From the Earth to the origins of the moon

Many of us likely took a moment this past Sunday to admire the full moon.

Diana Brueton, in her delightful 1991 book “Many Moons,” tells of the grip our nearest celestial neighbor has held on our imaginations throughout the ages. She says the earliest written stories involving the moon came from the Babylonians in 750 B.C. and then goes on to document lunar mythology, involving mostly fertility rites, down to the present day.

The moon lost much of its mystery when Apollo 11’s Neil Armstrong became the first person to walk on its surface July 16, 1969, and later when 243 pounds of lunar rocks were brought back for study by the Apollo 16 crew in 1972. Today police and emergency room physicians tell us we can still look for strange behavior at the full of the moon but it is astronomers who now find the moon most perplexing as they wrestle with the question: How did the moon come to be?

Robin Canup and Eric Asphaug have weighed in with the latest entry on lunar origins in the Aug. 16 issue of Nature.

There were three early theories as to the origins of the moon. In 1796, the French mathematician LaPlace proposed the “sister” theory, which holds that the Earth and moon grew side by side during the early days of the solar system. This was followed in 1879 by George Darwin’s “daughter” theory, proposing that the moon was torn from the primitive molten Earth by tidal forces exerted by the sun.

In 1959, Harold Urey’s “capture” theory, in which the moon was a wandering body drawn into its present orbit by Earth’s gravity, was widely adopted. In fact, so great was Urey’s prestige that the Apollo missions were planned, in part, to prove he was right.

Unfortunately, there are problems associated with all three theories, and analysis of the lunar rocks did not help Urey’s theory. The chemical composition of the moon is so similar to that of Earth’s mantle that they had to have formed from the same supply of raw materials. But how?

William Hartmann, writing in the November 1989 issue of Natural History, relates how he and Donald Davis, both of the Planetary Science Institute in Tucson, proposed a radically new theory in 1974.

Their theory, which they call the “collision” theory but promptly got dubbed the “Big Whack” theory, has the primitive Earth being struck by a Mars-sized body that had formed concurrently with Earth in the early solar system. The impact destroyed the impacting body and blasted enough debris from Earth into orbit that the moon eventually formed from it.

The collision theory could explain both the chemical similarity between Earth and the moon and the planet’s spin, both of which were sticking points with the earlier theories.

Immanuel Velikovsky, an American psychologist, was pilloried by the scientific community in 1950 when he proposed a similar scenario in a book titled “Worlds in Collision.” Velikovsky, however, made the collision between Earth and a comet that later became the planet Venus, a contention that has no evidence to substantiate it.

The collision theory sent theorists to their computers to see whether a model could be put together that would satisfy all of the physical and chemical conditions that had to be met.

Robin Canup’s model visualizes a collision between Earth and a Mars-sized body sometime in the first 100 million years after the solar system took shape. The glancing blow vaporized a large amount of Earth’s mantle while imparting enough energy to the planet to start its spinning motion.

The ejected material spewed into space where it cooled and eventually coalesced into the materials that now make up the moon. The end result was a system in which Earth and the moon were much closer together than today, Earth spinning much more rapidly because of the blow it had absorbed.

Canup’s computer model answers many of the questions associated with the collision theory and, far from being unlikely, she says, collisions could have been fairly common in the early stages of solar system formation.

One of the criteria for the collision theory is that the moon once was much nearer, and Earth was spinning faster, than today.

Charles Sonett and Erik Kvale say this was the case in the July 5, 1996, issue of Science. Studies of sedimentary rock dating back 900 million years show that the moon was more than 10 percent closer at that time and that the length of Earth’s day was only 18 hours.

They determined this from layers in the rock marking the time between successive neap tides. Today, by reflecting laser beams from reflectors left on the lunar surface, it is known that the moon is moving away at a rate of 3.8 centimeters per year.

During the period Sonett and Kvale studied, the rate was 4.3 centimeters per year. Earth is slowing, and the moon receding, due to gravitational tidal drag.

Fredrick Jueneman, in the July 1982 issue of Industrial Research & Development, offers another, beautifully elegant way of proving that the days and months are growing longer. The shells of a mollusk named the chambered nautilus exhibit daily growth lines and septums, or chambers, built monthly. A count of growth lines between septums gives the number of days per month at the time the animal was building its shell. Jueneman says that one shell from about 420 million years ago exhibited only nine growth lines per chamber. This nine-day per month time frame would be equivalent to a 21-hour day when the moon would have been only 146,000 kilometers away compared to today’s average distance of 384,000 kilometers. Data from various sources differ but they all indicate that the moon once was much closer to us than today.

Clair Wood taught chemistry and physics for more than 10 years at Eastern Maine Technical College in Bangor.