The moon, a natural satellite of the earth, was formed at approximately the same time as the earth, roughly 4.6 billion years ago. The origin of the moon remains the subject of debate; differing theories have been advanced, each with its merits and defects. Scientists have created a lunar geological timescale and used radiometric dating methods to aid in estimating the age of the moon.
Theories explaining the formation of the moon include the fission theory, capture theory, cocreation, and collision-ejection theory. The fission theory states the moon was a piece of the earth that broke away in an early stage of the planet’s development. The circumstances for this event to have occurred are considered somewhat implausible.
The capture theory states that the moon was an asteroid, or another similar spatial entity, that was pulled into orbit by the gravitational field of the earth. This theory is improbable given the size of the moon and the gravitational force needed to trap it.
The cocreation theory states the moon accumulated from the debris orbiting the earth as it, too, was forming. This theory succeeds in explaining the similar ages of the two bodies, but it fails to explain the difference in composition between the Earth and the moon.
The collision-ejection theory is the favored theory; it combines aspects of the different theories into one unified theory. It theorizes that an asteroid, possibly the size of Mars, collided with the earth, ejecting debris from both bodies and merging the fragments into one. The fragments fused over time while captured in an orbit around the earth. This theory is fairly consistent with the known facts about the moon.
The moon has its own geologic timescale consisting of five periods. Unlike the earth’s geologic timescale, there are no subdivisions of these periods due to the insufficient amount of information available. Two of the major reasons for this lack of information are the limited accessibility of the moon and the nonexistence of fossils, which helped to detail the geological timescale of the Earth. A lot of what we know about the moon is through distant observations and rock samples returned from numerous lunar missions.
Using relative and radiometric dating methods, scientists have been able to determine that the moon sustained a period of bombardments after the surface solidified. This accounts for the vast number of craters across the moon’s highlands. Toward the end of this period, known as the Nectarian period, asteroids 100 kilometers in diameter collided with the moon, leaving enormous craters. These craters were later filled with basaltic lava during the next 500 million years, also known as the Imbrian period. Early earthbound observers believed these lava-filled craters, long since solidified, to be large bodies of water and dubbed them maria, or seas.
Scientists tested the rock samples from lunar missions using radiometric dating methods. The age of these rocks varies between samples from the older highlands and the younger maria. The samples range from 3 to 4.5 billion years old. The older rocks confirm that the moon was formed when the earth was in a primordial state.
Mat T. Wilson
See also Earth, Age of; Eclipses; Moon, Phases of;
Nebular Hypothesis; Satellites, Artificial and Natural;
Universe, Age of
Dalírymple, G. B. (2004). Ancient earth, ancient sky: The age of earth and its cosmic surroundings. Palo Alto, CA: Stanford University Press.
Nicolson, I. (1999). Unfolding our universe. Cambridge,
UK: Cambridge University Press.