Apollo Expeditions to the Moon|
INCONCEIVABLE VIOLENCEThe cratered highland phase that followed was extremely, almost inconceivably violent. The debris left over from the creation of the planets bombarded the light-colored crust. These highland surfaces have survived as the bright portions of the full Moon we see today. They were pulverized, remelted, reaggregated, and, finally, saturated with craters at least 30 to 60 miles in diameter. The sheer violence of those times is difficult to comprehend.
The large basin phase was the time when very large basins were formed. This appears to have been the result of a distinctly more massive scale of bombardment than that which preceded their formation. These large basins dominate the surface character of the front side of the Moon and are responsible for the major chemical differences we have measured between various large surface regions.
The light-colored plains phase that followed was a brief, still controversial period in which most old basins appear to have been partially filled with debris largely derived from the surrounding light-colored crust. The events that created these plains are poorly understood partly because several different processes related to both meteor impact and internal volcanism may have produced similar plains.
The basaltic maria phase was the main period during which the accumulation of heat from radioactive elements within the Moon produced melting and volcanic eruptions. Those eruptions filled all of the large basins with thick masses of dark-colored basalt called the maria. (These sea-like regions are the dark portions of the full Moon.) The lunar basalts are very different from basalts on Earth; they contain much less sodium, carbon, and water and commonly have much more titanium. iron, and heavy elements. At least the upper parts of the maria are ancient lava flows up to 300 feet thick. Many flows differ significantly from each other in chemical and mineral characteristics, differences that vary with both the age and the region.
The quiet crust phase from about 3.0 billion years ago to the present was largely just that- quiet. Compared to the past, very little happened except for the formation of scattered, very bright craters like Tycho and Copernicus, the creation of regional fault systems like the Hyginus Rille, and the appearance of mysterious light-colored swirls like Reiner Gamma. Eruptions of basaltic maria also seem to have continued along a ridge and volcanic system that stretches for 1200 miles along the north- south axis of Mare Procellarum. Some of the events may be indications of continuing internal activity and stress beneath a now strong crust, such as the slow, solid convection of the lunar mantle.
For the most part, the surface of the Moon appears to have completed recording its history about three billion years ago. It has been largely unchanged except for the continued eroding rain of small meteors and now by the first primitive probings of men.
The Moon is as chemically and structurally differentiated as the Earth, lacking only the continued refinements of internal melting, solid convection, surficial weathering, and recycling of the crust. It moves through space as an ancient text, related to the history of the Earth only through the interpretations of our minds. It also exists as an archive of our Sun, possibly preserving in its soils much information of importance to man's future.
If we are to continue to read the text, we must continue to go there and beyond.