The USA Mariner 10 spacecraft encountered Mercury three times in 1974-1975. The 1st and 3rd encounters provided detailed observations of a well developed, detached bow shock wave which results from the interaction of the solar wind. The planet possesses a global magnetic field, and modest magnetosphere, which deflects the solar wind. The field is approximately dipolar, with orientation in the same sense as Earth, tilted 12° from the rotation axis. The magnetic moment, 5×1022 Gauss-cm3, corresponds to an undistorted equatorial field intensity of 350γ, approximately 1% of Earth’s. The origin of the field, while unequivocally intrinsic to the planet, is uncertain. It may be due to remanent magnetization acquired from an extinct dynamo or a primordial magnetic field or due to a presently active dynamo. Among these possibilities, the latter appears more plausible at present. In any case, the existence of the magnetic field provides very strong evidence of a mature, differentiated planetary interior with a large core, Rc ≈ 0.7RM, and a record of the history of planetary formation in the magnetization of the crustal rocks.

It was not generally anticipated by lunar scientists that the Moon rocks would prove to possess a natural remanent magnetization because the absence of a present field had already been demonstrated and it was almost universally believed that the Moon did not possess an iron core in which such a field could be generated. Consequently the initial investigation of Apollo 11 rocks was directed towards the use of rock magnetic studies as an adjunct to petrological examination. One of the most notable findings of the Apollo programme was the demonstration of the existence of a natural remanent magnetization (NRM) in Apollo 11 lavas and breccias. Experiments on the stability of the NRM enabled it to be concluded that the rocks were already magnetized when they were on the Moon. This conclusion was soon corroborated by the discovery of the existence of a steady magnetic field at the Apollo 12 site of the magnitude expected from the intensity of the NRM of about 36 γ (in addition to the field fluctuating solar wind. It was reasoned from this early work that the natural remanent magnetization had been acquired at the time of origin of the rocks or at least in their early history and a case was made out that this was a thermoremanent magnetization acquired as the lava flows cooled from magma flowing into the mare basin and as the high grade breccias cooled after the impacts. The origin of the natural remanent magnetization of the low grade breccias is more problematic and the role of other kinds of remanent magnetization processes has been investigated, particularly the effect of shock.

A brief descriptive summary of Jupiter’s magnetosphere is based on in situ observations with the spacecraft Pioneer 10 and Pioneer 11 in November-December 1973 and November-December 1974, respectively. Current interpretative work emphasizes particle acceleration and loss mechanisms, the determination of diffusion coefficients by satellite effects, the topology of the outer magnetosphere, the possible recirculation of energetic particles, and the controversial evidence for an extended magnetotail.

Available evidence on non-thermal radio emissions of the planet and on the solar wind flow at 10 AU is invoked to suggest that Saturn very likely has a large, well developed magnetosphere resembling that of Jupiter but with the important difference that a radiation belt can not exist interior to the outer edge of the A ring of particulate matter. The first in situ observations will be made by Pioneer 11 in August-September 1979.

The landers Venera 9 and 10 transmitted to Earth television pictures of the immediate vicinities of landing sites. The pictures were taken by optical-mechanical panoramic cameras looking from view-ports at about 0,9 meter above the ground. The nominal field of view is 40°x l80°. The width of one element of the picture (camera resolution) is 21 minutes of arc. The pictures were subsequently treated by computer enhancement. The first results of panorama analysis were published elsewhere (1,4).

Venera 9 landed at the point 32°N, 291°E on the rather steep (˜20°) slope covered by angular sharp-edged rock fragments. The fragments have a horizontal dimension of up to 50 or 70 cm, and a vertical dimension of no more than 15 to 20 cm. Some of the fragments demonstrate the evidence of layering approximately parallel to the flattening of fragments. The surface between fragments is darker than the fragment surfaces of the same orientation and seems to be composed mainly of particles of a size less than the resolution of the camera. The on-board gamma-spectrometer reveals that the radioactivity of the surface at Venera 9 site is similar to radioactivity of basaltic rocks of the Earth ( 2 ).

Analysis of cratering on all terrestrial planets and satellites has produced tools to study (1) the past meteoroid and planetesimal environment, (2) the erosive environments of planetary surfaces, and (3) the relative and absolute ages of planetary surface units. Important findings include a decline in lunar crater production rate from a value 4 x 109 years ago that was thousands of times higher than the present, to present values which have been relatively constant for 2 to 3 x 109 years; evidence for an erosive period or periods on Mars that degraded many Martian craters but declined substantially at some time in the past; and the concept of destruction of primeval planetary surfaces by early intense cratering and production of a mega-regolith.

Pioneer 10 flew by Jupiter in December 1973 and Pioneer 11, in a more polar trajectory, one year later. We present some of the pictures of Jupiter made with the spin-scan technique by the imaging photopolarimeter. The pictures are presented here without comment as a detailed description of Jupiter together with references concerning the instrument and its results, and were published in the book, Jupiter, edited by T. Gehrels (University of Arizona Press, 1976).

