Hot on the Jim Marrs’ post “Who Parked Our Moon” that resurfaced last week, now the highly respected science journal Nature gets in on the act of questioning just what the deal is with our moon. Author Robin Canup, associate vice-president of the Planetary Science Directorate of Southwest Research Institute, Boulder, Colorado, says current theories on the formation of the Moon owe too much to cosmic coincidences:
The Moon is more than just a familiar sight in our skies. It dictates conditions on Earth. The Moon is large enough to stabilize our planet’s rotation, holding Earth’s polar axis steady to within a few degrees. Without it, the current Earth’s tilt would vary chaotically by tens of degrees. Such large variations might not preclude life, but would lead to a vastly different climate.
Knowing how the Moon was made is central to understanding Earth and the formation of other planets. Since the 1980s, work on lunar origins has focused on the ‘giant-impact’ theory. This proposes that the collision of another planet-sized body with the forming Earth generated a disk of debris that coalesced into the Moon. Such giant collisions were common in the Solar System during the final stages of Earth’s formation 4.5 billion years ago.
But we still do not understand in detail how an impact could have produced our Earth and Moon. In the past few years, computer simulations, isotope analyses of rocks and data from lunar missions have raised the possibility of new mechanisms to explain the observed characteristics of the Earth–Moon system.
The main challenge is to simultaneously account for the pair’s dynamics — in particular, the total angular momentum contained in the Moon’s orbit and Earth’s 24-hour day — while also reconciling their many compositional similarities and few key differences. The collision of a large impactor with Earth can supply the needed angular momentum, but it also creates a disk of material derived largely from the impactor. If the infalling body had a different composition from Earth, as seems probable given that most objects in the inner Solar System do, then why is the composition of the Moon so similar to the outer portions of our planet?
So far, the solutions proposed appeal to extra processes — such as extensive mixing of materials from both bodies or a later gravitational resonance with the Sun — the feasibility of which are unclear. Planetary scientists need to double modelling efforts of the Earth–Moon system and identify chemical signatures in lunar and terrestrial rocks that could rule out some scenarios or suggest alternatives.
More alike than different
There are clear differences in the compositions of the Moon and Earth. Earth’s core is rich in iron, which comprises about 30% of the planet’s mass. By contrast, iron contributes less than 10% to the mass of the Moon. The Moon is also less rich in elements that vaporize readily, such as potassium, suggesting that they may have boiled off and been lost as the Moon formed from the hot disk…
[continues at Nature]
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