Earth's Moon is the brightest and largest object in the star-splattered night sky. It is an enchanting and lovely companion world, the fifth largest moon in our Solar System. Earth's Moon is mysterious and beckoning, an age-old symbol for love, fleeting beauty, and that which is feminine. But where did our Moon come from?
Earth's Moon is our nearest neighbor. More than 100 moons orbit the eight major planets in our Solar System. The majority of them are frigid worlds, made up primarily of ice and some rocky material, circling the giant planets inhabiting the outer limits of our Solar System--Jupiter, Saturn, Uranus, and Neptune. The inner Solar System is almost devoid of moons. Our own Moon is the largest one in our region of the Sun's family. Of the four relatively small, rocky inner planets that circle nearest to the Sun--Mercury, Venus, our Earth, and Mars--Mercury and Venus are moonless, and Mars is circled by two misshapen, miserable excuses for moons, Phobos and Deimos. The two Martian moons are quite small and are likely kidnapped asteroids from the Main Asteroid Belt that whirls around our Sun between the orbits of Mars and Jupiter.
A moon is usually defined as a natural object that is in orbit around a planet. The moon is kept in its orbit by the host planet's gravity, as well as the gravity of the moon itself. Some planets host moons; some do not.
At least five theories have been suggested explaining how our Moon was born. The first theory states that the Moon was once a part of our own planet, and then somehow budded off about 4.5 billion years ago, when our Solar System was first forming. The Pacific Ocean basin is the most frequently suggested site for where our Moon originated. A second theory suggests that the Earth and Moon formed together out of the original nebula that gave birth to our Solar System. The third theory states that the interaction of Earth-orbiting and Sun-orbiting planetesimals, early in the history of our Solar System, resulted in their disintegration. Earth's Moon then condensed from this debris. Planetesimals were the building blocks of the planets in the early Solar System, and they were rocky, icy, or both. They collided and then stuck together to form the planets. The fourth theory states that our Moon was actually born elsewhere in the Solar System, and was eventually kidnapped by our planet when it ventured too close to Earth's gravitational embrace.
However, the theory that is currently considered to be the most credible is usually called the giant impact theory, or alternatively (and playfully), the Big Whack or Big Splash theory. The fundamental suggestion of this theory is that a hypothetical Mars-sized protoplanet, named Theia by astronomers, crashed into the primordial Earth billions of years ago. The catastrophe resulted in a portion of the ancient Earth's crust to be shot off into space, hoisting a myriad of moonlets into the sky. Some of this debris was eventually captured into orbit around the ancient Earth about 4.5 billion years ago, where it was eventually pulled together by the force of gravity to become our lovely Moon!
Most of the giant impact theory was first presented back in 1975 by Dr. William K. Hartmann and Dr. Donald R. Davis of the Planetary Science Institute in Tucson, Arizona. Their theory is based on geological samples gathered by Apollo astronauts when they made their historic voyage to the Moon in July 1969. The oxygen isotopes found within the gathered Moon rocks proved to be almost identical to those of our own planet! An isotope refers to each of two or more types of the same element containing equal numbers of protons but different numbers of neutrons in their atomic nuclei. In addition, other pieces of evidence suggested strongly that the Moon is composed primarily of the same material that is found in the Earth's mantle.
Evidence presented in the October 18, 2012 issue of the journal Nature adds still more credibility to the Big Whack scenario. Planetary scientists, examining Moon-rocks gathered during NASA's Apollo Moon-landing missions, as well as a meteorite that is known to be a chunk of the Moon, searched for traces of zinc in the lunar samples. The ratios of heavy to light isotopes were revealed to be greater than on our planet. This indicates that the Moon experienced a violent evaporation event when our Solar System was first forming. This catastrophic event may well have been the ancient, violent collision of a Mars-sized world--Theia--with the primordial Earth. The study clearly provides additional evidence that Earth's Moon was formed in the wake of a gigantic impact, the scientists explained.
When our Moon was still forming, it is thought to have sported a fiery, global magma ocean that was hot enough to vaporize zinc. A catastrophic impact is one of the few events that would be capable of generating that immense quantity of heat. A second prediction of the Big Whack is that heavier isotopes would exist in greater abundance. This is because they would condense at hotter temperatures.
