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Venus has no seasons, slow rotation and a very massive atmosphere, which is mainly carbon dioxide with clouds primarily of sulphuric acid droplets. Infrared observations by previous missions to Venus revealed a bright 'dipole' feature surrounded by a cold 'collar' at its north pole (1–4). The polar dipole is a 'double-eye' feature at the centre of a vast vortex that rotates around the pole, and is possibly associated with rapid downwelling. The polar cold collar is a wide, shallow river of cold air that circulates around the polar vortex. One outstanding question has been whether the global circulation was symmetric, such that a dipole feature existed at the south pole. Here we report observations of Venus' south-polar region, where we have seen clouds with morphology much like those around the north pole, but rotating somewhat faster than the northern dipole. The vortex may extend down to the lower cloud layers that lie at about 50km height and perhaps deeper. The spectroscopic properties of the clouds around the south pole are compatible with a sulphuric acid composition.

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Mars' radio spectrum at centimeter wavelengths is produced by thermal radiation from the surface and sub-surface. Observations at 2.8 cm made in the 1975 and 1978 show variations of its radio brightness as a function of longitude on the planet (Doherty et al. , ApJ 233, 1979). In addition, an overall increase in the radio brightness observed in 1978 just at the start of a global dust storm. Recent theoretical studies show that radio emission can be produced by corona discharges between dust particles, as these become charged through triboelectric processes in dust lifting/transportation events (Renno et al. GLR v.30, 2003). In this work we explore the hypothesis that changes in Mars' radio emission are be linked to changed in the atmospheric dust content.

We have proposed to develop a polarization nephelometer for use on future planetary descent probes. It will measure both the scattered intensity and polarization phase functions of the aerosols it encounters descending through an atmosphere. These measurements will be taken at two wavelengths separated by about an octave, with one light source near 500nm and another near 1mum. Adding polarization measurements to the intensity phase functions greatly increases our ability to constrain the size distribution, shape and chemical composition of the sampled particles. There remain important questions about these parameters of the aerosols on Venus, the giant planets and Titan that can only be addressed with a nephelometer like ours. The NRC Planetary Sciences Decadal Survey has identified probe missions to Venus and Jupiter as a priority. On both of these missions, our proposed instrument would be an excellent candidate for flight. We also expect that future probe missions to Saturn, Uranus, Neptune or Titan would employ our instrument. It could also find use in Earth in situ aerosol studies. We will use a technique to simultaneously measure intensity and polarization phase functions that uses polarization modulation of a light source. This technique has been implemented in laboratory settings, but not with considerations to the environment on a planetary descent probe. We have proposed to design and build a flexible breadboard nephelometer to test the components and concepts of our approach. We would then test the device against well defined aerosols, ensuring that it accurately measures their expected intensity and polarization phase functions. With the knowledge gained in this flexible design, we would then design and build a breadboard polarization nephelometer more suited to integration on a planetary descent probe. To include traceability in the technical requirements of our device, we would also conduct an Observing System Simulation Experiment. In this study, we would determine what the performance trade-offs are for de-scoping each capability of our instrument. Additionally, it would aid us in optimizing the nominal design parameters to yield the most unambiguous aerosol microphysical information. All of these investigations will be carried out to enhance the likelihood of success and useful data return of our proposed instrument in its descent through a planetary atmosphere. Considerations will also be given to mass, volume, power and cost.

The spectrum of Saturn has been recorded between 4.5 and 16.0 mu m with the grating mode of the Short-Wavelength Spectrometer (SWS) of ISO. The resolving power is 1500. The main results of this observation are (1) the detection of CO2 CH3C2H and C4H2 in the stratosphere and (2) the detection of H2O in the troposphere. In the 4.5-5.5 mu m range, information is retrieved on the tropospheric composition (NH3, PH3, AsH3, GeH4, CH3D and H2O) down to pressure levels of several bars. Above 7 mu m, the Saturn spectrum probes the upper troposphere and the lower stratosphere, at pressure levels ranging from 0.5 bar to 0.4 mbar. The CH4 emission band at 7.7 mu m and the H-2-He continuum longward of 11 mu m are used to retrieve the thermal profile, which is then used to derive the vertical distributions of minor species: NH3, PH3, CH3D in the troposphere, and C2H2 and C2H6 in the stratosphere. Estimates of the CO2, CH3C2H and C4H2 mean mixing ratios (above the 10-mbar level) are 3 10(-10), 6 10(-10) and 9 10(-11) respectively. The retrieved disk-averaged thermal profile is found to be colder in the stratosphere than the Voyager 1 ingress radio-occultation profile by about 7 K at P = 0.5 mbar, and slightly warmer in the troposphere (about 5 K at 400 mbar and 3 K at 150 mb).

The spectrum of Jupiter has been recorded between 2.75 and 14.5 mu m with the grating mode of the Short-Wavelength Spectrometer (SWS) of ISO. The resolving power is 1500. The main preliminary results of this observation are (1) at 3 mu m, the first spectroscopic signature, probably associated with NH3 ice, of the Jovian cloud at 0.5 bar and (2) the first detection of a thermal emission at the center of the CH4-nu(3) band at 3.3 mu m, showing evidence for a high temperature in the upper jovian stratosphere (T=800 K at P=0.16 microbar). In addition, the R(2) HD line has been detected for the first time in Jupiter, using the Fabry-Perot (FP) mode of the SWS, with a resolving power of 31000. A preliminary analysis of the HD line indicates a D/H ratio of about 2.2 10(-5).

The spectrum of Saturn has been recorded between 2.3 and 15 mu m, on June 13, 1996, with the grating mode of the Short-Wavelength Spectrometer of ISO (Infrared Space Observatory). The resolving power is about 1500 and the sensitivity is better than 1 Jy. As compared to Jupiter, the spectrum of Saturn is characterized by a higher signal at 3 mu m, as compared to the 2.75 mu m value, and a weaker signal in the 5-mu m region, due to Saturn's lower temperature. The main differences between the Jupiter and Saturn spectra are mainly due to a lower NH_3 mixing ratio and higher mixing ratios of PH_3, CH_4, C_2H_2 and C_2H_6 in Saturn's atmosphere as compared to their values in Jupiter. A preliminary modelling will be presented and estimates of the mixing ratios and/or vertical distributions of minor atmospheric species will be inferred.

The spectrum of Jupiter has been recorded on April 12, 1996, between 2.75 and 14.5 mu m, with the grating mode of the Short-Wavelength Spectrometer of ISO (Infrared Space Observatory). The resolving power is 1500 and the sensitivity limit is better than 1 Jy. The corresponding S/N ratio is better than 1000 at 2.75 mu m, 4000 at 5 mu m and 20000 at 12 mu m. The main preliminary results of the grating observations are (1) at 3 mu m, the first spectroscopic signature, probably associated with NH_3 ice, of the Jovian cloud at 0.5 bar and (2) the first detection of a thermal emission at the center of the CH_4 nu_3 band at 3.3 mu m, showing evidence for a high temperature in the upper jovian stratosphere (about T=800 K at P=0.16 mu bar). A modelling of the whole spectral range will be presented, with implications on the jovian thermal profile and on the mixing ratios and/or vertical distributions of the atmospheric constituents. A preliminary analysis of these data can be found in Encrenaz et al (submitted to Astron. Astrophys., 1996).