The origin and age of the seemingly volcanic features in the Marius Hills region are of considerable importance in understanding the evolution of the lunar surface. Terrestrial volcanic features are built up in very short times compared with the entire history of the earth. Even an extensive region such as the volcanic chain constituting the Hawaiian Islands represents a period of less than 70 million years. The absolute age and the length of time involved in building up the Marius Hills domes will be of great interest in the characterization of lunar volcanism.
The Marius Hills region is far too extensive to be covered in a single manned expedition to the moon. Fortunately a number of characteristic features of smaller scale can be visited in several areas that are no more than 70 kilometers in diameter. A mission to such an area would be able to sample and study a number of small domes 50 to 100 meters in elevation with convex slopes; steep-sided domes with rough, intricate surfaces; steeply convex or bulbous domes that are smooth and generally symmetrical; steep-sided cones with linear depressions at the summit; narrow, steep-sided ridges, and a variety of impact features.
The fourth candidate site is the region of the Apennine Mountains, which roughly form the southeastern boundary of Mare Imbrium and also the northwestern leg of a triangular highland area bounded by Mare Imbrium, the southwestern boundary of Mare Serenitatis and the northern part of Sinus Aestuum. The Apennines are among the most impressive of the lunar mountain ranges. The Apennine front rises 4,800 meters above the adjacent mare level to the west.
What can be learned about the moon by visiting this area? The Apennine front is a major physical feature of the moon, exposing an extensive vertical section several thousand meters thick for sampling and examination. Here is an opportunity to assess what may be a long period of lunar history. Are the rocks in form or physically and chemically heterogeneous? How old are they? Are they stratified? Answers to such questions could have a profound effect on our understanding of lunar history.
Two landing sites have been proposed near the Apennine front that are within five kilometers of important lunar features. One such feature is the rille, or canyonlike configuration, known as Rima Hadley. Is it a surface-flow channel or a collapsed lava tube? If it was formed by water, as has been speculated, where did the water come from and what prevented its immediate evaporation?
The Significance of Rima Hadley
Close examination of the Lunar Orbiter photographs of this rille reveals that fresh exposures of rock are visible along its walls and that blocks have fallen down the walls to the floor of the rille. Rima Hadley cuts into the Boor of a mare and thereby yields a depth and perhaps a cross section of the history of a major lunar feature. Hence it might provide answers to such questions as whether the maria are bedded deposits of lava or ash flows, sedimentary deposits that contain a sequential history of formation or simply an agglomeration of cold particulate matter accreted from space.