During the Apollo era, I remember gazing at the Moon to find the areas where astronauts were exploring at the time. Even with the most powerful telescopes, we are unable to detect the flags and equipment left behind, but it still is an interesting challenge to pick out these spots and not a bad way to learn about the Moon as well. Recently, the Lunar Reconnaissance Orbiter has been able to image these landing spots and made some discoveries that point to some other interesting potential landing regions should we return.
The Moon is divided into two types of terrain, the highlands which are the bright regions and the maria which are the darker areas. The highlands are very old, about 4-4.5 billion years and thus, heavily cratered. This makes the highlands geologically rich but challenging to land on. The maria are younger and thus easier to make a landing attempt as the terrain is smoother. The maria were formed 3-3.5 billion years ago when large impacts flooded basins with lava eventually to solidify into the dark, iron rich basaltic surfaces we see today. Maria is derived from the Latin word for seas which ancient astronomers thought these dark areas were. Of course, there are no large bodies water to be found in the maria. Like the first Apollo landings, a return to the Moon will likely begin in the maria and expand outward into more challenging landing zones.
Neil Armstrong and Buzz Aldrin spent 21.5 hours on the lunar surface on the Sea of Tranquility. Just before the famous Christmas Eve reading of Genesis by the Apollo 8 crew, William Anders noted the Sea of Tranquility was selected as a future landing site in order to preclude dodging mountains. While there were not any mountains to dodge, there were several large boulders causing Neil Armstrong to take manual control of the lunar module, eventually finding a safe landing spot with 25 seconds of fuel to spare. Apollo 11 returned 22 kg (48 lbs) of samples back to Earth. As would be expected from landing in a mare region, the rocks were mostly basalt created from lava when the region formed. There were also breccias which are smaller fragmented rocks fused together over time.
Launched during a rainstorm, the crew of Apollo 12 had to experience the adventure of getting hit by lightning before reaching orbit and proceeding to the Moon. Like Apollo 11, this mission landed in a mare region. The Ocean of Storms (or Oceanus Procellarum in Latin) was the landing site of Surveyor 3 in 1967. NASA wanted to aim for a precise landing near Surveyor 3 and examine samples in this region which appeared younger than the Sea of Tranquility. Astronauts Pete Conrad and Alan Bean made two excursions on the lunar surface reaching half a mile away from the lunar module. The samples were mostly basalt and, as expected, were 500 million years younger than the Sea of Tranquility establishing a range for lunar volcanic activity. The crew also visited the Surveyor 3 and retrieved its television camera which is currently on display at the Smithsonian Air & Space Museum.
After the Apollo 13 landing was aborted due to an explosion in a service module oxygen tank, its intended landing site in the Fra Mauro formation was slated for the Apollo 14 mission. This was the first landing to occur in the lunar highlands. This region contains rocks ejected by the formation of the Imbrium basin and it was hoped to capture samples that originated deep under the lunar surface. The plan was also to capture samples from the nearby Cone Crater but the rugged terrain prevented the astronauts from reaching the rim. Alan Shepard and Edgar Mitchell collected almost 42 kg (92 lbs) of rock samples most of which were breccia formed by rocks fragmented by the impact event. The crew did collect some basalts which clocked in at 4-4.3 billion years old, significantly older than the earlier basalts collected. Apollo 14 also had perhaps the most humorous event of the program with Alan Shepard’s attempt to play golf on the Moon.
Apollo 15 began the J-series missions for the program. These missions were more ambitious with longer duration stays and with the lunar rover, the ability to travel longer distances from the lunar module. Apollo 15 was the first landing to stray away from the equatorial region. David Scott and James Irwin spent some 18 hours exploring the lunar surface and traveled 28 km (17 miles-compared to 2 miles on foot for Apollo 14) on the rover. A major target was the Hadley Rille. Rilles are sinuous features on the Moon thought to be ancient lava tubes whose ceilings have since collapsed. Indeed, the rocks returned from this region were basaltic in nature. By the time Apollo 15 landed on the Moon, the final three missions (Apollo 18-20) were cancelled due to budget cuts, meaning there were only two trips to the Moon left for the program.
