258 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



T. Gold [6] has suggested that the maria consist of deep layers of 

 dust produced by the eroding effects of sunlight and particle radia- 

 tion from the sun. He believes that such particles would move over 

 the surface because of cliarges on them. A hopping motion is assumed 

 to have moved them over great distances in the past and to move them 

 in the same way at the present time. He points out that the many 

 craters, large and small, in the southern regions have smooth, gray 

 material within and between them, and that neither lava from the 

 interior nor molten material resulting from collisions appears to be a 

 reasonable explanation. Such reasoning prompts two questions. Do 

 we think that liquid from the interior seeped up in all these places? 

 If there were ever large quantities of liquid beneath the lunar surface, 

 would not the more dense solid phase sink steadily down into the 

 liquid until it was completely submerged? 



However, the hypothesis that erosion was the main origin of the 

 dust is unreasonable, for if this were so, all parts of the moon surface 

 should have the same appearance, or this should at least be true of 

 all equatorial regions. It is not true of the hemisphere that faces 

 the earth and, as we have lately learned, emphatically not true of 

 the back hemisphere. These facts make Gold's general hypothesis 

 untenable, although some erosion of this type may be present and may 

 modify the effects of other more dominant processes. 



If spread over the moon's surface, the Imbrian planetesimal, sup- 

 posedly some 200 km. in diameter, would make a layer 110 m. deep, 

 and if it contained 1 percent of water by weight this would be enough 

 to cover the moon to a depth of 3.9 m. This is a larger percentage of 

 water than is contained in the meteorites or on and within the earth. 

 Other planetesimals wliich produced the other circular maria could 

 have supplied water also. Such planetesimals would be moderately 

 explosive if they contained water or other volatile substances. These 

 possibilities suggest that in addition to the materials which have 

 already been discussed above as coming from these planetesimals, 

 there may have been a vast dust cloud which spread finely divided 

 materials over the moon's surface. This material, of course, would 

 fall on the mountain slopes, but it does not appear to be there now. 

 Did temporary rains wash it off? Or did the violent moonquakes 

 cause it to slide into the valleys, as "fluidized" mixtures of gases and 

 solids used in industrial chemical processes do? Gilbert remarks 

 that many lunar features look as though they had been covered by a 

 pasty material, and thought it was partially solidified lava. Perhaps 

 he was right, and it is neither dust nor rubble. 



To resolve the problem we should look for objective evidence. Any- 

 one looking at the moon through even a small telescope is immedi- 

 ately imprcvssed with the smooth character of the maria, but I am 



