Study the Moon

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Article 6 of 6 on Studying the Moon

Observing the Age of the Moon and Its Craters

Michael Packer

Comments: m·DOT·packer·AT·Yahoo·DOT·com


Amateur astronomers like to know the basic facts of deep sky objects they observe - distance, age, size, a property that makes the object noteworthy. The same thing goes for the Moon and its features. But age, one of the most important data points for understanding lunar terrain (and the major impact events in the inner solar system for that mater), is hard to recall both because there are a lot of lunar features to remember and no good method presented on recollecting them. This article on studying the Moon tries to solve this dating puzzle by first reviewing the formation of the Moon, presenting the a 6 Major Lunar Epochs that date important features and finally listing the Dates Of Craters worth remembering.


The Lunar Age Story: A cataclysmic collision between a Mars sized body scientists have come to call Theia and a proto-Earth is the best model that fits the conditions of our 12,700 km planet having a relatively large 3500 km moon. The impact theory was first proposed in the 1940’s and gained new life after rock samples were brought back to Earth.


The collision with Theia likely marked the final stages of planetary accretion across the solar system and contributed roughly to the last 10% of Earth’s mass. Ancient lunar rocks are dated to 4.56–4.29 Gyr (Gyr = billion years). But the exact age of the Moon is unknown.


Both the rock samples and computer impact scenarios, that meet the high angular momentum conditions of the Earth-Moon system, suggest that the Moon formed mostly from the mantles surrounding the iron cores of Earth and Theia. What was left was a molten Earth, rich in heavy and light elements, and a molten Moon, iron deficient and gravitationally unable to hold on to the many light gaseous volatiles (such as nitrogen, water, carbon dioxide, ammonia, hydrogen, methane) that persisted before it cooled. The Moon was a globular magma ocean. It became Earth’s satellite continent but it was destined to be a gray volcanic badlands.


Not too gray of a place for geologists however. As the Moon cooled, the fraction of heavy elements that remained crystallized into iron and magnesium silicates and sank. Less dense material then crystallized, floated and formed an anorthositic crust about 50 km in thickness. For what it’s worth, this anorthositic rock is composed of more than 90% calcic plagioclase feldspar which means “calcium-rich oblique fracture shaped crystalline minerals” an example of which is CaAl2Si2O8. But the point is that this chalk looking rock represents the original crust of the moon and, after billions of years of horrendous impacts, it can still be recognized all over the lunar surface. It has a high albedo and makes up the bright white terrae or lunar highlands (Example: southern highlands around Tycho). It contrasts markedly with the dark iron-rich basaltic maria - the lava seas or extrusive magma that found its way to the surface a billion years later by erupting through the floors of the impact basins.


Six Lunar Epochs: Mentioning maria moves this age story to the epochs of cratering. And once again to the delight of geologists, a badly cauterized moon would suitably record the major impact episodes right up to the present day. This enduring impact record continues to aid planetary geologists’ understanding of how solar system bodies were formed, or transformed, into their present state.


Maps of the 6 epoch impacts make a great observing plan for an evening.

4.53 to 3.92 Billion Years

3.92 to 3.85 Billion Years

3.85 to 3.75 Billion Years

3.75 to 3.2 Billion Years

3.2 to 1.1 Billion Years

1.1 to Present Day

A crater for each epoch. Compare the poor deffinition of crater rims for oldest craters left to progressively fresher craters right.


Pre-Nectarian: 4.53 to 3.92 Gyr

The Pre-Nectarian period is defined from the point at which the lunar crust formed to the time of the Nectaris impact event. The Pre-Nectarian period started within an intense cratering period associated with the accretion of the Moon. Fragments of ruined craters on the Moon are the oldest and most difficult to recognize. The impact basins Procellarum, Tranquillitatis, and Fecunditatis likely formed in this epoch. Their basin walls are respectively demolished.


Nectarian: 3.92 to 3.85 Gyr

The Nectarian period is named for the impact basin that contains the relatively small Mare Nectaris, which is one of about 12 multi-ring basins that formed in this brief interval. This epoch likely corresponds to the Late Heavy Bombardment period of the inner solar system. These old basins are all heavily degraded by subsequent impacts.


Lower Imbrian: 3.85 to 3.75 Gyr

The short interval beginning with the Imbrium impact and ending with the Orientale impact is called the Lower Imbrian period. The formation of the Imbrium and Orientale basins provides the most important and widespread stratigraphic boundary between the ancient, heavily cratered Moon and the more recent Moon dominated by lava flows and a great reduction in impact cratering.


Significant Craters And Basins That Formed Under The Lunar Epochs



Lower Imbrian

Upper Imbrian



4.53 to 3.92 Gyr

3.92 to 3.85 Gyr

3.85 to 3.75 Gyr

3.75 to 3.2 Gyr

3.2 to 1.1 Gyr

1.1 to Present Gyr













Fecunditatis Basin

Crisium Basin





Procellarum Basin

Nectaris Basin


Sinus Iridium



Tranquillitatis Basin

Serenitatis Basin


Oriental Basin




Upper Imbrian: 3.75 to 3.2 Gyr

2/3's of the Moon's mare basalts erupted within the Upper Imbrian with many of these lavas flows filling the impact basins. What did not crystallize and sink or crystallize and rise in the formation of the moon remained in a magma state. This material called KREEP is rich in rare-earth elements including uranium and thorium. Consequently radioactive heating was believed to be intense during this time and is attributed to the formation of most of the lunar maria particularly around Imbrium and Procellarum. It should be noted however that an interesting flooding theory, aside from the above, postulates that a major far-side impact sent shockwaves through the moon and instigated massive magma flooding on the near side - much in the same way that a rock striking an outside car windshield transfers energy and blows off a cone shaped piece of glass on the inside of the car [1].


