Like many others, my wife and I enjoy getting away and visiting new places. A great way to take it all in and learn more is by taking a road trip —driving through beautiful landscapes, navigating new terrain to various points of beauty, and stopping to visit places of historical interest. I always try to work in points of geological interest on our trips (a geologist’s and history buff’s education never ends). Often, both history and geology combine at chosen destinations. Could such a sojourn be contemplated for the Moon? In the future, people might be traveling across the Moon, living and working there, as well as venturing out to sightsee at various significant locales. What places would I set out to see on a lunar road trip? Having studied the Moon for over 40 years, I have a few places in mind.
I suggest a road trip that starts on the lunar near side, near the equator in Mare Tranquillitatis, and slowly travels north, up through Maria Serenitatis and Frigoris, ending in the highlands surrounding the North Pole. Along the way, we’ll stop to examine the geology and some historical landmarks important to the history of lunar exploration and development. As with all good road trips, this itinerary is only an outline. The real adventure unfolds along the way. Let’s transport our imaginations into the future and see what unfolds before us on the Moon.
We’ll begin our trip at the greatest historical landmark on the Moon —the Apollo 11 landing site. Of course, we can’t get close to the descent stage of the Lunar Module Eagle to read the plaque on its landing leg, but we still have a good view of the site from the viewing platform a few dozen meters away. Cameras in hand, we start snapping away. It is somewhat annoying to gaze at this site from a distance (fighting the understandable urge to go over and put the toppled-over American flag back up) but mankind’s first steps on another world should be preserved as long as possible. It’s startling to look at this place and realize that our first visit to the Moon was restricted to such a small area —the crew did not roam much farther from the Eagle than the outline of a baseball diamond. But here is where the future began.
Moving on, we encounter the vast helium-3 mining fields of Tranquillitatis. From the Apollo samples, we found that the highest concentration of this gas correlated with the finest fraction of dust from the highest titanium content mare regolith. Thus, the largest prospect for that rare gas is here in Mare Tranquillitatis, where we find the highest titanium lava flows. However, the richness of an “ore deposit” is relative, as one realizes that about 35 cubic meters of regolith (about 50 metric tons) must be processed to extract a single kilogram of helium-3. But that single kilogram can produce 19 megawatt-years of electrical energy on Earth. The mining process barely registers on our consciousness from afar and as we draw closer we see why —the mining drones only plow and turn over the mare regolith, actually smoothing out the cratered surface, rather than scarring the landscape with deep pits. An open-pit mine on the Moon comparable in size to the Bingham Canyon copper pit in Utah wouldn’t even be visible from the Earth, even through the most powerful telescopes.
Passing northwards, we stop to examine several interesting volcanic features of Mare Tranquillitatis, including sinuous rilles (or lava channels) and small shield volcanoes. A most interesting feature is the large collapse pit of Mare Tranquillitatis (MTP), about 20 km NE of the crater Sinas A. This pit, discovered from high-resolution images of the LRO orbiter back in 2010, leads to the largest underground cave found so far on the Moon. It wasn’t until we arrived on the Moon that we discovered that this was the entrance to an extended lava tube tunnel. Because this cave provides constant protection from the harsh thermal and radiation environment at the lunar surface, it has the potential to be a nearly perfect shelter for human habitation.
Crossing the boundary between Tranquillitatis and Serenitatis, we next visit the site of the last Apollo landing on the Moon, the Taurus-Littrow landing site of the Apollo 17 mission in 1972. Gene Cernan and Jack Schmitt spent three days covering this little valley from corner to corner. While they discovered many interesting things here, perhaps the most startling was the orange and black glass found at Shorty crater. After holding a sample in hand, they recognized large deposits of orange soil all around the rim of the crater and even saw it on the surface from orbit. These glasses are volcanic ash. They were created when hot, liquid rock under high pressure squirted into space quickly, where it cooled into spheres of glass during ballistic flight. They later landed back on the lunar surface, forming bedded deposits. These glasses contain volatile elements from deep within the lunar mantle and tell us much about both the early history of the Moon and its bulk composition. Some of these deposits contain fragments of the lunar mantle, ripped from the walls of the conduits that transported the magmas up to the lunar surface.
