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• 1 media/596px-Moon-apollo17-schmitt_boulder copy_crop.jpg 2016-06-06T14:28:54+00:00 The surface of the Moon 64 image_header 2022-02-03T20:18:51+00:00 In the previous module, you learned about the leading theory for the formation of the Moon by a giant impact and magma ocean.  In this section, you will learn about the exploration and geology of the Moon's surface.
  
  As our natural satellite, the Moon is tantalizingly close and recognizable.  The Moon has been studied ever since humans could stare at the night sky. We hope that after reading this section, you will go outside with a pair of binoculars to look closely at the features that are visible on the Moon’s surface. You will see a heavily cratered surface with dark splotches that resemble a face or a rabbit. The face of the Moon is so familiar in part because we always see the same side, the nearside. 
  
  The farside of the Moon is less recognizable and is only visible outside the Moon’s orbit around the Earth. Orbiting spacecraft such as the Lunar Reconnaissance Orbiter allow us to study the farside.
  The reason why we always see the same side is because the Moon is tidally locked to the Earth. Tidal locking occurs when the gravitational gradient makes one hemisphere of a revolving body constantly face the partner body.  More simply, the mass of the Moon is more concentrated on one side than the other and this is the side that we see. The 2 minute video at right explains how this came to be for the Earth-Moon system. Tidal locking is common to many moons in the Solar System.

  Humans reach for the Moon 

  Long before John F. Kennedy launched the Apollo program with the goal of “landing a man on the Moon and returning him safely to the Earth,” humans have dreamt of visiting the Moon. The beautiful 1902 silent movie Le Voyage Dans la Lune captures this dream and is regarded as the earliest example of the science fiction film genre. The version at right was hand colored and the sound track was added during remastering in 2011. 
  
  
  The Apollo program lasted from 1966 to 1972, first proving the ability to successfully launch and land space craft followed by demonstrating the ability to orbit the Moon.  The program culminated with several missions that saw humans landing on the Moon’s surface. Lunar landings occurred from 1969 to 1972 (Apollo 11 to 17). The scale of technological development was unprecedented and the accomplishment of landing a man on the surface of the moon astounded the world.

  Mariek had asked me (FF) to write a brief personal account of my reaction to the Apollo 11 landing on the moon on July 20^th, 1969. Her excuse seems to be that she was not alive and has nothing to report.  Well, I was a little kid of 10, growing up in Germany. Nevertheless, here it goes:  There are a few instances in world history that punctuate the normal noise of world events.  Events, where a disproportionate number of human beings pay attention and for which they still remember where they were much later.  Most of these events are bad, such as 9/11.  The Apollo 11 landing is one of these events and it is a positive example.  I have forgotten some of the details of that event, like exactly what time we watched it.  But I do recall being up, either beyond my normal bedtime, or because I had been woken early. Time conversion suggests the later.  I remember being in my pajamas, watching fuzzy video on a black and white television.  I don’t recall the sound, but I do vividly remember that first step.  It had a huge impact on me; for much of my childhood I wanted to be an astronaut.
  
  In an odd twist I came in contact with the Apollo missions again much later.  When I did my PhD at the University of Toronto, my work was done in a lab that had participated in Apollo mission’s related research.  There was still a gold-foil wrapped module that had been a backup to one that went to the moon.  That led me to appreciate the delicate construction of scientific instruments that made it to the moon.  We also had a large safe that still contained lunar rock samples, which I felt very lucky to be able to examine. 
  
  And since this is a personal reflection, I will close with one more personal comment.  I always feel pity when I hear about conspiracy nuts who believe the lunar landing was staged in some studio. These people only display their ignorance of the true accomplishments of the Apollo program.  The lunar missions might have been conceived as a cold-war exercise. But the scientific data obtained, both from the instruments deployed on the moon and the rocks returned from it have provided very important insights into the early history of this planet and the solar system.  No Hollywood director would have been able to fake that.

  
  The video below presents some of the highlights of Apollo 11, the first lunar landing:
  
   

  Moon rocks

  Unfortunately, humans have not been back to the lunar surface since 1972. But we have the legacy of Apollo.  Rock and regolith samples collected by the Apollo astronauts at six different landing sites dotted around the nearside of the Moon are curated at NASA Johnson Space Center in Houston, Texas. These samples are invaluable and scientists continue to study their chemistry and mineralogy to understand the origin and history of the Moon. 
  Around the same time as the Apollo program, the Soviet Union also explored the surface of the Moon in its Luna Programme (1959-1976).  Luna sent a series of robotic spacecraft to the Moon. These missions studied the Moon’s chemical composition, gravity, temperature, and surface radiation.  In addition, Luna 16 (1970), Luna 20 (1972), and Luna 24 (1976) successfully returned 0.326 kg of lunar soil to Earth. The Luna missions were the first sample return missions to rely solely on advanced robotics.
  
