Member Since: 3/15/2013
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TIME DRAGS Mercury for clocking Earth, Venus and Mars
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Planets shouldn't be tiny and full of iron, but that's exactly how Mercury wound up. A very rough childhood may explain its strange state.
But Mercury doesn’t quite fit with the other rocky worlds, says Erik Asphaug, a planetary scientist at Arizona State University. “Most, including the Earth, have a composition that is about one-third metallic iron and two-thirds rock. Mercury is the other way around.”
As for Mercury, it saw plenty of action, but it would be the one that got hit mostly by glancing blows, with the outer layers getting stripped away and a successively smaller planet surviving. “The original Mercury,” Asphaug says, “might have been maybe three times the mass of present-day Mercury, but lost it rocky mantle when it impacted proto-Venus or proto-Earth.”
Not only is that the likely explanation, it’s the all-but statistically inevitable one. Given 20 Mars-size objects to begin with, says Asphaug, “you expect to end up with a repeatedly-stripped freak, a planetary core without its mantle.” Earth and Venus would have readily gobbled up some of what Mercury lost.
The same shooting-gallery period in the solar system’s history also explains the formation of the moon, which is Mercury’s compositional opposite, with lots of rock and very little iron. Once Earth had formed from the Mars-sized protoplanets that were whizzing about the solar system, it was slammed hard by one more of them—but instead of just adding to Earth’s already formidable bulk, this collision vaporized some of the impactor and some of the Earth’s outer layers.
More at http://time.com/2958731/mercury-peculiar-solar-system/
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Mercury, ha Paula era!
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