You would not know it by looking at it, but moon is a time capsule.
The surface has been completely exposed to vacuum for almost 4.5 billion years; meanwhile it has been soaked with particles from sun and beyond the solar system. These particles are left, buried beneath the lunar surface, and provide a detailed overview of the history of our solar system and even our entire galaxy.
It’s okay there. We just need to dig it up.
Related: Amazing lunar images from NASA’s Lunar Reconnaissance Orbiter
Here comes the sun
In addition to light, our sun constantly emits a steady rain of high-energy particles, collectively known as solar wind. The solar wind is made up mainly of electrons and protons, but the occasional heavy core also slides out of the sun̵
The solar wind flows through the entire solar system, but very few of these particles reach the earth’s surface, where we can more easily study them. It’s because of our magnetic field – which does a fantastic job of redirecting the paths of the 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 beautiful northern lights light show.
The moon has none of these functions. At least it has not the last 4.5 billion years: Back when the moon was melted, it may have had a temporary magnetic field, but it is in the distant past. In all these billions of years, the moon has regularly sucked up solar wind particles and absorbed them in its regolith.
Faced with the direct attack, is regolitt has changed. The high-energy particles may have disturbed the chemical composition of the moon’s surface. Elements such as potassium, which are to be found in abundance, appear to have been converted into other elements, which then floated away.
The moon dust has also been sunburned: Although each particle is super small, the moon has no atmosphere and therefore no erosion, leaving the same dirt to meet the sun again and again. Each small solar particle tears a microscopic hole in the dirt, so by studying the structure of the regolith we can see an overview of the sun’s glare.
Sometimes the sun flares up and emits an extreme eruption of high-energy particles – far above the usual drip of sunshine. The moon has had to face these eruptions again and again for billions of years. The higher the energy of the event, the deeper the solar wind particles can settle into the regolith. So digging will tell us when the sun threw tantrums in the past.
The sun is not the only source of small high-energy particles that swim through the solar system, but particles come outside our system and get a different name: cosmic rays. They are not rays at all, but a mixture of protons and heavier nuclei coming in from all directions, usually with more energy than the solar wind – they managed to cross the interstellar gorges after all, which is none of it.
Cosmic rays come from a number of super-powerful processes in the galaxy, especially the infamous ones supernova explosions that mark the ultimate death of massive stars. These titanic eruptions can cross entire galaxies and release a truly unholy flood of cosmic rays.
Fortunately, we are nowhere near a supernova event that is about to happen; even candidates like the red giant Betelgeuse is too far away to harm us. But that has not 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 180-440 million years. (The great uncertainty stems from our difficulty in measuring the rotational speed of the arms.)
The spiral alarms are places with intense star formation in 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 do not live very long, and when they die, they tend to go up in a supernova flash.
So over the last billion 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 soaked by Earth’s atmosphere, and if someone came to the surface and implanted themselves in our earth’s crust, erosion and tectonic activity would eventually erase any memory of the accident.
But the moon remembers. High-energy cosmetic rays can leave small traces in the moon’s regolith that can be seen under a microscope. The cosmic rays can also change the molecular composition of the regolith, crush nuclei and transform them. And in the end, the cosmic rays can only … sit there, silent, locked in the moonlight after their explosive birth and long journey.
Digging up tiny fossils
Humans have collected moon samples before: NASA’s six landed Apollo missions in the 1960s and ’70s brought back souvenirs, and China’s Chang’e 5 lands brought home the first fresh moonbells for decades earlier this month.
But it is not enough to put together the great story scientists are looking for. According to a paper posted on the preprint server arXiv in november we need more moonstones. We must dig down at least one meter and collect samples from as many places as possible, in order to reliably use the moon as a record holder for these solar and galactic events.
It’s a good thing that NASA and other space agencies will be building long-term habitats on the moon – we need these facilities to begin studying lunar orbits in more detail and unlock the history of our solar system and our passage through the galaxy.
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