Human beings have been studying the moon closely, if remotely, since night first fell on our species. But until the 1960s, scientists could do little more than guess at answers to the biggest questions about our nearest celestial neighbor.
Namely, how was it created, and what processes formed its oddball topography?
To the naked eye, the lunar surface is a gray-on-gray pastiche of dark regions called maria (Latin for seas) that reflect about 10 percent of the sunlight that strikes them and make up about one-sixth of the visible area. The rest, composed of a lighter material and collectively called highlands (in Latin, terrae, or lands), reflects approximately 15 percent.
Ever since people first trained telescopes on the moon four centuries ago, it has been clear that the highlands actually are higher than the seas and consist of a rough, pocked landscape of hills and craters within craters.
Galileo (1564-1642) usually gets the credit for being the first to examine the lunar surface with a telescope in 1610. Actually, Englishman Thomas Hariot or Harriot (1560-1621) probably beat him, but failed to publish first. Johannes Kepler (1571-1630) first used the names terrae and maria.
There are far fewer craters in the maria, suggesting that the rock surface there is considerably younger than the surrounding hills. How could this peculiar arrangement be explained?
Before the Space Age, there were two equally plausible interpretations -- meteoric and volcanic. The former argued that impacts from itinerant space objects sculpted the craters. These objects initially were leftover lumps of the stony "planetesimals" that originally formed Earth and other planets in the coalescing solar nebula and later were asteroids and other debris. They also supposedly produced the low-lying maria from local melting caused by the stupendous heat of hitting the moon at several dozen miles per second.
The rival volcanic theory posited that the vast basins and individual craters were the result of eruptions from a molten-rock hot zone beneath the lunar crust.
Both concepts had serious problems. On one hand, if impacts had formed all of the lunar surface features, why was each crater almost perfectly circular? Wouldn't some be elliptical where meteoroids struck the surface on an angle? And why were there so many fewer craters in the maria?
(Scientists later would determine that no matter what the angle, the enormous energy of collision would vaporize much of the surface rock, causing an explosion that produced a circular crater. Nobody was quite sure of that in the 1960s.)
On the other hand, if volcanism was responsible for the moon's appearance, then it must have been plenty hot under the surface; indeed, the entire moon might have been molten at one point. (Some believe it may still have a tiny iron core that may be partially molten.) But that seemed unlikely because in the mid-'60s, unmanned lunar probes clearly detected irregularly spaced areas of abnormally high gravity.
This meant that the interior was not homogenous, as one might expect if it had been thoroughly melted; in that case, all of the heaviest elements would have migrated to the center, as they did within the nascent Earth.
Instead, the moon had gigantic wads of high-density material stuck randomly here and there like chocolate chips in a cookie. These concentrations of mass, or "mascons," suggested to many leading scientists that the moon had never been hot enough to melt more than a fraction of the surface.
Which theory -- if either -- was correct? Nobody knew, and scientists were constrained by major problems. For one thing, terrestrial telescopes cannot resolve lunar features much smaller than one-third of a mile. For another, they can observe only about half of the moon's surface.
Over eons, gravitational action has forced our satellite into a situation in which it spins about its own axis in the same amount of time that it takes to circle Earth. As a result, the moon always keeps the same face toward our planet.
The latter problem was eliminated in 1959 when the Soviet Union conducted the first successful lunar fly-around. In short order, U.S. Ranger, Surveyor and Lunar Orbiter unmanned probes provided vastly more detail about the surface on both sides, culminating in human flight around the moon in December 1968.
The first views of the lunar backside were shocking. There were practically no maria there, and the cratered highlands appeared higher and rougher. On the near side, the maria were one to three miles below the average surface elevation; on the far side, hills extended miles above the average.
But pictures and data from unmanned probes could tell only so much. Soil analyzers on Surveyor 5 and 6, which landed in the maria in 1967, had detected basalt, a kind of volcanic rock congealed from a molten state and found widely on Earth. Other sensors determined that the moon was covered with a fine "rock powder."
