The events that led to the formation of the moon may not have been as improbable as previously thought, argue two papers in the same edition of Nature. However, the papers resolve the puzzle in very different ways.
Either solution would be something of a relief to astrophysicists, who until now have relied on an explanation generally admitted to be quite a stretch. It is also good news in the search for extraterrestrial intelligence, since several theories propose that a large moon was essential for advanced life on Earth.
The favored theory of lunar formation requires several things, none of them impossible, but cumulatively challenging. Dr. Alessandra Mastrobuono-Battisti of the Israel Institute of Technology has tackled one of these: the question of why the moon and Earth have such similar chemical compositions.
As astronomers began to reveal the solar system, they inevitably noticed how unusual the moon is. Other satellites in the inner solar system are tiny. Some gas giants have large moons, but these objects are all much, much smaller in comparison to the planets they orbit.
While various theories about the moon's origins have come and gone, the only one that seems to explain what we can observe is that an object, named Theia, roughly the size of Mars slammed into the Earth early in its development. The impact was so great that huge amounts of rock from both Theia and the proto-Earth's crust were thrown into orbit. Some eventually returned to the planet, but most coalesced to become the moon.
The problem, however, had been that the conditions required for such an event appeared unlikely. While big objects slamming into each other was pretty much the order of the day at this point in the solar system's history, models suggested that the collision required a planetoid of just the right size, moving at just the right speed and angle to produce something like the moon as the outcome. Moreover, Theia had to have a sufficiently similar chemical composition to the proto-Earth that we can't detect the difference in moon rocks. Some estimates put the chances of such an event at 1%, which expert on the topic Dr. Robin Cannup of the Southwest Research Institute describes as “uncomfortably rare” in a review article.
The Mastrouono-Battisti still thinks we are lucky to have the moon, but put the chances at a more comfortable 20-40%. “We track the feeding zones of growing planets in a suite of simulations of planetary accretion,” the paper reports, “in order to measure the composition of Moon-forming impactors. We find that different planets formed in the same simulation have distinct compositions, but the compositions of giant impactors are statistically more similar to the planets they impact.”
The object that collided with Earth was almost certainly formed at a similar distance from the sun as us. Consequently, Mastrobuono-Battisti and her co-authors argue, it is not so surprising it resembled Earth much more than Venus or Mars, which were formed at different distances.
In the same edition of Nature, however, researchers from the University of Maryland present evidence that they claim indicates that Theia did not have the same composition as Earth. Instead, they think that the material from Theia and Earth mixed so thoroughly that the Earth's mantle and the moon each received a similar portion, explaining their resemblance.