Trivia 087: Returning to an old sample

Back in 2017, when I was just getting started blogging, I did a post on sample-return missions. These are space missions designed to bring material back from space so that we can study it here on the Earth. While sample-return missions are complicated, it allows us to use larger and more capable instruments than we can pack into a space probe.

The rocks on the Earth have been affected by billions of years of geologic processes such as weathering and erosion, sedimentation, volcanic activity and plate tectonics. As a result, it is quite difficult to find rocks on the earth whose compositions have not changed since they formed. The oldest rocks on Earth are about four billion years old, and are mostly found in Australia, Canada, and Greenland. However, rocky materials from asteroids and comets have remained largely unchanged since the formation of the solar system. Studying these materials can teach us about how the solar system (and by extension, the Earth) formed.

And as it turns out, the oldest Earth rock ever discovered was actually found during a sample-return mission – one of the most famous ones at that! The rock, nicknamed “Big Bertha“, was collected by Allan Shepard during the Apollo 14 lunar mission. This sample is a kind of rock known as “breccia“. These rocks are rather distinct, as they consist of fragments of other rocks held together by a conglomerate of fine grains. They can be formed in a variety of ways – through sedimentation, volcanic activity, or even from meteorite or comet impacts. Big Bertha was found near the rim of a crater, and likely formed from the impact.

In 2019, researchers studying the rock discovered that one of the fragments embedded in it was made out of granite and quartz. These minerals are quite common on the Earth, but rare to find in lunar samples. When the geologists analyzed this fragment, they determined that the quartz and zircon grains it was made of were very close in composition to similar samples on the Earth, but completely different from quartz and zircon grains in any other lunar sample. Their conclusion was simple – this rock must have come from the Earth. Most likely, the meteorite which formed the crater where this sample was found was created by a chunk of rock that was blasted off the Earth by some other impact.

As strange as this sounds, it is actually more likely than not. This sample dates back to a period of the solar system’s history known as the Late Heavy Bombardment. During this time, early in the solar system’s history, there was a lot of instability, as well as many asteroids leftover from the solar system’s formation. This resulted in both the Earth and Moon being struck many times by large impactors. In addition, the moon was about three times closer to the Earth than it is now, making it easier for material to be blasted from the Earth to the moon. This is also a phenomenon which has happened many times before. There have even been meteorites found on the Earth which likely originate from Mars.

While studying Big Bertha, the geologists estimated its age to be around 4.011 billion years old. This outdates any other Earth rock, making Big Bertha the oldest known Earth rock. And it was found on the Moon.

As you may have noticed, I’ve switched my upload schedule to Tuesday evenings. I think that this will work better with my current job schedule, so I am planning to continue trying it out It does take a little while to research and write these, so I do have to invest some time into it..

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