Structure of the Venus atmosphere

ARTICLE IN PRESS

Planetary and Space Science 55 (2007) 1712–1728 www.elsevier.com/locate/pss

Structure of the Venus atmosphere L.V. Zasovaa,b,, N. Ignatieva,b, I. Khatuntseva, V. Linkina a

IKI RAS, Moscow, Russia b IFSI INAF, Rome, Italy

Accepted 10 April 2006 Available online 1 February 2007

Abstract The structure of the Venus atmosphere is discussed. The data obtained in the 1980s by the last Soviet missions to Venus: orbiters Venera 15, 16 and the entry probes and balloons of Vega 1 and 2 are compared with the Venus International Reference Atmosphere (VIRA) model. VIRA is based on the data of the extensive space investigations of Venus in the 1960s and 1970s. The results of the IR Fourier Spectrometry experiment on Venera 15 are reviewed in detail. This instrument is considered as a precursor of the long wavelength channel of the Planetary Fourier Spectrometer on Venus Express. r 2007 Elsevier Ltd. All rights reserved. Keywords: Venus; Temperature; Clouds; Wind; Tide; Water vapor; Sulfur dioxide

1. Introduction All in all 15 Soviet and seven American missions have explored Venus since 1962. The full history was described and chronologically tabulated by Moroz et al. (2002) and Huntress et al. (2003). Venus was investigated most extensively in the decades of the 1960s and 1970s. The American flyby missions (Mariner 2, 5 and 10), the Soviet flybys which delivered entry probes (Venera 4–8 and 11–14) and the orbiters with entry probes Soviet Venera 9 and 10 and American Pioneer Venus (PV), together collected a lot of data about the planet. Those concerning the atmosphere were systematized to produce the Venus International Reference Atmosphere (VIRA). It was published in Advances in Space Research in 1985 (Kliore et al., 1985). The first chapter of VIRA is devoted to the structure of the atmosphere below 100 km (Seiff et al., 1985). During the 1980s and 1990s investigations of Venus were not so extensive, however several spacecraft added some important data about the Venus atmosphere. In 1983–84 Venera 15 and 16 orbiters investigated the planet using an important atmospheric instrument, the Fourier spectrometer Corresponding author. IKI-RAS, Moscow, Russia. Tel.: +7095 3333466 fax: +7095 3334455. E-mail addresses: [email protected], [email protected] (L.V. Zasova).

0032-0633/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2007.01.011

(FS) for the thermal IR spectral range (Oertel et al., 1985, 1987; Moroz et al., 1986). Both spacecraft also carried out radio occultation experiments (Yakovlev et al., 1987a, b, 1991). In 1984 Vega 1, 2 missions delivered the entry probes and balloons to Venus (Linkin et al., 1986, 1987; Crisp et al., 1990). During the Galileo flyby of Venus in 1990 the measurements by the near infrared experiment NIMS obtained important information about the atmosphere and lower clouds on the nightside of Venus (Belton et al., 1991; Carlson et al., 1991; Drossart et al., 1993; Grinspoon et al., 1993; Roos-Serote et al., 1995). Earth based observations of the nightside of Venus have provided a lot of information about the atmosphere below the clouds (Bezard et al., 1990; Pollack et al., 1993; Meadows and Crisp, 1996; Taylor et al., 1997). In the early 1990s the radio occultation experiment of the Magellan mission obtained high resolution temperature/ density profiles of the atmosphere from 35 to 95 km (Jenkins et al., 1994; Hinson and Jenkins, 1995). The most recent investigations of the nightside of Venus from a spacecraft were carried out by Cassini/VIMS (Baines et al., 2000). 1.1. Thermal structure of the atmosphere from the surface up to 60 km The temperature profile below 60 km (VIRA, Seiff et al., 1985) was constructed using data from Veneras 10–12

ARTICLE IN PRESS L.V. Zasova et al. / Planetary and Space Science 55 (2007) 1712–1728

(Avduevskiy et al., 1983) and PV probes (Seiff, 1983). Below 12 km the only available data were based on Venera 10 measurements. Analysis of the temperature differences between the experiments (typically several Kelvins) showed that these discrepancies can be a result of differences in latitude, local Venus time, and possibly due to different gravity wave amplitude and phase. Nevertheless no clear evidence was found for a steady variation of temperature with latitude or with Venus local time. For this reason a universal model was proposed to represent the deep atmosphere for all latitudes. Actually, the local temperature may differ from the model by up to 10 K, but more typically by 5 K. The last Soviet Venus entry probe VEGA-2 provided new temperature and pressure measurements between 62 km and the surface (Linkin et al., 1987; Moroz et al., 1996). These measurements have a higher accuracy than similar experiments on other Soviet probes. The temperature sensors on the PV probes failed below 12 km. So, high precision temperature measurements at these altitudes were provided by VEGA 2 for the first time. The temperature profile obtained by VEGA 2 is consistent in general with the VIRA basic model (see Fig. 1a). However, there are some peculiarities found below 15 and above 58 km. A difference between the VIRA model temperature profile and those obtained from VEGA 2 is presented in Fig. 1b. One may see that the temperature measured by VEGA 2 is systematically lower, besides the narrow altitude range around 45 km and above 60 km altitude: at 60 km an inversion in the temperature profile is observed by VEGA 2. The differences in the lower layers may be explained by the uncertainties of the VIRA profile. However, real variation with local time and local position are not excluded even in the deep atmosphere. The maximal difference from the VIRA model, 8 K, is reached at 52–57 km altitude, in the middle cloud layer, and above 60 km in the upper clouds. Below the clouds,

1713

from 40 to 48 km altitude this difference is in the range of 4 to þ2 K. The static stability profile calculated from the temperature profile of VEGA 2 entry probe is compared (in Fig. 2) with the VIRA static stability profile. Both profiles are in a good agreement: the atmosphere is generally stable except in two altitude intervals, 49–55 km, and below 30 km. So the atmosphere of Venus (unlike the Earth) has two convective zones. The lowest possesses a double structure, because a peak of stability is observed around 15 km. However, such strong fluctuations of stability factor (up to 2 K=km) may be not realistic: it is known that in dense atmosphere its lowest values should be near 0. The peak of high stability around 15 km exists also in the model profile, however it is not so pronounced there. Below this peak the atmosphere is slightly unstable down to the surface in the case of the VIRA profile, but in the VEGA 2 profile

Fig. 2. Comparison of the static stability profile obtained from VEGA 2 temperature profile (solid line) with the VIRA model.

Fig. 1. (a) Comparison of the temperature profile for lower atmosphere from VIRA with those measured by VEGA 2 probe and (b) difference between Vega and VIRA temperature profiles.

ARTICLE IN PRESS 1714

L.V. Zasova et al. / Planetary and Space Science 55 (2007) 1712–1728

the highest instability was observed in the range of 2–4 km altitude. Measurements on VEGA 1 and 2 balloons were carried out successfully during their 40 h journey in the middle clouds (Linkin et al., 1986). They covered a local time range from midnight to 8 AM, and the full length of their travel path was about 11 000 km. The VEGA I balloon moved almost exactly along the parallel of 8 North latitude, and VEGA 2 was meridionally shifted at about 500 km southward. The extreme values of pressure and temperature along the trajectories of the balloons are the following. It was found that temperature vs. pressure obtained along the trajectories of the VEGA balloons (Linkin et al., 1986) have a systematic difference of a few K, which may be considered as an evidence of the extended non-mixing atmospheric regions. VEGA 1 balloon was floating in more stable atmosphere (the static stability changes from 0 to þ2:5 K=km) than the VEGA 2 balloon (Crisp et al., 1990), which met instabilities, especially, at the beginning of its travel (the obtained values of static stability changes in the interval þ2:5 to 9 K=km, however, the latter value seems to be physically unreal). The vertical temperature profiles measured by both balloons have vertical gradient comparable to that of 96% CO2 and 4% N2 adiabatic lapse rate (Crisp et al., 1990). Several temperature and static stability profiles were also obtained by the Magellan radio occultation experiment with high vertical resolution. Three of them were published (Hinson and Jenkins, 1995). They have a wave-like shape, evidently due to gravity waves. The static stability profiles show that the atmosphere is stable to convective overturning above the middle clouds (that is above 57–60 km). For all three temperature profiles an instability may exists in the range of 50–57 km altitude (inside of the middle clouds). This is in a good agreement with both the VIRA model and VEGA 2 entry probe data. The difference between the temperature profiles below 60 km (typically less than 7 K) may be explained by local variation: Vega 1, 2 balloons floating in the middle clouds found up to 10 K temperature variation at isobaric levels (Crisp et al., 1990). We may conclude that the temperature profile below 60 km in the VIRA model is rather representative and the static stability profile includes most of the features, stable and unstable layers, which were confirmed later by more accurate experiments. The differences are in the range of possible local time, spatial and temporal variation. 1.2. Middle atmosphere (60–100 km) VEGA 2 entry probe found a new feature in the lowest several kilometers of the middle atmosphere (Linkin et al., 1987): the temperature inversion above 60 km (Fig. 1). This inversion is typical for latitudes 450 . In the equatorial region it look like a local narrow feature. The temperature profile obtained by the Radio Occultation experiment (RO) on Venera 15 (V15) in the equatorial region (Yakovlev

