The location of the proposed landing site at the boundary between a highland and a mare provides the opportunity for another promising investigation. Deployment of a multiple-axis seismometer and recording of seismic waves from different directions should reveal something about any deep structural differences between the maria and the highlands. Thus, one might answer the question of whether or not the maria and highlands are analogous to the oceans and continents on the earth, which show major structural differences.
The Long Traverse
After the early fixed-station landings at a wide variety of lunar sites some form of long-range, mobile surface exploration will be necessary to overcome the limitations of men on foot. The answer lies in vehicular traverses, which would make it possible to study cross-country variations on the moon and so form the bridge between the intensive observations that can be made in the vicinity of a landing site and the extensive averaging observations that can be made from orbit. The technique that is attracting particular interest at present is called the dual-mode lunar surface roving vehicle system. The term dual mode refers to the fact that the vehicle can be used by the astronauts while they are working on the lunar surface and can be operated remotely from the earth after they depart. The present plan entails two separate lunar landings 500 kilometers apart at sites chosen to maximize the amount of information returned from the unmanned, automatic traverse.
Such an operation would proceed as follows. Near the end of surface activities in the first landing the men would start their unmanned vehicle on an automatic traverse. The vehicle, guided from the earth, would move across the moon toward a distant point that is within the second landing area. There the men participating in the second landing would meet the vehicle several months after it had started its journey. During its traverse the vehicle would collect samples of rock, transmit television pictures and conduct geophysical experiments yielding data that would be transmitted to the earth by telemetry. After the rocks had been retrieved by the astronauts at the second site the vehicle could be used by them in the exploration of that site. If the vehicle was still in satisfactory condition, they could start it on another long traverse.
A typical traverse might go from Rima Hadley into Mare Imbrium and thence into Mare Serenitatis. Along the way it would provide continuous profiles of the variations in gravity, magnetic and electric fields and depths of the surface layer. This particular traverse crosses one of the largest of the mascon areas and would cover enough ground to explore the phenomenon adequately with geophysical techniques.
The continuous monitoring of gravity along the traverse would provide information on the regional isostatic balance on the moon, that is, whether the higher topographic features are compensated by a deficiency of mass below them or whether they represent loads on the surface. An answer to this question would tell a great deal about the mechanism of formation of such features. Gravity information will also yield clues about the maximum depth of variations in density. If the moon has a crust analogous to the earth's, how does it vary between the lunar highlands near Rima Hadley and the center of the Imbrium basin?
The value of gravity measurements is increased if they can be combined with seismic information. Seismic measurements could be expected to resolve details of any layering in the lunar substrata. Along the traverse we have been describing a properly executed seismic experiment would quickly reveal the presence of giant iron asteroids buried in the mascons.