[vii] This is the third document in a chronological series. The first appeared in 1970 at the request of NASA's former Planetology Program under the title, "A Strategy for Geologic Exploration of the Planets,'' edited by M. H. Carr and published as United States Geological Survey Circular No. 640. Then, as now, the objectives were to define the major goals of planetary geology and to set forth methods of meeting these goals. In 1976, the Carr study was updated with the publication of A geological basis for the Exploration of the Planets, edited by R. Greeley and M. H. Carr (NASA SP-417).
The purpose of the present report is to once more provide an update and a future projection, this time into the 1980s.The basic objectives of the original strategy laid out in 1970 and in 1976 have changed little. Rather it is our approach to realizing these goals that has matured and expanded. Now that Voyager has ventured beyond the asteroid belt into the realm of the other planets, we have become keenly aware that the detailed study of satellites can teach us much about the geologic evolution of solid bodies in general. When the 1976 report was written, the bizarre nature of Titan's surface was hardly suspected: we were not aware of the intense volcanic activity of Io, or of the possibility that Neptune's Triton may have an ocean of liquid nitrogen. Another new emphasis that will be found in this report concerns the developing perception that the investigation of small bodies-asteroids, comets, and small satellites-can reveal much about the processes that were important during the earliest stages of the solar system's history. The enhanced emphasis on satellites and small bodies does not imply that our traditional interest in the geology of Earthlike planets has decreased; rather, it is a sign that as our discipline grows we are able [viii] to assimilate more and more diverse information in building a truer picture of the geologic evolution of the planets. We certainly expect that the 1980s will see a continuation of the vigorous effort to decipher the complex geologic history of Mars revealed by the Viking data. We also anticipate that as the quality of our geologic information concerning Venus continues to improve a major goal of planetary geology research will be to understand the distinct evolutionary paths of Earth and Venus-two planets which at first sight appear to be similar in many of their bulk physical properties.
As before this report is not concerned with advocating any particular spacecraft mission or series of missions nor does it outline a sequence of solar system exploration programs. Rather it is restricted to defining the kinds of experiments observations and measurements that need be made by Earth-based Earth-orbit and spacecraft exploration techniques to address major issues of concern to planetary geology. It also discusses important goals of data analysis and data synthesis in the area of planetary geology.
This document was written by the Planetary Geology Working Group during 1981-1982. The Group attempted to produce a consensus document based on drafts of individual chapters and sections written by specific members (the individual contributions are identified in the Acknowledgments). We also received numerous comments on several draft versions of the report from many of our colleagues; we hope that this helpful criticism has enabled us to maintain a balanced and a representative perspective throughout this document.