et al., 1987a, b) has shown a similar inversion at close altitudes, although the quality of their figure in the paper is not good enough to digitize the temperature profile for comparison. So, a temperature inversion in the upper cloud layer may exist also in the equatorial region. Being a narrow feature, it is not available for observations at the vertical resolution of the FS on Venera 15, however variation of temperature gradient was detected in some of the temperature profiles retrieved from FS data at these altitudes at low latitudes. This narrow inversion may be a result of gravity wave activity. The most extensive set of FS data, which appeared after publication of the VIRA model, was provided by (Oertel et al., 1985, 1987; Moroz et al., 1986). More than 1500 spectra in the range from 6 to 40 mm were obtained. Typical spectra for different latitudes are shown in Fig. 3. The shape of the thermal IR spectra is defined by the following factors: (1) temperature profile; (2) aerosol vertical profile, which defines the level from which the radiation escape outside of the gaseous absorption bands; (3) vertical profiles and mixing ratios of the absorbing gases, among them are the CO2, which is the main constituent, and two minor: H2O and SO2. The most pronounced CO2 spectral feature is the 667 cm1 (15 mm) fundamental band. Other CO2 features are the 961 and 1064 cm1 hot bands, and the 1259 and 1366 cm1 isotopic (12C16O18O) bands. The water vapor features are visible in two parts of the LW channel spectrum: the 280–475 cm1 H2O rotational band and the 1590 cm1 (6:3 mm) roto-vibrational fundamental band. There are three SO2 fundamental bands: n2 (519 cm1 Þ, n1 (1150 cm1 Þ and n3 (1360 cm1 Þ. Absorption features belonging to liquid sulfuric acid are at 450, 580, 900, 1150 cm1 (the 580 cm1 feature is in the wing of the 667 cm1 CO2 band and is not seen in the spectra, see Fig. 3). Spectral profiles of the 15 mm CO2 band and spectral ranges free from gaseous absorption were used to retrieve the vertical temperature profile from 55 to 95 km and aerosol profiles in the upper clouds layer (Schafer et al., 1987, 1990; Spankuch et al., 1990; Zasova and Moroz, 1992; Zasova, 1995; Zasova and Khatuntsev, 1997; Zasova et al., 1993, 1999, 2000, 2002, 2004). Most of the observations covered latitude range from 20 to 87 N, but equatorial and south latitudes (up to 65 S) were observed only once in a special session of measurements. Venera 15 worked in a near polar orbit and for each orbit a wide range of latitudes was observed practically simultaneously at almost the same local time. For two months of observations the morning sector from 3:30 to 10:30 AM and evening sector from 3:30 to 10:30 PM were covered by measurements, but the local time near noon and midnight

ARTICLE IN PRESS L.V. Zasova et al. / Planetary and Space Science 55 (2007) 1712–1728

1715

Fig. 3. Averaged spectra, obtained for some typical regions: 1—10 ojo þ 10 , LS ¼ 75 ; 2—warm areas, j ¼ 60280 ; 3—‘cold collar’, j ¼ 60280 ; 4—‘hot dipole’, j ¼ 75285 , 5—j485 . The arrows at the top show positions of spectral channels, free from gaseous absorption, which were chosen for aerosol soundings.

remained uncovered. The field of view (FOV) during the near-pericenter measurements was about 100 km. The lowest level, for which a temperature can be retrieved, depends on the upper clouds structure. The upper clouds are inhomogeneous in the thermal infrared, especially at high latitudes: in the most transparent spectral range the optical depth, equal to unit, reaches at the levels between 54 and 59 km and in most opaque spectral range it takes place at 55–70 km. We compare typical FS temperature profiles with VIRA. We remind that in VIRA for model of middle atmosphere temperature profiles five latitudinal ranges were separated: o30 , 45 , 60 , 75 and 85 . During the equatorial measurements FOV of FS was about 500 km. By this reason the data were averaged over 5 of latitude. In Fig. 4 the IR V15 temperature profile, obtained at 7:30 AM and averaged in the latitude range 0–5 N, is compared with the temperature profile, measured by the PV Sounder probe (at 4 N latitude at 7:38 AM local time). Actually, the local measurements of PV Sounder probe has much higher space and vertical resolution, while the Venera 15 temperature profile in Fig. 4a is averaged over about 500 km FOV. However, a reasonable correspondence is observed: one may compare for example, the position of the temperature inversion (above 80 km) in morning hours. We considered several spectral groups separately, trying to follow the VIRA latitude division. The spectra of these groups one may find in Fig. 3 and the corresponding temperature profiles are shown in Fig. 4b–d. Comparing the morning (2) and the afternoon (3) low latitudinal temperature profiles in Fig. 4b, one can see that the temperature is lower at the altitudes above 85 km in the afternoon, is higher between 75 and 85 km, and becomes lower again below 70 km.

The profiles averaged over the coldest part of the cold collar ((3) in Fig. 4c), observed on the day morning side, and over the warm areas at the same latitudes, observed in the afternoon (4), are compared with the corresponding VIRA 60 and VIRA 75 profiles. One may see (Fig. 4c) that the VIRA 60 temperature profile is representative for the warm areas at the latitudes 55–75 . At higher latitudes the warm areas are associated with the hot dipole. VIRA 75 temperature profile is about 10 K warmer than the observed one in 65–75 km altitude interval. The ‘hot’ dipole temperature profile differs by its higher temperatures below 60 km from the surrounding polar region (Fig. 4d). However, near 65 km altitude its temperature in most cases is lower. This difference is the latitudinal effect in the inversion, which exists near 65 km at lower latitudes. VIRA 85 temperature profile gives up to 10 K higher temperature in the interval 70–75 km. Being practically isothermal up to 75 km, it does not agree with the shape of the FS polar spectra. They are isothermal in the continuum ((4) in Fig. 3), however, the weak gaseous absorption bands, like the water lines and the hot CO2, SO2 bands are present. It indicates that the temperature profile is not isothermal near the cloud tops. The isothermal shape of the continuum is explained by a sharp upper boundary of the clouds rather than isothermal temperature profile. However, some radio occultation measurements give an isothermal temperature profile up to 90 km (see Yakovlev et al., 1991). Might it be a spatial and time variable phenomenon? We may conclude that for the middle atmosphere the VIRA 30 temperature profile is representative as averaged over local time in corresponding latitude range. VIRA 60 presents pretty well a temperature profile in warm areas in the 55–75 range. VIRA 75 and VIRA 85 gives too high

ARTICLE IN PRESS L.V. Zasova et al. / Planetary and Space Science 55 (2007) 1712–1728

1716

a

b

100 4

3

1

4

2 90

80 2

2

2

0 200

240

280

80

2

70

60

1

60

50

0 150

70

1

160

4

3

1

H, km

3

-log (P, b)

-log (P, b)

90

320

50 200

Temperature, K

c

250

300

Temperature, K

d

4

4 90

90 3 80

1 2

70

2 4

1

3

0 150

200

250

-log (P, b)

-log (P, b)

3

80

1

50

0

Temperature, K

70

3

60

300

1

2

60

2

50 150

200

250

300

Temperature, K

Fig. 4. (a) A comparison between the temperature profile (1), obtained from averaged over latitude range 0–5 N FS-spectra at 7:30 AM and measured by Pioneer Venus Sounder and (2) at 7:38 AM and 4 N (Seiff et al., 1985). (b) (1) VIRA fo30 ; (2) FS, jo35 N, LS ¼ 20290 ; (3) FS, jo35 N, LS ¼ 2702310 ; (4) FS, 10 Sojo10 N, LS ¼ 75 . Temperature profile (4) was obtained from spectrum (1) in Fig. 3. (c) (1) VIRA f ¼ 60 and (2) VIRA f ¼ 75 , (3) FS, the coldest part of the cold collar—spectrum (3) in Fig. 3, (4) FS, warm areas at the latitudes of cold collar—spectrum (2) in Fig. 3. (d) (1) VIRA f ¼ 85 , (2) FS, ‘hot’ areas at 75285 N—spectrum (4) in Fig. 3, (3) (dashed line)—FS, f485 N—spectrum (5) in Fig. 3.

