INFERENCES ABOUT THE MARIA

With four mare sites in the Apollo zone visited, distributed more or less uniformly in longitude from 23°E to 43°W, it was possible to draw some generalisations.

Surveyor 1 inspected a level plain in an ancient 100-km-diameter crater known as the Flamsteed Ring that had been ‘inundated’ in some way by Oceanus Procellarum; Surveyor 3 landed in a subdued medium-sized crater situated on the open plain of Oceanus Procellarum; Surveyor 5 provided a detailed inspection of a very small irregularly shaped crater in Mare Tranquillitatis; and Surveyor 6 inspected the plain of Sinus Medii within sight of a mare ridge. All four sites were very similar in terms of topography, and in terms of the structure of the surface layer and its mechanical, thermal and electrical properties; and the surfaces at the latter two sites were similar in terms of elemental composition and the content of magnetic material. ft seemed unlikely that terrestrial sites situated thousands of kilometres apart and selected in a manner similar to that by which the lunar targets were chosen would prove to be so similar.

At all sites, the undisturbed fine-grained surface material was lighter toned than the subsurface. This difference was as much as one-third for Surveyors 1, 3 and 6, but less for Surveyor 5. The fact that the albedo of the subsurface was the same at all sites meant the exceptional case of Surveyor 5 was due to the surface material being less bright. Observations of the erosion of the fine-grained surficial material by the vernier efflux during the ‘hop’ performed by Surveyor 6 and of the tracks left by the

fragments that were rolled across the surface, indicated that the bright surficial layer was limited to the uppermost few millimetres. The existence of such a well-defined ‘contact’ in a nominally undisturbed surface at four widely spaced sites on the maria implied the action of a process (or combination of processes) which had the effect of increasing the albedo of the material at the surface, for otherwise such a fine layer would be destroyed by the gardening of meteoritic bombardment. Furthermore, the fact that the material at all depths below the surface was uniformly dark, as opposed to there being a gradation in albedo, indicated that whenever an impact mixed the lightened surficial material into the subsurface, it became dark. Perhaps the process which altered exposed material had not had long to act on the material in the small fresh-looking crater in which Surveyor 5 landed. At all sites, the bright rounded rock fragments visible on the surface had textures featuring knobs and pits, whereas these were absent on the highly angular faceted blocks. This hinted that the process which produced the rounding – undoubtedly the relentless meteoroid bombardment – also gave rise to the ‘worn’ texture.

At all sites the fine-grained material was cohesive, and whilst the surficial layer was mildly compressible, its bearing strength increased rapidly with depth. But there was no observable variation in grain size with depth – evidently it was simply a case of the porosity decreasing with depth. It was estimated that the bulk density of the upper centimetre of undisturbed material was in the range 0.7 to 1.2 g/cm3, and that by a depth of several centimetres this had increased to 1.6 g/cm3.

The size-frequency distribution of small craters at all sites matched that expected for a steady-state population resulting from the protracted bombardment of primary meteoroids and the fall of ejecta from such impacts. Furthermore, this distribution was independent of individual differences in the mare surfaces and of the population of craters larger than several hundred metres in size.

The thickness of the fragmental debris layer on the mare plains was clearly related to the abundance of craters with diameters ranging between 1 and 10 km. In the part of Mare Tranquillitatis where Surveyor 5 landed the size-frequency distribution of such craters was twice that of the Oceanus Procellarum inundation of the Flamsteed Ring where Surveyor 1 landed, and the minimum size of the blocky rimmed craters at those sites indicated that the fragmental debris layer was several times thicker in Mare Tranquillitatis than in Oceanus Procellarum. Of all the maria, Sinus Medii had one of the highest size-frequency distributions of craters with diameters larger than several hundred metres, and the fragmental debris layer on the plain near Surveyor 6 was thicker to match. The fact that the cratering indicated the surface of Sinus Medii to be older than the other maria was evidence that the older the surface the thicker its fragmental debris layer. The size distribution of the material on the surface was also related to the thickness of the fragmental debris layer. When the mare lava flow was fresh and its rocky surface was exposed, small impacts were able to excavate it. As a layer of fragmental debris accumulated, it took larger and larger impacts to reach the substrate. Over time, the loose fragments were both reduced in size and increased in number. The trend was therefore towards a thickening layer of ever finer fragments. That is, the regolith ‘matured’.

The implication for Apollo was that an older surface would be a safer landing site.

When viewed from afar, an older surface might look rough by virtue of having large craters with blocky rims, but since only large craters would be able to excavate the substrate this meant that the plethora of small craters (which must be present) would not possess blocky rims. The task for the site selectors was therefore to measure the smallest craters with blocky rims on a mare surface to measure the thickness of the fragmental debris layer, and then seek a flat patch of open ground situated between such craters where it was likely to be relatively free of blocks.