The Earth’s moon may be a present from Venus, which once had a moon and then lost it, a new theory suggests. Under the theory, Earth’s gravity captured Venus’ old moon, giving our planet its big natural satellite.
This idea contrasts to the thinking of the vast majority of moon researchers, who believe that the Earth’s moon formed some 4.5 billion years ago when a planet-size body slammed into nascent Earth at high speed.
This giant impact hypothesis, however, has its own issues, as did all the alternative moon formation theories discussed this week at the Origin of the Moon conference at the Royal Society here.
“I think part of the key to [understanding] the moon may be that Venus has no moon, and we certainly have to study it (Venus) more,” said Dave Stevenson, professor of planetary science at Caltech University, who proposed the Venus idea at the conference. In an interview with SPACE.com after his presentation, Stevenson said that he himself favored the impact theory on moon formation, but unfortunately this theory did not yet answer all the questions.
How did Earth get its moon?
The “moon capture” theory assumes that Earth used its gravitational pull to attract a pre-formed space body into its orbit, thus making a satellite of this object. [How the Moon Formed: A Lunar Tour (Video)]
However, the geochemical composition of the moon and Earth likely trips up this theory. Analyses of the lunar rocks brought back by NASA’s Apollo moon landing missions have shown that the satellite has an isotopic composition very similar to that of Earth.
Isotopes refer to varieties of chemical elements that have the same number of protons, but different numbers of neutrons. Two isotopes behave the same chemically.
And if both moon and Earth have very similar isotopes, it makes the capture theory difficult to maintain, said Alex Halliday, head of science at Oxford University. Such isotopic similarities suggest that “the material that makes up the moon did actually either come out of the Earth, or that the stuff that was in the disk that formed the moon got completely mixed up with the stuff in the Earth.”
Nonetheless, some aspects of the idea that the moon may have come from Venus are intriguing, he said.
“The reason why it’s interesting is that Earth and Venus are close to each other. They have similar mass, and people think they have probably formed in a similar way,” he said. “So the question is, if Earth and Venus formed in similar ways, how come the Earth has a Moon and Venus doesn’t?”
Dr. Stevenson’s idea would answer that question, Halliday said, “throwing a new twist into the whole capture theory.”
There are many theories for what might have caused such a large moon for a planet as small as Earth. The most popular theory assumes an impact, where the debris of the collision — a mix of the material from Earth and the other body — gave birth to the moon. This body then stayed in orbit about the Earth, forever bound to its new home.
Another posits that the moon “fissioned” from the Earth’s crust and mantle due to the centrifugal force of a rapidly spinning early Earth.
Another theory, called binary accretion, assumes that the moon was born at the same time and place as Earth.
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