Continuing northward, we travel across the strange fractured floor of the crater Posidonius. This ancient feature has an unusual sinuous rille (lava channel) that runs partly between the wall and floor of the crater. Perhaps this relation indicates that hot lava eroded parts of a pre-existing debris deposit. The issue of lava eroding the surface of the Moon was controversial many years ago when it was proposed and remains so now. Lava channels are primarily constructional features, but sometimes, very hot lava can partly melt and remove pre-existing surfaces. The fractures and rilles of Posidonius are being studied to address this contentious issue. It is instructive to stand here and allow one’s eye to take in all the pieces of this geological puzzle.
As we move north out of Mare Serenitatis, we notice that the highlands take on an unusual, knobby texture, similar to that of the unit we saw back in Taurus-Littrow that the Apollo 17 crew called Sculptured Hills. They should look similar —this rolling landscape is all part of the same unit, an immense sheet of debris called the Alpes Formation, thrown out from the gigantic Imbrium basin (diameter ~1200 km), over 500 km west from this locality. Traveling further, we come across the unusual “paired craters” Atlas and Hercules, two impressive fractured floor craters with nice euphonious mythological names. Large crater “pairs” occur all over the Moon and also on Earth. They do not appear to be simultaneous double-impacts, as they typically possess no septum ridges between them, as many secondary craters do. However, the close spacing of large, comparably sized craters so often seems to defy random chance. Perhaps these craters form by the collision of mutually orbiting asteroid twins, possibly more common among the Earth-crossing objects than had been thought.
It is instructive to see this undisturbed historical record, so unlike our Earth where much of the record has been lost by erosion and an active, ongoing geologic history. Fortunately, the Moon serves as a template, a Rosetta Stone for our understanding of the solar and impact history of the Earth. We’ve now begun our traverse into the highlands terrain of the polar region, having passed through Mare Frigoris, the most northerly mare deposit, a monotonously flat and featureless terrain. Passing crater after crater, we slowly cross the rugged terrain, happily finding that it’s not as difficult to travel over as we’d envisioned from earlier views of the area from orbit, and as we’d seen it in pictures. The terrain is all quite smooth —gentle, rolling hills as far as the eye can see.
As the lunar day ends and the surface cools, we can use our infrared sensors to detect the presence of water molecules on the soil. Where does this water come from? Years after its discovery, this is still debated. Perhaps the water is made from solar wind protons that reduce metal oxides in the soil. Or, we may be seeing the beginning of the migration of cometary water towards the pole. In any event, there is much too little here to be of practical use; only in the deep, cold traps of the polar wastes do we find massive accumulations of water, gradually added molecule by molecule over hundreds of millions of years.
As we approach the dark regions near the North Pole, the appearance of the sun circling around the horizon is a powerful image —never rising or setting at the poles, it continually skims the horizon on its roundabout path. At our final stop, we’re treated to breathtaking views of the largest mining operation of all. We watch as robotic diggers and haulers, working deep within the crater, process polar soil to extract water. Hundreds of tons of water are mined and stored each day. This water has many uses —as life support for residents of the lunar base near Peary crater, and as shielding to protect the human crews that go out to explore various regions of the Moon. Most importantly, the water is sent to local cracking plants, where it is broken down into its constituent hydrogen and oxygen atoms, which are then frozen into liquid form to create rocket propellant. This valuable, locally obtained rocket propellant fuels vehicles throughout cislunar space. It allows us not only to come and go on the Moon at will, but also to conduct routine operations between Earth and Moon, and to fuel missions to other planets being conducted by many nations.
Mining of lunar volatiles drives the economic engine of cislunar space. Just as on Earth, major industry on the Moon spawns the necessary supporting infrastructure, which in turn, also profits. And of course, we’ll pack up and bring home all the souvenirs we’ve found on our 2,730 km (1,700 mile) road trip on the Moon. Future road trips will certainly follow. You can begin planning your own route by using this interactive map of the Moon.