NASA scientist Jennifer Heldmann describes the most popular theory of how the solar system and Earth’s moon was formed. Below you can watch a short four minute video of her explanation of the accretion theory, see a computer simulation of the hypothesis, or watch the whole 45 minute video as recorded during the “Ask a Scientist” event in San Francisco, CA, on Oct 7th, 2008.
There may be much more water on the moon than we thought. And that could change everything.
How did the moon become the moon? Where did it come from? How did it first form?
We don’t know in any fully satisfying way, but we do have a compelling theoryin the form of the giant impact hypothesis. Per the theory, early in Earth’s history — when Earth was still, technically, proto-Earth — an object the size of Mars slammed into the planet. The impact of the collision generated a ring of debris — and the planetary detritus coalesced slowly (very slowly: over the course of millions of years) into the gray, glowing spheroid that has since been a source of fascination to scientists and poets alike.
The giant impact hypothesis, as its name suggests, is just that: a hypothesis, a theory, an educated guess. Some of the evidence for it comes from computer simulations suggesting how Proto-Earth could become Proto-Earth Plus Moon. Some of the remaining evidence, though, comes from the unique geological properties of the moon: dusty, dry, aggressively barren. Which means that the rocks of the moon, per the giant impact theory, hold some of the answers to the mystery of its creation. One of the assumptions made by the theory is that the impact itself would have essentially de-gassed the proto-moon, dispersing, among other gases, hydrogen — the element, along with oxygen, required to form water... Here’s the problem, though: A string of recent research has suggested that there’s more water on the moon than we previously thought. (Though liquid water can’t persist on the moon’s surface, scientists can search for “water” in the form of ice, and also in the form of hydrogen and oxygen atoms extant in the lunar landscape.) In 2008, Space Ref notes, an analysis of Apollo lunar samples conducted by ion microprobe detected indigenous hydrogen in the moon rocks. And in 2009, NASA’s Lunar Crater Observation and Sensing Satellite slammed into a permanently shadowed lunar crater, ejecting a plume of material that ended up being rich in, yep, water ice. Hydroxyls — chemical functional groups consisting of one atom of hydrogen and one of oxygen — have also been detected in other volcanic rocks and in the lunar regolith, the layer of fine powder that coats the moon’s surface... The team found about six parts per million of water — quantities large enough to suggest that the water didn’t just come from elements in errant comets or asteroids. “The surprise discovery of this work is that in lunar rocks, even in nominally water-free minerals such as plagioclase feldspar, the water content can be detected,” researcher Youxue Zhang put it. And given the age of the sample and the amount of hydroxyl discovered, the scientists say, the water-forming elements must have been on the moon since its formation. “Because these are some of the oldest rocks from the moon, the water is inferred to have been in the moon when it formed,” Zhang told TG Daily.
The team’s findings — “Water in lunar anorthosites and evidence for a wet early Moon” — were published this weekend in the journal Nature Geoscience.
.. None of that means the impact formation theory is invalid — that the moon theory is moot. It means, though, that the giant impact theory now has another layer of complexity, in the form of evidence that would seem to contradict it. We’ve long known of the presence of water — and of water-forming elements — on the moon; the question is how it got there. Previous research suggested that those elements might have been brought to the moon from outside sources like comets and meteorites after the moon’s crust formed and cooled. But evidence of water-forming elements in lunar-native rocks complicates that theory. “I still think the impact scenario is the best formation scenario for the moon,” study leader Hejiu Hui, an engineering researcher at the University of Notre Dame, noted, “but we need to reconcile the theory of hydrogen.”
The giant-impact hypothesis, sometimes called the Big Splash, or the Theia Impact suggests that the Moon formed out of the debris left over from a collision between Earth and an astronomical body the size of Mars, approximately 4.5 billion years ago, in the Hadean eon; about 20 to 100 million years after the Solar System coalesced. The colliding body is sometimes called Theia, from the name of the mythical Greek Titan who was the mother of Selene, the goddess of the Moon. Analysis of lunar rocks, published in a 2016 report, suggests that the impact may have been a direct hit, causing a thorough mixing of both parent bodies.
- Earth’s spin and the Moon’s orbit have similar orientations.
- Moon samples indicate that the Moon’s surface was once molten.
- The Moon has a relatively small iron core.
- The Moon has a lower density than Earth.
- There is evidence in other star systems of similar collisions, resulting in debris disks.
- Giant collisions are consistent with the leading theories of the formation of the Solar System.
- The stable-isotope ratios of lunar and terrestrial rock are identical, implying a common origin.
However, there remain several questions concerning the best current models of the giant-impact hypothesis. The energy of such a giant impact is predicted to have heated Earth to produce a global magma ocean, and evidence of the resultant planetary differentiation of the heavier material sinking into Earth’s mantle has been documented. However, as of 2015 there is no self-consistent model that starts with the giant-impact event and follows the evolution of the debris into a single moon. Other remaining questions include when the Moon lost its share of volatile elements and why Venus—which experienced giant impacts during its formation—does not host a similar moon.
The origin of the Moon is usually explained by a Mars-sized body striking 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, Synestia), 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 ratio (50Ti/47Ti) 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.