This mission was specifically designed to bring back samples from the highlands. Landing in the Descartes formation, Apollo 16 would return 96 kg (211 lbs) of Moon rocks that would fundamentally alter our understanding of the highlands. Previously thought to be of volcanic nature, the sample contained very few basalt rocks. Instead, the samples were breccia in nature. Rocks on the Moon are fragmented when impacts occur. These fragmented rocks are then fused together to form breccia rocks by the heat caused by subsequent impacts. The age of the highland are 4.5 billion years old. This dates back to the origin of the Moon as it cooled from a molten to a solid state.
As this was the final Apollo mission, a sense of urgency was placed on obtaining a high scientific yield. At one point, a landing on the far side was considered but rejected as it would require the additional cost of a communication satellite. The far side is often confused as the dark side of the Moon. However, during the new Moon phase the far side is facing the Sun and experiences daylight. The far side differs from the near side as it is mostly highlands and has very little maria regions as can be seen below.
The site selected was Taurus-Littrow Valley, a very geologically diverse region that required a precision landing. For this mission, Harrison Schmitt was moved up from the cancelled Apollo 18 mission to become the first astronaut-scientist. Three days after the only night launch of the Apollo program, America made its final Moon landing. Three excursions extending 25 km (15 miles) brought back a haul of 111 kg (245 lbs) of samples including highland rocks ranging from 4.2-4.5 billion years old, basaltic rocks from the valley floor indicating volcanic activity about 3.7 billion years ago, and ejecta from the Tycho crater that was 100 million years old. By lunar standards, the Tycho crater is a relatively young feature even though dinosaurs were walking on Earth when created.
The scientific phase of the Apollo program, which was to be 18-20, was cancelled by President Nixon as the economy began to experience its first bout of both high inflation and unemployment that would plagued the economy during the 1970’s. Two of the unused Saturn V’s are on display at the Kennedy and Johnson Space Centers. NASA began to develop the space shuttle and its planetary exploration program. The public lost interest in lunar exploration as there was a sense of been there, done that. However, the lunar surface covers an area 38 million square km (14.6 million square miles), about four times the surface area of the United States. As NASA began to recommence unmanned lunar exploration in the 1990’s, the Moon began to offer some surprises.
Lunar exploration was started again in 1994 with the Clementine mission that globally mapped the Moon, in particular, the 15 km (9 mile) deep South Pole-Aitken Basin. This was followed by the Lunar Prospector in 1998. The Moon had been thought to be completely water free but the Lunar Prospector detected the presence of 300 million tons of water mixed in the soil at both polar regions. How could water exist on the Moon? The Moon’s axis is only tilted 1.5 degrees. This means the Sun in these regions can only reach 1.5 degrees above the horizon, roughly the same as the Sun about ten minutes after sunrise in the mid-latitudes on Earth. Hence, large craters remain in permanent shadow so that any water there will not evaporate into space.
However, the Lunar Reconnaissance Orbiter (LRO), launched in 2009, discovered the presence of hydrogen beyond shadowed areas of the Moon. The water could have been delivered to the Moon very early in its history via comets. It is also thought the solar wind, which carries hydrogen, could interact with oxygen embedded in silicates on the surface to form water. To be sure, we’re not talking lakes or even underground springs here. The water amounts to about 45 parts per million, but given the cost to lift material from Earth into space (about $10,000 per ounce), any long-term settlement on the Moon will require the use of raw material situated there. This gives some promise that the Moon could be used as a base to colonize space.
Above – LRO’s high-resolution tour of the Moon
NASA is currently developing the Orion crew module along with the heavy lift Space Launch System which will make an unmanned test run past the Moon in 2018. The ultimate goal of this program is to land humans on Mars although a lunar program to test mission systems beforehand is not out of the question. Going to the Moon is only a three-day hop compared to seven months for Mars. Using the Moon as a testbed could make sense before making the leap to Mars. It is often argued that unmanned missions are less expensive, and less hazardous than crewed spaceflight. However, humanity is hardwired to explore and expand its presence. That is how we expanded beyond our origins on the African continent across the oceans to all corners of the Earth. Hopefully, in the near future, children will once again gaze at the Moon and ponder the about the people exploring our nearest celestial neighbor.