In 2002, in the journal Icarus I published an essay with two students from the University of Washington. It was titled "Searching Earth's Soil for Remnants of Ancient Life". Like a loft in a house, our moon serves as a long-term storage for old things. In the paper, we have estimated the amount of earthly material we can expect on the moon.
Wait. Continents on the moon? The moon is far away. In addition, Earth's gravity stubbornly sticks to its matter, requiring an asteroid or cometary impact to blow parts of it into space. Seems like an unlikely way to bring parts of the earth to the moon. This is true for the recent past, but the odds are much better if we look at the early Earth.
Two Things to Consider
Four billion years ago, the Earth-Moon system had two things that greatly influenced the country's opportunities for matter transfer. First, the Moon was much closer to Earth, about one third of its current distance. This means that a larger part of the ejection material has reached the moon. Second, the rate of impact on Earth was much greater. We know that from the crater record on the moon.
We estimated that the near lunar surface should contain about 7 parts per million of earth material. Among the rock samples brought by the Apollo astronauts should be about 3 grams of earth material. We suggested that you search the Apollo samples before performing a direct search on the Moon. However, we did not follow our own advice and shortly thereafter submitted a proposal for a moon mission to JPL to search for these rare materials. It was not funded.
Fast-forward to January 24, 2019. Results of a study of an Apollo-rock sample called "Big Bertha" were published in the journal Earth and Planetary Science Letters ]. The authors concluded that a small 2 gram inclusion in the rock, which the astronauts of Apollo 14 had taken from the Cone crater, most likely originated on Earth.
This discovery confirms, assuming it holds further studies, our predictions. It not only shows that pieces of earth can be transported to the moon, but also that our estimate of the amount of material in the Apollo samples is in the right range.
It is not surprising that this little piece of earth was undergoing changes when it traveled to the moon. The researchers conclude that it was formed at a depth of about 20 kilometers below the surface of the earth 4.0 to 4.1 billion years ago, was bombed on the moon and buried. Eventually, it was excavated on the lunar surface by the impact that formed the Cone Crater, 26 million years ago. This small sample gives us chemical and isotopic clues to their origins, but it is unlikely that anything about the early life of the earth will be revealed directly.
The Discovery Principle
It was the question of early earth life and the origin of life. that motivated me in particular to carry out this research project. While working on it, I also wrote The Privileged Planet . The basic thesis of the book is that the universe is designed for scientific discovery. An implication is a very science-friendly idea, which I call Discovery Principle. That is, we should be optimistic to answer important questions of nature through observation and experiment.
The origin of life seemed to be an important question that should be accessible to empirical tests. Due to the continued geological and hydrological activity of the Earth, it is unlikely that we will ever find unadulterated samples of first life on Earth due to the alteration of ancient rocks by multiple processes. The Discovery Principle made me look at the moon to empirically investigate the origin of life.
Return to the Moon
True, most of the pieces that were blown up from the early Earth and the moon landing would have been at least partially melted and vaporized. A small part, however, would emerge at the surface of the earth and would be the least changed. This material could include traces of early earth life. To find it, you have to look for a large amount of lunar soil, probably as part of a permanent lunar village. In fact, the search for meteorites on the moon is now cited as one of the most important scientific justifications for a return to the moon.
I do not claim that without the discovery principle, no one would think of meteorites on the moon as a way to investigate the origin of life. In fact, a Spanish scientist suggested the same idea independently (as far as we know) and almost simultaneously with our own proposal. What I mean by that is that the fruits of research have reinforced a specific design-based idea.
Image: A planetoid collides with the early Earth, by Don Davis (commissioned by NASA) [Public domain] via Wikimedia Commons.