temperature values between 70 and 75 km and may agree with some radio occultation temperature profiles, but not with the IR data. 1.2.1. Temperature fields in the middle atmosphere The temperature field, globally averaged and averaged over quadrants of the solar longitude, are presented in Figs. 5 and 6. The data are also averaged over 5 of latitude. In the global average temperature field (Fig. 5) it is clearly seen that the temperature increases from low latitudes to the pole in the altitude range 65–90 km. The temperature minimum is observed around 95 km altitude (at low latitudes) which is associated with the temperature inversion at these altitudes. At high latitudes temperature profiles are close to isothermal below 70 km. Below 65 km the temperature decreases with latitude, up to 65–70 , where the temperature minimum in the cold collar is found at the altitude of 64–65 km (100 mb level). In the polar region (f480 ) between 58–70 km the temperature is practically constant (within 5 K). The global average temperature field, obtained from OIR PV data, was also included in VIRA. Its main features are confirmed by FS measurements. The temperature fields averaged over the measurements inside the quadrants of solar longitudes are shown in Fig. 6. During the daytime Ls ¼ 90220

(LT ¼ 6–10:30 AM) a strong dependence of temperature vs. latitude at isobaric levels is clearly visible. For example, near 80 km of altitude the temperature changes from 180 K at low latitudes (20–35 ) to 210 K at 87 . At upper levels the high values of temperature appear to be typical for this morning time. Temperature minimum of 175 K is observed near 85 km (1 mb level). The cold collar is very pronounced with temperature minimum at 65 latitude and 100 mb level. A low temperature is observed in the polar region (235 K) below 70 km. In the afternoon (Ls ¼ 3102270 , LT ¼ 3:30–6 PM) we have a different picture: a growth of the temperature to high latitudes is observed only below 75–78 km. At 80 km in the polar region temperature of 200 K (compare to 210 K in the morning) is observed. At low latitudes it is 205 K (compare to 185 K in the morning). A temperature minimum (of 155 K) appears at 95 km altitude at low latitudes. Hence, above 85 km the atmosphere cools during the daytime. At high latitudes the atmosphere becomes warmer below 70 km. The cold collar is not pronounced. The temperature in the isothermal (within 5 K) part of the profile (below 70 km) above the N-pole is about 245 K, exceeding by 10 K the morning temperature. During the daytime, at low latitudes atmosphere heats up in the altitude range of 75–85 km, so that at 80 km altitude the temperature increases from 175 K in the

ARTICLE IN PRESS L.V. Zasova et al. / Planetary and Space Science 55 (2007) 1712–1728

1717

4

3 80

H, km

-log10 (P, bar)

90

2 70

1 60 30

40

50

60

70

80

Latitude, deg

Fig. 5. Global averaged temperature field obtained from the Venera 15 IR spectrometry data.

a

b

Ls=20-90°

Ls=270-310°

4

4

90

80

80 2

2 70

70 1

1 60 30

40

c

50 60 Latitude, deg

70

60

80

30

40

d

Ls=90-130°

4

50 60 Latitude, deg

70

80

Ls=200-270°

4 90

-log10 (P, bar)

H, km

3

3

3

90 3

80

80

H, km

-log10 (P, bar)

90

2

2 70

70 1

1 60 30

40

50 60 Latitude, deg

70

80

60 30

40

50 60 Latitude, deg

70

80

Fig. 6. Temperature fields averaged over the quadrants of solar longitude: (a) and (b) correspond to the dayside, (c) and (d) to the nightside. The crosses show the position of the upper boundary of the clouds at n ¼ 1218 cm1 . (Note that the solar longitude at Venus Ls ¼ 0 correspond to noon, Ls ¼ 90 — morning terminator, Ls ¼ 180 —midnight, Ls ¼ 270 —evening terminator.).

ARTICLE IN PRESS 1718

L.V. Zasova et al. / Planetary and Space Science 55 (2007) 1712–1728

morning to 200–205 K in the afternoon. At upper levels the atmosphere cools down so that at 95 km the temperature decreases from 175–180 K in the morning to 155–160 K in the afternoon. Around 60 km altitude the atmosphere is warmer in the morning (by about of 10 K) than in the afternoon. At high latitudes the main changes are observed below 65 km altitude. In the afternoon the mean temperature in the cold collar and polar region exceeds the morning value by about 10–15 K. At the nightside, Ls ¼ 270–200 (LT ¼ 6–10:30 PM) and Ls ¼ 130–90 (LT ¼ 3:30–6 AM) the cooling of the atmosphere below 70 km altitude and the heating above 85 km is observed. The position of the upper boundary of the clouds changes in such a way that it becomes higher when the temperature increases and vice versa. 1.2.2. Thermal tides Thermal tides are very important for the Venus atmosphere. They are generated by the solar energy, absorbed in the middle atmosphere. About 50% of all solar energy, absorbed by Venus, deposit in the upper clouds (Tomasko et al., 1980; Bertaux et al., 1996). Dissipation of the tides produces the energy to support the superrotation (Gierash et al., 1997). A polar orbit of Venus is very suitable for the thermal tide investigations. As it was shown above, the thermal structure of the middle atmosphere of Venus strongly depends on latitude and local time at most levels of the atmosphere. Fifteen levels between 0.1 and 600 mb (55–95 km) were chosen for a detailed study (Zasova et al., 2002). A distance between the levels is chosen about 1–2 km in the clouds and 3–3.5 km above the clouds. The temperature at isobaric levels was presented as a Fourier expansion vs. solar longitude (or local time), containing the first five components Tðp; f; Ls Þ ¼ T 0 ðp; fÞ þ T 1 ðp; fÞ cosðLs þ j1 ðp; fÞÞ þ T 2 ðp; fÞ cosð2Ls þ j2 ðp; fÞÞ þ T 3 ðp; fÞ cosð3Ls þ j3 ðp; fÞÞ þ T 4 ðp; fÞ cosð4Ls þ j4 ðp; fÞÞ,

ð1Þ

where p is the pressure, f is the latitude, T i , ji are the amplitude and phase of the ith harmonics, T 0 is the mean temperature, T 1 is the amplitude of the diurnal tide, T 2 is the amplitude of the semidiurnal tide and so on. The amplitudes and phases depend on latitude and pressure (altitude). The temperature of the atmosphere vs. solar longitude relationships, obtained for several levels (namely: 0.1 mb (95 km), 1 mb (86 km), 10 mb (76 km), 100 mb (65 km), 300 mb (58 km) for low (35 ) and for high (75 ) latitudes) together with the fitting curve, described by Eq. (1), are shown in Fig. 7. The dependence of temperature vs. local time at 0.1 mb level at 35 reveals two minima near the morning (168 K) and evening (158 K) terminators and two maxima near 9 AM and 9 PM, with the morning temperature systematically higher than the evening one. At 75 the

temperature variation is not so pronounced (it remains constant within 6 K interval), being systematically higher on the morning side. At 1 mb level at 35 there are two temperature minima near 10 AM (176 K) and 9–10 PM (174 K), while maximal values reach at 4–5 AM (182 K) and 4 PM (185 K), being higher in the evening. The phase of the thermal tide changed here by about 180 , if to compare with the 0.1 mb level. At 75 a temperature maximum is observed near morning terminator (192 K). In the evening the temperature increases from 186 K at 4 PM to 192 K at 10:30 PM. At 10 mb at j ¼ 35 there are two temperature maxima observed near the morning (218 K) and evening (221 K) terminators. At 100 mb level these maxima are shifted to 10 AM (242 K) and 10 PM (246 K) keeping higher temperature at night. At 300 mb level we observed minimal temperatures near the terminators and maximal ones at night. At 75 at 10 mb level the maximal temperatures took place at 9 AM and the minimal ones—at about 4 AM and 4 PM. At 100 and 300 mb maximal temperature reaches in the afternoon, after that it decreases with time and reaches minimum in the morning near 10 AM. At high latitudes in the cold collar the temperature in the afternoon is higher than in the morning by more than 30 K (see also Table 1). The behavior of the T 1  T 4 amplitudes vs. latitude and pressure is illustrated in Fig. 8a–d. All four components reach maxima in the upper clouds. This is the region of the maximum deposit of solar energy and one can see (Fig. 8c and d) that even the 13 and 14 day components have significant amplitudes there, reaching 2.5 and 1.5 K at 400 mb level correspondingly. The amplitudes of these components reach values of 2 K at high latitudes below 200 mb. Above the clouds, the diurnal component, T 1 , reaches its maximum above 90 km, the semidiurnal, T 2 —at 83–86 km and the T 3 component—at 72–76 km (at low latitudes). Diurnal and semidiurnal amplitudes at chosen isobaric levels are shown in Fig. 9a,b as a function of latitude. At low latitudes at the upper levels 0.1, 0.2 mb (above 90 km), the diurnal component exceeds the semidiurnal one and has the amplitude of 4–5 K. The amplitude decreases below this level having a minimum of 1–2 K at the levels 65–80 km, after that it increases up to a maximum of 5–6 K at 300 mb (58 km). From 0.5 mb (90 km) down to 300 mb (58 km) wave number 2 predominates. It has a maximum amplitude of 7 K at 1–2 mb levels, then its value decreases below these levels and has the amplitude of 2–2.5 K at 50 mb (68 km), after that the amplitude of wave number 2 increases again, reaching a maximum of 8 K at 200 mb. At high latitudes above 72 km both tides have the amplitudes typically o2:5 K. Below 50 mb the amplitude of diurnal tide increases and reaches the maximal value 13 K near 400 mb level in the cold collar. At high latitudes the wave number 2 amplitude (T 2 Þ increases below 200 mb and reaches its maximum near 400 mb (8 K). Both tides have the amplitude minimum between 40 N and 50 N.

ARTICLE IN PRESS L.V. Zasova et al. / Planetary and Space Science 55 (2007) 1712–1728 75°

Temperature, K

0.1 mb

190 186 182 178 174 225

178 176 174 172 170

0.1 mb

1 mb

194 192 190 188 186

1 mb

10 mb

230 228 226 224

10 mb

220 215 210 250 246 242 238 234

Temperature, K

35° 180 175 170 165 160

1719

100 mb 230 100 mb 220 260 250 240 230 220

300 mb

273 268 263 258 253

300 mb

0

100

200

300

0

100

200

300

Solar longitudes, degrees

Fig. 7. Some examples of the temperature behavior at isobaric levels (0.1, 1, 10, 100, 300 mb) averaged over 10 of latitude and 10 of solar longitude for low (35 ) and high (75 ) latitudes. The fitting curves follow the Fourier expansion Eq. (1) for 75 .

Table 1 Floating parameters of the VEGA 1 and 2 balloons Insertion

VEGA 1 VEGA 2 a

T. P. at the floating height

Date

UT

Lat.

Long

Loc. time

P (mb)

T (K)

June 1984

(h)

(deg)

(deg)

(h)

a

b

a

b

11 15

2.1 2.1

8N 7.5S

77 180

0.3 1.0

540 535

630 900

308 302

322 338

At maximal floating height (54 km). At minimal floating height (VEGA 1—53 km, VEGA 2—50 km).

b

The results by Zasova et al. (2002) may be compared with those obtained from OIR (Schofield and Taylor, 1983; Elson, 1983). In the latter paper the components with wave numbers 1 and 2 are shown in their Fig. 1 for altitudes 90, 80, 70, and 65 km (brightness temperatures in four OIR channels were attributed to the levels, where corresponding weighting functions reach their maxima). Elson (1983) concluded that the wave number 2 is dominant in most cases. In turn we found from the Venera 15 data that T 2 exceeds T 1 in the altitude range 58–90 km at low latitudes. At high latitudes Elson claimed a dominance of wave number 1 at 70 and 65 km, which is also in a good agreement with our results: T 1 exceeds T 2 at high latitudes in the range of 55–72 km. We have not only qualitative but also good quantitative agreements. Taking into account the 5 years time interval between the observations of Venera 15 and PV, it means that the solar-related structures are quite

stable in the middle atmosphere. A comparison, made in paper by Zasova (1995) between the effective temperature distributions (Taylor et al., 1983) and temperature at 100 mb level (FS) vs. solar longitude and latitude, confirm that the main solar related features (position of maxima and minima in the cold collar and at low latitudes) are the same in both pictures. 1.2.3. Thermal zonal wind The geometry of the observations of Venera 15 allows to obtain the map of the thermal wind without global averaging (Zasova et al., 2000). In Fig. 10 we show the zonal wind fields (b) for the morning day (20–90 ) and night (90–130 ), and for the evening night (200–270 ) and day (270–310 ) sides. One can see that the midlatitude jet, connected with the cold collar, changes its shape and position. The strongest wind was observed in the morning

ARTICLE IN PRESS L.V. Zasova et al. / Planetary and Space Science 55 (2007) 1712–1728

1720

b

a

3.0

2.0 70

1.5

H, km

2.5

80

2.5

- log (P, b)

80

2.0 70

1.5 1.0

1.0

60

60 0.5

0.5 20

30

40

50

60

70

20

30

40

Latitude,deg

50

60

70

Latitude,deg

d

c

90

3.5

90

3.5

3.0

2.0 70

2.5

80

60

H, km

2.5

H, km

80

- log (P, b)

3.0 - log (P, b)

H, km

3.0 - log (P, b)

90

3.5

90

3.5

2.0 70

1.5

1.5

1.0

1.0

60

60 0.5

0.5 20

30

40

50

60

70

20

30

40

Latitude,deg

50

60

70

Latitude,deg

Fig. 8. Amplitude of the harmonics (Eq. (1)) vs. the solar longitude for wave numbers 1(a), 2(b), 3(c), and 4(d). Approximate altitudes are shown along the right y-axis.

0.1

4

0.2

3

1

2

5 0.5

1

Temperature, K

Temperature, K

8 5

T1 2

0.1

4

2

2 5

10

0

0

1

20

0.5

0.2

10

10

T2

1

6

30

40

50

60

70

80

1

10

20

30

500

T1

Temperature, K

Temperature, K

8

400

12

300

8 200 100

4

40

50

60

70

50

400

T2

300

200

6

500 100 50

4 2

20

20

0

0

1

10

20

30

40

50

80

Latitude

Latitude

60

70

80

Latitude

1

10

20

30

40

50

60

70

80

Latitude

Fig. 9. Amplitudes T 1 and T 2 as a function of latitude for chosen pressure levels. Pressure values (mbars) are given for every curve.

(Fig. 10). The detailed analysis of these variations one may find in Zasova et al. (2000). A thermal zonal wind field was obtained for each individual session. This approach enables to restore the

meridian structure of zonal wind. The results of determination of tidal components in the midlatitude jet are presented in Fig. 11. The wind speed variation with local time has a periodic solar related character. We obtained the

ARTICLE IN PRESS L.V. Zasova et al. / Planetary and Space Science 55 (2007) 1712–1728

1721

Fig. 10. Zonal wind speed (m/s) in the middle atmosphere in coordinates (latitude–altitude), averaged over quadrants of solar longitude. The intervals of solar longitude for each quadrant are given along vertical axis.

Zonal wind speed, m/s

-120

-110

-100

-90

0

60

120

180

240

300

360

Solar longitude, deg

Fig. 11. Zonal wind speed in the midlatitude jet, obtained from each individual session, vs. solar longitude (local time) together with the approximating curve, which gives the amplitude and phase of each harmonic (Zasova et al., 2000). —from individual orbits, þ averaged over 10 of solar longitude.

amplitudes and phases of the components with periods of 1, 12, 13, 14 Venus days. The maximal amplitude (about 10 m/s) has the semidiurnal tidal component. Wind speed is maximal at 9–10 AM and minimal in the afternoon. It was also discovered that parallel with variation of the wind speed, the jet changes its position, both by latitude and altitude. As a boundary conditions we took a wind speed at Pref ¼ 280 mb (at about 58 km altitude) using the equation, proposed by Newman et al. (1984), for which the coefficients were adjusted to satisfy the measurements of Veneras, VEGA 1, 2, PV (more details see in Zasova et al., 2000). For the morning sector, Ls ¼ 20–130 (‘þ’ in Fig. 12a) wind speed in the midlatitude jet may be described by the following equation: umax ðjÞ ¼ 77:75  cosðjÞ  55:76.

(2)

For the evening sector Ls ¼ 200–310 (‘’ in Fig. 12a) umax ðjÞ ¼ 72:06  cosðjÞ  60:51.

(3)

Correlation between the wind speed in the jet and its altitude is described by the following exponential law: umax ðhÞ ¼ 83:86  1:25  eðh57:46Þ=4:11 .

