A giant "hit and run" impact with another planet billions of years in the past may explain the unexpected and mysterious internal makeup of Mercury, the solar system's closest planet to the sun, researchers say.
The mystery is that a small planet shouldn't have as much iron as has been observed on Mercury, they say. The Earth, Mars and Venus -- the solar system's other rocky worlds -- are about 30 percent iron.
So why is Mercury almost 60 percent iron?
The answer may be down to a particularly violent "childhood" in which Mercury was involved in a number of impacts that stripped away much of the materials other than iron that we would expect to see there, planetary scientists Erik Asphaug of the University of Arizona says in a study published in the journal Nature Geoscience.
While still a developing proto-planet, Mercury could have lost as much of its mantle, covering its iron-rich core, in a glancing hit-and-run collision with a bigger planet, possible the proto-Earth or proto-Venus, Asphaug and his study co-author Andreas Reufer say.
The majority of the material stripped off both bodies -- mostly lighter mantle materials -- will fall back onto the larger of the colliding bodies, leaving the smaller one more metal-rich, they say.
Asphaug and Reufer modeled the merging and growth of planets using a common assumption that holds that Mars and Mercury represent the last leftover relics of the original group of perhaps 20 solar system bodies that for the most part came together, or accreted, in a series of collisions that formed the Earth and Venus.
Mercury and Mars are lucky survivors of all that crashing and banging, they say.
"How did they luck out? Mars, by missing out on most of the action -- not colliding into any larger body since its formation -- and Mercury, by hitting the larger planets in a glancing blow each time, failing to accrete," Asphaug says.
'We propose one or two of these hit-and-run collisions can explain Mercury's massive metallic core and very thin rocky mantle," he says.
And where is Mercury's mantle -- or at least the material from it -- today?
We may be standing on it, Asphaug says, as the Earth (and Venus) would have readily been the winners in any cosmic fight to gobble up mantle material stripped from Mercury in a grazing collision.
"The implication of the dynamical scenario explains, at long last, where the 'missing mantle' of Mercury is -- it's on Venus or the Earth, the hit-and-run targets that won the sweep-up," Reufer says.
Given an initial 20 bodies careening and colliding in the early solar system, a planet such as Mercury is an almost foregone conclusion, he says.
In such a shooting-gallery environment, "you expect to end up with a repeatedly-stripped freak, a planetary core without its mantle," he explains.