$$\require{cancel}$$

# 5.24 Geological History of the Moon

The lunar samples returned to Earth in the 1960s by Apollo astronauts and robotic missions revealed a complex, foreign geology unlike what scientists were accustomed to looking at in terrestrial rocks. The geologic history these lunar samples revealed told a story of early catastrophes followed by tranquil eons, punctuated only by the occasional meteoric impact. Today's moon has an ancient surface; a mirror of the Earth’s history without the influences of water, atmosphere, or life erasing the deadly details of past bombardments.

Lunar geology is characterized by two very different types of terrain that can be distinguished with the unaided eye. The round, dark-colored "maria" (singular mare, Latin for “sea”) contrast with the light-colored "highlands." Together they form the "man in the moon" or "rabbit in the moon" visible from Earth. The highlands are aptly named, for they are higher in elevation than the maria. They are also more heavily cratered than the relatively smooth maria. This is a reflection of the age of the two surfaces — the highlands are much older and have had more time to accumulate craters. Radioactive dating of lunar samples of both types of surface tell the same tale as the cratering record: rocks from the highlands are mostly around 4 billion years old, with the oldest being about 4.4 billion years, while maria rocks date from 3.1 to 3.8 billion years old, about the same as the oldest terrestrial rocks we’ve found.

The highlands and maria aren't evenly distributed. When spacecraft first traveled to the far side of the Moon, we found a surprise: the maria are mostly located on the Earth-facing side of the Moon. The far side of the moon is almost entirely covered with highlands. Ironically, this means the "dark side of the moon," not only is illuminated as often as the side we see almost everyday, but it is also made of lighter colored material.

In studying the first lunar samples, scientists found that the differences between these two terrains go much deeper than appearance. Maria rocks are basaltic, a type of volcanic rock common on the Earth. These rocks are rich in magnesium and iron, whereas highlands rocks are composed almost entirely of the mineral feldspar, which is rich in calcium and aluminum. The feldspar-rich rock is called anorthosite, and it gives the highlands both its characteristic light color and also a low density. In piecing together these facts, geophysicists built a picture of the moon forming about 4.5 billion years ago as a molten ball. According to leading theories, the necessary rock melting energy came in part from the moon's violent formation, and partly from the early heavy bombardment of asteroids and other space debris that plagued the early Solar System. When these interplanetary bodies slammed into the young Moon’s surface, their kinetic energy was converted into thermal energy.

The Pre-Nectarian Period span from 4.5 to 3.9 Billion years ago. It's the earliest distinct geologic period of the Moon. During this era, the molten Moon began to solidify. The first thing to crystallize out of the “magma ocean” was feldspar. Because these crystals were less dense than the liquid magma, as they crystallized out they floated to the top, forming a solid crust of anorthositic rocks. It took the moon more than 200 million years to form the solid highlands, although rocks dating back to that period are extremely hard to locate.

In studying the rocks returned to Earth, we don’t find many from the very earliest part of the Moon’s history. In fact, rocks more than 3.9 billion years old are extremely rare. This could be because billions of years of steady cratering have obliterated the most ancient rocks. Alternatively, a “spike” in the cratering rate — a sharp increase in the number of impacts over a short period of time — could also have erased the lunar surface.

Termed the "Late Heavy Bombardment," this violent period spanned from approximately 4.1 to 3.8 billion years ago. During this time, a great deal of unused planet-building material — asteroids and frozen bodies — had highly eccentric orbits that carried them through the inner Solar System where their orbits sometimes intersected with planets and moons, causing collisions. The exact reason for this period is still unknown, but some believe it may have been due to the migration of the gas and ice giants to their current locations.

The Nectarian Period runs from 3.9 to 3.8 Billion years ago. Toward the end of the heavy bombardment period, a few large objects created huge impact basins on the Moon. These basins are huge multi-ring features, and this geologic period is defined as beginning with the formation of the Nectaris Basin and ending with the formation of the Imbrium basin.

Next comes the Imbrian Period, from 3.8 to 3.2 Billion years ago. The filling of giant impact basins with Mare is actually disconnected in time with the creation of the craters. For reasons that aren't fully understood, it was during the Imbrian period that cracks in the Moon's solid surface allowed still hot liquid rock to flow from the still-molten mantle into low-lying areas created during Nectarine Period cratering events. Basaltic lava flooded the round impact basins, forming the maria. Eventually this volcanism ceased, as the moon cooled and solidified. Further adding to the mystery of this period, the maria is primarily on the Earth-facing side of the moon, with some of the lowest lying areas on the Moon's far side — the Aitken basin — being very mare poor. The majority of Moon rocks thus far collected come from the Imbrian period.

The Eratosthenian Period follows, from 3.2 to 1.1 billion years ago. Without internal heat to drive further geologic activity, the Moon has evolved into an essentially dead world. While low levels of volcanic activity are believed to have occurred throughout this period, they appear to have come to a halt toward the end of the Eratosthenian period. This period in general is delineated as the period of time that begins with the formation of the Eratosthenes Crater, which was coincident with the end of the great lava flows that filled older craters.

The last interval is the Copernican Period, from 1.1 billion years ago until today. This period begins with the formation of the Copernican crater and the bright rays of material radiating from the crater which clearly mark older features from younger features. This modern geologic era is best described as a period of calm after aeons of storm. While cratering continues at modest rates, and there are some hints at volcanism in this period, in general the geologic activity on the moon is very low.

Although the Moon essentially "died" geologically, the innumerable interplanetary objects that continue to impact its surface do leave a mark. These impacts melt part of the surface rock and throw up lots of tiny droplets of molten material. The majority of these impactors are small, but occasionally larger ones have also hit the surface leaving moderate and large sized craters. These continued impacts are pulverizing the lunar surface into a special soil called regolith. When we look at the regolith in the lab, we find microscopic glassy spherules that formed when those molten droplets cooled and solidified quickly. We also find bits of solid rock pulverized by impacts. Most of these rock bits are less than a millimeter across, giving the regolith a powdery texture, almost like ash. The regolith layer varies from several meters thick over the maria, to hundreds of meters thick in the highlands.

Today, data from satellite missions launched by many different countries are being studied to try and gain an even fuller understanding of the Moon's mineralogy, chemical composition,and history. Each mission uses its own unique suite of instruments to explore the problem. From their combined results scientists are finding water, oxygen, and a wealth of interesting compounds embedded in the learner surface waiting to be explored by future sampling missions.

The maria terrain on the moon is the darker parts and the highland is the lighter parts. Click here for original source URL.