Mercury was Born from Hit-and-Run Interplanetary Collisions

Mercury, Venus, Earth and Mars are known as the rocky planets, in contrast the Solar System’s gas giants-Jupiter, Saturn, Uranus and Neptune, writes Time. Yet Mercury doesn’t quite fit with the other rocky worlds, Erik Asphaug, a planetary scientist at Arizona State University, said in a statement.

Mercury’s unusual metal-rich composition has been a long-standing puzzle in planetary science. The origin of Mercury has been a difficult question in planetary science because its composition is very different from that of the other terrestrial planets and the moon.

This 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.

How do we get Venus, Earth and Mars to be mostly “chondritic” — having a more-or-less Earth-like bulk composition — while Mercury is such an anomaly? For Arizona State Univ.’s Prof. Erik Asphaug, understanding how such a planet accumulated from the dust, ice and gas in the early solar nebula is a key science question.

 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.

To explain the mystery of Mercury’s metal-rich composition, ASU’s Asphaug and Andreas Reufer of the Univ. of Bern have developed a new hypothesis involving hit-and-run collisions, where proto-Mercury loses 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, leaving behind a mostly-iron body and satisfying a number of the major puzzles of planetary formation – including the retention of volatiles – in a process that can also explain the absence of shock features in many of the mantle-stripped meteorites.

Mercury from 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.

‘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.

These simulations are of great relevance to meteoritics, which, just like Mercury’s missing mantle, faces questions like: Where’s all the stripped mantle rock that got removed from these early core-forming planetesimals? Where are the olivine meteorites that correspond to the dozens or hundreds of iron meteorite parent bodies?

“It’s not missing – it’s inside the mantles of the planets, ultimately,” explains Asphaug. “It got gobbled up by the larger growing planetary bodies in every hit-and-run series of encounters.”

The study appeared online in the journal Nature Geoscience.

Source: http://goo.gl/WxtJfO