|
The next time you enjoy gazing at the full Moon, you might want to consider just how remarkable our moon truly is.
Of all the moons in the Solar System, only ours substantially affects its host planet. As a fraction of its host planet’s mass, our Moon is 50 times greater than any other moon. The gravitational acceleration it exerts on its host is also 50 times greater than any other moon.
Our Moon stabilizes Earth’s rotational axis, and the tides it creates on Earth are thought to have facilitated the origin of life.
So how did our remarkable Moon come to be? Scientists are still debating.
Some Early Ideas:
Capture
One possibility is that the Moon formed elsewhere in our Solar System, and later happened to venture close enough to be captured by our planet.
But capturing a passing body isn’t easy in a system governed by gravity. Nearly all close encounters result in either a collision or an exchange of momentum that deflects both bodies from their initial trajectories. The latter occurs because objects approaching Earth from far away have enough energy to escape Earth’s gravity. To be captured, something other than Earth’s gravity must slow down the smaller object. While not impossible, this seems highly unlikely.
Additionally, everything in our Solar System formed from the coalescence of a primordial disk of gas and dust that was inhomogeneous — concentrations of different types of molecules and atoms was quite different in different places. If the Moon formed far from Earth’s orbit, its composition would likely be different from Earth’s. But we now know that Earth and the Moon have nearly identical concentrations of several key elements and isotopes. Caltech researchers have shown that the likelihood of another Solar System body matching Earth’s isotopic signature is less than 1%.
Fission
George Darwin (son of Charles) proposed that Earth initially was spinning so fast that a large piece tore loose from Earth’s surface, flew out into space, and became our Moon. Some said the Pacific Ocean is a scar of this fission. They also claimed this event initiated Earth’s continental drift.
But we now know that the basin of the Pacific Ocean is only about 200 million years old, whereas radioisotope dating of zircon fragments collected by Apollo 14 astronauts shows the Moon is 4.51 billion years old.
Accretion
Another suggestion was that the Earth and Moon formed together from the primordial dust and gas — a big brother and a little sister. But this idea cannot explain the high angular momentum of the Moon’s orbit around Earth, nor why the Moon has so much less iron than Earth.
Giant Impact
A theory that gained general acceptance, proposed principally by Dr. Robin Canup of the Southwest Research Institute, says Earth was struck in a glancing blow by a body the size of Mars. The collider was named Theia, who was the mother of Moon goddess Selene in Greek mythology. Much of Theia and some of Earth’s upper layers were thrown off into nearby space, and later coalesced forming the Moon.
Importantly, Theia’s iron core fell into Earth’s core, the bottom of the local gravitational potential well. This explains why Earth has much more iron than the other terrestrial planets. Its massive iron core gives Earth a strong magnetic field that protects our atmosphere, thereby enabling life.
The collision might have produced temperatures of up to 18,000ºF, and melted or vaporized trillions of tons of rock. It also tilted Earth’s rotation axis, creating our seasons.
This hypothesis requires a collision between a target about 90% of the present size of Earth, and an impactor half Earth’s size and 10% of its mass. These ratios are needed to provide sufficient mass and angular momentum to form our Moon.
As material ejected in the collision fell inward upon itself, forming the Moon, enough energy was released to melt much of the nascent Moon’s surface, explaining its lack of volatiles (low-mass atoms and molecules).
Scientists believe the Moon initially orbited at 10% of its current distance, and gradually spiraled outward as tidal forces transferred angular momentum from the rotations of both bodies to the Moon's orbital motion. The Moon also became tidally-locked to Earth, with one side always facing Earth.
Although the giant impact hypothesis explains much, some unresolved problems remain.
The Earth and Moon have very similar oxygen and tungsten isotopic ratios, and their titanium isotopic ratios differ by only 4 parts per million. These facts suggest the Moon contains very little of any body that formed independently of Earth.
Also, computer simulations show that a giant impact of the right type occurs in 8% or less of all encounters, whereas lesser impacts are much more frequent.
Dual Impact
To help resolve these problems, Canup published a new theory in 2012 that has two bodies, each five times the size of Mars, colliding and forming a debris disk that later coalesced into the Earth and Moon.
Mini-Moons Pre-Giant Impact
Another idea is that Earth initially had one or more small moons that shared its composition. Following a giant impact, a more massive moon formed closer to Earth and then spiraled outward, merging with the smaller moons. (More massive satellites have more tidal effect, and spiral outward faster.) The resulting Moon became covered with material with the Earth-like composition of the original moons.
Many Lesser Impacts
In 2017, scientists at Israel’s Weizmann Institute suggested Earth was repeatedly hammered over millions of years, with the ejected material forming many small moons that eventually merged into our Moon. They estimate substantial impactors hit Earth every few million years, while collision ejecta could coalescence in 100 years or less. Our Moon may have formed from a series of small moonlets that ultimately merged. This early heavy bombardment era must have predated the so-called late heavy bombardment period (4.1 to 3.8 billion years ago), because the Moon bears scars of that latter period. Since then, Earth has been impacted much less often.
)
So many possible answers!
All this is enough to make one loony.
Maybe the real answer is a blend of several ideas.
|