More complete understanding, however, would require much more evidence, chiefly including lunar surface samples. Without them, it was impossible to know how individual features were formed or to choose among what then were the three leading theories of how the moon was created.
Scientists agreed that Earth was formed about 4.6 billion years ago from remnants of a titanic swirling cloud that somewhat earlier created the faint newborn sun and the solar system. The oldest known Earth rocks are barely 4 billion years old; anything older has been swept under the geological rug by the rise and fall of tectonic plates that make up the planet's restless surface. Presumably, the moon arose in much the same way. But how, where and when?
In the "co-accretion" model, the moon formed in the same gradually condensing cloud of dust, gas and rocky detritus that formed Earth. An alternative "capture" model posited that the moon had been a derelict mass of rock that became trapped in Earth's gravitational field. Finally, the "fission" model proposed that a loose lump of the whirling early Earth spun off to become the moon.
Rocks would tell the story. If the moon co-evolved with Earth, its mineral content and distribution of elements should be virtually identical to those found throughout the planet. If it was flung off Earth's surface, its rocks should be as deficient in heavy elements -- notably iron, nickel, gold and platinum -- as Earth's crust.
If it wandered in from elsewhere in the solar system, its geochemistry should be distinctively different from Earth's.
So although Apollo 11 was not a scientific mission, the first thing Neal Armstrong was scheduled to do after planting his boots on the moon was scoop up a "contingency" sample from the surface. That way, even if the astronauts had to leave immediately, some specimens would return to the home planet.
As it happened, the Apollo crew brought back 47.5 pounds of rock and dust from their landing site in the Sea of Tranquility. And the surprising analysis of those samples prompted a revolution in the way scientists viewed the moon.
* The lunar surface -- which is about 7 percent the area of Earth's -- is covered with a kind of fine, compacted rock dust called regolith. It was formed by a billion or so years of meteoroid impacts that gradually ground much of the uppermost layer of rock into powder.
In the highlands, the regolith is light in color. Its predominant mineral is a form of feldspar called plagioclase, which is rich in lightweight elements including aluminum, calcium, silicon and oxygen. In the darker sea-like basins, called maria, the regolith is chiefly made up of basalt containing the heavier minerals pyroxene and olivine, both of which have large amounts of iron and magnesium.
Apollo 11's landing site was a flat section of the Mare Tranquillitatis (Sea of Tranquility). Later Apollo missions, notably numbers 15, 16 and especially 17 -- the last time human beings walked on the moon -- explored much rougher terrain.
* Decades ago, some scientists believed that water once flowed through river-like valleys called "rilles," which are now known to be volcanic features. Indeed, until very recently it was assumed that the moon held no water. But comets may have deposited some ice that is now trapped in dark areas near both poles.
* The moon has practically no atmosphere beyond a trace of carbon dioxide, carbon monoxide and methane. And as far as is known, it contains no organic material of any kind. Once this became clear, NASA stopped the practice of quarantining Apollo astronauts because it seemed unlikely that they could be carrying lunar germs.
The moon rotates on its axis in exactly the same amount of time that it takes to circle Earth. Therefore, the same side of the moon always faces Earth. Most sections get 14 days of daylight each month. But there is an abnormally deep depression near the south pole, and much of it remains in perpetual shade.
Temperature on the surface ranges from about 230 degrees F in full sunshine at the equator to around -315 degrees F in the dark at the poles. The timing of the Apollo 11 moonwalk was planned so that the surface temperature would not be far from 25 degrees F.
CAPTION: Gemini 4 astronaut Edward H. White II's spacewalk on June 3, 1965, was the first by an American. Gemini missions tested methods used in Apollo flights.
CAPTION: First stage of Saturn V rocket falls away during April 4, 1968, launch of unmanned Apollo 6. Its second- and third-stage engines malfunctioned.