The Significance of Lunar and Planetary Exploration

Having accomplished a preliminary exploration of the solar system, we now possess a general understanding of the problems of and constraints on planetary origin and evolution. Much of our information has been derived from the study of the Moon. We have been able to establish the following sequence of events from the lunar evidence: Differentiated planetesimals accreted to form the Moon within 100 million years of the isolation of the solar nebula. The Moon became partially to completely molten and large-scale crystal-liquid differentiation produced a feldspathic crust and a complex mineralogically zoned mantle, from which some 20 differing types of mare basalts were subsequently erupted. Whether the initial melting produced a magma ocean or several smaller-scale melted regions is partly a semantic problem, since the bulk of the Moon went through this process on a timescale of 100-200 million years. By 4.4 aeons, the interior had crystallized, as shown by the isotopic data, and the feldspathic crust was in place. For the next 500 million years, the crust was subjected to the full fury of bombardment by planetesimals and lesser objects. The resulting chaos in the upper crust explains much of the petrologi-cal and chemical complexities of the highland samples. Possibly the crust did not become stable against the raining impacts until 4.2 aeons. The declining stages of the bombardment are preserved in the majestic ringed basins and large craters of the highlands. The basin-forming events were over by 3.8 aeons, with the formation of the Imbrium and Orientale multi-ring basins. A "terminal cataclysm" does not appear to be required. This date (3.8 aeons) provides a younger limit for heavily cratered surfaces , which are common throughout the solar system, and is perhaps the