(4)

Eq. (4), which describes this variation, has a dominator under exponent close to the scale height of the atmosphere at these altitudes of 4–5 km. 1.3. Upper clouds The main clouds lie below 70 km. It consists of three layers: upper (roughly, 60–70 km altitude), middle (50–60 km) and lower (48–50 km). Sulfuric acid, which is the main compound of the clouds, is created by photochemistry in the upper clouds. SO2 and H2O are the gases

ARTICLE IN PRESS L.V. Zasova et al. / Planetary and Space Science 55 (2007) 1712–1728

1722

-120

Zonal wind speed,m/s

-120

a

b

-110

-110

-100

-100

-90

-90

-80

-80 40

45

50

55

60

65

70

Latitude, deg

60

62

64

66

68

70

72

Altitude, km

Fig. 12. Dependence of the wind speed in the midlatitude vs. its position, latitude (a) and altitude (b).

precursor of the clouds particles (Yung and De More, 1982; Esposito et al., 1997). Upper clouds are homogeneous without details in the visual spectral range: contrasts (brightness variation) do not exceed several percent. In the UV the contrast reaches 30%. An ‘unknown UV absorber’ is responsible for the contrasts at l40:32 mm (Esposito et al., 1997). Radiation comes from the upper boundary of the clouds (68–70 km) in this spectral range. In the thermal IR the contrasts (related to 100 mb level) reaches three times, between the coldest part of the collar and polar hot dipole. A warm polar region is surrounded by the cold collar. It was discovered (Taylor et al., 1980) by experiment OIR on PV Orbiter and investigated later by FS on Venera 15. The temperature differences between hot dipole and cold collar near upper boundary of the clouds may exceed 50 K. In the UV images obtained by Mariner 10 the polar vortex looks like as a single feature, centered at 86 N (Limaye, 1985). UV images from PV does not reveal a polar vortex, may be due to a thick haze at high latitudes, observed during PV mission (Esposito et al., 1983, 1997; Esposito, 1984). The dipole structure is conserved at least up to late 1983 in the Venera 15 observations. In the upper clouds the particle sizes distribution is bimodal: mode 2 with a mean radius of 1 mm and mode 1 (submicron haze) with mean radius of o0:2 mm. The latter may be found up to 90 km. The haze is variable in time: ground based polarimetric observations of late 60th and early 70th of a whole Venus disk showed very narrow particle sizes distribution with effective radius of 1 mm, which means the absence of submicron haze. However, it was found by the polarimetric experiment on PV that in 1978 the whole disk of Venus was covered by submicron haze with visual optical depth of 1 (Kawabata et al., 1980). During the following years the haze reduced significantly (Esposito, 1984; Esposito et al., 1997). In the middle cloud layer a three-modal particle size distribution was found by the LCPS (PV) experiment: mode 3 particles have a mean radius of 3:8 mm (Knollenberg and Hunten, 1980). Nevertheless, experiment ISAV on VEGA descent probes did not find big mode 3 particles. Data of Venera 15 suggest the presence of mode 3 particles below 57 km at low latitudes and in the coldest area of the

cold collar. The mode 2 particles are composed of concentrated (about 75–85%) H2SO4. Actually, mode 1 may have another composition. These particles are practically invisible in the thermal IR spectral range and no constrains on their composition may be given from Venera 15 data. In Fig. 13 the calculated extinction coefficient vs. altitude in the atmosphere is shown for three spectral ranges: visible (18 000 cm1 Þ and thermal IR (1218 cm1 —close to maximum absorption and 365 cm1 —near minimum absorption in the clouds). Four modes of particles sizes distribution are used according to model of Pollack et al. (1980). Mode 20 particles with effective radius of 1:4 mm were introduced below 65 km to fit the PV measurements (Knollenberg and Hunten, 1980), which showed that effective size of mode 2 particles increases with decreasing altitude. Number of particles of each mode was updated to satisfy the Venera 15 IR observations (Zasova et al., 1985). The log-normal particle sizes distribution is taken according to Pollack et al. (1980) with parameters given in Fig. 13. One may see that extinction coefficient for mode 1 particles even at 1218 cm1 is 10 times less compared to the visible and negligible at 365 cm1 . Sulfuric acid is white in the visual spectral range (o0 ¼ 0:99999). The spherical albedo of Venus is of 0.77. It is not equal to 1, because the cloud opacity is finite although very big (30–40) in the visual spectral range, and part of radiation reaches the surface and is absorbed there. It cannot be excluded that the UV absorber is slightly absorbing in the visual and have absorption bands in NIR spectral range also. Sulfuric acid is pretty black in the thermal IR (single scattering albedo is shown in Fig. 14) this fact encourages us to neglect the multiple scattering during retrieval of temperature aerosol from the thermal IR. Low line for single scattering albedo at n44000 cm1 includes the absorption of the weak solution (o1%) of FeCl3 in sulfuric acid, it may be considered as ‘unknown’ UV absorber (Zasova et al., 1981; Esposito et al., 1997). Normalized extinction cross sections for all four modes are given in Fig. 15. To describe the aerosol vertical profile we used the number density of ‘equivalent particles’. Mode 2 particles

ARTICLE IN PRESS L.V. Zasova et al. / Planetary and Space Science 55 (2007) 1712–1728

1723

Fig. 13. Vertical structure of the Venus clouds at low latitudes. Extinction coefficient in km1 is given for three spectral ranges.

Fig. 14. Single scattering albedo for mode 2 particles. At n44000 cm1 one may see two curves: upper curve is for 75% H2SO4, lower curve for 1% admixture of FeCl3 in 75% H2SO4, which reproduce the shape of UV absorption in Venus spectrum (Zasova et al., 1981; Esposito et al., 1997).

were taken as the ‘equivalent particles’ (Zasova et al., 1999). In Fig. 15 the normalized extinction efficiencies Qðni Þ=Qð1218Þ vs. wave number are shown for different modes of particles from Fig. 13. The adoption of ‘equivalent particles’ is used, because to retrieve the aerosol vertical profile it is necessary to take a priori some spectral dependence of extinction coefficient, which may be corrected in the process of retrieval. There is no crucial difference of spectral dependences of relative extinction between mode 1 (r0 ¼ 0:15 mm), mode 2 (r0 ¼ 1:05 mm) and mode 20 (r0 ¼ 1:4 mm) particles (Fig. 15), thus it is not possible to make choice between them from the FS spectra. The behavior of normalized extinction for mode 3 big particles is quite different (see curve 4, Fig. 15). It does not fall down so sharply at no1100 cm1 . Hence, mode 3 particles may be distinguished from mode 2 or 1 particles on the basis of thermal IR observations, because the shape of spectra outside of the gaseous absorption bands

is defined by the spectral dependence of the aerosol extinction. One may see (Fig. 15) that extinction cross section changes significantly in the thermal IR spectral range. This means that position of the upper boundary of the clouds, which we define as the altitude or the pressure level, where aerosol opacity reaches 1, depends on wave number. In Fig. 16 we show the altitude of the upper boundary of the clouds at two wave numbers corresponding to maximum and minimum absorption coefficient for sulfuric acid. The altitude difference between them depends also on the scale height of the clouds. The sharp upper boundary of the clouds at high latitudes becomes rather diffused at low latitudes (scale height changes from 1 to 5 km). The altitude of the upper boundary of the clouds at 1218 cm1 reaches 70 km at low latitudes and decreases to less than 57 km in polar region. The lowest levels, for which the temperature profile may be retrieved independently of aerosol is H 0 ¼ 54259 km

ARTICLE IN PRESS L.V. Zasova et al. / Planetary and Space Science 55 (2007) 1712–1728

1724

isothermal shape of the continuum as a result of the sharp upper boundary of clouds. Near the observed upper boundary of the clouds the main component is sulfuric acid (mode 2 and mode 3 particles). During the Venera 15 observations the upper cloud layer looked very inhomogeneous and some times nearly negligible over the cold collar and hot dipole.

1.5

Normalized extinction

4

1.0 3

2 0.5 1

0.0 200

400

600

800

1000

1200

1400

1600

1800

Wavenumber

Fig. 15. Normalized extinction efficiency, Qn =Q1218 cm1 , for 75% H2SO4 particles, having log-normal sizes distribution. Curve 1—mode 1, r0 ¼ 0:15 mm, s ¼ 1:91; 2—mode 2, r0 ¼ 1:05 mm, s ¼ 1:21; 3—mode 20 , r0 ¼ 1:4 mm, s ¼ 1:23; 4—mode 3, r0 ¼ 3:85 mm, s ¼ 1:30 (Pollack et al., 1980).

75 1218 cm-1 Altitude, km

70

65 365 cm-1

60

55 20

40 Latitude, deg

60

80

Fig. 16. Position of the upper boundary of the clouds (t ¼ 1) vs. latitude at n ¼ 365 and 1218 cm1 , obtained from individual FS spectra.

