Study of the Moon has documented that it is an ancient world— a planetary object whose intense geologic evolution in the early years of the Solar System led to rapidly declining activity and its current quiescence. These are the very properties that allow us to use its largely unchanging surface to recover the record of events that occurred in the distant past.
New research from Imperial College of London has focused attention on an interesting possibility. One of the most important discoveries of recent years is that planetary objects exchange material through the process of impact. An object colliding with a planet or moon can dislodge and eject material at high velocities, in some cases greater than escape velocity. These ejecta, traveling throughout the Solar System, can eventually be swept up by another world – a process first discovered when we recognized that certain meteorites with unusual compositions are actually pieces of other planets. In meteorite collections here on Earth, pieces of the Moon and Mars have been identified. Additionally, several rare and unusual meteorite groups may eventually be determined to have come from a planet (Mercury is the leading contender of origin for some stones).
If we have found meteorites from the Moon on the Earth, is it not then possible that some day we may find meteorites from the Earth on the Moon? Some physical issues working against this possibility should be noted. It is relatively easy to eject material from the Moon (a body with a land size equal to the continent of Africa) due to its low gravity (any object accelerated to 2.4 km per second will leave the Moon forever). The Earth is a much larger object with high gravity and a very high escape velocity (a bit less than 12 km per second); this high gravity makes material harder to eject. Moreover, in its early stages of flight, the Earth’s atmosphere would also tend to slow down impact-tossed ejecta. However, it may be that large impacts temporarily evacuate the atmosphere around the point of impact, mitigating this drag. It was originally thought that Mars (which also has an atmosphere) likewise was too large an object to allow rocks to be ejected, but that idea was abandoned when an overwhelming mass of data demanded a Martian origin for a certain class of meteorites, causing scientists to re-examine the possibility. It was a case of “settled science” being overturned by new facts.
So we should not rule out the possibility of the ejection of significant amounts of terrestrial debris during large-body impacts on the Earth. If this occurred, what might have happened to this material? We know that anything ejected at less than orbital velocity would eventually fall back to Earth. We have evidence of this process from pieces of glass called tektites. Tektites are fragments of molten ejecta (created during an impact event on the Earth) that were thrown through the atmosphere into space before falling back to Earth in large strewn fields. But what about the material that is thrown off at very high orbital velocities—or at escape velocity or greater?
This “escaped” material typically goes into orbit around the Sun, but the first object it might encounter in space is our nearby Moon. Material from an impact on Earth could travel to cislunar space, where it might subsequently collide with the lunar surface. Because these objects are traveling through space along with the Earth-Moon system, they would hit the Moon at fairly low velocities and hence, might not vaporize on impact (as does virtually all other matter hitting the Moon). Given these facts, we must consider the interesting possibility that pieces of the Earth, thrown into space from large impacts, exist on the Moon. Unfortunately, a rock on the lunar surface doesn’t just sit there. Though not as dynamic as Earth’s geologic mixing bowl, the continuous rain of micrometeorites bombarding the Moon—a gigantic “sandblaster” that’s pulverized the surface over eons—has crushed, buried and mixed fragments into a complex mixture of rock and fine-grained powder. Over time, a rock from Earth would eventually become dust, along with the surrounding native lunar rocks.
Suppose this Earth rock had not been destroyed, but in fact, had been preserved? What if this terrestrial (Earth) meteorite lying on the Moon was covered by an insulating layer sometime before that long pulverization process had done its work, say, within several tens of millions of years? If an Earth rock on the Moon was thrown below the surface by a nearby impact and then covered by a lava flow, it would be protected. The lunar regolith is an excellent thermal insulator and heat from molten rock diffuses only a few centimeters into the subsurface (so lava would not melt and destroy the meteorite). After the lava solidified, the thick, overlying rock sheet would protect this buried regolith (a “paleoregolith,” or ancient soil) and this meteorite fragment of Earth’s crust could survive undisturbed, from all but the largest subsequent impacts.
One more key fact about these suppositions—large lava flows on the Moon stopped erupting at least a billion years ago. Thus, any fragments of terrestrial rock on the Moon would date from that era—the distant geological past, when life on Earth first emerged. Thus, a record of ancient Earth life (in the form of tiny fossils embedded in rock) might be found on the Moon, like “flies” caught in lunar “amber.” All of this may sound very speculative, and in fact, it is such an unlikely concatenation of events as to seem ludicrously unlikely. But the past 50 years of planetary exploration have certainly taught us that the most unlikely of events not only have occurred, but in some cases, have occurred more frequently than we ever imagined. This new study indicates that the delivery and protection of terrestrial rock fragments on the Moon are possible and likely—thus opening up an entirely new line of inquiry into the origins and evolution of life—and strongly suggests the need for more search and discovery on the Moon.
Once again, we find that the very properties that lead some to ignore the Moon (“It’s a boring rock!”) actually make it a unique and valuable resource. A protected paleoregolith could hold evidence for the emergence of life on Earth, billions of years ago. We already knew that these ancient soils contain a record of the output of the ancient Sun, as particles from the solar wind are implanted into the dust grains. Study of Apollo samples indicated that the ancient solar wind had a composition slightly different than the current Sun (a contrast not predicted by current models of stellar evolution). Additionally, cosmic rays also impact the Moon, and ancient dust grains from a paleoregolith could preserve a record of ancient supernovae, induced by high-energy particles hitting the Moon billions of years ago.
It was once said the Moon acts as a cosmic tape recorder, quietly and continuously collecting a detailed history of the Earth-Moon corner of the universe over the last 4 billion years. Now we are discovering that this geologic tape recorder holds an even more impressive collection of “tunes” than we had previously imagined. A growing chorus of scientists, engineers and policy makers recognize that the Moon is not a roadblock to a human mission to Mars, but rather a bridge from Earth to everywhere else – as well being a spectacular window to Earth’s distant and otherwise unrecoverable geologic past.