You wouldn’t know looking at it, but omoon it is a time capsule.
Its surface has been completely exposed to the vacuum for almost 4.5 billion years; however, it was soaked by particles from the Sun and beyond the solar system. These particles remain buried under the lunar surface, providing a detailed record of the history of our solar system and even of our entire galaxy.
Is everything okay over there. We just need to dig it up.
Related: Incredible photos of the moon from NASA’s Lunar Reconnaissance Orbiter
Here comes the Sun
In addition to light, our sun is constantly emitting a constant drizzle of high-energy particles, collectively known as solar wind. The solar wind is mainly made up of electrons and protons, but the occasional heavy nucleus also escapes the sun’s gravitational embrace.
The solar wind flows throughout the solar system, but very few of these particles reach the Earth’s surface, where we can study them more easily. This is because of our magnetic field – which does a fantastic job of redirecting the paths of these charged particles, forcing them to follow specific routes around our planet – and our atmosphere, which absorbs most of the solar wind in the form of our adorable Aurora light shows.
The moon has none of these characteristics. At least, it hasn’t done so in the past 4.5 billion years: by the time the moon was melted, it may have had a temporary magnetic field, but that is in the distant past. For all these billions of years, the moon has continuously absorbed the particles of the solar wind, absorbing them in its regolith.
In the face of this continuous attack, the regolith changed. The high-energy particles may have disturbed the chemical composition of the lunar surface. Elements like potassium, which should be found in abundance, seem to have turned into other elements, which then fluctuate.
Lunar dust has also been burned by the sun: although each individual particle is super tiny, the moon has no atmosphere and therefore has no erosion, leaving the same dirt to face the sun continuously. Each small solar particle tears a microscopic hole in the dirt, so when studying the structure of the regolith, we can see a record of the sun’s brightness.
Sometimes the sun shines, sending an extreme explosion of high-energy particles – far above the normal drizzle of the solar wind. The moon had to face these explosions over and over again for billions of years. The higher the energy of the event, the deeper the particles of the solar wind can be incorporated into the regolith. So digging will tell us when the sun has sparked tantrums in its past.
Galactic fingerprints
The sun is not the only source of tiny, high-energy particles that swim through the solar system, but particles that come from beyond the boundaries of our system are given a different name: cosmic rays. They are not rays, but a mixture of protons and heavier nuclei coming from all directions, usually with more energy than the solar wind – they managed to cross interstellar gulfs, after all, which is no small feat.
Cosmic rays come from a variety of superpowerful processes in the galaxy, most notably the infamous supernova explosions that mark the final death of massive stars. These titanic explosions can overshadow entire galaxies and unleash a truly profane flood of cosmic rays.
Fortunately, we are nowhere near an event that will soon become a supernova; even candidates like the red giant Betelgeuse they are too far away to harm us. But it was not always so. Due to our orbit around the center of the Milky Way, the solar system passes through a galactic spiral arm every 180 to 440 million years. (The great uncertainty comes from our difficulty in measuring the rotation speed of the arms themselves.)
The spiral arms are places of intense star formation within galaxies. That is why the spiral arms stand out so much when we look at distant galaxies: they are home to huge, bright and blue stars. But huge, bright, blue stars don’t live long, and when they die, they tend to rise in a supernova flash.
So, in the past billions of years, our solar system has probably come close to more than a few nasty supernova surprises. The cosmic rays released by these explosions would only be absorbed by the Earth’s atmosphere, and if any reached the surface, implanting themselves in the crust of our planet, erosion and tectonic activity would eventually erase any memory of the calamity.
But the moon remembers. High-energy cosmic rays can leave tiny tracks in the lunar regolith, which can be seen under a microscope. Cosmic rays can also change the molecular composition of the regolith, breaking nuclei and transforming them. And finally, cosmic rays can just … stay there, silent, trapped in the lunar land after its explosive birth and long journey.
Digging up tiny fossils
Humans had collected lunar samples before: NASA’s six Apollo missions in the 1960s and 1970s brought souvenirs, and China’s Chang’e 5 landing module took home the first fresh moon rocks in decades earlier this month.
But it is not enough to piece together the general story that scientists are looking for. According a paper posted on the arXiv prepress server in November, we need more moon rock. We need to dig at least one meter and collect samples from all possible locations, in order to use the moon reliably as a recorder of these solar and galactic events.
It’s a good thing that NASA and other space agencies want to build long-term habitats on the moon – we’re going to need these facilities to start studying lunar dirt in more detail and unraveling the history of our solar system and our passage through the galaxy.
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