A new approach is suggested to the problem of the theory of Jovian decametric radiation and physical conditions at the point of origin. This depends upon a comparison of the characteristics of fast pulse decametre-wave radio emission from both Jupiter and the Sun and invokes the deductive procedure which Sagan (1971) has called ‘Propositional Calculus’.

Fast polarized pulses in radiation from Jupiter and the Sun have been studied at fixed frequencies in the range 18 to 26 MHz with time resolutions from one to five msec; a number of similarities between the pulses from both sources have been noted. A comparison of some of the pulse characteristics is being made in order to decide whether or not they are sufficiently alike to be regarded as having a common mechanism of origin at both Jupiter and the Sun. From this ‘decision’ it is proposed to establish boundary conditions for theoretical study. Fast pulses in the Jupiter radiation are generally supposed to be a source phenomenon although their actual mechanism is not understood. The reasons for this are to some extent inferred rather than proven and so, to check the possible (if unlikely) role of the interplanetary medium, observations are also being made using the large 26 MHz array at the University of Florida to search for possible fast pulses in the radiation from the more distant source Taurus A.

The results of recent studies of the optical properties, temperature, chemical composition and a probable structure of Saturn's atmosphere are reviewed.

A theoretical model of Saturn's rings is investigated which includes the shadowing effect and realistic anisotropic phase functions for the ring particles. The effects of multiple scattering and the finite size of the Sun, including the penumbra, are rigorously included. The permissible range of the relevant parameters, including optical thickness, single scattering albedo, volume density, and phase function are investigated by comparing the theoretical results to observations of the ring brightness vs phase angle, wavelength, and elevation of the Sun and Earth. Anisotropic scattering by the ring particles is necessary in order to match the observations. The colour dependence of the opposition effect is interpreted in terms of the albedo spectrum of the ring particles.

The attention is called to the fact that the discovery by Feibelman (1967) of the rarefied outer ring of Saturn is confirmed by the observations of Kuiper (1972). It is proposed to designate this object as E-ring (exterior) in order to avoid confusion with the innermost, also rarefied, D-ring observed by Guérin (1970) and earlier by Barabashov and Semejkin (1933). The effects of the interaction of E-ring with inner Saturn's satellites are briefly discussed. The conclusion is drawn that in cosmogonic time scale these effects are small. It is also shown that the optical thickness of E-ring is lower than 1/20000; the available photometric estimations of the geometric thickness of A- and B-rings need not be corrected for the light scattering and absorption by E-ring.

The subject of this paper is an attempt to predict what can be seen in the vicinity of Saturn through a UV photometer on board the Mariner-Jupiter-Saturn mission. We will restrict ourselves to the study of atomic hydrogen and OH and to H2O which can be the parent molecule and will consider only the particles ejected from the rings and from Titan.

It is hypothesized that constituents escaping the atmosphere of Titan are unlikely to have sufficient velocity to escape from Saturn. Thus most of the material will enter into elliptical orbits which, at some point, cross Titan's orbit at a small angle. In total, this matter will form a ‘doughnut’ shaped feature, roughly centered on Titan's orbit and having dimensions of about 20 Rs (Saturn radii) in the Saturnian-equatorial plane and 10 Rs normal to this plane. It is assumed that the material is lost from the doughnut when it is ionized or is recaptured by Titan's atmosphere. The upper limit on the density of hydrogen in the ring is estimated to be about 103 cm−3, with up to 99% of the particles in the doughnut being recaptured by Titan.

A brief summary of the work which has been done on this problem is given, and areas needing further research are suggested.

We have re-examined the visible and near-IR regions of the spectra of Uranus and Neptune to provide additional data for constructing atmospheric models. We find that a true continuum exists only at wavelengths below 4700 Å, that the 6420 Å absorption previously attributed to hydrogen is probably caused by methane, that there is no evidence for ammonia, ethylene or ethane absorptions in our spectra, and that the abundance of methane is probably much higher than previous estimates suggest. This last finding implies that the value of H/C in the atmospheres of both planets is much less than 1/10 the solar (or Jovian) value. Clear, Rayleigh-scattering model atmospheres are not compatible with the observations, but more work is needed to establish viable alternatives.

The first measurement of Neptune's quadrupole H2 lines is reported. The equivalent widths of the S(0) and S(1) lines of the (4–0) band are given along with the corresponding widths measured from comparison spectra of Uranus taken on the same nights. These are interpreted in terms of both an inhomogeneous atmosphere overlying a reflecting layer and a homogeneous, semi-infinite, scattering atmosphere. Only the scattering model proves to be consistent with Neptune's spectrum in this wavelength region. The H2 abundance along the scattering mean free path is found to be less than the value for Rayleigh scattering in pure H2. This result is interpreted in terms of the presence of H2, CH4 and at least one other gas, instead of the more conventional interpretation in terms of the presence of an aerosol mixed with H2. Weak features in the continuum were observed. Their widths and the strength of the H2 features indicate that H2 is more abundant than the sum of the remaining gases in these atmospheres.

The four Galilean satellites of Jupiter have been observed frequently during the past 20 years with the high resolution telescopes of Pic-du-Midi. Reliable features are seen and photographed at their surface. They have been mapped on planispheres. Their identification proves that the rotation period is synchronous with the revolution for each satellite, with an axis not significantly departing from the normal to the orbital plane. The transits of satellites in front of the limb-darkened Jupiter disk provide a technique for accurate contrast and albedo determinations of the features observed. For Callisto, the albedos at 5° phase angle range from 0.19 to 0.09 with a contrast 0.5, indicating a lunar type surface, despite the low density involving a different internal structure. For Europa, the albedos are very high, between 0.73 and 0.52, with low contrasts of 0.3, readily explainable by a snow or frost deposit, the spectroscopic evidence indicating water. For Ganymedes, the very high contrasts reach 0.7, and albedos span from 0.50 to 0.15; patches of snow on Callisto-type terrains are suggested. Io has very high albedos of 0.83 for the equatorial zone to 0.46 for the two darkened poles; the yellow color and absence of H2O bands in the spectrum indicate a special surface composition.

In early summer of 1972, we discovered an anomalous brightness in the spectrum of Io near the sodium D-lines. This effect is revealed to be time-varying emission by free sodium atoms on Io.

The behaviour of the available light curves of Jupiter and Saturn satellites is shown to change steadily from inner to outer satellites.

The change in reflectivity of satellites with orbital motion could be explained assuming that dark meteoroidal material accumulates on a given portion of the satellite's surface, which was once mainly covered with ice or snow. The preferential impact location on the satellite's surface could be a consequence of the perturbing action of the planet on the motion of interplanetary material.

The interest of a spectroscopic investigation of the upper atmosphere of Venus in the ultraviolet is briefly shown. The conditions for the observations to be made are defined and the main properties of the necessary instrument are deduced and described. The proposed package is a quadruple miniature spectrometer covering the entire spectral range extending from 1200 Å to 8000 Å with a resolving power of 500.

Polarimetry is reviewed in a new book of which I am the editor (Gehrels 1974) while detailed papers of our group in Arizona continue to appear in a series ‘The Wavelength Dependence of Polarization’ in the Astronomical Journal.

NASA program of future solar system exploration in the years 1973–1990 is presented.