(depends on cloud structure). We fix temperature at 50 km equal to VIRA model for corresponding latitude. It may influence the temperature below H 0 , and consequently leads to systematic errors in the position of H ðt ¼ 1Þ level at 365 cm1 , the most transparent spectral range (error is estimated equal 1–2 km). However, it does not influence the local time variation of the altitude of t ¼ 1 level at 365 cm1 (Fig. 17). Mode 3 particles were identified in the cold collar in its coldest part, where the lowest position of the upper boundary of the clouds is observed. The observed clouds are composed mainly of sulfuric acid: the sulfuric acid bands at 450 and 900 cm1 are clearly seen in the spectrum of the cold collar (3) in Fig. 3. In the coldest areas of the cold collar the upper clouds, if they exist, have to be transparent in the thermal IR. We cannot define the particle size in the polar region because of the practically

1.3.1. Tides in upper boundary of the clouds The three-dimensional field of aerosol has been studied from the thermal IR. The altitude of the upper boundary of the clouds, H ðf; Ls Þ, vs. solar longitude is presented in a form similar to Eq. (1), with the amplitudes H i ; i ¼ 0; 1; 2; 3, and 4, which depend on latitude. Below we discuss the position of the upper boundary of the clouds for two spectral ranges: 1218 cm1 , where aerosol extinction coefficient is maximal and comparable with that in the visible, and 365 cm1 , where the extinction coefficient in sulfuric acid is minimal. The zonal mean altitude (H 0 Þ of the upper boundary of the clouds decreases with latitude from 69 km at 15 to 59 km at 75 latitude at 1218 cm1 . The highest value of the component with the period of one Venusian day was found in the cold collar, where H 1 reaches 1.5 km. At low latitudes both amplitudes H 1 and H 2 are about 0.8–1 km. Fig. 17 illustrates how the expansion (Eq. (1)) presents the behavior of the altitude of the upper boundary of the clouds (t ¼ 1) vs. solar longitude at two wave numbers, Hð1218Þ and Hð365Þ. At low latitudes (25 ) the difference between Hð1218Þ and Hð365Þ exceeds 10 km. The minimal altitude of the upper boundary of the clouds for both wave numbers reaches at 4 AM (67 and 56 km) and the maximal one is observed at 9–10 PM (70 and 59 km, respectively). At 45 the upper boundary of the clouds at 365 cm1 goes down by 1 km from 4 to 10 AM. For higher latitudes the lowest position of the clouds shifts towards the morning terminator (6 AM) at 55 , to 8–10 AM at 65 and to 10:30 AM at 75 . The coldest parts of the cold collar are observed in the latter case. At 365 cm1 a minimal altitude of the cloud upper boundary is observed on the morning side. It moves toward the dayside parallel with the increasing of latitude, so that at about 10:30 AM the minimum is observed at 75 . Curiously, a second minimum appears at 8 PM. At 45 the difference in the position of the clouds for two wave numbers is around 10 km. This value decreases with latitude, reaching about 8–9 km at 55 , 4–5 km at 65 and 2–3 km at 75 . 1.3.2. Gases precursor of the cloud particles Sulfur dioxide and water vapor are the chemical precursors of H2SO4 and the resolved absorption features of both of them were observed by Venera 15 for the first time (Moroz et al., 1985, 1990; Zasova et al., 1993, Ignatiev et al., 1999). The composition of Venus atmosphere is reviewed in paper de Bergh et al. (2006). In the thermal IR spectral range the n1 , n2 and n3 bands of SO2 and the H2O rotational (40 mm) and

ARTICLE IN PRESS L.V. Zasova et al. / Planetary and Space Science 55 (2007) 1712–1728 1218 cm-1

1725

365 cm-1 60.2

70.2 ϕ=25°

59.2 58.2 57.2 56.2 57.8

68.6 67.8

Altitude, km

67.0 68.2

ϕ=45°

67.4

57.4

66.6

57.0

65.8

56.6 56.2 56.8 56.5 56.2 55.9 55.6 56.5 56.0

65.0 66.4 65.3

ϕ=55°

64.2 63.1 62.0 64.0

ϕ=65°

62.5 61.0

55.5

59.5

55.0

58.0 60.8 59.8 58.8 57.8 56.8

ϕ=75°

0

90

180

270

360 0

90

180

270

Altitude, km

69.4

54.5 55.6 55.4 55.2 55.0 54.8 360

Solar longitudes, degrees

Fig. 17. Position of the upper boundary of the clouds vs. solar longitude together with the fitting curves (Zasova et al., 2002).

vibro-rotational (6:3 mm) absorption bands were observed (see Fig. 3). The shape of the band depends not only on the gaseous absorption coefficient, but also strongly on the temperature and aerosol vertical profiles. An advantage of the thermal infrared spectrometry method is that both the temperature and aerosol profiles (needed for retrieval of the vertical profiles of minor compounds) are obtained from the same spectrum as minor compounds. Using three SO2 bands a vertical profile may be retrieved in the altitude range, which depends on clouds structure. One can see (Table 2) that the SO2 absorption coefficients coincide at n1 and n2 wave numbers, however the aerosol absorption coefficients differ by factor 10. Hence, despite of the equal SO2 absorption coefficients, the effective level of these bands formation may be different. Because of high aerosol absorption coefficient in the n1 band, it is observed only at high latitudes, where upper boundary of the clouds has the lower position. It is most pronounced in the spectra of the cold collar and hot dipole (See Fig. 3). The SO2 absorption coefficient in the n3 band is about 15 times higher than that in n1 and n2 and in this band the radiation comes from the higher levels of the atmosphere. Use of all these bands together gives a physical basis for the vertical SO2 profile retrieval. Now consider the sensitivity of the IR spectrum to the vertical SO2 profile. Comparing the hottest and coldest areas at high latitudes (Fig. 3) one can see that the hot and

Table 2 Relative opacity of the SO2 and aerosol near the centers of the SO2 bands

SO2 sa

n1 ; 1150 cm1

n2 ; 519 cm1

n3 ; 1366 cm1

1 1

1.02 0.1

15 0.4

isotopic CO2 bands are very pronounced in both cases, indicating the low position of the clouds. However, being in absorption in the hot dipole spectrum, they are observed in emission in the cold collar spectrum. Neighboring SO2 bands are observed in both spectra in absorption. This is an argument for quite different vertical distribution of SO2 and CO2. In Zasova et al. (1993) we discuss this question in detail. Comparison of the spectra, obtained in the cold collar at different local time: in the coldest part (in the morning) and in the warmest (in the afternoon) shows the different shape of the SO2 band, which indicates the different scale heights and different levels, from which radiation escapes: a low scale height in the morning (1 km) and high value in the afternoon (4–5 km), which coincides with the cloud scale height. The SO2 vertical profiles were obtained using three fundamental bands (Zasova et al., 1993). It was found that on the average the SO2 abundance increases with latitude.

ARTICLE IN PRESS 1726

L.V. Zasova et al. / Planetary and Space Science 55 (2007) 1712–1728

However, the mean total SO2 column abundance (gas+ aerosol) at low latitudes above 62 km is approximately equal to the SO2 gas abundance at high latitudes (4–5eþ19 cm2 ). The vertical profiles of the clouds and SO2 we obtained demonstrate that at low latitudes the SO2 vertical distribution is controlled by the photochemical processes of cloud formation (SO2 scale height is about a half of gaseous one). In the cold collar the SO2 scale height is about 1 km in the range of 55–65 km, while in the warmest areas at these latitudes it is up to 5 km (55–72 km) and is comparable with the cloud scale height there. Above the clouds at high latitudes the scale height of SO2 reduces to 1 km. The H2O abundance in the range of 55–65 km was also determined demonstrating the local time and spatial variation (Ignatiev et al., 1999). Most of the measurements fall in the range of 5–15 ppm. The effective altitude of sounding is approximately equal to the altitude where the optical depth t ¼ 1, which takes place (on the average) near 62 km (jo55 ) and near 55 km (j460 ). At low latitudes a weak dayside maximum and a weak nightside minimum were observed. High latitudes are rather inhomogeneous (it relates to the temperature, cloud structure, SO2 profiles) and the H2O mixing ratio also changes from the detection level of 1 ppm up to 30 ppm there. The H2O distribution obtained by experiment PV OIR (Schofield et al., 1982) contradicts the FS data. It is not clear whether it is spatial and temporal variation or because of observations made under different conditions. The spectral experiments on the Venus Express (VEX) mission should answer this question. With the spectral resolution of the long wavelength channel (LWC) of planetary Fourier spectrometer (PFS) of 1:8 cm1 the significant progress for H2O and SO2investigations may be achieved. However, the results, obtained from the spectral data of FS Venera 15, related to SO2 and H2O distribution in the middle atmosphere of Venus remain unique up to now. 1.4. PFS VEX The paper Formisano et al. (2006) is devoted to description of PFS VEX and its scientific goals. We reviewed the results of FS Venera 15 to show how effective the IR Fourier spectrometry may be from a polar orbit for Venus middle atmosphere investigations. The LWC PFS has a higher spectral resolution and joint analysis of data of both LWC and the dayside observations of the Short Wavelength Channel (SWC) should give new possibilities for middle atmosphere studies. The LWC PFS has a spectral resolution of 1:8 cm1 (with apodisation), which allows us to sound the Venus atmosphere up to 100 km. Several weighting functions in the 15 mm CO2 band are shown in Fig. 18 for the case of pure gaseous atmosphere for nadir observation. One may see that at 100 km altitude the value of highest weighting function is about 70% of its maximal value (at 97 km). It means that emission from these levels gives significant

