That’s one small step for a man, one giant leap for mankind.
—NEIL ARMSTRONG
ONE GIANT LEAP….
Like many Americans older than 55, I was glued to the TV set on July 20, 1969. I was visiting my cousins’ ranch outside Hot Springs, South Dakota, at the time, getting a month’s experience in ranch life: collecting eggs from the chickens, riding horses and tractors, taking care of the daily chores, and getting to know my extended family. We’d been hearing the buildup to the Apollo 11 mission all week, but now we were about to witness something extraordinary: the first man to walk on the moon, and even more amazing for that time, the world would be able to watch it live on television! We all clustered around the TV in the small living room that afternoon as the networks began to broadcast the preparations for the first moonwalk. Then, finally, the magic moment arrived, and millions of people all over the world simultaneously saw one of the most stirring achievements in human history.
The race to the moon was launched by President John F. Kennedy in 1961, challenging the United States, and especially its space program, to land a man on the moon before the end of the decade. We had been trailing the Soviets badly in the space race ever since they first launched the Sputnik satellite in 1957, long before we could do so. Then they launched the first animals into space, and then the first man into space, Yuri Gagarin, in 1961 (a month before American astronaut Alan Shepard). Between 1959 and 1963, the Mercury program launched the first Americans into space, and we were all riveted when John Glenn was the first American to orbit the earth in 1962. From 1965 to 1966, we moved on to the Gemini program, with two astronauts doing even more daring missions, including space walks and docking between spacecraft. From 1968 to 1972, the Apollo program with its three-man crews was building up the expertise to land on the moon and then return, each mission flying longer times and farther distances around the moon than before.
Finally, on that fateful day in 1969, Apollo 11 reached the moon. While the third astronaut, Michael Collins, remained in orbit around the moon, Neil Armstrong and Buzz Aldrin flew the lunar lander to the moon’s surface, then did a short moonwalk (figure 11.1) before blasting off and returning to the mother ship for their voyage back to Earth. Each successive Apollo mission (Apollo 12 through Apollo 17, except for the ill-fated Apollo 13, which had an explosion in space and barely returned its astronauts alive) took longer and longer moonwalks and brought back more and more samples. By the time Congress canceled the Apollo program in 1973, the six moon missions had landed 12 men on the moon, collected a huge amount of data about the moon, and returned with 381.7 kilograms (842 pounds) of lunar samples. The only scientist ever to walk on the moon was a geologist, Harrison Schmitt, who was on the last mission, Apollo 17, which spent several days on the moon in December 1972.
Figure 11.1
Photograph of Apollo astronaut Edwin Eugene “Buzz” Aldrin on the moon, taken by Neil Armstrong, July 20, 1969. (Courtesy of NASA)
The space program generated a huge program of research that produced enormous technological breakthroughs not only in space, but in all sorts of other fields as well. It jump-started the race for smaller and faster computers and hugely improved telephone communication, especially satellites for communication and GPS navigation. The robots assembled to build spacecraft eventually made our assembly lines for cars and many other products more efficient. A wide variety of products were developed based on NASA research, including artificial hearts, thermal blankets, strong but light metal alloys and lightweight composite materials, better drugs grown in zero gravity, smoke detectors, air-purification systems, small practical lasers, high-capacity batteries, UV sunglasses, Teflon-coated fiberglass, better fire protection gear for firefighters, solar power systems, artificial limbs, MRI and CAT scanning, LED technology, joysticks for video games, better golf balls, the TACS system that aircraft use to avoid collisions, virtual reality simulators, hydroponics, DirecTV, pacemakers, and even disposable diapers.
In another sense, the space program was immensely important not only in giving us satellite images of Earth to study all sorts of processes happening on our planet, but in providing the humbling views of Earth from space that transformed the way we think about our “pale blue dot.” All this and more was produced for less than 1 percent of the federal budget, a trivial amount compared with what we spend on other things that provide much fewer benefits.
SISTER, DAUGHTER, OR PICKUP?
Perhaps the biggest scientific benefit of all was the definitive answer to the long-standing scientific question: How did the moon form? What was it made of? Various legitimately scientific ideas beyond the “green cheese” hypothesis had been floating around in the planetary geology and astronomy community for more than a century. The ideas fall into three broad categories, which (thanks to the male-dominated community of astronomers) acquired sexist names that are no longer acceptable:
1. The “pickup” or “capture” hypothesis: For decades, some scientists had suggested that the moon was a foreign body from far outside the earth’s orbit that was captured as it flew by and pulled into orbit by the earth’s gravity. But there were numerous problems with this model from the very start. For one thing, the moon’s orbit moves in the same plane as the earth’s orbit around the sun, which would be unlikely if an object coming at any angle from outer space were captured. Such an orbit would swing around the earth in any plane except the plane of the earth-sun system. In addition, the usual consequence of gravitational capture of a large body is either collision, or else the object flies back off into space with an altered orbit. For the moon to have been slowly captured by the earth’s gravity, staying in orbit without collision or escape, the earth would have needed to have a very thick atmosphere that extended much farther than it does now. No evidence supports this. Finally, if the moon were an exotic object captured by Earth’s gravity, its composition would be radically different than that of the earth. The moon rocks could be used to test this idea.
