Apart from the earth, the near environment of only one other planet, Venus, has been the subject of sustained in situ investigations from space. Owing to the similarity in the physical characteristics of the two bodies, this is most appropriate in that it provides the opportunity to test the credo that through comparative analysis, planetary exploration holds the promise for better understanding of our earth environment.
The Pioneer Venus Orbiter was placed into orbit about Venus in December 1978 and has provided a wealth of observations of the planet surface, its atmosphere and clouds, the ionosphere, and the interaction of the solar wind and interplanetary magnetic field with the planetary environment. Among these observations, perhaps the most potentially significant and yet controversial result has been the claimed evidence of extensive lightning and the coupled assertion that the lightning is stimulated by volcanic activity. This lightning- volcanism scenario has become widely publicized in films, books, and numerous journal and magazine articles. Also, these results have had considerable influence in stimulating ongoing research on the basis that active volcanic output might explain otherwise unanswered questions in wide-ranging disciplines, including atmospheric and cloud physics, geology, and plasma physics. Quite naturally, the question of whether Venus is geologically either living or dead is fundamentally significant for comparative planetary analysis.