Fig. 18. Several weighting functions in the 15 mm CO2 band.

input into the measured intensity at corresponding wave numbers. We calculated S/N for the LWC of PFS, using the value of NER, from Formisano et al. (2005) (Report 1 on Calibration of PFS VEX). The expected signal-to-noise ratio is shown in Fig. 19. Five typical Venusian spectra (Fig. 3) from Venera 15 data are used as signal being divided by value of NER. The S/N value exceeds 100 in the long wavelength part of the spectra and permanently decreases to the short wavelength edge. Hence, the n3 SO2 (1360 cm1 ) is in the range of low value of S/N, especially for the case of cold collar. However, averaging over several spectra will allow us to use this important band for the SO2 vertical profile retrieval. 2. Conclusion The main features of the structure of the Venus atmosphere below 100 km, presented in VIRA, are confirmed by the latest measurements. New data obtained in the 1980s and 1990s allow us to know much more about the local time and space variations. Vega balloons provided the first measurements along horizontal paths in the middle clouds. The Vega 2 probe measured for the first time the precise temperature profile from the clouds top to the surface. It was confirmed that the troposphere of Venus has not one but has a minimum two convective zones, one of which is close to the surface and another in the lower clouds. More than 1500 vertical temperature profiles of the middle atmosphere (from about 55 to 95–100 km) were retrieved from the Venera 15 IR spectra. They cover a wide range of latitudes and local time. A complicate picture of latitudinal and solar time variation of the temperature and upper clouds at different levels was analyzed. A time dependent models set of vertical temperature profiles in the middle atmosphere is tabulated (Zasova et al., 2006); there is no such description in VIRA. It was shown that the clouds, observed in the thermal IR at all latitudes, are

ARTICLE IN PRESS L.V. Zasova et al. / Planetary and Space Science 55 (2007) 1712–1728

1727

Fig. 19. Expected signal-to-noise for several typical spectra of Venus, shown in Fig. 3 for the case of signal spectrum. Curve numbers correspond to Fig. 3.

composed mainly of sulfuric acid. Big particles (mode 3), which are observed at high latitudes in the middle clouds, are also composed of sulfuric acid. A comparison of solar related structures with those observed earlier by PV shows a stable pattern in the middle atmosphere over the 5-year period between the observations. References Avduevskiy, V.S., Marov, M.Ya., Kulikov, Yu.N., Shari, V.P., Gorbachevskiy, A.Ya., Uspenskiy, G.R., Cheremukhina, Z.P., 1983. Structure and parameters of the Venus atmosphere according to Venera probe data. In: Hunten, D.M., Collin, L., Donahue, T.M., Moroz, V.I. (Eds.), Venus, pp. 681–765. Baines, K.H., Bellucci, G., Bibring, J.-P., Brown, R.H., Buratti, B.J., Bussoletti, E., Capaccioni, F., Cerroni, P., Clark, R.N., Cruikshank, D.P., Drossart, P., et al., 2000. Detection of sub-micron radiation from the surface of Venus by Cassini/VIMS. Icarus 148, 307–311. Belton, J.S., Gierasch, V., Smith, M.D., Helfenstein, P., Schinder, J., Pollack, J.B., Rages, K., Ingersoll, A., Klaasen, K.P., Veverka, J., et al., 1991. Images from Galileo of the Venus Cloud Deck. Science 253, 1531–1536. Bertaux, J.-L., et al., 1996. VEGA-1 and VEGA-2 entry probes: an investigation of local UV absorption 220–400 nm in the atmosphere of Venus (SO2 aerosols, cloud structure). J. Geophys. Res. 101, 12709–12745. Bezard, B., de Bergh, C., Crisp, D., Maillard, J.-P., 1990. The deep atmosphere of Venus revealed by high-resolution nightside spectra. Nature 345, 508–511. Carlson, R.W., Baines, K.H., Encrenaz, Th., Taylor, F.W., Drossart, P., Kamp, L.W., Pollack, J.B., Lellouch, E., Collard, A.D., Calcutt, S.B., et al., 1991. Galileo infrared imaging spectroscopy measurements at Venus. Science 253, 1541–1548. Crisp, D., Ingersoll, A.P., Hildebrand, C.E., Penston, R.A., 1990. VEGA balloon meteorological measurements. Adv. Space Res. 10, 109–124. de Bergh, C., Moroz, V.I., Taylor, F.W. , Crisp, D., Bezard, B., Zasova, L.V., 2006. The composition of the atmosphere of Venus below 100 km altitude: an overview. Planet. Space Sci. 54 (N13–14), 1389–1397. Drossart, P., Be´zard, B., Encrenaz, Th., Lellouch, E., Roos-Serote, M., Taylor, F.W., Collard, A.D., Calcutt, B.S., Pollack, J., Grinspoon, D.H., Carlson, R.W., Baines, K.H., Kamp, L.W., 1993. Search for

spatial variations of the H2O abundance in the lower atmosphere of Venus from NIMS-Galileo. Planet. Space Sci. 41, 495–504. Elson, L., 1983. Solar related waves in the Venusian atmosphere from the cloud tops 100 km. J. Atmos. Sci. 40, 1535–1551. Esposito, L.W., 1984. Sulfur dioxide: episodic injection shows evidence for active Venus. Science 223, 1072. Esposito, L.W., Knollenberg, R.G., Marov, M.Ya., Toon, R.B., Turko, R.P., 1983. The clouds and hazes of Venus. In: Hunten, D.M., Colin, L., Donahue, T.M., Moroz, V.I. (Eds.), Venus. The University of Arizona Press, Tucson, Arizona, pp. 484–4588. Esposito, L.W., Bertaux, J.-L., Krasnopolsky, V., Moroz, V.I., Zasova, L.W., 1997. Chemistry of lower atmosphere and clouds. In: Bougher, S.W., Hunten, D.M., Phillips, R.J. (Eds.), Venus II. The University of Arizona Press, Tucson, Arizona, pp. 415–458. Formisano, V., Angrilli, F., Arnold, G., et al., 2005. The Planetary Fourier spectrometer (PFS) onboard the European Mars Express Mission. Planet. Space Sci. 53 (N10), 963–974. Formisano, V., Angrilli, F., Arnold, G., et al., 2006. The planetary Fourier spectrometer (PFS) onboard the European Venus Express Mission. Planet. Space Sci. 54, (N13–14), 1298–1314 Gierash, P.J., et al., 1997. The general circulation of the Venus atmosphere and assessment. In: Venus II. The University of Arizona Press, Tucson, Arizona. Grinspoon, D.H., Pollack, J.B., Sitton, B.R., Carlson, R.W., Kamp, L.W., Baines, K.H., Encrenaz, Th., Taylor, F.W., 1993. Probing Venus’s cloud structure with Galileo-NIMS. Planet. Space Sci. 41, 515–542. Hinson, D., Jenkins, J., 1995. Magellan radio occultation measurements of atmospheric waves on Venus. Icarus 114, 310. Huntress, W.H., Moroz, V.I., Shevalev, I.L., 2003. Lunar and planetary robotic and exploration missions in the 20th century. Space Sci. Rev. 107 (3), 541–649. Ignatiev, N.I., Moroz, V.I., Zasova, L.V., Khatuntsev, I.V., 1999. Water vapor in the middle atmosphere of Venus: an improved treatment of the Venera 15 IR spectra. Planet. Space Sci. 47, 1061–1075. Jenkins, V.M., et al., 1994. Radio occultation of the Venus atmosphere with the Magellan spacecraft. 2. Results from the October 1991 experiment. Icarus 111, 79. Kawabata, G., et al., 1980. Cloud and haze properties from Pioneer Venus polarimetry. J. Geophys. Res. 85, 8129. Kliore, A.J., Moroz, V.I., Keating, G.M. (Eds.), 1985. The venus reference atmosphere, Adv. Space Res. 5 (11), 1–303.

