Free Astronomy Magazine May-June 2018

7 PLANETOLOGY those on the Moon, with a few meters of regolith, the typical dusty material that surrounds the surface of our satellite. This solution is certainly suitable for defense against harmful radi- ation and micromete- orites, but it may be an unsuitable choice to counteract the ef- fects of impactors several tens of cen- timeters in diameter or larger. Moreover, an engi- neering problem arises, that of having to send to the lunar surface (worse on the martian one) all the machines necessary to dig, carry and lay hundreds or thousands of tons of regolith. The placement of that much regolith over bases is a significant obstacle to the realiza- tion of projects of this type, a problem that grows proportionally with the size of the base to be installed and protected. Habitation would be much simpler if there were geological structures on the Moon and Mars that would naturally shield future human settlements. Incredibly, such fea- tures seem to exist on both bodies, and in recent times researchers have gathered more and more convincing evidence for them. The geological structures in question are long under- ground tunnels that may be large enough to accommo- date the population of a small city. The evidence that those types of formations are present on the Moon dates back to the 60s, when the surface of our satellite was photographed and exam- ined with particular atten- tion as part of the planned Apollo landings. Researchers noticed in the images many narrow and long channels, meandering in vast areas af- fected in the very distant past by massive effusions of lava (essentially short-lived lava seas and the floors of major impact craters). A n example of a lava tube: the floor was the crust on a former lava river that fell in- ward as it drained from beneath. [Dave Bunnell] Below, this cav- ern in Mare In- genii is almost twice the size of the one in the Marius Hills. [NASA/Goddard/ ASU]