According to recent reports, China plans to be the first to land on the “dark side” of the Moon, using hardware left over from the recent Chang’E 3 lander/rover mission. Although there is a “dark side” (the nighttime hemisphere, which changes constantly because the Moon rotates on its axis once every 708 hours), what is meant here is the far side, the hemisphere of the Moon that is permanently turned away from the view of Earth. This arrangement results from the fact that the Moon’s day equals its “year” —it takes the same amount of time for it to rotate once on its axis as it does to revolve once around the Earth. Although several spacecraft have “landed” on the far side (specifically, by crashing into it), no mission has ever soft-landed and sent information about the site back to Earth.
Until the flight of the Soviet Luna 3 mission in 1959, no one had seen the far side. Speculation prior to this flight was that the lunar far side, although of course probably differing in detail, had terrain and features just like the near side—large circular, dark maria, rayed craters and rough, cratered highlands. The pictures returned by Luna 3 were abysmal in quality, but as bad as they were, we could see that something was different about the far side—it looked largely bland and featureless.
Upon further study, it gradually became clear that the far side is almost all highlands, the heavily cratered, rough uplands that, on the near side, occur primarily in the southern sector of the lunar disc. Very few dark maria are present. A particularly spectacular deposit fills a large 180 kilometer-diameter crater on the far side that the Soviets named “Tsiolkovsky” (the Russian father of astronautics who developed the rocket equation around the turn of the 20th century). In addition, a fairly large region in the central far side contains a significant dark deposit; the Soviets named this feature “Mare Moscoviense,” the Latinized form of “Sea of Moscow.” Aside from these two obvious areas, at that time, few maria were seen.
In time, many other spacecraft flew to the Moon and returned ever more detailed images of the far side. Other maria were found, but none as extensive as the vast plains covering the near side. Through mapping, we discovered several small “patches” of maria concentrated in the central, southern hemisphere of the far side. These deposits do not constitute a large, contiguous region of maria, as do the Oceanus Procellarum on the near side, but rather, fill highland craters and occur in a roughly circular pattern.
We found from the Apollo missions that the dark, smooth lunar maria are made of basaltic flows that erupted billions of year ago. Basalt is lava, rich in the elements iron and magnesium; some lunar basalts contain large amounts of titanium as well. Basalt is created when molten rock (magma), produced by partial melting of the deep interior of a planet (the mantle), ascends to the surface and erupts from fissures or cracks in the crust. The lava then spreads out across the surface and cools as deposits of smooth plains. On the Moon, lava erupted from about 4 billion to less than 1 billion years ago (with the vast bulk extruded between 4 and 3 billion years ago). Mare basalt tends to occur within large, regional depressions called basins, impact features hundreds of kilometers across that formed very early in lunar history. Because they are so old, basins are difficult to recognize. It took careful geologic mapping to document their existence and locations.
As the Moon was mapped globally (using data obtained by orbiting spacecraft), we learned that while basins occur in equal numbers on the near and far sides, infilling by mare basalt is scarce on the far side. If basin-filling by lava was a ubiquitous feature of the Moon’s geologic style, why is it not more widespread on the far side? This relation presented lunar scientists with a bit of a puzzle and remains an unsettled issue, although some ideas (mostly speculative) for the paucity of maria have been advanced.
What could a lander mission to the far side tell us about lunar history? That depends on where the spacecraft is sent. Most of the terrain there seems fairly unremarkable—an endless series of large, overlapping craters in highlands terrain. Remote sensing data show that most of this crust is very low in iron, meaning that it is made of feldspathic rocks rich in aluminum and calcium. These rocks are called anorthosites, and make up the primitive, early crust of the Moon. If the lander was sent to a mare area, it could tell us whether far side lavas are similar to, or different from, those on the near side.
Much of the scientific interest in the far side centers around the largest impact crater on the Moon, the 2,500-km-diameter South Pole-Aitken basin. Amazingly, this interesting feature was first recognized before we had even seen the far side clearly. In 1962, while mapping the southern highlands of the Moon using a telescope, astronomer Bill Hartmann noticed several large mountains near the pole. He knew that the only process that could make mountains on the Moon was impact (basin creation ringed by uplifted and collapsed mountains). Based on the spacing and accurate trend of these mountains, Hartmann postulated that a very large basin must exist on the far side. When the Apollo missions orbited the Moon in the early 1970s, a laser altimeter recorded a topographic low as the spacecraft passed over the southern central far side, near the crater Aitken. This low area was on the opposite side of the basin that Hartmann had seen in the telescope, implying a basin greater than 2,000 km in diameter! It was initially called the “Big Backside Basin” but as this term was considered undignified, geologist Don Wilhelms later gave it the formal name South Pole-Aitken (SPA) basin, after the two, unrelated features that spanned its diameter.
We have since found that SPA basin is the principal compositional anomaly of the far side; the basin not only contains almost all of the far side maria, but its floor is more mafic (iron-rich) than the rest of the highlands. This relation suggests to scientists that the basin-forming impact excavated much of the crust in this region of the Moon, exposing either the deep crust or the upper mantle of the Moon at the surface. A sample return mission has been proposed to this area that would collect rocks that might answer this question, as well as allow us to determine the absolute age of the basin (considered to be the oldest impact feature on the Moon).
While China is focusing the upcoming Chang’E 5 mission on sample return from the near side, a properly instrumented lander and rover (for the future, unnamed far side mission) could make chemical and mineral measurements of the surface, and this information could guide site selection of future sample return missions. For example, if lunar mantle material is exposed on the surface within SPA basin, it should have a distinct composition (detected by a properly instrumented spacecraft). It may even be possible to age-date materials using remote techniques (although this has yet to be attempted with robotic spacecraft). As there is no line-of-sight to the Earth on the far side of the Moon, any mission there will require placing a relay satellite in orbit to maintain communication with and control of the vehicle.
Yet again, China is showing foresight and leadership in space by attempting missions and activities designed to create cislunar presence and dominance. While the Moon is not a priority for America, other nations are asserting their superpower status by planning and flying missions to and around the Moon—demonstrating, through concrete actions, that they are establishing a critical, strategic presence beyond low Earth orbit. With a mission to the “dark side” of the Moon, China not only achieves a new milestone in the history of space exploration, but also advances itself to become the leading world power of lunar spaceflight. Their efforts (which, unlike the U.S., are current) are remarkable and significant; they have flown spacecraft to all regions of cislunar space (including landing on the Moon), parked and loitered at libration points, returned to lunar orbit, and left orbit for flights to asteroid close encounters. By ignoring or denigrating the importance of these developments, we willingly relinquish our place in space and our obligation to continue what others (who gave so much) accomplished to give us a lead 50 years ago.