ARTICLE IN PRESS 1728

L.V. Zasova et al. / Planetary and Space Science 55 (2007) 1712–1728

Knollenberg, R.G., Hunten, D.M., 1980. The microphysics of the clouds of Venus: results of the Pioneer Venus particle size spectrometer experiment. J. Geophys. Res. 85, 8039–8058. Limaye, S.S., 1985. Venus atmospheric circulation: observations and implications of the thermal structure. Adv. Space Res. 5 (9), 51–62. Linkin, V.M., Kerzhanovich, V.V., Lipatov, A.N., Shurupov, A.A., Seiff, A., et al., 1986. Thermal structure of the Venus atmosphere in the middle cloud layer. Science 231, 1420–1422. Linkin, V.M., Blamont, J., Devyatkin, S.I., Ignatova, S.P., Kerzhanovich, V.V., Lipatov, A.N., Malik, K., Stadnyk, B.I., et al., 1987. Thermal structure of the atmosphere of Venus from the results of measurements taken by landing vehicle Vega-2. Cosmic Res. 25, 501–512. Meadows, V.S., Crisp, D., 1996. Ground-based near infrared observations of the Venus nightside: the thermal structure and water abundance near the surface. J. Geophys. Res. 101, 4595–4622. Moroz, V.I., et al., 1985. Infrared experiment on VENERA-15 and VENERA-16 orbiters 4 of analysis of spectra in the ranges of H2 O and SO2 absorption bands. Kosm. Issled. 23 (2), 236–247 (in Russian). Moroz, V.I., Spankuch, D., Titov, D.V., Schafer, K., Dyachkov, A.V., Dohler, W., Zasova, L.V., Oertel, D., Linkin, V. M., Nopirakowski, J., 1990. Water vapor and sulfur dioxide abundances at the Venus cloud tops from the Venera-15 infrared spectrometry data. Adv. Space Res. 10 (N5), 77–81. Moroz, V.I., Oertel, D., et al., 1986. Venus spacecraft’s infrared radiance spectra and some aspects of their interpretation. Appl. Opt. 25, 1710–1719. Moroz, V.I., Zasova, L.V., Linkin, V.M., 1996. Venera-15, 16 and Vega mission results as sources for improvements of the Venus Reference atmosphere. Adv. Space Res. 17 (11), 171–180. Moroz, V.I., Huntress, W.H., Shevalev, I.L., 2002. Planetary missions of the 20th century. Kosmicheskie issledovanija 40 (N5), 451–481 (in Russian). Newman, M., Schubert, G., Kliore, A.J., Patel, I.R., 1984. Zonal winds in the middle atmosphere of Venus from Pioneer Venus radio occultation data. J. Atmos. Sci. 41, 1901–1913. Oertel, D., et al., 1985. Infrared experiment on VENERA-15 and VENERA-16 orbiters. 1. Methods and first results. Kosmicheskie issledovanija 23, 191–205 (in Russian). Oertel, D., Moroz, V.I., Spankuch, D., Linkin, V.M., et al., 1987. Infrared spectrometry from Venera-15 and Venera-16. Adv. Space Res. 5, 25. Pollack, J.B., Toon, O.B., Whitten, R.C., et al., 1980. Distribution and source of the UV absorption in Venus atmosphere. J. Geophys. Res. 85, 8141–8150. Pollack, J.B., et al., 1993. Night infrared light from Venus’ night side: a spectroscopic analysis. Icarus 103, 1–42. Roos-Serote, M., Drossart, P., Encrenaz, Th., et al., 1995. The thermal structure and dynamics of the atmosphere of Venus between 70–90 km from the Galileo-NIMS spectra. Icarus 114, 300–309. Schafer, K., Zasova, L.V., Spankuch, D., et al., 1987. Structure of the middle atmosphere of Venus from analyses of Fourier-spectrometer measurements aboard Venera-15. Adv. Space Res. 7 (12), 17. Schafer, K., Dubois, R., Haus, R., et al., 1990. Infrared Fourier Spectrometer experiment from Venera-15. Adv. Space Res. 10 (5), 57. Schofield, J.T., Taylor, F.W., 1983. Measurement of the mean solar fixed temperature and cloud structure of the middle atmosphere of Venus. Q J. R. Meterol. Soc. 109, 57–80. Schofield, J.T., Taylor, F.W., McCleese, D.J., 1982. The global distribution of water vapor in the middle atmosphere of Venus. Icarus 52, 263–278. Seiff, A., 1983. Thermal structure of the atmosphere of Venus. In: Hunten, D.M., Colin, L., Donahue, T.M., Moroz, V.I. (Eds.), Venus. The University of Arizona Press, Tucson, Arizona, pp. 215–279.

Seiff, A., et al., 1985. Models of the structure of the atmosphere of Venus from the surface to 100 km altitude. in: Kliore, A.J., Moroz, V.I., Keating, G.M. (Eds.), The Venus Reference Atmosphere, Adv. Space Res. 5(11), 3–58. Spankuch, D., Matsygorin, I.A., Dubois, R., Zasova, L.V., 1990. Venus middle atmosphere temperature from Venera 15. Adv. Space Res. 10 (5), 67–75. Taylor, F.W., Beer, R., Chahine, M.T., Diner, D.J., Elson, L.S., Haskins, R.D., McCleese, D.J., Martonchik, J.V., Reichley, P.E., 1980. Structure and meteorology of the middle atmosphere of Venus: Infrared remote sensing from the Pioneer orbiter. J. Geophys. Res. 85, 7963–8006. Taylor, F.W., Hunten, D.M., Ksanfomality, L.V., 1983. The thermal balance of the middle and upper atmosphere of Venus. In: Hunten, D.M., Colin, L., Donahue, T.M., Moroz, V.I. (Eds.), Venus. The University of Arizona Press, Tucson, Arizona, pp. 650–680. Taylor, F.W., Crisp, D., Bezard, B., 1997. Near infrared sounding of the lower atmosphere of Venus. In: Bougher, S.W., Hunten, D.M., Phillips, R.J. (Eds.), Venus II. The University of Arizona Press, Tucson, Arizona, pp. 325–352. Yakovlev, O.L., Gubenko, V.N., Matyugov, S.S., YakovIeva, G.D., Vaganov, L.R., 1987a. Atmosphere of Venus in south subpolar region. Kosm. Issled. 25 (25), 258–266 (in Russian). Yakovlev, O.L., Matyugov, S.S., Efimov, A.I., Gubenko, V.N., Kucheriavenkov, A.I., 1987b. Atmosphere of Venus in north polar region. Kosmich. Issled. 25 (25), 267–274 (in Russian). Yakovlev, O.L., Matyugov, S.S., Gubenko, V.N., 1991. Venera-15 and 16 middle atmosphere profiles from radar occultations: polar and near polar atmosphere of Venus. Icarus 94, 493–510. Yung, Y.L., De More, W.B., 1982. Photochemistry of the stratosphere of Venus: implications for atmospheric evolution. Icarus 51, 197–212. Zasova, L.V., 1995. The structure of the Venusian atmosphere at high latitudes. Adv. Space Res. 16, 89–98. Zasova, L.V., Khatuntsev, I.V., 1997. Thermal zonal wind in the Venus middle atmosphere according to Venera 15 IR-spectrometry. Adv. Space Res. 19 (N8), 1181–1190. Zasova, L.V., Krasnopolsky, V.A., Moroz, V.I., 1981. Vertical distribution of SO2 in the upper cloud layer of Venus and origin of the UV absorption. Adv. Space Res. 1(N9), 13–16. Zasova, L.V., Moroz, V.I., 1992. Latitude structure of the upper clouds of Venus. Adv. Space Res. 12, 79–90. Zasova, L.V., et al., 1985. Infrared experiment on VENERA-15 and VENERA-16 orbiters Some conclusions about structure of clouds based on the analysis of spectra. Kosm. Issled. 23, 221–235 (in Russian). Zasova, L.V., Moroz, V.I., Esposito, L.W., Na, C.Y., 1993. SO2 in the middle atmosphere of Venus: IR measurements from VENERA-15 and comparison to UV data. Icarus 105, 92–109. Zasova, L.V., Khatountsev, I.V., Moroz, V.I., Ignatiev, N.I., 1999. Structure of the Venus middle atmosphere: Venera 15 IR Fourier Spectrometry data revisited. Adv. Space Res. 23 (N9), 1559–1568. Zasova, L.V., Khatuntsev, I.V., Linkin, V.M., 2000. Thermal zonal wind in the middle atmosphere of Venus. Cosmic Res. 38 (N1). Zasova, L.V., Khatuntsev, I.V., Ignatiev, N.I., Moroz, V.I., 2002. Local time variations of the middle atmosphere of Venus: solar-related structures. Adv. Space Res. 299 (N2), 243–248. Zasova, L.V., Moroz, V.I., Formisano, V., Ignatiev, N.I., Khatuntsev, I.V., 2004. Infrared spectrometry of Venus: IR Fourier sspectrometer on Venera 15 as a precursor of PFS for Venus Express. Adv. Space Res. 34, 1655–1667. Zasova, L.V., Khatuntsev, I.V., Linkin, V.M., Mayorov, B., 2006. Thermal structure of the Venus atmosphere from the surface to 100km. Cosmic Res. 44, (N4), 364–383.