Dr. Frederic Moynier, assistant professor of Earth and Planetary Sciences at Washington University in St. Louis, Missouri, told the press on October 17, 2012 that "What we found is that the depletion [of lighter isotopes] of zinc is probably due to evaporation." Dr. Moynier is one of the study's co-authors.
Dr. Moynier, the paper's lead author Dr. Randal Paniello and Dr. James Day of the Scripps Institute of Oceanography in California, discovered that the ratio of zinc-66 to zinc-64 contained in the lunar rocks is approximately three to four times greater than that found on either Earth or the planet Mars. On Mars it is 0.27 parts per thousand, while on Earth it is 0.25 parts per thousand. In contrast, on the Moon, it was a difference of 1.3 to 1.4 parts per thousand--a very significant difference!
Almost all of the lunar samples gathered from the Moon showed similar ratios of heavier to light isotopes, even though they were collected from different regions scattered all over the Moon.
The extremely high temperatures suggest that water vaporized. This further indicates a depletion of other volatiles, which are elements such as sulfur, chlorine, and hydrogen, which all vaporize at much lower temperatures. However, it must be noted that there are several studies suggesting that water is still present in some Moon-rocks.
Exactly how much water and other volatiles Earth's Moon contains is still a key question. Planetary scientists who suggest that there is water in the lunar mantle say that the Moon may have large quantities of those chemicals. However, other planetary scientists argue that the volatiles are primarily located in the upper layers of the Moon's soil, carried in by meteorite impacts and mostly dating from the ancient era of lunar formation.
Water and volatiles most likely resulted from impacts and solar wind, Moynier told the press in October 2012. "[The results] show that all this water they found on the face of the Moon is secondary water," he added.
Still another model, devised by Dr. Matija Cuk and Dr. Sarah Stewart of Harvard University in Cambridge, Massachusetts, explains the amazingly similar chemistry of our own planet with its lovely Moon. A giant impact onto a fast-spinning Earth shoots material from Earth into orbit, giving birth to a Moon that is just like our own--depleted in iron, with a composition similar to Earth's mantle. After the impact, the once-rapidly spinning Earth is slowed down by a gravitational dance between the Moon and the Sun, termed an orbital resonance.
Of late, the Big Whack theory has lost some credibility because the Earth and its Moon have been shown to be "isotopic twins". The original Big Whack theory predicted that most of Earth's Moon was composed of Theia-stuff rather than Earth-stuff, and therefore should have shown a different isotopic composition.
Therefore, the giant impact theory was in trouble because, even though it could match the mass of the Moon and the rotation rates of the Earth and its Moon, it could not correctly predict the lunar chemical composition. Currently, tides between the Earth and Moon have slowed down Earth's orbit, and have also pushed the Moon further and further away. In the early days of our Solar System, the Moon was much closer to our planet than it is today--a gigantic companion world taking up a large part of the sky. The ancient Earth possessed a mere five-hour day when the Moon was born. If the Earth had a spin of a mere five hours after the impact, the crash of Theia could not have shot enough Earth-stuff into orbit to create a Moon with a chemistry that matched that of Earth.
However, Cuk and Stewart were able to demonstrate that if Earth's initial angular momentum were higher, corresponding to an Earth day of between two and three hours, a catastrophic impact could indeed shoot enough Earth-stuff into orbit to create a Moon that has the same isotopic composition as Earth. With this extremely fast rotation rate, it is much easier to shoot Earth-stuff into orbit as the result of a giant impact.
Cuk and Stewart also calculated that the ancient Earth could well have possessed such a short rotation period after the crash and then later have attained its current much more sluggish spin-rate by transferring angular momentum to the Sun by way of evection resonance. Evection resonance refers to the gravitational dance between Earth's orbit circling the Sun and the lunar orbit around Earth. This paper has revealed for the first time that it is indeed possible for the ancient Earth to have possessed a spin rate of only two or three hours after the giant impact.
But where did Theia go after it had smashed into the ancient Earth, causing the formation of our enchanting large Moon? That is the question! No trace of this hypothetical ancient world has ever been observed--and it has not been for lack of trying. If Theia or its remnants and relics are ever observed by astronomers it will at long last explain the mysterious origin of Earth's luminous, captivating companion.
I am a writer and astronomer whose articles have been published since 1981 in various magazines, newspapers, and journals. Although I have written on a variety of topics, I particularly love writing about astronomy because it gives me the opportunity to communicate to others the many wonders of my field. My first book, "Wisps, Ashes, and Smoke," will be published soon.