It would be fine if astronomers would identify for us which stars could be accompanied by planets, and it would be staggering if through some technologic tour de force they were able to demonstrate the positive presence of planets in a few such cases. Yet the next step-to determine whether life as we know it could exist there-involves many parameters whose interplay is poorly understood even under optimum observational circumstances and which seem utterly indeterminate at interstellar distances. Everyone recognizes that this is so. Yet the fact that this Conference is being held is evidence of our belief or hope that somehow or someday the subject may open up, probably in a way no one here can anticipate.
That may be too slender a thread for most scientists to hang their professional energies on, but the subject of the viability of planetary surfaces in our Solar System is fully worth our attention. The exploration of the Solar System by spacecraft is opening up a vast field of investigation which did not exist as a serious subject 15 years ago. We cannot help but gain a better feeling for the possibilities-not the probabilities-of life near other stars if we are able first to understand the processes that have operated on the surfaces of our own planets. We begin with an examination of what may or may not be a very special case, our own Earth.