Concluding remarks

A predominant aim of planetary exploration is to discover as much as possible about the origin of the Solar System. Therefore it is appropriate that the closing paper by Dr H. Reeves should be explicitly about this. Of course, almost every paper has implicitly provided constraints upon models for the formation; many are already familiar, but the recent work reported here renders them more severe or precise than before. The discussion prompts, in particular, the following considerations. In regard to terrestrial planets and the Moon and other large satellites, much is being learnt from recent comparative studies of surface features, of possible effects of volcanism, and of the physics of impact cratering. After taking account of differences due to the presence or absence of various sorts of atmosphere and of the consequences of the different values of surface gravity, the ‘geologies’ of the various bodies are still surprisingly different. Some of the differences that have been described are undoubtedly owing to the presence or absence of water in various states. This feature must surely be an important clue to the way in which the bodies were assembled. For instance, one cause of the difference between Earth and Venus is probably the circumstance that, if as generally supposed the solar luminosity had about 70 % of its present value in the early stages of the Solar System, a water-ice coating on a dust grain (assumed to behave as a small blackbody) at the Earth’s distance would remain frozen, but at the distance of Venus it would be melted. This must make a considerable difference to the way in which water was originally incorporated into the structures of these two planets