What made Mercury the 'iron planet'? Researchers say collision with Earth-sized planet led to massive metal core
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It has baffled boffins with its metal rich core.
Now, researchers say they have solved the mystery of how Mercury became the 'iron planet' - with its massive core making up 65% of its mass.
They key was a giant interplanetary 'hit and run' with either Earth or Venus.
New simulations show that Mercury and other unusually metal-rich objects in the solar system may be relics left behind by hit-and-run collisions in the early solar system with Earth or Venus.
MERCURY: THE MINI METAL PLANET
Mercury is the smallest and closest to the Sun of the eight planets in the Solar System, with an orbital period of about 88 Earth days.
Unlike Earth, which as several plates of crust, Mercury has just one rigid, rocky layer which bears telltale cliffs and chasms caused by global contraction.
The small, innermost planet has more than twice the fraction of metallic iron of any other terrestrial planet.
Its iron core makes up about 65 percent of Mercury's total mass; Earth's core, by comparison, is just 32 percent of its mass.
According to a study published online in Nature Geoscience July 6, Mercury and other unusually metal-rich objects in the solar system may be relics left behind by collisions in the early solar system that built the other planets.
Mercury, the small, innermost planet has more than twice the fraction of metallic iron of any other terrestrial planet.
Its iron core makes up about 65 percent of Mercury's total mass; Earth's core, by comparison, is just 32 percent of its mass.
Arizona State University professor Erik Asphaug said Mercury may have lost half its mantle in a grazing blow into a larger planet (proto-Venus or proto-Earth).
One or more hit-and-run collisions could have potentially stripped away proto-Mercury's mantle without an intense shock, thy say, leaving behind a mostly-iron body.
Mercuryfrom the Messenger spacecraft. The bright crater just south of the center of the image is Kuiper, identified on images from the Mariner 10 mission in the 1970s.
Asphaug and Andreas Reufer of the University of Bern developed a statistical scenario for how planets merge and grow based on the common notion that Mars and Mercury are the last two relics of an original population of maybe 20 bodies that mostly accreted to form Venus and Earth.
These last two planets lucked out.
'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,' explains Asphaug.
The main bodies of the solar system, the Sun, Mercury, Venus, the Earth, from left in foreground, Uranus, Neptune, Saturn, Jupiter and Mars, from left in background. The Moon, the Earth's natural satellite, is seen at right in foreground, as the relative size of the orbits of the planets is not respected.
'It's like landing heads two or three times in a row – lucky, but not crazy lucky.
'In fact, about one in 10 lucky.'
'The surprising result we have shown is that hit-and-run relics not only can exist in rare cases, but that survivors of repeated hit-and-run incidents can dominate the surviving population.
'That is, the average unaccreted body will have been subject to more than one hit-and-run collision,' explains Asphaug.
'We propose one or two of these hit-and-run collisions can explain Mercury's massive metallic core and very thin rocky mantle.'
According to Reufer, who performed the computer modeling for the study, 'Giant collisions put the final touches on our planets.
'Only recently have we started to understand how profound and deep those final touches can be.
'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.'
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