APOD: GRAIL Maps the Moons Gravity (2013 Mar 19)

[neufer]

A simple question: If a major object collided with the earth which broke off the moon from the earth, it seems logical that some of the object that hit the earth should still be present. Do we have any physical evidence? Is the composition different from the earth? Or, is it now impossible to differentiate?

Computer simulations suggest that Earth rocks & Moon rocks should be somewhat different amalgamations of the mantle of that object [i.e., Theia] and of the original Earth. However there is little evidence that Moon rocks & Earth rocks significantly differ.

<<Theia is thought to have struck the Earth at an oblique angle when the latter was nearly fully formed. Computer simulations of this "late-impact" scenario suggest an impact angle of about 45° and an initial impactor velocity below 4 km/s. Theia's iron core would have sunk into the young Earth's core, and most of Theia's mantle accreted onto the Earth's mantle, however, a significant portion of the mantle material from both Theia and the Earth would have been ejected into orbit around the Earth. This material quickly coalesced into the Moon (possibly within less than a month, but in no more than a century). Estimates based on computer simulations of such an event suggest that some twenty percent of the original mass of Theia would have ended up as an orbiting ring of debris, and about half of this matter coalesced into the Moon.

The Giant impact hypothesis does not explain well the similar composition of Earth and the Moon. Especially, the indistinguishable relation of oxygen isotopes cannot be explained by the classical form of this hypothesis. According to research on the subject that is based on new simulations at the ETH Zürich by physicist Andreas Reufer [no relation - Neufer] and his colleagues, Theia collided directly with Earth instead of barely swiping it. The collision speed may have been higher than originally assumed, and this higher velocity may have totally destroyed Theia. According to this modification, the composition of Theia is not so restricted, making also a composition of up to 50% water ice possible.>>

[stephen63]

Perhaps there was no Theia at all.
Alternative hypothesis: http://www.lpi.usra.edu/meetings/lpsc2012/pdf/1738.pdf