You wouldn't know it by looking at it, but the moon is a time capsule.
Its surface has been completely exposed to vacuum for almost 4.5 billion years; meanwhile, it has been soaked by particles from the sun and beyond the solar system. Those particles remain, buried under the lunar surface, providing a detailed record of the history of our solar system and even our entire galaxy.
It's all right there. We just need to dig it up.
Here comes the sun
Besides light, our sun is constantly emitting a steady drizzle of high-energy particles, collectively known as the solar wind. The solar wind is made of mainly electrons and protons, but the occasional heavy nucleus also slips out of the sun's gravitational embrace.
The solar wind flows through the entire solar system, but very few of those particles reach the surface of the Earth, where we can study them more easily. That's because of our magnetic field — which does a fantastic job at redirecting the paths of those charged particles, forcing them to follow specific routes around our planet — and our atmosphere, which absorbs the bulk of the solar wind in the form of our lovely aurora light shows.
The moon has neither of those features. At least, it hasn't in the past 4.5 billion years: Back when the moon was molten it may have sported a temporary magnetic field, but that's in the distant past. For all these billions of years, the moon has been steadily soaking up solar wind particles, absorbing them into its regolith.
Faced with that nonstop onslaught, the regolith has changed. The high-energy particles may have disrupted the chemical composition of the lunar surface. Elements like potassium, which should be found in abundance, seem to have been turned into other elements, which then floated away.
The lunar dust has also been sunburnt: Even though each individual particle is super tiny, the moon has no atmosphere and so no erosion, leaving the same dirt to face the sun again and again. Each little solar particle tears a microscopic hole in the dirt, so by studying the structure of the regolith, we can see a record of the sun's glare.
Sometimes the sun flares up, sending out an extreme burst of high-energy particles — far above the usual drizzle of the solar wind. The moon has had to face these outbursts again and again for billions of years. The higher the energy of the event, the deeper the solar wind particles can embed in the regolith. So digging will tell us when the sun threw tantrums in its past.
The sun isn't the only source of tiny high-energy particles swimming through the solar system, but particles come from beyond the confines of our system get a different name: cosmic rays. They're not rays at all, but a mix of protons and heavier nuclei coming in from all directions, usually with more energy than the solar wind — they did manage to cross the interstellar gulfs, after all, which is no mean feat.
Cosmic rays come from a variety of super-powerful processes in the galaxy, most notably the infamous supernova explosions that mark the ultimate deaths of massive stars. Those titanic outbursts can outshine entire galaxies and release a truly unholy flood of cosmic rays.
Luckily, we're nowhere near a soon-to-be-supernova event; even candidates like the red giant Betelgeuse are too far away to harm us. But that hasn't always been the case. 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 large uncertainty is from our difficulty measuring the speed of rotation of the arms themselves.)
The spiral arms are places of intense star formation inside galaxies. That's why the spiral arms stand out so much when we look at distant galaxies: they are home to massive, bright, blue stars. But massive, bright, blue stars don't live very long, and when they die they tend to go up in a supernova flash.
So in the past few billion years, our solar system has likely come close to more than a few nasty supernova surprises. The cosmic rays released by these explosions would just get soaked up by the Earth's atmosphere, and if any made it to the surface, implanting itself in our planet's crust, then 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 that can be seen under a microscope. The cosmic rays can also change the molecular makeup of the regolith, smashing apart nuclei and transforming them. And lastly, the cosmic rays can just … sit there, silent, locked in the lunar dirt after their explosive birth and long journey.
Digging up tiny fossils
Humans have collected lunar samples before: NASA's six landed Apollo missions in the 1960s and '70s each brought back souvenirs, and China's Chang'e 5 lander carried home the first fresh moon rocks in decades earlier this month.
But it's not enough to piece together the big-picture history scientists are looking for. According to a paper posted to the preprint server arXiv in November, we need more moon rock. We need to dig down at least a meter and collect samples from any many locations as possible, in order to reliably use the moon as a record-keeper 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'll need those facilities to start studying lunar dirt in more detail and unlock the history of our solar system and our passage through the galaxy.
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