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.[1] 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.[2] 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.[3]
The giant-impact hypothesis is currently the favored scientific hypothesis for the formation of the Moon.[4] Supporting evidence includes:
- Earth’s spin and the Moon’s orbit have similar orientations.[5]
- 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.[6]
However, there remain several questions concerning the best current models of the giant-impact hypothesis.[7] 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.[8] 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.
Origin of the 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.[1][2] Other proposed scenarios include captured body, fission, formed together (condensation theory, Synestia), planetesimal collisions (formed from asteroid-like bodies), and collision theories.[3]
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.[1] The Moon’s oxygen isotopic ratios seem to be essentially identical to Earth’s.[4] Oxygen isotopic ratios, which may be measured very precisely, yield a unique and distinct signature for each solar system body.[5] 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.[6] 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.[7]
When and How Did the Moon Form?
New studies offer contrasting scenarios for making the Moon. One argues for a one big splat early in solar-system history; a second envisions a score of lesser blows that built up the Moon over time; and a third suggests water was involved.
Given the trove of lunar samples in hand and the power of modern laboratory analyses, you’d think that by now geochemists should have completely nailed exactly how the Moon formed. But not so — in fact, there’s still lots of debate on how Earth formed.
Artwork of a Mars-sized object colliding into the Earth early in solar system history. Many planetary scientists believe that an impact such as this threw off the debris which eventually formed the Moon.
Lynette Cook / Getty ImagesHere’s the basic problem: about 30 years ago, dynamicists showed that a body roughly the mass of Mars could have struck Earth a glancing blow and ejected enough debris into orbit to collect into a Moon-size object. In virtually all of those simulations, most of what ends up in the Moon came from the impactor rather than from Earth.
Throwing Water on the Problem
As if the How and When of the Moon’s formation weren’t complicated enough, a third new analysis argues that — despite its extreme dryness today — the Moon likely contained a lotof water when it formed. In the same issue of Nature Geoscience, Yanhao Lin (Vrije Universiteit Amsterdam) and three others describe their experimental attempts to mimic how the Moon’s magma ocean solidified. Lower density minerals would have floated to the top, forming a crust.
They find that the suite of minerals found in the lunar crust today — combined with its thickness — argue that water was part of the mix at a concentration of 270 to 1,650 ppm. This might not seem like much — but if proven true there’d be significant implications.
“A wet start of the Moon, coupled with the strong similarities between the composition of the Moon and the composition of the silicate Earth,” Lin’s team concludes, “suggests that equally high concentrations of water were present in the Earth at the time of the Moon-forming event.”
Christ in Paul’s Eyes: A Bigger Story (Richard Rohr)
As a rule, Christians were more interested in the superiority of our own group or nation than we were in the wholeness of creation. Our view of reality was largely imperial, patriarchal, and dualistic. Things were seen as either for us or against us, and we were either winners or losers, totally good or totally bad—such a small self and its personal salvation remained Christianity’s overwhelming preoccupation up to now. This is surely how our religion became so focused on obedience and conformity, instead of on love in any practical or expanding sense.
Without a Shared and Big Story, all humans retreat into private individualism for a bit of sanity and safety.
Perhaps the primary example of Christians’ lack of attention to the Christ Mystery can be seen in the way we continue to pollute and ravage planet Earth, the very thing we all stand on and live from. Science now appears to love and respect physicality more than most religion does! No wonder that science and business have taken over as the major explainers of meaning for most people today (even many who still go to church). Christians did not take this world seriously, I am afraid, because our notion of God or salvation didn’t include or honor the physical universe. And now, I am afraid, the world does not take Christianity seriously.