The origin of the Moon
is usually thought to be that a Mars
-sized body struck the Earth, making a debris ring that eventually collected into a single natural satellite
, the Moon
, but there are a number of variations on this giant-impact hypothesis
, as well as alternate explanations, and research into how the Moon came to be continues. Other proposed scenarios include captured body, fission, formed together (condensation theory), planetesimal
collisions (formed from asteroid-like bodies), and collision theories.
The standard giant-impact hypothesis suggests the Mars-sized body, called Theia
, impacted Earth, creating a large debris ring around Earth, which then accreted to form the Moon. This collision also resulted in the 23.5° tilted axis
of the earth, thus causing the seasons. The Moon's oxygen isotopic
ratios seem to be essentially identical to Earth's. Oxygen isotopic ratios, which may be measured very precisely, yield a unique and distinct signature for each solar system body. If Theia had been a separate protoplanet
, it probably would have had a different oxygen isotopic signature from Earth, as would the ejected mixed material. Also, the Moon's titanium isotope
Ti) appears so close to the Earth's (within 4 ppm) that little if any of the colliding body's mass could likely have been part of the Moon. Read more...