Mercury, the innermost planet of our solar system, has always been an oddball. It has an unusually large iron-rich core, a silicate mantle, and not much of a crust. It’s the only one of the terrestrial planets with no atmosphere to speak of, and is tidally locked—it rotates three times on its axis for every two orbits around the Sun.
Life on Mercury would be all but impossible, but it’s still an intriguing place to study. NASA’s MESSENGER spacecraft was sent there in 2004, and has been returning intriguing data ever since it went into orbit around the planet in 2011. Timothy Goudge and his colleagues have just published some astounding results based on their analysis of volcanic deposits seen by MESSENGER.
Volcanism appears to have continued on Mercury for longer than we had suspected—perhaps from the formation of the planet to about a billion years ago. The new finding begs another question: Where did Mercury actually form?
We know that at least some of the giant gas planets of the outer solar system did not originally form where they are located today. Could Mercury have moved to its current location as well? The thinness of the crust has always suggested something odd in the planet’s early life. One possibility is that a major impact devastated Mercury and stripped most of its crust away.
Mercury also has a very eccentric (less circular) orbit, which usually indicates that it might have been captured into its current orbit. If this interpretation is correct, it may be one of the millions of “runaway” planets we think exists between the stars in our galaxy. In this scenario Mercury wandered in to our solar system, and was forced by gravity to remain here. But where? Interestingly, it has about the same overall density as Earth. Could Mercury have possibly formed in the vicinity of Earth?
It is generally assumed that a Mars-sized object collided with the early Earth, and that our Moon formed from a combination of Earth’s mantle material and material from the impactor. Could that impactor possibly have been early Mercury, which then lost most of its crust to the Moon? It’s only a wild speculation, but it might bear looking into. Fortuntely, MESSENGER is still going strong, and another mission called BepiColombo, a joint venture of the European and Japanese space agencies, is due to reach Mercury in 2024.