km. It is also possible to admit that there
is one more jet, which locates at low latitudes near cloud tops, being related to not very pronounced
temperature inversion there. This inversion was found the first time by VEGA descent probe (Linkin et al.
1987).
CONCLUSION
The new analysis of Venera-15 data is in progress now. We plan to analyze the obtained temperature and
aerosol database in several directions. One of them, which we consider as the most important, is the
investigation of the solar related structures found in the temperature fields, in the cloud distribution and in
the thermal zonal wind fields.
10
20
30
40
50
60
70
4365
55
51
5
25
45
5
10
80
25
5
80
115
35
11
5
1911505
35785 455 35 15
25
85
75
70
215515 355
5
4
2
55
65
75
85
90
105
5
10
95
60
35
70
Altitude, km
90
25
Altitude, km
55
90
60
170
180
90
190
200
210
80
80
220
230
70
70
10
240
250
260
60
20
30
40
50
60
60
70
Lalitude, deg
Fig. 4. Global averaged thermal zonal wind field (upper) and temperature fields (lower).
This work is supported by grant 98-02-17363 of Russian Foundation of Basic Research.
REFERENCES
Chahine M.T. Inversion problem in radiative transfer:determination of atmospheric parameters. J.Atmosph.
Sci., 27, 960 (1970).
Dubois R., L. V. Zasova, D. Spankuch, V. I. Moroz, K. Schafer et al. Thermal structure of the middle
atmosphere of Venus from Venera-15 data. Veroffenlichungen des Forschungsbereichs Geo- und
Kosmoswissenschaften, 18, 99-119 (1990).
Ignatiev N.I., Moroz V. I., Zasova L.V., Khatuntsev I. V. Venera-15: Water vapor in the atmosphere of
Venus. COSPAR, Nagoya,1998. Adv. Space Res. 1999 (be printed).
Khatountsev I.V., Zasova L.V.; Wind on Venus: a new interpretation of Venera-15 data; IX Rencontres
de Blois; (1997).
Knollenberg, R. G., and Hunten, D. M. The microphysics of the clouds of Venus: Results of the Pioneer
Venus particle size spectrometer experiment, J.Geophys. Res., 85, A13, 8039-8058 (1980).
Linkin, V. M., J. Blamont, S. 1. Devjatkin, S. P. Ignatova, V. V. Kerzhanovich, et al., Thermal Structure
of Atmosphere of Venus According Results of Measurements on VEGA-2 landing probe, Kosmich.
issled., 25, N5, 659-672 (1987)
Moroz V. I., Spankuch D., Linkin V. M., Dohler W., Matsygorin I. A. et al. Venus spacecraft infrared
spectra. Applied Optics, 25, N10 (1986)
Moroz V.I., D. Spankuch, D.V. Titov, K. Schafer, A.V.Dyachkov, W. Dohler, L. V. Zasova, D. Oertel, V.
M. Linkin, and Nopirakowski. Water vapor and sulfur dioxide abundances at the Venus clouds tops
from the Venera-15 Infrared spectrometry data. Adv. Spacve Res., 10, N5, 77-81 (1990)
Oertel D., D. Spankuch, H. Jahn, H. Becker-Ross, W. Stadthaus, J. Nopirakowski, W. Dohler, K. Schafer,
J. Guldner, R. Dubois, V.I.Moroz, V/M/ Linkin, V.V. Kerzhanovich, I. A. Matsygorin, A. N.
Lipatov, A.A. Shurupov, L. V. Zasova, E.A. Ustinov. Infrared spectrometry from Venera-15 and
Venera-16. Adv. Space Res., 5, N9, 25 (1985)
Pollack J.B., Toon, O.B., Whitten, R.C., Boese, R., Tomasko, M., Esposito, L.,Travis, L., and Wiedman,
D. Distribution and source of the UV absorption in Venus atmosphere, J. Geophys. Res., 85, A13,
8141-8150 (1980).
Schafer K., Zasova,L.V., Spankuch,D., et al. Structure of the middle atmosphere of Venus from analyses
of Fourier-spectrometer measurements aboard Venera-15. Adv. Space Res., 7, N12, 17 (1987).
Schafer K., Dubois,R., Haus,R., K. Dethloff, H.Goering, D. Oertel, H. Becker-Ross, W. Stadthaus, D.
Spankuch, V. I. Moroz, L.V.Zasova, I.A. Matsygorin. Infrared Fourier Spectrometer experiment
from Venera-15. Advances in Space Res., 10, N5, 57 (1990).
Schofield J.T. and Taylor F.W. Measurements of a mean , solar-fixed tenperature and cloud structure of
the middle atmosphere of Venus. Quart. J.R. Meteor. Soc.,109, 57-87 (1983)
Seiff A.J., Schofield J.T., Kliore A.J. et al. Model of the structure of the middle atmosphere of Venus
from the surface to 100 km. Adv. Space Res., 5, N11, 3-60 (1985)
Spankuch D., L.V. Zasova, K. Schafer, E.A.Ustinov, J.Guldner et al.Infrared experiment aboard the
automatic interplanetary stations Venera-15, Venera-16. Preliminary results of temperature retrieval.
Veroffenlichungen des Forschungsbereichs Geo- und Kosmoswissenschaften, 18, 28-46 (1990).
Twomey S., D.Herman, R.Rabinoff. An extension of the Chahine method of inverting the radiative transfer
equation. J. Atmosph. Sci., 34, 1085 (1977).
Zasova L.V., Spankuch, D., Moroz, V. I., R. Dubois, V. M. Linkin et al. Venusian clouds from Venera-15
data. Veroffenlichungen des Forschungsbereichs Geo- und Kosmoswissenschaften, 18 (1990).
Zasova L.V. and V.I.Moroz. Latitude structure of the upper clouds of Venus. Adv. Space Res., 12, 79-90
(1992).
Zasova L.V., V.I.Moroz, L.W.Esposito and C. Na. SO2 in the middle atmosphere of Venus: IR and UV
data. .Icarus, 105, 92-109, (1993).
Zasova L.V. Structure of the middle atmosphere of Venus and its upper clouds. Bull.Amer.Astron.Soc.,26,
N3, 1145, (1994)
Zasova L.V. The structure of the Venusian atmosphere at high latitudes.Adv. Space Res.,16, 89-98 (1995).
Zasova L.V. and I.V. Khatountsev. Zonal wind in the Venus middle atmosphere, Adv .Space Res., 19, N8,
1191-1190 (1997).
Zasova L.V., V.I. Moroz , I.A.Khatountsev and N.I.Ignatiev. Structure of the Venus mesosphere from
reanalyzed Venera 15 IR-spectrometry data. Bull. Amer. Astronom. Soc., 30, N3, p.1105, (1998a)
Zasova L.V., V.I.Moroz, N.I. Ignatiev and I.V.Khatountsev . Water Vapor in the middle atmosphere if
Venus from Venera-15 Fourier specrtometry data. Kosmich. Issled., 36, N6 (1998b)
Zasova L.V., V. M. Linkin, I. A. Khatountsev . Thermal zonal wind in the Venus middle atmosphere.
Kosmich. Issled. (1999) (be printed).