Apollo 11 , the Moon, and the Future of Space ExplorationDespite Apollo’s robust scientific legacy, there were still fundamental questions that were left unanswered for decades after the last human left the moon in 1972 and the last Soviet lander departed shortly thereafter. A robot didn’t touch the surface again until 1993, when Japan’s Hiten lunar probe was intentionally deorbited. But in the late 2000s, a series of missions launched by NASA, China, India, and Japan inaugurated what Brett Denevi, a planetary geologist at Johns Hopkins University, has called “the second era of lunar exploration.” Indeed, 14 missions launched by four different space agencies have successfully placed spacecraft on or around the moon in the past 10 years. This includes a historic first by China, which last year placed a rover on the moon’s far side . And with NASA gearing up to send astronauts to the moon’s south pole , there’s never been a better time to be a lunatic.
The surge of interest in lunar exploration is great news for planetary scientists hoping to learn more about Earth’s rocky sidekick. These are the burning questions they’re dying to find the answers to.
Why Aren't Moon Rocks as Old as the Moon?
The moon is just over 4.5 billion years old, which makes it a mere 60 million years younger than the solar system itself. The early days of the inner solar system were chaotic and defined by the constant collision of solid materials as they whipped around the nascent sun, gradually forming larger and larger bodies in a process known as planetary accretion. Analysis of the rocks collected by the Apollo astronauts shows that most were created by impact events about 3.9 billion years ago, but almost none of them were dated to the moon’s first 600 million years of existence. This is weird because impact events should have become less frequent as the planetary accretion process wound down, so you’d expect to find a lot more rocks formed from earlier collisions.
This led scientists to hypothesize that the moon was subject to intense collisions about 3.9 billion years ago, a period known as the late-heavy bombardment or, more poetically, the lunar cataclysm. While this theory nicely accounts for the Apollo moon rocks, it also raises a big question: What caused all these rocks to start pummeling the moon? The leading model suggests that the outer planets used to orbit much closer to the sun and, as they moved outward, sent big rocks on a collision course with the moon. But an alternative theory posits that the cataclysm never happened and that the preponderance of rocks dating to 3.9 billion years ago is due to sample bias.The last three Apollo missions all took samples from three major impact craters—Imbrium, Serenitatis, and Nectaris. New evidence suggests that the samples used to date the age of each of these craters, which is crucial to determining whether a period of heavy bombardment occurred, may actually just be debris from the impact that formed the largest crater—Imbrium—about 3.9 billion years ago.“We're pretty confident that when Imbrium formed, it spattered the nearside collection areas with its ejecta,” says Nicolle Zellner, a planetary scientist at Albion College. “So when the Apollo astronauts landed in these regions and collected samples, they were very likely to collect samples of Imbrium.”Zellner says the best way to settle the lunar cataclysm debate will be to visit craters where samples aren’t likely to have been contaminated by the Imbrium impact, such as the south pole or the far side of the moon. If most of those new samples are older than 3.9 billion years, it will cast the theory of the lunar cataclysm in serious doubt and also help scientists better understand conditions in the early solar system.
What Creates the Lunar Ionosphere?High up at the outer reaches of Earth’s atmosphere is a region of electrically charged particles called the ionosphere. It’s created when the solar wind strips electrons from atmospheric gasses, turning them into ions. In the 1970s two Soviet lunar orbiters discovered that ions also existed in the moon’s ultra-thin exosphere, and scientists have been trying to explain this observation ever since.The fact that the moon has an ionosphere is not particularly surprising, says Jasper Halekas, an associate professor of physics and astronomy at the University of Iowa. Any planet that has an atmosphere, even one as diffuse as the moon’s, will produce ions when gasses interact with the solar wind. What is surprising, however, is the discrepancies in measurements of how dense the lunar ionosphere is. The figures range from about 1,000 ionized particles per cubic centimeter to about a tenth of a particle per cubic centimeter. As Halekas notes, “Four orders of magnitude is a pretty wide range of discrepancy for measurement, even when it comes to astronomy.”Better measurements will help scientists understand how the lunar ionosphere is produced. Only a decade ago, some scientists believed that the lunar ionosphere might be created by ionized dust in the atmosphere, which would make the moon’s ionosphere much different from Earth’s. Yet in 2013, when the Lunar Atmospheric Dust and Environment Explorer failed to detect an appreciable amount of dust in the upper lunar atmosphere, this theory was cast into serious doubt. The problem is that if there really are 1,000 ions per cubic centimeter, the ionization of gas in the lunar exosphere can’t account for such a high concentration—there just isn’t enough gas.Halekas is the co-investigator on the Lunar Surface Electromagnetics Experiment, which was recently selected by NASA to be one of 12 experiments that will hitch a ride to the lunar surface on a commercial lander. The experiment will measure oscillations in different types of electromagnetic fields, which can be used to determine the density of the ionosphere with unprecedented accuracy. Halekas predicts that the experiment will find low enough concentrations of ions to match the amount of gas present, which would put an end to the debate. But if the experiment detects high concentrations, Halekas says it will be necessary to “go back to the drawing board” to explain how these ions were produced in such large quantities.
