The Moon's Story: Nectaris to Apollo and beyond
Geology is essentially history on the grandest scale. It is the history not just of a single nation or even a single species but of an entire planet and all that it experiences over its lifetime, from asteroid impacts to volcanic eruptions to the formation and evolution of life and the rise of civilization. Just as each major civilization (China, Mesopotamia, Mesoamerica, etc.) had its separate history which eventually merged into the history of our global civilization, each planet has its separate geologic history which is now merging into one history as the solar system enters the Anthropocene, the age of humans, and eventually the “Clawd-ocene“ the age of post-biological life. In this article, I explore the Moon’s version of this story in light of the upcoming Artemis II mission.
When I first decided I wanted to be an astronaut at the age of twelve, I knew that I needed to major in a STEM field in college but couldn’t decide. My other major interests at the time were history and archaeology. Although I wanted to be an archaeologist on Mars, I ended up settling for the closest thing, a geology. This is because geology is history. As a geologist you study the history of Earth through rocks that record the evolution of life and the environment. If you know how to read the rocks, they reveal a history of ancient ecosystems, rivers, oceans, volcanoes pole shifts, and deserts. These aren’t just fun but otherwise irrelevant facts either. For example, finding fossil fuel deposits requires an understanding of the environments that form coal and oil deposits to identify the correct rock types and geologic settings.
This same fact applies to the geology of other planets. The surfaces of the Moon, Mars, and other planetary bodies record the history of the solar system, a record of catastrophic impacts, volcanic eruptions, the formation and drying out of oceans and rivers, and, in one case at least, the formation of life.
The Moon for example, is divided into four periods. These are the Pre-Nectarian, the Nectarian (geologists are ever-imaginative with naming time periods), the Eratosthenian, and the Copernican.
The Pre-Nectarian period dates from the Moon’s formation about 4.5 billion years ago to the formation of the Mare Nectaris basin, about 3.9 billion years ago. Understandably not much is known of this very early period in the Moon’s history. The Moon had only recently formed and was probably still hot enough to have volcanic eruptions, which may have led to the outgassing of water vapor that could have contributed to some of the Moon’s modern polar ice deposits and ancient transient lunar atmosphere. Of course, comet strikes and impacts by water-rich asteroids likely still contributed to the lunar water inventory.
The origin of lunar water shows how, like with fossil fuels on Earth, a vital resource for human civilization to be sustained on the Moon may have ancient geologic origins that also reveal significant details about the Moon’s evolution and geologic history. The Pre-Nectarian is also probably when the lunar highlands, the lighter colored and rougher terrain visible on the Moon’s surface, formed. The current idea is that the lunar highlands formed from a “magma ocean” where lighter minerals, like anorthite, accumulated at the surface while denser minerals, like olivine and pyroxene, sank into the core and mantle.
The Pre-Nectarian is believed to be dominated by frequent impacts events from the higher number of space rocks flying around the primordial solar system, compared to today. Giant crater basins older than the ones prominent on the Moon’s surface today probably formed as result of giant impact events common in the pimordial solar system, but they were degraded over time by later impact events. The heavily catered lunar highlands likely hide the eroded ghosts of ancient crater basins from collisions which may have almost destroyed the early Moon.
The Nectarian period starts around 3.9 billion years ago and is marked by formation of Mare Nectaris and other large basins that formed at the time. The surface of the Moon during the Nectarian was likely dominated by this impact crater basin and its extensive debris deposits associated with Crater Janssen and the valley Vallis Rheita as well as othe large basins. The Nectarian is more or less a continuation of barrage of impacts that defined Pre-Imbrian periods of the Moon’s history. It also a continuation of the Pe-Nectarian in tems of geologic activty since there is geologic evidence that the Moon may have had a weak magnetic field at this time.
The Imbrian period begins around 3.8 billion yeas ago with the formation of the Mare Imbrium basin. The Imbrian period is also significant in marking a drop off in the rate of impact events compared to Pre-Imbrian times. This could be related to the end of the late heavy bombardment. The late heavy bombardment is a solar system-wide event or period during which the frequency of impact events from large >1-10 km was much higher than it is today. The Imbrian period is also probably when most of the lunar maria form. The lunar maria are the darker regions visible on the Moon. They are vast plains of basaltic rock which were once magma seas from molten rock filling recently carved impact basins.
The Eratosthenian lasted from about 3.8 billion years ago to about 1 billion years ago. It is name for Eratosthenes crater and represents the transition from the geologically active Moon of Pre-Imbrian and Imbrian times, with a continuous rain of large impactors (the rain continues today but >1 km sized objects are far less frequent), magma seas, sporadic volcanic eruptions, transient atmospheres, and primordial magnetic fields, to the Moon of geologically dead Moon of today. If Pre-Imbrian times represented a geological golden age for the Moon. The Eratosthenian marks the silver age, when there is still a sign of ancient glory but also decline. The Eratosthenian contains the last evidence of volcanic activity on the Moon. Also, most post-maria craters that lack crater rays are considered Eratosthenian. The Eratosthenian period could be seen as the the period over which the Moon slowly dies geologically as it runs out of heat.
The current geological epoch of the Moon’s history is the Copernican epoch named after Copernicus crater, one of the rayed craters. Rayed craters have bright streaks or “rays“ that are eroded in geologically brief periods of time by subsequent impacts. Since the dawn of the Copernican, the Moon has changed little.
The geology of the Moon during the Copernican period is dominated by impact processes. Large impact events are much rarer though and the most important impact process governing the moon today are micrometeorite impacts. The tiny meteorites have rained down on the lunar surface for billions of years, eroding its mountains and crater rims as efficiently as wind and rain erode geologic features on planetary bodies with atmospheres.
Since the earliest organisms with eyes probably only evolved in the last 600 million years or so, it could be said that Moon has looked the way it does since before there were eyes (on Earth anyways) to see it.
The Moon’s surface remained relatively unchanged for a billion, until September 13, 1959. This is the that the Soviet Luna 2 spacecraft crash-landed into the ancient plains of Mare Imbrium. Since then, human activity has been altering the lunar surface in unique and rapid ways. In the future, this may involve the mysterious self-organizing matter that we call life and civilization.
As humans set out into space, possibly to establish settlements and prospect resources, life may begin to play a major role in the geologic history of the Moon, as it has on Earth for 4 billion years. On Earth, geologists talk about the Anthropocene, a proposed name for a new epoch in Earth’s history where humans are a significant driving force in Earth’s geology. The Anthropocene may very well be spreading across the solar system through our robotic probes and eventually human missions. This is likely to be especially true for the Moon in the near-term. The Anthropocene may not last for long, however. The Anthropocene may only be a transitional phase in the history of the solar system before the solar system becomes colonized by post-biological intelligence. Perhaps the solar system is in the Anthropocene but about to enter the “Clawd-ocene.”
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