The large dark areas on the Moon, the so-called Lunar Seas, formed three to four billion years ago when most of the large impact basis filled with layers of a very low viscosity lava and cooled. Some such basins on the Farside of the Moon did not fill with lava and are called thassaloids ( from the Greek word for sea).

While an initial Lunar Base might be built just about anywhere, once more extensive settlements are built, the maria are clearly preferable. The regolith, the loose surface material, composed of rock fragments and soil, which overlies consolidated bedrock, has a very variable thickness in the highlands, from zero to 30 meters. On the mare, however, the regolith has a more uniform depth of about 10 meters, which makes construction easier. While Lunar concrete relying on calcium rich highland soil and upported hydrogen will be a lot cheaper for initial base construction than pre-built modules brought from Earth, once a lot of construction is planned, even that method will be too costly. The only way to go is site-extrusion, building the structures from the fused soil on the site itself. Mare soils melt 200C ( 360F ) lower than highland soils and so will require significantly less energy either in fusing rammed soil or in making panels of cast basalt. The melt’s lower viscosity will also help in some applications.

The levelness of the mare surface will also be an asset to laying out any extensive settlement. And importantly, the average atomic number and weight of mare soils, as compared to highland soils, makes them preferable for shielding against cosmic rays, etc.

But the best mare sites will be just offshore, so to speak, so that highland soil, richer in aluminum and calcium, will also be available for manufacturing and processing. Finally, such a site will offer more scenic and recreational interest.

[Subsequent articles in later issues of MMM call for coastal sites, in the spirit of this last paragraph.]