Where Did Lunar Water Come From?Last year, NASA scientists used data from India’s Chandrayaan-1 spacecraft to definitively prove that water ice is present at the lunar poles. Most of this ice exists in permanently shadowed craters at the south pole, where temperatures never rise above -250 degrees Fahrenheit. This is good news for future expeditions to the moon, which plan to use this water ice for everything from life support to rocket fuel. Although it’s unclear what form the water ice is in—big blocks or crystals mixed with lunar regolith—for many scientists the big question is how it got there in the first place.According to Paul Hayne, a planetary scientist at the University of Colorado, Boulder, there are three main theories for how water originated on the moon. The most “obvious” theory, Hayne says, suggests that the water ice was deposited by asteroid and comet impacts, where it vaporized and eventually made its way to the poles. It’s also possible that ionized hydrogen from solar winds binds with oxygen trapped in regolith and is eventually released as vaporized water due to temperature fluctuations on the surface. Finally, there’s a possibility that water was present in the material that originally formed the moon and was forced to the surface by volcanic eruptions. It could be that all three processes were at work, which makes it a question of how much water each mechanism contributed.“So we have some ideas about how water got there, but the competing theories have not really been tested yet,” Hayne says. Still, there has been some promising initial data. In 2009, NASA launched the Lunar Crater Observation and Sensing Satellite on a mission to impact the lunar surface at the south pole. LCROSS not only detected the presence of water, but it also identified a mix of other materials that are common in comets, suggesting at least some of the water hitched a ride on space rocks.To get a better idea of how much of the moon’s water was brought to the lunar surface by comets, asteroids, or solar winds, Hayne says it will be necessary to send a robot or human to take a sample and examine its isotopic composition. “That’s really the only way we can definitively associate that material with a source,” he says.
But even if scientists can determine the origins of lunar water, there’s still the question of how it came to be concentrated in the poles, a “controversial topic,” according to Hayne. Currently the lunar science community is divided on whether water that is vaporized during comet and asteroid impacts can travel across the surface of the moon or whether it becomes trapped in the regolith. The only way to know for sure is to return for further tests.
What Can the Moon Teach Us About the Early Solar System?The moon lacks much in the way of an atmosphere and hasn’t been volcanically active for billions of years, which means its surface has remained relatively unchanged across the eons. In this sense, says Prabal Saxena, a postdoctoral researcher at NASA’s Goddard Flight Center, the craters are like the pages of a history book of the early solar system—if only we could figure out how to read them.
As mentioned above, a prevalent theory of lunar formation says that our planetary neighbor was bombarded by space rocks about 3.9 billion years ago. If the samples from the surface confirm that there was a lunar cataclysm, this could also tell us a great deal about how the solar system formed. Not only would it suggest that the outer planets were once much closer to the sun, it would likely mean that the Earth was bombarded too. This would have vaporized any water on the Earth’s surface and killed any life that may have existed there.Strangely enough, the moon also appears to have recorded early solar history. Earlier this year, Saxena and his colleagues used the composition of the lunar crust to determine that our sun likely rotated 50 percent slower than similar newborn stars during its first billion years of life. The moon and Earth are largely composed of similar materials, but the moon has notably less sodium and potassium. Using this evidence, Saxena and his colleagues ran simulations that showed how solar activity can either deposit or strip the moon of these minerals, and then incorporated data about the relationship between solar flares and stellar rotation rates. According to the simulations, the sun must have been rotating slowly to account for the levels of potassium and sodium observed on the moon today. This data about the sun’s early history can also help explain things like how quickly Venus lost its water, how quickly Mars lost its atmosphere, and how it impacted atmospheric chemistry on Earth.As NASA and other space agencies lay the foundations for a permanent human presence on the moon, there will be more big questions to answer. “We understand the moon better than so many other places, and yet we still have these really important unanswered questions,” Denevi says. “The moon is really a stepping stone to other planets and even though it’s kind of become cliché, it’s totally true.” Indeed, the moon is something like a Rosetta stone for our solar system. If we hope to understand—and eventually travel to—far more distant planets, the best place to start is our own backyard.
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