2. The “daughter” or “fission” hypothesis: This scenario, first proposed by astronomer George Darwin (son of Charles Darwin) in the late 1800s, argued that the moon is made of matter from the original rapidly spinning earth. During this rapid spin, molten material spun off from the earth into space to form the moon. Some astronomers even suggested that the Pacific Ocean Basin was the remnant scar of that event. In 1925 Austrian geologist Otto Ampherer proposed that the spinning off of the moon caused continental drift. This scenario was plausible for many years, although by the 1960s plate tectonics had shown that the Pacific Basin is not an ancient scar but floored by very young lavas less than 160 million years old. This model also didn’t account for the angular momentum of the earth-moon system. Once again, the crucial test would be the moon rocks. If they were the same composition as the primordial earth (before it separated into core, mantle, and crust), then this concept would be plausible.
3. The “sister” hypothesis: Similar to the “daughter” hypothesis, this model suggests that the original earth-moon system started out as two large blobs of matter that became locked into gravitational attraction with each other. Again, there are problems with the angular momentum of the earth-moon system in this model. But like the “daughter” hypothesis, it predicts that moon rocks would have a composition very similar to the primordial earth.
These ideas and more were hanging in the balance when Apollo 11 and later moon missions brought lunar samples back to labs to study. To everyone’s surprise, their composition did not support any of the previous ideas. Instead, it suggested a new idea that no one had ever imagined.
IMPACT!
The lunar rocks brought back by the Apollo missions (figure 11.2) were not similar to the early earth in composition. Nor were they some exotic composition, as if they had been a body from outside the earth captured by gravity. Instead, they were made of anorthosite and its volcanic equivalent, the familiar black lava known as basalt. In other words, their composition was very much like parts of the upper mantle where the lavas that erupt onto the seafloor or from volcanoes like Kilauea on Hawaii have their source.
Figure 11.2
Sample of typical lunar anorthosite. (Courtesy of Wikimedia Commons)
This was a shock. If the moon was made almost entirely of mantle material, it must be a piece of the earth’s mantle that formed after the primordial earth had separated into a core of iron and nickel (chapter 10) and a mantle made of silicate minerals. In other words, the moon was formed long after the earth had cooled and coalesced and its layers had differentiated and separated.
Even more startling, the only way to get a lot of mantle material into space was to blast the early earth with a giant impact from another body (figure 11.3). Planetary geologists now call this body Theia (the Greek name for the mother of Selene, the moon goddess) and postulate that it was a Mars-sized protoplanet that hit the earth with an impact that blew material sideways off the earth and into orbit. Once this debris began to orbit the earth (at one-tenth the distance from the earth that the moon is today), it would have gradually coalesced. The energy of this collision would have been amazing! Trillions of tons of material would have been vaporized, and the temperature of the earth would have risen to 10,000°C (18,000°F).
Figure 11.3
An artist’s conception of the impact that blasted the mantle and formed the moon. (Courtesy of Wikimedia Commons)
The heat from its own radioactive minerals would have melted the moon completely, and most of the moon would have remained the same composition as the earth’s mantle, while the melting would also have caused huge eruptions of basaltic lava, forming magma oceans that now make the dark “maria,” or “seas,” on the moon’s surface (figure 11.4). Meanwhile, the moon has a tiny iron core, only 330–350 kilometers in diameter, thought to be a relic of the core of Theia left behind after the collision; most of its iron-nickel core accreted to the earth’s core. By contrast, if the “sister” or “daughter” models (favored before Apollo 11) were correct, the moon would have a large core, roughly proportional to the size of the earth’s core relative to its mantle.
Figure 11.4
The near side of the moon, which always faces Earth, showing its impact craters and dark lava flows (“maria”). (Courtesy of NASA)
When did this all occur? Once again, moon rocks give the answer. Using the same uranium-lead and lead-lead dating methods discussed in chapter 11, many labs have dated moon rocks. Most are at least 4 billion years old, suggesting that the moon’s surface formed early and has not been modified much since. After all, it has none of the forces that change the earth’s surface—it has no atmosphere, no water, no weathering, very reduced gravity, and no plate tectonics. The only major modification of its surface has been huge impacts that left craters (figure 11.4), and most of the crater debris has been dated at older than 3.9 billion years, so most of the impacts occurred early, and not much has happened since then.
The oldest pre-impact rock dates from the moon are currently 4.527 ± 0.0010 billion years. This is about 30 million years younger than the meteorites that date back to the origin of the solar system, so the moon is definitely younger than the events that formed the solar system and the earth and the melting and differentiation episode that separated the earth’s core from its mantle.
Since the initial proposal of the giant impact hypothesis, much additional evidence has come out of analysis of moon rocks to support the mantle source of the moon. Nearly all the geochemical isotopes (oxygen, titanium, zinc, and many others) that have been studied in the past 48 years since the moon rocks were collected have shown that the moon and the earth’s mantle have identical chemical compositions. There are also many refinements to the impact model, with some versions having more than one impacting body or positing different-sized impactors or different impact mechanics. But no matter which version is currently favored by scientists, the Apollo samples inescapably point to the moon as being a chunk of the earth’s mantle.
