How Did the Earth Get its Moon?

We, as inhabitants of planet Earth, take the presence of the Moon in the night sky for granted.  Yet, the evidence tells us that at one time in the Earth's 4 and ½ billion year history there was no satellite in Earth orbit.  The generally accepted explanation among astronomers and cosmologists is that at a time early in the Earth's history there was a collision between Earth and another large planet resulting in the ejection of massive amounts of material into Earth orbit forming a disk of debris.  It is further postulated that from this disk, the Moon was formed in a process not unlike the formation of the planets of our solar system around the sun.  This is referred to as the giant impact theory.

According to this paradigm, a low-velocity impact of a planetary body (impactor) roughly the size of Mars could produce an iron-poor debris disk with sufficient total mass and energy in the form of angular momentum – a measure of the rotation of a body that is the product of its inertia and angular velocity - to produce an iron-poor Moon.  In addition, this model also predicts that the debris disk would contain material primarily from the impactor's mantle.   This data is where the inconsistency lies; for, the Earth and Moon, in fact, share many similarities in regards to composition, including the isotopes of oxygen, chromium and titanium.  It is unlikely that any postulated impactor would share these similarities.

Dr. Robin M Canup from the Planetary Science Directorate at the Southwest Research Institute in Boulder, Colorado has proposed a solution to this apparent dilemma.  Canup has postulated through the use of sophisticated and computer-assisted simulations that if a larger-sized impactor than the one proposed in the giant impact theory was involved, then the resulting collision with Earth would produce a disk with the same composition as the Earth's mantle.  The actual size of the impactor used in these simulations was comparable in mass to the Earth.

This proposed scenario demonstrates just how chaotic and disruptive the environment of our solar system was during the early stages of its evolution.  The cosmos is, in fact, ever-changing in its past, its present and for the foreseeable future.