The moon has a new timeline that reflects what we have learned

Earth’s geological timeline is an essential tool for understanding how it has changed, and planetary scientists have tried to replicate it for planets and moons we’ve examined. However, the existing timeline for the moon emerged when we knew much less about its history, and is increasingly dated. A new effort attempts to offer a timeline that makes more sense.

Earth’s timeline reflects the fact that several catastrophic events are found in the geological record, and these are used to delineate units of geological time. In some cases the definitions are simple, such as the layer of material separating the Cretaceous from the Paleogene. Others are more controversial, as evidenced by the ongoing controversy over when to define the start of war The Anthropocene, if we recognize it as a geological epoch at all.

The moon requires something different. On Earth, biological changes are just as important as geological changes when it comes to defining boundaries in time, something that is of course not possible on the moon. A lunar timeline was proposed before we even visited our satellite, and refined based on the Apollo results, but some of what we have learned since has made it obsolete. In particular, it almost completely ignores the older far side, and does not take into account what we have now learned about the moon’s early years.

Therefore, a team of authors, mainly from the Chinese Academy of Sciences, have proposed an update, dividing the moon’s timeline into three eons and six periods.

The proposal is based on the observation that the moon had very hot origins that dominated its early development, but that these faded over time, leaving the moon to the whims of the wider universe.

What the authors call the Eolunary Eon is the time when changes on the Moon were driven primarily by internal forces. The extreme heat produced by the moon’s formation created a magma ocean that slowly solidified into primary crust. Large objects collided with the moon, but left no lasting marks thanks to its nearly liquid state.

The Paleolunary Eon occurred while internal and external processes were of similar significance – both were dying, but the supply of asteroids was declining much more slowly than the internal heat sources. Major formations remain from this era, beginning with the Das Formation, formed from ejecta near the South Pole by the creation of the South Pole-Aitken (SPA) Basin. Not only is the SPA basin now considered the largest and oldest impact structure on the moon, but it is also likely to be the site of the first lunar bases, giving the unique terrain a double meaning.

The Neolunar Eon occurred when volcanic activity had largely ceased and most of the changes came from outside. Although Chang’e 5 proved that volcanic activity lasted at least 2 billion years ago, by then such events were rare and overwhelmed by asteroid impacts. That’s why the authors chose not to make the last eruption the dividing line between centuries.

Instead, the authors date the boundaries between the eons at 4.31 and 3.16 billion years ago.

The dates, eons and periods of major events in the proposed new moon timeline

The proposed new moon timeline (from left) dates, eons, periods and major events.

Image credit: Science China Press

Just as Earth’s four major eons are subdivided into finer ages, periods, and epochs, the authors propose that the two most recent eons should have subdivisions.

They call the current period the Copernican, dating from 800,000 million years ago, when the prominent crater Copernicus was formed. The first part of the Neolunarium is called the Eratosthenic, after one of the oldest existing craters. There is no suggestion in the proposal that the Moon should now be considered its own Anthropocene.

The Paleolunarium has three periods: the Imbrian period, during which most of the great seas were formed, and earlier short (by lunar standards) Nectarian and Aitkenian periods; these lasted only 70 and 390 million years respectively.

Whether the majority of lunar scientists will adopt this timeline, adapt it, or choose something else remains to be seen, but as the race for the moon heats up, a good timeline will be in high demand.

The study was published in the journal Science China Earth Sciences.

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