Two ways to make the moon
The composition of the moon and the earth are so similar that most planetologists now beleive that the moon must have formed from ejection of material from the earth. This is thought to be caused by the collision of a different planet with the proto-earth. Many problems arise in this scenario. This week there are two possible detailed answers to how the moon was made.
Science published a paper by Robin Canup from the University of Boulder, Colorado which says:
In the giant impact theory, the Moon forms from debris ejected into an Earth-orbiting disk by the collision of a large planet with the early Earth. Prior impact simulations predict that much of the disk material originates from the impacting planet. However, the Earth and Moon have essentially identical oxygen isotope compositions. This has been a challenge for the impact theory, because the impactor’s composition would have likely differed from that of the Earth. Here, we simulate impacts involving larger impactors than previously considered. We show that these can produce a disk with the same composition as the planet’s mantle, consistent with Earth-Moon compositional similarities. Such impacts require subsequent removal of angular momentum from the Earth-Moon system through a resonance with the Sun, as recently proposed.
The figure on the right shows that after the initial impact, the planets re-collided, merged, and spun rapidly. Their iron cores migrated to the center, while the merged structure developed a bar-type mode and spiral arms (24). The arms wrapped up and finally dispersed to form a disk containing about 3 lunar masses whose silicate composition differed from that of the final planet by less than 1%.
Back-to-back with this in Science is an article by Matija Cuk and Sarah Stewart of Harvard University, Massachusetts, which claims:
A common origin for the Moon and Earth is required by their identical isotopic composition. However, simulations of the current giant impact hypothesis for Moon formation find that most lunar material originated from the impactor, which should have had a different isotopic signature. Previous Moon-formation studies assumed that the angular momentum after the impact was similar to the present day; however, Earth-mass planets are expected to have higher spin rates at the end of accretion. Here, we show that typical last giant impacts onto a fast-spinning proto-Earth can produce a Moon-forming disk derived primarily from Earth’s mantle. Furthermore, we find that a faster-spinning early Earth-Moon system can lose angular momentum and reach the present state through an orbital resonance between the Sun and Moon.
This figure shows an example of an impact of a light planet with a proto-Earth spinning with a period of 2.3 hours. The colors denote the silicate mantles and iron cores of the Earth and impactor. The disk is dominated by material originating from Earth’s mantle near the impact site. After material is ejected from the earth, it condenses into a moon. The high speed of rotation (technically, angular momentum) is transferred to the sun through a mechanism known as an orbital resonance.
The main difference between the two mechanisms is that the first requires a very large mass impactor, whereas the second can make do with a light impactor.