From Venera to PFS on Venus Express
Since the beginning of the space era, Venus has been an attractive target for planetary science. The first phase of Venus spacecraft exploration (1962-1985) discovered an exotic world hidden behind a curtain of dense clouds. The earlier exploration of Venus included a set of Soviet orbiters and descent probes, Venera 4–16, the US Pioneer Venus mission, the Soviet Vega balloons, the Venera 15, 16 and Magellan radar orbiters, the Galileo and Cassini flybys, and a variety of ground-based observations. But these studies provided not more than a basic knowledge of the conditions on the planet, since the amount of data was very limited in space and time coverage, and, prior to the discovery of the near infrared spectral windows, lacked the capability to sound the lower atmosphere of Venus.
In July 2002, the European Space Agency has started the work on the Venus Express mission, which was selected in the framework of a re-use of the Mars Express bus. The Planetary Fourier Spectrometer (PFS) is included in the VEX scientific payload. It is a key instrument in the study of the middle and lower atmosphere. PFS has two spectral channels, one for the long wavelengths (LWC, 5-50 µm), and the second for the short wavelengths (SWC, 0.9-5 µm). The spectral resolution is 2 cm -1 in both channels.
Fig. 1 shows the principal composition of the Venusian atmosphere. It consists mainly of CO 2 and N 2 with small amounts of other gases. Although there is very little observational data, the chemistry of the lower atmosphere is expected to be dominated by the thermal decomposition of sulfuric acid, and cycles that include sulfur and carbon compounds and water vapor. The key species involved, such as SO 2 , CO, and COS , can be mapped from orbit in the near IR windows.
The surface of Venus is obscured by a 20 km thick cloud layer whose opacity varies between 20 and 40 in the UV, visible and infrared. The clouds are almost featureless in visible light, but display prominent markings in the UV-blue spectral region. Earlier observations showed that at least the upper cloud consists of micron size droplets of 75% H 2 SO 4 , which is produced by photochemical reactions at the cloud tops. The physical and chemical processes forming the lower clouds are virtually unknown, including major problems like (1) the nature of the UV-blue absorber which produces the features observed from space and absorbs half of the energy received by the planet from the Sun, and (2) the origin of the large solid particles detected by the Pioneer-Venus probe. PFS on VEX will address the structure, composition, dynamics, and variability of the cloud layer, including cloud structure and opacity variations and the distribution and nature of the UV-blue absorber.
The Fourier spectrometer experiment onboard the Venera-15 satellite (FSV15) was the direct precursor of PFS long wavelength channel measurements. This instrument worked for about 2 months and recorded about 1700 thermal emission spectra of Venus. Despite the short duration of the mission and a limited coverage in latitude and local solar time (the whole southern hemisphere and local times around noon and midnight were unexplored), that experiment demonstrated the power of thermal emission spectrometry at Venus. The complete coverage of the near IR part of the spectrum (SWC) has never been done before from Venus orbit.
Fig. 2 shows examples of measured FSV15 spectra that have been averaged from 5-10 individual spectra. Characteristic temperature profiles and the global averaged temperature field which have been retrieved from FSV measurements are illustrated in Fig. 3 and 4, respectively.
1: VIRA 30°, 2: j < 35 ° , L S =20–90 ° , 3: j < 35 ° , L S =270–310 ° , 4: -10 ° < j < +10 ° , L S =75 °
PFS-VEX will strongly enhance the capabilities of temperature measurements, including a much better coverage in space and local time. Higher spectral resolution will help to obtain more accurate temperature profiles, especially at high levels in the atmosphere.
PFS will cover the following scientific objectives:
As in case of PFS studies on Mars, the calculation of synthetic spectra and their iterative comparison with measurements is the fundamental task in the retrieval of atmospheric and surface parameters. Figure 5 shows a complete synthetic spectrum for Venus day side. It consists of two main parts, thermal radiation for n < 3000 cm -1 and reflected solar radiation at n > 3000 cm -1 . In both parts, the radiation comes from the cloud top, and the spectrum shows the characteristic spectral features of atmospheric gases above the cloud and the clouds themselves. All pronounced spectral features are CO 2 bands. There are many other spectral features that belong to minor constituents like CO, H 2 O, etc. and can be used to estimate their abundances at different altitudes.
Night side spectra of Venus are even more interesting. They show weak thermal emissions penetrating from the atmosphere below the clouds in windows between CO 2 bands. The near IR spectrum expected from PFS on the night side is shown in the Figure 6. Since the “windows” sound different altitudes, these observations will give information about abundances of the trace constituents in the lower atmosphere (especially of H 2 O vertical profiles) and will help to discriminate between the surface and cloud contribution.