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A section of this lunar rock (arrow) has a composition strangely similar to that of Earth – probably because it started here before being projected onto the Moon by an asteroidal impact 4 billion years ago. This would make it the oldest known intact sample of our planet. (Credit: NASA / USRA)
It was a great week for humans to hit things around the solar system. The Japanese probe Hayabusa2 landed and took a sample of the asteroid Ryugu; NASA's InSight is hammering the surface of Mars; and an Israeli private spacecraft called Beresheet is heading for an April landing on the moon. But we are just beginners to the game. Nature strikes and moves things in the solar system for billions of years and does so with impressive efficiency.
Example: a rock nicknamed Big Bertha and officially known as NASA catalog number 14321. It was collected during the Apollo 14 mission by astronaut Alan Shepard, who chose it because it contained a fragment of unusual appearance. His instinct was perfect. This piece of Big Bertha is really not like the surrounding lunar rocks, probably because it is not native to it. According to a study by David Kring of the Lunar and Planetary Institute and his colleagues, the fragment actually began on Earth.
This is the first clear example of terrestrial material traveling to another world – and further evidence of the interplanetary highway crossing the solar system.
If you follow recent scientific news, you may already know how the story is going. The fragment probably began as a block of rock that crystallized about 20 kilometers below the surface of the young Earth. There are between 4.0 and 4.1 billion years ago, it was projected into space by a huge asteroid impact, which eventually landed on the moon.
Then, about 3.9 billion years ago, it was heated and buried under another impact, the latter striking the lunar surface. 26 million years ago, when another impact was dug and brought to the surface of the moon, Shepard discovered it on February 6, 1971. The unusual composition of the rock, rich in quartz, is a strong indication of its earthly source. Planetary scientists know 360 meteorites on Earth that originated on the Moon, but it is the first moon rock convincingly dating back to Earth.
Kring's discovery is based on one of the deepest discoveries of the space age: the recognition that the solar system is geologically dynamic at the interplanetary scale and that materials flow from one world to another. other.
It is no secret that this interplanetary highway was active in the first solar systems. The Moon's footprint has a long history of impact (although, until the landing of Apollo, there were still some people who claimed that most lunar craters were of volcanic origin). Scientists have also realized that the cratering process continues to this day, but were nevertheless surprised to note the abundance of new impacts appearing on images of the Lunar Reconnaissance Orbiter, which has been monitoring the lunar surface since 2009.
Telescopic gratings now monitor the moon to detect the bursts of light that occur when a small asteroid strikes, providing another measure of the level of activity. In just 89 hours of observation, the European project NELIOTA observed 57 lunar lightning bolts. The meteorite impacts on the Moon lead to much larger deformations than the scientists predicted. Neil Armstrong's emblematic prints will be completely erased by meteorite alterations in a few tens of thousands of years, far from the millions or even billions of years imagined by many researchers at that time.
The impacts of asteroids and comets on other worlds also prove more commonplace than expected. A dramatic example occurred in the summer of 1994, when comet Shoemaker-Levy 9 hit Jupiter. Since then, fans have noticed several other flashes on Jupiter, most often the result of collisions with other small comets. A recent study suggests that Jupiter is hit once a week by an impact at the Chelyabinsk scale!
Significant impacts are much rarer, but when they hit rocky worlds, they launch materials that join the interplanetary highway. This is how these more than 300 lunar rocks found themselves on Earth, and that's how Big Bertha found on the Moon. Another 227 meteorites on Earth were found until March. No one has yet positively identified a Venus meteorite, but some of them surely exist. An earth rock has even been tentatively identified as a piece of the planet Mercury.
The impact craters have completely returned to the surface of the Earth. It is probably thus that a stone of the Earth was found on the moon; that is how life on Earth could have been on other worlds. (Credit: Simone Marchi et al)
Land rocks have certainly landed on all these neighboring worlds. Also note that traffic on the interplanetary highway was much denser at the beginning of the solar system, when impacts were much more frequent. Many researchers have hypothesized that life would first have originated on Mars, and then went on Earth aboard a Martian meteorite spray, sowing life on our planet. It could have gone in the other direction, with the seeding of the Earth by Mars. This is one of the main reasons why it would be so fascinating to find life on Mars: is it totally different from earthly life or do both seem to have the same origin?
The story does not stop with Earth and Mars. Terrestrial rocks have undoubtedly bombed Venus as well, and Venus may have been a habitable ocean world during the two billion years of its existence. Perhaps it was once a living planet. Perhaps only a few vestiges of life remain in its clouds; it's worth taking a look at. The distant Earth rocks can also provide the most direct evidence of what our planet looked like at the time when life appeared here.
Big Bertha seems to contain the oldest intact land rock ever found. Moon surface materials (including what is in NASA's Apollo Missions archive) could include many other Earth specimens dating back about 4 billion years ago. They could reveal the composition of the ancient atmosphere, the structure of the first continental crust, even chemical clues to the primordial organisms of this planet.
And even if I think big, I'm even bigger. Interplanetary highways should be a generic feature of all planetary systems with multiple rocky worlds. In some cases, the transportation system will be much more efficient than it is here. This is particularly the case for systems around red dwarf stars, which tend to have small, closely spaced planets. The Trappist-1 system, which contains seven known planets, approximately the same size as the Earth, is an extreme case.
Two of the Trappist-1 worlds are considered potentially habitable. If life appeared on one of these worlds, it would spread quickly and efficiently to the other. The same goes for all compact planetary systems and there is no reason to think that Trappist-1 is unique. In this limited sense, at least panspermia (the assumption that life can survive in space and spread from one world to another) can be real in many places from the universe.
The highway that brought Big Bertha to the Moon could lead us to a broader vision of planetary systems as dynamic incubators, dispersing life wherever it appears.
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