Data on the surface distribution of galaxies in a number of clusters has been given by several authors (see, for example, Shane & Wirtanen 1954; Zwicky 1956, 1957; Omer, Page & Wilson 1965; Clark 1968; Noonan 1971, 1974; Bahcall 1971, 1972a, b, 1973a, b, 1974, 1975; Rudnicki 1963; Rudnicki & Baranauska 1966a, b; Rood et al. 1972; Rood & Sastry 1972; Oemler 1974; Austin & Peach 1974a, b; Chincarini & Rood 1975, 1976; and references quoted therein). The distributions show a relatively smooth fall-off from a high density at the center of the cluster to a low density tail at the outskirts of the cluster. Secondary maxima or subclustering are sometimes superimposed on an otherwise smooth and monotonic profile.

Clusters of galaxies are easily identifiable collections of galaxies, all at the same distance and all observed under similar conditions of galactic obscuration, etc. They are, therefore, very convenient samples with which to study the matter content of the universe. However, clusters are also very particular physical environments, and from this latter point of view it is their atypical character which is of interest. The differences in the contents of one cluster from another, and of each from the contents of small groups and the “field” can teach us much about how the properties of galaxies depend on the environments in which they were born and have evolved.

Because of the interrelatedness of these two points of view, one cannot really understand the galaxy populations of clusters until one also understands the populations of galaxies which are not in clusters. Therefore, while this review will concentrate on the contents of rich clusters of galaxies, it will also be necessary to discuss the properties of non-cluster galaxies.

31 redshifts have been obtained for A1367, 34 for A262 and 61 for the Centaurus cluster (HMS 1247-4102). Full details of this work have been and will be presented in M.N.R.A.S. Both A1367 and A262 are spiral-rich in Oemler’s classification, while Centaurus is intermediate in type between spiral-rich and poor. In all 3 cases, the distributions of E and L galaxies are centrally concentrated, whilst the spirals are distributed in a more extensive and ragged fashion. The mean corrected redshifts and velocity dispersions for 2 morphological subsets are given in Table I. There is no significant difference in the mean velocities of the 2 subsets for each cluster. However, the velocity dispersions for the spirals are significantly greater than those for the E,L galaxies in both A1367 and A262. The differences in velocity dispersions of the 2 subsets for Centaurus are not statistically significant. Gott and Gunn have suggested that irregular clusters of the sort presently described have not undergone collapse. However, all 3 clusters show morphological separation and are x-ray sources. This is consistent with a collapsed core surrounded by a shell of infailing spirals.

A very faint intergalactic background light component (to be called IBL) was photographically discovered by Zwicky (1957) in the centre of the Coma Cluster. More recently Welch & Sastry (1971, 1972) have mapped the area using isodensity tracings of photographic plates. In order to provide an accurate calibration of the intensity and to determine the colour of the IBL in the Coma Cluster, photoelectrical observations have been made of a region located ˜2′ east of IC 3949 and ˜12′ south-west of the cluster centre. The observations were made using a 60-cm Ritchey-Chretién telescope at the Metsähovi Observatory near Helsinki. The diaphragm size was 115″. In Table I the results of these observations are given together with three other available photoelectrical values for the IBL intensity in clusters. For comparison, the surface brightness due to visible galaxies has been estimated. The resulting values are: IV = 11.6 S10 (≙ 25.1 mag sec-2), IB = 4.6 S10 (≙ 26.1 mag sec−2), and thus the IBL intensity amounts to 25 percent and 39 percent in V and B, respectively, of the light of visible galaxies. For the colour index of the IBL one obtains using the values in Table I: B-V = 0.54 ± 0.18. Two mechanisms have been hitherto proposed in the literature to explain the IBL in the Coma Cluster: (1) light from dwarf galaxies, intergalactic globular clusters or individual intergalactic stars, and (2) thermal bremsstrahlung from a hot intergalactic gas with Te = 0.5 − 10 × 105 K. A third possible mechanism can still be mentioned, namely the scattering of the light of galaxies by intergalactic dust grains.

Typical orbital periods for galaxies in rich clusters are observed to be several billion years. Consequently these large systems have had little time to come to equilibrium, and indeed if they formed from small initial density perturbations they must have taken a substantial fraction of the age of the Universe to separate from the Hubble flow and recollapse (Gunn and Gott 1972). Despite the relatively short time available, dynamical effects can cause appreciable evolution both in the distribution of galaxies within clusters and in the observable features of individual galaxies. The overall aspect of a cluster changes as violent relaxation irons out in homogeneities and establishes a relaxed core-halo structure. At the same time collisional relaxation causes massive galaxies to lose energy to lighter objects in an attempt to reach equipartition of kinetic energy, and to spiral towards the cluster centre. These processes are relatively easy to analyse in any particular scheme for cluster formation, and below we concentrate on comparing their predicted effects with observation.

