In summary, then, we have reason to speculate that the moon is hot today in its deep interior. In the distant past, it must also have been hot much closer to the surface. Vast sheets of lava in the maria, such as. Serenitatis, Tranquillitatis, and Imbrium, testify to widespread volcanism long ago. Such flows, scientists say, could easily have come from depths of as much as 300 miles, finding their way to the surface through the massive networks of fractures caused by the impacts that created the circular basins.
Scientists conclude that the moon’s heat engine seems to be cooling, at least in its upper levels. About the only hint of contemporary volcanism comes from suspected venting of gases. Orbiting instruments in the Apollo spacecraft have detected radon, a radioactive gas, possibly seeping from the vicinity of the craters Aristarchus and Grimaldi. Such gases may be linked to elusive bright spots sometimes seen on the moon.
As far back as the 1500′s,observers reported brief sightings of such spots, sometimes glowing with color. Just ten years ago at Lowell Observatory, three reddish patches were seen near Aristarchus, and they remained visible for more than an hour.These transient phenomena seem to be closely connected to tidal forces which are many times more powerful than those affecting earth. Actually, the distortions caused by these forces are also believed to be the chief source for Dr. Latham’s quakes. The moon displays a number of peculiar anomalies, or irregularities. For one thing, its mass is mysteriously off center.
When a Ranger spacecraft plunged into the moon on a photographic mission in 1964, scientists monitoring its passage were perplexed because it crashed slightly later than expected. Careful analyses of radar measurements, photographs, and spacecraft orbits showed that the center of the moon’s mass and the geometrical center were not the same. In fact, the center of mass is displaced toward the earth, and the near side of the moon is about two miles farther away from earth than expected, as confirmed by Apollo’s laser altimeter. Small as it seems, it is significant to geodesists.
No one has a certain explanation of this anomaly. However, some scientists believe that the crust on the moon’s far side may be thicker than the 35 miles indicated by seismometer readings in Oceanus Procellarum. If so, the two irregularities probably have some connection.
In several other ways the near and far sides of the moon exhibit remarkable differences. The front side, as the most casual glance at the full moon reveals, displays vast flat stretches of dark maria, many of which, scientists believe, are enormous impact basins filled with lava. These are interspersed with heavily cratered highlands, the brighter areas of the moon’s face.
By contrast, the far side, which we never see from the earth, is almost entirely highlands. Only a few of the far-side basins and craters, such as Moscoviense and Tsiolkovsky, have been filled with lava. Why is this? Why did lava in enormous quantities rise to fill the big basins on one side of the moon but not on the other? Perhaps a thicker crust on the back made it harder for lava to break through.