Eratosthenian 3.2 to 1.1 Gyr

After the core formation of lunar seas, the Eratosthenian period is demarked by fresh looking craters with no impact blemishes but with no rays structure either. Eratosthenes crater is a reference example. This was the longest period in Lunar history. During this era, late-stage volcanism still filled low-lying regions in and around Mare Imbrium and Oceanus Procellarum. Consequently secondary craters from some Eratosthenian impact craters are flooded or embayed by mare lavas. Bright ray systems would have undergone space weathering and faded away over the long 3 billion year (to present day) time scale.


Copernican  1.1 to Present Gyr

Copernican period is demarked by fresh craters with rays. The Copernicus impact (.800 Gyr) itself is a large impact with well defined walls, central peaks, secondary impacts, extensive ejecta and a well defined ray system. It has all the fresh detail of a relatively new impact. We are currently in the Copernican period of lunar history. There are no definite mare lava flows in this era but small numbers of superposed impact craters suggest that a few lava deposits in northern Oceanus Procellarum are of Copernican age. The most significant geologic activity on the Moon during the Copernician period has been the continuing (but infrequent) impact cratering. The Copernican craters Tyco (.108 Gyr) and Chicxulub (.065 Gyr) on Earth are believed to be formed form a group of  asteroids called Baptistina which itself was created from a collision within the Asteroid Belt about 160 million years ago (.160 Gyr). The Chicxulub crater has been strongly linked to the extinction of the dinosaurs 65 million years ago. Hence Tycho is known as the Dinosaur crater by its association with Chicxulub.


Observing The Age Sequence Of Lunar Terrain: Archimedes, Autolycus and Aristillus: Grouped together in the northeastern sea of Mare Imbrian lie the craters Aristillus (northeast), Autolycus (middle) and Archimedes (southwest). With a diameter of 55 km, Aristillus is a complex crater. It has terraced walls, a crenulated rim, a large flat floor and a massive clump of central peaks. It is also fairly easy to see that Aristillus is a fresh crater with a well defined wall, central peak system and bright rays of ejecta. It therefore is a Copernician period crater, under 1.1 billion years old.



The middle crater Autolycus is a little more tricky to pin down. Sitting on the bench of the nearby Apennine Mountains, 39 km Autolycus is smaller crater with less complex morphology. It does not have a discernable central peak but does have a floor filled with lava material and collapsed wall debris. Age wise, Autolycus shows distinct signs of erosion around its wall particularly to the north. Inspection with a telescope at or near the terminator shows radiating debris from Aristillus draping over Autolycus’s outer structure. On the other hand, high angled light observations show that Autolycus has a faint ray structure itself - part of which lies over Archimedes. Hence Autolycus is likely a Copernician period crater, younger than Archimedes but older than Aristillus.


83 km Archimedes (flat floored and only 1.6 km deep) seemingly does not fit within the crater size scale. Its floor should be more like 4 km deep and have a peak system 2 km high. It was recognized early on that it, and Plato 600km north, must be partially filled with mare lavas from the Imbrium basin. And in fact, shadows cast from Archimedes rim on its floor and surrounding mare show the lava inside is about the same level as the lava outside. There are no breaches around the rim of Archimedes so the lava must have risen through fractures in the floor of the crater. Since Archimedes-sized craters are 4 km deep (not 1.6 km) these means that 2.4 km of lava filled the crater and buried its 2 km peak system. That’s why it’s not there!


Pulling these observations together in context with the 6 epochs, a chronology of this area can be assessed. 3.85 billion years ago the Imbrium basin formed creating the Apennine Mountains. A billion years later, basaltic lava filled the basin. But sometime in between these two events the Archimedes crater formed and its floor was subsequently flooded along with the Imbrium basin. After the flooding, at least two billion years passed by before the Autolycus impact crater formed and then, sometime later, the Aristillus crater formed.


These kind of observations taken in context of the epochs (which in turn were estimated with the aid of lunar rock samples) can allow amateur astronomers to better recount the age of the terrain they’re seeing and sharing at the scope. It takes some practice, but after a while the countless detail in the Moon's surface is replaced by a story told by a more manageable number of craters and maria. The age of the lunar surface gives one a profound sense of inner solar system development dating back 4.5 billion years. And our connection with this detached land mass may be thought of as a remote continent rather than a unrelated satilite. Thanks for sharing my interest in studying the Moon. (Sources: Chuck Wood’s LPOD and book, various articles on the net including "The Geologic History of the Moon" by Don Wilhelms and the Virtual Lunar Atlas)


Study the Moon

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Michael Packer