Despite remaining just an ominous floating object in the sky, the moon has played a large role in human history. One of the few things reliably visible during a cloudless night, the moon was enshrouded in mystery for much of history. For a period, it was revered as a reflection of gods and goddesses, and then it was viewed as another planet with seas and land. Eventually, Galileo and his telescope got a more modern picture of what we know the moon to be today: a natural satellite with mountains, craters and expanses of flat ground. In the 1960s, the moon became popular again as the Americans faced off against the Russians in the race to the moon, which ended with Neil Armstrong and Edwin “Buzz” Aldrin stepping foot on its surface in 1969. Even though the moon had been thoroughly studied and researched for many generations, it wasn’t until the mid-1970s that a moon-origin hypothesis caught traction.
The Impact Trigger Hypothesis: The Modern View
Dr. William K. Hartmann and Dr. Donald R. Davis, of the Planetary Sciences Institute in Tucson, Arizona, published a paper in 1975 which laid out a hypothesis that when the Earth was forming, 4.5 billion years ago, other planets were are well. When Earth was at the end of its growing period, a smaller planet collided with Earth, sending debris into the Earth’s orbit which eventually banded together to become the moon we know today. This is known as the Impact-Trigger Hypothesis.
Hartmann and Davis began their research and hypothesizing based on information collected by the Soviet Union in the 1960s, namely from astrophysicist V. S. Safronov. Safronov wrote “Evolution of the Protoplanetary Cloud and the Formation of the Earth and Planets” which was a building block for the work of Hartmann and Davis. After Hartmann and Davis first presented their hypothesis in the 1970s, two more scientists, William Ward and A. G. W. Cameron of Harvard, said they were also working on a similar theory which was attempting to prove that the Earth had been impacted by an object about a third the size of Mars which produced enough debris to make the moon. For the next ten years, the two pairs formulated theories on the size of the planet which struck Earth as well as when during Earth’s development it happened. During this time, the theory was heavily critiqued by peers. However, at a conference on the moon’s origin in Kailua-Kona, Hawaii in 1984, it became apparent the Impact Trigger hypothesis was the leading theory on the moon’s formation and has been since.
The Sister World Hypothesis
The Impact-Trigger Hypothesis took into account many aspects of the moons origin that earlier hypothesis did not explain. An early hypothesis claimed the moon originated as a “sister planet” which formed similarly to Earth and the other planets. However, this was disproven by the Impact-Trigger Hypothesis because of the density difference between moon and the Earth. The moon is only 1-3% iron, and therefore has a very light density. Contrasting this, the other planets in Earth’s solar system, as well as Earth itself, are about 30% iron. Because Hartmann and Davis hypothesis hinged on the idea the moon was constructed from debris off the outer mantle of the Earth. This explained why the moon does not contain much iron, since most of the Earth’s iron is at its core.
The Capture and Fission Theories
A couple of other outdated hypotheses are that the moon originated elsewhere in the solar system and was captured by the Earth’s orbit as it floated by or that the moon was formed from debris that spun off the Earth at a point in time when it rotated rapidly. These theories fell flat when a magma ocean was discovered on the moon. Based on calculations, scientists proved that the capture of the moon alone and Earth’s effects on the moon’s surface would not cause enough heat to produce a magma ocean. Under Impact-Trigger Hypothesis, the violence of the colliding debris from the Earth and the other unknown smaller planet, as well as extra debris the moon picked up over time in orbit, did produce enough of an impact to produce the previously unexplained oceans.
The “origin of the moon” debate has certainly not been laid to rest and many respected scientists continue researching and testing the Impact-Trigger Hypothesis as new technology, information and data become available. Dr. Robin Canup of SwRI Instrumentation and Space Research Division is well-known for her efforts today to continue researching how the moon and other planets formed. Discovering the origin of the moon will give us answers to many questions about not just the Earth’s past, but also humanities’ future. The moom plays a significant role in Earth’s ability to host human life. Without the moon’s gravitational pull on its surface, the Earth may rotate so wildly that extreme weather conditions would make it uninhabitable. As the science community continues to research Earth’s past to create a future full of opportunities, including possibly creating a life planet elsewhere, hypothesis on the moon’s origin will become more important than ever.