So what can all this tell scientists about the origin of the moon? After all, the two bodies have significant differences: unlike the moon, Mercury has a global magnetic field; its density is higher; there is an abundance of volatile elements such as sulfur; its surface contains much less iron, particularly in its crustal silicates — which indicates that the processes deep inside both bodies have been very different.
But Solomon says we should look at the many commonalities of their geological evolution.
"It means that many aspects of the moon are common to rocky bodies that are similar in size despite being different in bulk composition," he said. "Therefore, we have to focus on those aspects of the moon that are special, if we want to gain traction on the question of what made the moon as opposed to what made the planets."
One stumbling block of the giant impact theory of moon formation has been the incredible isotopic similarities between the Earth and the moon.
Indeed, if a large object that hit the Earth was different from it, why then are the planet and its satellite so isotopically alike?
One thing that could really help solve this riddle, is a sample from Mercury — from a meteorite or a future mission, Solomon said.
If Mercury happens to be isotopically similar to Earth, it would mean that most of the material in the early disk spinning around the Sun shared isotopic characteristics — and that the objects that collided to grow the final stages of the Earth and created the moon started out with a similar composition.
However, such a scenario would leave an odd one out: Mars.
Mars is isotopically different from Earth. If Theia had had Mars' isotopic composition, the oxygen isotopes on Earth and moon would be quite distinct.
But if Mercury turns out to be similar to Earth, it could imply that there was some process that homogenized the isotopes of all planets between the sun and Mars — and this is "the big question right now," said Jay Melosh, Distinguished Professor of Earth, Atmospheric and Planetary Sciences at Purdue University.
"If there was [such a process] then Theia might have had a nearly identical isotopic composition to Earth and all our puzzles about why the Moon and Earth are so similar isotopically then evaporate," he added. "The process obviously did not affect Mars, but it did affect the planets closer to the sun."
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