You would not 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 is soaked by particles sun and beyond the solar system. These particles remain behind, buried beneath the lunar surface, and provide a detailed account of the history of our solar system and even our entire galaxy.
It’s right there. We just have to dig it up.
Related: Incredible lunar images from NASA’s Lunar Reconnaissance Orbiter
Here comes the sun
In addition to light, our sun constantly emits a rainstorm of high-energy particles, collectively known as the solar wind. The solar wind consists mainly of electrons and protons, but occasionally a heavy nucleus also slips out of the sun’s gravity.
The solar wind flows through the entire solar system, but very few of the particles reach the earth’s surface, where we can study them more easily. This is due to our magnetic field – which does a fantastic job of diverting the paths of the charged particles, forcing them to follow specific paths around our planet – and our atmosphere, which absorbs most of the solar wind in the form of our sweet aurora light showers.
The moon does not have any of these characteristics. At least that has not happened in the last 4.5 billion years: when the moon melted, it may have had a temporary magnetic field, but that is in the distant past. During all these billions of years, the moon has gradually taken up solar wind particles and taken up in its revival.
Faced with the incessant onslaught, the regoliet has changed. The high-energy particles may have disturbed the chemical composition of the lunar surface. Elements such as potassium, which must be found in abundance, were apparently changed into other elements which then drifted away.
The lunar dust is also sunburn: although each individual particle is super small, the moon has no atmosphere and therefore no erosion, leaving the same dirt to the sun again and again. Each small sun particle tears a microscopic hole in the dirt, so by studying the structure of the regolith, we can see a record of the sunshine.
Sometimes the sun flares up and sends an extreme eruption of high-energy particles – far above the usual drizzle of the solar wind. The moon has had to experience these eruptions time and time again for billions of years. The higher the energy of the event, the deeper the solar wind particles can embed in the regolith. To dig, therefore, will tell us when the sun cast tantrums in its past.
Galactic fingerprints
The sun is not the only source of small high-energy particles that swim through the solar system, but particles come from outside the boundaries of our system and get another name: cosmic rays. It is 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 have managed to cross the interstellar gorges, which is no great achievement.
Cosmic rays come from a variety of super-powerful processes in the galaxy, notably the infamous supernova explosions marking the eventual death of massive stars. Those titanic eruptions can overshadow entire galaxies and release a true unholy flood of cosmic rays.
Fortunately, we are nowhere near a supernova event; even candidates like the red giant Betelgeuse is too far to harm us. But this has not always been the case. Because of 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 big uncertainty is because we have a hard time measuring the arm speed of the arms.)
The spiral arms are places of intense star formation in galaxies. That is why the spiral arms fall out so when we look at stellar stars: it is 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.
In the last few years, our solar system has probably come across more than a few nasty surprises from supernovae. The cosmic rays released by these explosions will only be Earth’s atmosphere, and if someone had brought it to the surface and implanted it in the crust of our planet, erosion and tectonic activity would eventually erase the memory of the disaster.
But the moon remembers. High-energy cosmic rays can leave small traces in the lunar orbit that can be seen under a microscope. The cosmic rays can also change the molecular composition of the regolith, which crushes and transforms nuclei. And lastly, the cosmic rays can only … sit there, silent, trapped in the moonlight after their explosive birth and long journey.
Dig up small fossils
People have previously collected lunar monsters: NASA’s six landed Apollo missions in the 1960s and ’70s each brought back souvenirs, and China’s Chang’e 5 lander brought home the first fresh lunar rocks earlier this month.
But it is not enough to put together the big picture that scientists are looking for. According to a paper posted on the preprint server arXiv in November we need more moon rock. We need to dig at least a meter and collect samples from as many places as possible to reliably use the moon as a record holder 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 will need those facilities to study the lunar filth in more detail and unlock the history of our solar system and our transit through the galaxy.
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