Moon born out of cosmic head-on: study
Hit-and-run A new twist in the theory of how the moon formed claims Earth was the victim of a cosmic hit and run, and the offender may still be out there.
Most scientists believe a Mars-sized planet collided with Earth about 4.5 billion years ago. The impact destroyed the impactor, causing most of its core to sink and merge with the Earth's core, while lighter materials were blasted into orbit to form the Moon.
However isotopic oxygen ratios on the lunar surface are identical to those on Earth, implying they came from the same source and not a planetary collision.
Now astronomers including Andreas Reufer from the University of Bern in Switzerland, have developed an alternative impact hypothesis to explain the identical isotopic readings.
Reufer and colleagues used computer simulations involving up to 550,000 virtual particles, to examine the dynamics of the impact under a range of different angles, speeds and planetary compositions.
In a paper to appear in the journal Icarus, and published on the pre-press website ArXiv.org, Reufer and colleagues claim the impactor collided with Earth at a higher speed and steeper angle, which would have heated debris to almost 10,000°C.
They claim this scenario could have created a mixture of material from the Earth and the impactor closely matching the isotopic signatures observed on the Moon.
The higher velocity impact, which the researchers dub a "hit-and-run collision", also results in some of the debris forming into an object that may still be floating around in the solar system.
Astronomer Dr Michael Ireland from Sydney's Macquarie University says the hypothesis is interesting because it allows the impactor to come from much further away than the more accepted model.
But he adds that either scenario would produce enough heat to mix material from the early Earth and its impactor.
"There were such high energies in this planetary collision anyway, that rocks melted and vaporised," says Ireland. "So you don't need the hit-and-run impact theory to achieve the temperatures and degree of mixing to explain the ratios on both worlds."