By Thomas

Most of my posts here on the blog are related to the Moon, which stems from the fact that my PhD project is on lunar samples. I therefore could devote a big deal of my time in the last two years to read about, what a lot of people, much smarter than me, found out about the Moon.

That means, when I write about some new and/or interesting piece of research about the Moon, I do that on the rag rug of information, that I have mentally stitched together in my head. I try to provide the necessary information, why the topic I`m writing about is “new” or “interesting” (and I hope I succeed at least from time to time).

One of the most vital pieces of information normally needed, is the time or respectively the timeframe of the topic in question. A while ago, I came across the Video “Evolution of the Moon” from NASA that provides a nice summary of key events in lunar history. I think it can provide a nice Guideline through Lunar History, and allow you to place the topics I`m writing about in a broader context.

Source: NASA

The video starts after the formation of the Moon through the Giant Impact of a planet called Theia, about 4.5 Billion years ago. At that time the Moon was likely covered in a Magma Ocean. When this Magma Ocean cooled the Crust of the Moon formed on top.

The oldest feature we can identify on the Crust is the South Pole-Aitken basin, formed by a big impactor slamming into the Young Lunar Crust on the southern farside, that might have even had big effects on the nearside of the Moon. The video states that the impact happened “~4.3 billion years ago”.

You can mentally format the “~” in a bold font and underline it – twice.

The age of this basin could be 4.4 billion years or 4.0 billion years for all we know. It`s actual age could tell us a lot about the next important time frame:

The “Basin Formation/Heavy Bombardment” somewhere between ~4.1-3.8 billion years ago.

This “Late Heavy Bombardment”1 might have been caused by impactors that were the leftovers of the planet formation in the Early Solar System, which randomly hit the Moon over a long timeframe of a few hundred million years.

Or they swept in a wave (or several waves) through the Inner Solar System, caused by migration of the Big Planets in the Outer Solar System. In this case the impactors would hit in a much shorter timeframe (million to tens of million of years), the so-called “Lunar Cataclysm”.

The impacts of the Late Heavy Bombardment formed large basins, that subsequently were flooded by lava, rising up from the still hot Interior of the Moon. The result of this “Mare Volcanism” are the dark areas we still see on the Moons surface today.

Over the next 3.8 billion years the Moon was (and still is) constantly bombarded by meteorites, forming numerous smaller craters. The youngest ones can be identified by the rays that extend from them. These rays are formed from ejected material. The ray systems of older craters were destroyed through the following bombardment.

Of course, the events were not as distinct as in the video. Small Meteorites bombarded the Moon from the Beginning, but they had a minor effect during the time of the big impacts. Volcanism occured as soon as the Crust had formed. The video shows the dominant force acting at the timeframe in question – and it does a very good job at that.

1 “Late” because it happened after the formation of the Lunar Crust.