  The Apollo and Luna samples complement lunar meteorites, which are meteorites known to have originated from the Moon. The first lunar meteorite was found in Antarctica in 1982 (named Allan Hills ALH 81005) and was linked to the Moon by its similar mineralogy and chemical and isotopic composition to rocks returned by Apollo astronauts. Today, there are about 134 known lunar meteorites.  These meteorites were likely launched from the Moon’s surface by impacts, although no source craters have been identified. The lunar meteorites could come from anywhere on the Moon’s surface, including the far side, where we have no Apollo samples. These meteorites gives us a more widely distributed sampling of rocks on the Moon’s surface. Many of the lunar samples are breccias, which are rocks composed of broken rock fragments.  On the Moon, the breccias were most likely formed by impact processes, which should not be surprising considering its ancient and heavily cratered surface.

  Surface geology of the Moon

  The surface of the Moon has three main physiographic provinces that are easily seen in the topography. Topographic differences between the near and far hemispheres of the Moon is referred to as a dichotomy.  The colors mentioned below refer to the topographic map of the Moon above.
  • Lunar highlands: Distributed around the Moon, the light-toned highlands are thickest at the far side (yellows and reds).
  • South Pole–Aitkin Basin: A huge impact basin (2,500 km in diameter and 13 km deep; dark blue feature) on the far side of the Moon.
  • Lunar Maria: The dark-toned (blue) splotches visible on the near side are basaltic lavas. 
  More detailed descriptions of these regions are below.

  Lunar highlands 

  The lighter-toned (high albedo) surfaces of the Moon make up the highlands. Highland crust is primarily made of the low density plagioclase feldspar mineral anorthite (rock is called anorthosite). Anorthite is rich in calcium and aluminium.  The ancient highland crust is heavily cratered and thought to have originated from the lunar magma ocean. (Recall in the last module that when the Moon formed, a large portion is thought to have once been a molten ocean of magma.)
   
  The highlands are thickest on the far side. One hypothesis for why this may be suggests that the Moon collided with a a smaller, companion moon that also originated from the Theia collision (the Moon-forming impact). This collision would have been relatively slow and affected the structure of the Moon’s crust.  You can read the paper summarizing this hypothesis by clicking here.

  Lunar maria

  The lunar maria (singular: mare) are large, dark-toned, low-lying plains of basaltic lavas that were formed by volcanic eruptions. They were named maria, which in Latin means “seas” by early astronomers who mistook them for actual bodies of water. The subcircular shapes of many of the maria are a product of the lavas filling ancient impact basins. A good example is Plato crater, where dark-toned lavas forms a smooth floor of a pre-existing circular structure. Most maria occur on the near side, although a few are identified filling large craters on the Moon’s far side. 
   
  Compositionally, the mare basalts are very rich in iron compared to basalts from the Earth.  In addition, the mare basalts have very, very low abundances of water.
   
  Radiometric dating of returned Apollo samples and lunar meteorites range in age from 3.16 to 4.2 billion years. The majority of the mare basalts were erupted between 3 and 3.5 billion years. The youngest mare has relatively few impact craters and was dated by crater counting as about 1.2 billion years old. Thus, mare volcanism was active for roughly 3 billion years of the Moon’s history.
   
  The origin and distribution of the mare basalts is still debated by the scientific community. Hypotheses must account for both their greater abundance on the near side as well as the long-lived volcanic activity.  A single large impact event does not explain the long-lived activity. Two leading possible hypotheses include: 1) a higher concentration of radioactive (heat-producing) elements (potassium, thorium, and uranium) has been identified in some mare regions (Oceanus Procellarum) and may have provided a heat source for long-lived volcanism. And 2) because the crust is thinner on the Moon’s near side (see above), large impacts may have been able to break though crust more readily, allowing outpourings of lava. 

  South Pole-Aitkin Basin

  At about 2,500 km across, the South Pole-Aitkin Basin is the second largest known impact crater in the Solar System and the largest, oldest, and deepest basin recognized on the Moon. The lowest elevations of the Moon (-6000 m) are found within the basin, and the highest (about +8000 m) are found on the basin’s north-eastern rim.
  
  The composition of rocks within South Pole-Aitkin basin has elevated iron, titanium, and thorium, and is very different from typical Lunar Highlands, which are rich in aluminium and calcium (mineral anorthite). More importantly, the composition of the basin is unlike any of the identified lunar meteorites or any of the returned samples by the Apollo or Luna missions. Hypotheses to explain its anomalous composition include:
  1) widely distributed ponds of basaltic lavas in this region, similar to the lunar maria;
  2) a large impact melt sheet that formed during the basin-forming impact event; and
  3) uplifted mantle materials that were dug up by the basin-forming impact event.
   
  The unusual composition and the possibility of finding water ice at the Moon’s south pole (to be discussed in the next module) make South Pole-Aitkin Basin an attractive landing site for future lunar missions.
  
  Update June 2019: The Chinese Moon lander Chang'e-4 and rover Yutu-2 did a soft landing in South Pole-Aitkin basin on January 3, 2019, making it the first mission to ever visit the far side of the Moon. Observations by Chang'e-4 of rocks containing abundant olivine and low calcium pyroxene are most consistent with model 3 above - that uplifted mantle materials are exposed in South Pole-Aitkin Basin. This very exciting result demonstrates how planetary science is driven by discovery, technology, and gumption to boldly go where no man has gone before! (Cue Star Trek theme.)
  
  So now we wish you a good night, Moon.