Abstract: Terrestrial Evolution--Insights from the Moon and Planets
During the past 2 decades we have investigated the Moon in great detail, Mars intensively, and Mercury less so. The studies of Venus and the satellite of the outer planets are just beginning. Apparently these bodies all record (1) an early episode of intense bombardment, possibly the end of the accumulation phase; (2) planetwide differentiation and formation of an ancient, heavily cratered crust; (3) redistribution of crustal material, possibly by early convection; (4) emplacement of widespread younger lava flows in the low areas of the planet; and (5) continued sparse cratering, mass wasting, and other geologic processes. Some returned lunar samples are more than 4 b.y. old confirming the preservation of very ancient rocks. These rocks also record strong magnetic fields n the past whereas the field is very weak now. Mars data apparently record former fluvial episodes when the temperature and pressure of the atmosphere and surface must have differed radically from those now observed.
These data and their attendant hypotheses must be based, at least partly, on nonuniformitarian thinking. That is, events are recorded that differ dramatically from those processes operating now. It is possible, however, to infer reasonable scenarios of early planet formation based on, and extended from, processes now observable. Future spacecraft observations of outer-planet satellites will further strain our capability to devise processes that control the evolution of bodies that differ radically in temperature, density, and therefore composition from Earth.
AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma