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Featured Image - 07/22/2008
Lunar Highs and Lows

When we look at the full Moon in the evening sky, two regions of light and dark material are immediately apparent. As children, our imaginations were able to take this interplay of light and dark and construct images of the 'Man in the Moon' or 'Bunny in the Moon'. But just why does the Moon look like this? What are the dark and light regions of the Moon? How did they form? How do they differ?

Figure 1.  Lunar Orbiter image of the lunar nearside showing the highlands (light colored) and the mare (dark colored).
Figure 1. A mosaic of images collected by the U. S. Clementine spacecraft in 1994, showing the lunar nearside. The lunar highlands (light colored) and the mare (dark colored) can be easily distinguished [NASA/USGS/ASU].

We think that almost 4 billion years ago, after the moon coalesced, a global ocean of molten rock covered the entire lunar surface. As this "global magma ocean" cooled and the magma differentiated, a "global flotation crust" formed. This primodial lunar crust was composed of a refractory, light-colored, low-density mineral named anorthosite that floated to the surface of the global magma ocean.

Figure 2.  Visualization of the Lunar Global Magma Ocean, showing the formation of the Global Floatation Crust.
Figure 2. Artists' concept of the lunar magma ocean, illustrating the formation of the primordial lunar crust [courtesy Planetary Science Research Discoveries].

For reasons not well understood today, the farside crust formed in a manner that made it significantly thicker than the nearside crust. Later, over a period of many hundreds of millions of years, it is thought that the Moon experienced a cataclysmic bombardment of large asteroids and comets, culminating in a period referred to as the Late Heavy Bombardment. These impacts created giant basins. Some of these giant basins are more than 1,000 kilometers in diameter and very deep, with basin floors tens of kilometers below the mean surface level. Hundreds of millions of years later, these impact basins filled with the lavas that solidified into the dense, dark-colored mare basalts that you can still see today. This massive asteroid and cometary bombardment slacked as the population of large impactors decreased, leaving thse lava surface (relative) smooth and undisturbed. Today, we call these lava-filled impact basins "mare," because they appeared to early astronomers to be oceans or seas. The light-colored, predominatly anorthsitic terrains are called the "highlands" because their ancient, intensely-cratered surfaces (the battered remnants of the primordial lunar crust) have more topographic variation than the mare surfaces.

It is interesting to note that most mare exist on the nearside of the Moon. This may be because the ancient nearside crust was thin enough that the gargantuan impact events of the Late Heavy Bombardment thinned the nearside crust enough that later lava flows could fill the basins. Basins also formed on the farside, but there are very few mare deposits on the farside, perhaps because the rising plutons of magma solidified before they were able to push through the thicker farside crust.

Figure 3.  Apollo Metric image (frame ID AS15-M-0464) centered at 23.7 S / 152.3 E  on the farside of the moon, is a fine example of lunar highland terrain. Composed primarily of anorthosite, a light-colored mineral that is less dense than mare basalt, the crust on the farside is significantly thicker than on the nearside.
Figure 3. Apollo Metric image (frame ID AS15-M-0464) centered at 23.7 S 152.3 E on the farside of the Moon is a fine example of lunar highlands terrain. Composed primarily of anorthosite, a light-colored mineral, the crust on the farside is significantly thicker than on the nearside. It is also older than the lava intrusions that filled the impact basins. This can be deduced from the fact that the highland material is heavily cratered, while the surfaces of the various mare are relatively smooth and undisturbed.  
(Apollo Image AS15-M-0464 [NASA/JSC/Arizona State University])

Figure 4.  Anorthosite, a light-colored mineral that comprises the primordial lunar crust.
Figure 4. This is Apollo lunar sample 15415 - the famous "Genesis Rock" collected by Astronauts David Scott and James Irwin on the Apollo 15 mission. This lunar sample is one of the first large examples of anorthosite, a light-colored, low-density mineral composing the lunar Global Floatation Crust and the present lunar highlands collected on the Apollo missions [NASA/Johnson Space Center Photograph AP15-S71-42951]

Figure 5. Apollo Metric image (frame ID AS15-M-1156) centered at 25.5 N / 335.2 E, shows the lava-filled Mare Imbrium whose surface is very smooth relative to the lunar highland.
Figure 5. Apollo Metric image (frame ID AS15-M-1156) centered at 25.5 N / 335.2 E, shows the lava-filled Mare Imbrium whose dark surface is very smooth and lower in elevation, relative to the lunar highlands. This is due to the fact that the mare lavas did not fill the giant impact basins until after the end of the Late Heavy Bombardment period.  
(Apollo Image AS15-M-0464 [NASA/JSC/Arizona State University])
Figure 6.  Basalt, a fine-grained, dense, dark mineral composing the lunar mare.
Figure 6. Apollo 12 basalt 12008. Basalt is the dark material that fills the nearside impact basins and comprises the lunar mare. [NASA/Johnson Space Center photograph S170-44091]

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