MOONSTRUCK!
It’s funny. When we were alive we spent much of our time staring up at the cosmos and wondering what was out there. We were obsessed with the moon and whether we could one day visit it. The day we finally walked on it was celebrated worldwide as perhaps man's greatest achievement. But it was while we were there, gathering rocks from the moon’s desolate landscape, that we looked up and caught a glimpse of just how incredible our own planet was. Its singular astonishing beauty. We called her Mother Earth. Because she gave birth to us, and then we sucked her dry.
—Jon Stewart, Earth (The Book): A Visitor’s Guide to the Human Race
People have stared up at the moon for centuries and imputed mysterious powers to it. In the hit movie Moonstruck, the characters behave strangely under the influence of the full moon. Supposedly full moons cause werewolves to change out of their human form or real humans to act crazy. In astrology, the moon’s position at the moment we are born supposedly affects our personality and future, although this is pure bunk. Many cultures blamed the moon for odd events or worshiped it as a god. Early science fiction imagined men on the moon or aliens from the moon invading Earth. Lots of cultures have looked up at the blotchy pattern of dark and light surfaces and imagined a “face” or seen the “man in the moon.” In one of the earliest (1902) silent movies ever made, A Trip to the Moon, the main characters are loaded into a cannon and shot to the moon, where the cannon shell sticks in the “eye” of the “man on the moon.” It was influenced by Jules Verne’s science fiction novel, From the Earth to the Moon (1865), which blasted the explorer to the moon with a cannon. None of these ideas makes sense any more with our current understanding of the moon.
However, there are real and surprising ways the moon does affect us. Astronomers and physicists point to an interesting dynamic: the relatively large size of our moon (compared to the satellites of other planets) acts as a stabilizer, keeping the earth’s rotation fairly steady so it doesn’t flop on its side, like Neptune has. Many also think that the reason the rotational axis is tilted 23.5° from the plane of the orbit around the sun is due to the impact as well. When the impact occurred, it knocked the earth’s rotational axis 23.5° off vertical, so now it wobbles like a spinning top (see chapter 25).
Just as amazing as the original impact is the story of how the earth-moon system got to be the way it is now. Today, the tidal pull of the earth has completely stopped the moon’s rotation on its axis, so it is tidally locked to always show the same face to the earth. This is the only side of the moon that any human saw until the Apollo 8 mission first flew around the opposite side of the moon (figure 11.5) and photographed it. (This is not the same as Pink Floyd’s “Dark Side of the Moon,” which is a myth; there is no permanent “dark side,” since both sides experience darkness and light depending upon the position of the sun.) Meanwhile, the moon’s tidal pull on the earth has been gradually slowing the earth’s rotation, making it lose about 1.5 milliseconds every century, and more than a minute in a few thousand years. At the beginning of each New Year (especially after the millennium in 2000), the world’s most precise clocks need to be adjusted to account for this, since otherwise they would be out of synch with the atomic clocks.
Figure 11.5
The far side of the moon, only visible to spacecraft that orbit the moon, starting with Apollo 8 in 1968. (Courtesy of NASA)
A few milliseconds a year might not seem like much, but over millions and billions of years, it adds up. Physicists have done the calculations, and found that the earth slowed down so much that there were far more rotations of the earth (Earth days) in the geologic past than there are now.
Confirmation of this startling idea came from the paleontology of humble corals. In the early 1960s, paleontologist John W. Wells of Cornell University was looking at fossil corals that had both daily growth lines and larger marks that showed the annual cycle of the seasons. He was able to slice the corals very thin, polish them, and then count the growth rings under the microscope. Sure enough, in the Devonian Period (about 400 Ma), the earth spun on its axis so much faster that it had 400 turns (400 days) in one revolution around the sun (1 year). About 600 Ma, the day was only 21 hours long, not 24 hours, and there were 430 days in a year. And only 150 Ma, there were 380 days in a year.
What does this mean? Although it’s extremely slow by human standards, the earth is gradually slowing down, so some day about 20 billion years from now, it will also be tidally locked, and only one side will face the moon and one side will never see the moon. The energy of the entire earth-moon system is also decreasing, so both systems are slowly pulling apart. When the moon debris first was blasted out into space, the moon was only 10 percent of its current distance away from the earth, and it has been slowly receding since then. Back when the trilobites roamed (600 Ma), the moon was much closer and would have looked huge in the sky. Its tidal pull at that distance would have been so powerful that immense true tidal waves (not tsunamis formed by earthquakes, which have nothing to do with tides) would have swept across the earth as the tides rose and fell.
Eventually, the two bodies will not only be tidally locked and much farther apart, but their motion could come to a halt. However, this is billions of years in the future, and the sun will probably explode before then and wipe out the inner planets, so it will never actually get to occur.
It’s pretty humbling when you think about it. Just a few pounds of rocks brought back by the Apollo spacecraft have revolutionized our understanding of the moon, the earth, and the solar system. The next time you read poems about the moon, or hear romantic lyrics about the “moon in June,” you’ll never think about our only natural satellite the same way again.
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