There are several problems with studying the planet Mercury. It's so close to the sun that space probes have to follow indirect routes in order to avoid being pulled by the sun's gravity. The journey also very hot, which requires very hardy spacecraft. So scientific probes are few and far between. But the data that we have about the tiny planet shows us that it may have once been much larger and much further from the sun than it is now. This is mainly evident in Mercury's core, which is unusually large for a small planet, covered with a small mantle and an extremely thin crust. Mercury's crust contains thorium, which should have been blown away by the heat of the early solar system -unless Mercury wasn't as close to the sun then as it is now.
Scientists have been studying the composition and the relative mineral makeup of Mercury from both the data gleaned by probes and by rare analogue minerals found on earth that may have come here from an event in Mercury's history. The hypothesis is that Mercury was once a much larger planet, and was knocked off its orbit by some event like a collision, then settled in as a remnant of itself in its current orbit close to the sun. Read how the evidence points to this possibility at BBC Future. -via Damn Interesting
(Image credit: A loose necktie)
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