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Credit: Hinode JAXA/NASA/PPARC
Mercury is notoriously difficult to study, thanks to its proximity to the scorching hot and blindingly bright sun. Thus, mysteries abound. For example, Mercury has a giant core — perhaps because its outer, lighter layers got brushed off by planetary collisions long ago, but scientists aren't sure. It also has a magnetic field and an atmosphere, both of unknown origin. In fact, the little planet leaks a steady stream of atmospheric particles, suggesting its atmosphere is somehow constantly regenerated. The biggest boggler of all: Mercury's highly elongated orbit is growing more oval-shaped all the time, and it could someday crash into Venus or the sun. Will its changing path (and resultant changing gravitational field) disrupt the orbits of Earth and the other inner planets, causing chaos?