I would like to report on some work that Ed Turner and I have done on groups of galaxies and how groups may be used to obtain useful cosmological information. Of course the classic study of groups of galaxies was done by de Vaucouleurs (1975) who made a catalogue of groups and loose associations of galaxies in the local supercluster. Partly to obtain a larger sample of groups with uniform and well defined statistical properties and partly to obtain a sample of denser groups suitable for virial analysis, Ed Turner and I decided to make a new catalogue of groups of galaxies based on picking the groups as surface density enhancements in the Zwicky catalogue (Turner and Gott 1976) (TGI).

The problem of the existence of high-order clustering of galaxies is a basic one in extragalactic astronomy and cosmology. In a conventional formulation this problem consists of indicating the largest configurations in the spatial distribution of galaxies.

All treatments are carried out generally on the material presented in the catalogues of Abell (1958) and Zwicky et al. (1961-1968). However catalogues of the Jagellonian type (Rudnicki et al. 1973) may turn out to be extremely valuable. The nature of the observational material demonstrates that only statistical methods may be applied effectively. It is already clear that the expected effects are not large even if high-order clustering is a basic property in the Universe.

A new updated list of clusters of galaxies found within x-ray error boxes is presented. Since the last published list (Vidal 1975) four new clusters A478, A2142, A2255 and an anonymous cluster were found within 3U0405+10, 3U1555+27, 3U1639+40 and MX1329−31, respectively. This extensive list is used to look for phenomenological correlations between x-ray luminosities and other physical cluster parameters. We analysed 3 such correlations: (a) The x-ray luminosity-velocity dispersion diagram is presented using newly determined velocity dispersions in the clusters A1060 (Vidal & Peterson 1975), A1367 (Tifft & Tarenghi 1975) and the Centaurus cluster (Vidal & Wickramasinghe 1976). First, it is shown (Yahil & Vidal 1976) that the observed velocity distribution of galaxies in clusters in general is consistent with a gaussian and, therefore, the standard deviation of the distribution can be used as a good statistical measure for the velocity dispersion. But, inspecting the Lx-∆V diagram, it is shown that a linear relationship between these parameters is not compelling yet. Furthermore the slope of such an assumed linear relationship is relying too much on the isolated Perseus point. Consequently the velocity distribution in the Perseus cluster is critically analysed.

The discovery of the extended sources of X-ray emission associated with clusters of galaxies was undoubtedly one of the most significant observations carried out by the Uhuru satellite (Gursky et al (1971), Giacconi et al (1974)). At the present time, there are more than 30 identifications and suggested associations of X-ray sources with clusters of galaxies although extended emission has been directly observed in less than 10 cases.

The mechanism for X-ray emission from such large objects is of considerable interest. X-rays could be generated by the inverse Compton interactions of microwave background photons with populations of relativistic electrons distributed throughout the clusters. Alternatively the radiation may be due to Bremsstrahlung from hot (T ≃ 108 °K) gas which constitutes an intracluster medium. Progress in understanding the extended X-ray sources and in determining the emission mechanism has come from observations of the X-ray structure and spectra of the cluster sources and it is the purpose of this review to present the current status of these observations.

Sixty-five radio sources in the 4C catalogue lie within 0.3 of the radius, Rc, defined by Abell (1958), of the centre of an Abell cluster of galaxies. Statistically few of these are expected to be chance coincidences and hence they provide a well defined statistical sample of sources in rich clusters of galaxies. Over the last 6 years sources from this sample having declinations greater than 10° have been observed with high resolution using the Cambridge One-Mile telescope by Slingo (1974(a) and (b)), Riley (1975) and by myself (1977, in preparation). The number of sources observed and the number expected by chance at different distances from the cluster centre are shown in Table I; the radio positions used are from the 4C catalogue.

“The properties of radio sources in clusters” presupposes that we know something about radio sources out of clusters, or that we even know whether a radio source is “in” or “out” of a cluster. Thus we are faced with the problem of defining what we mean by a cluster. Most of us use Abell’s catalogue of RICH clusters and assume that we are really “in” a cluster. However, most radio sources are identified with faint, distant objects and it is often difficult to know whether the remark “galaxy in a group” or “galaxy in a cluster” indicates a cluster such as the Coma Cluster, a cluster similar to an “open” Zwicky cluster, or a group of galaxies which may be gravitationally bound.

This uncertainty must not be forgotten, and in the following discussion, we will try to limit the effects of this by concentrating on catalogued clusters; ignoring most distant radio galaxies, many of which may be in rich clusters; and also by neglecting quasars, some or all of which may be in clusters.

The idea that gas may be present in clusters of galaxies originated with the missing mass problem—the observation that galaxies in clusters were moving so fast that the inferred galaxy masses (using conventional mass to light ratios for the galaxies) were not enough to hold them together caused astronomers to speculate that the “missing mass” was in the form of gas. Subsequent observations in the 21 cm and La lines as well as radio, optical and x-ray continuum observations have effectively ruled out this possibility in a large number of clusters. However, there is evidence for the presence of gas in smaller quantities in many clusters.