MESSENGER, the first spacecraft ever to orbit Mercury, continued returning data through November 2011 that upended assumptions about the solar system's innermost planet. Scientists found evidence of extensive ancient volcanism and a magnetic field oddly offset from the planet's center, findings that reshaped planetary science's understanding of how rocky worlds form and evolve.
Mercury had been visited only briefly before, by Mariner 10 in the 1970s, which managed to photograph less than half the planet's surface during three flybys. MESSENGER, launched in 2004 after a circuitous seven-year journey using gravity assists to shed enough speed to be captured by the sun's innermost major planet, finally achieved orbit in March 2011.
By November, the mission was producing a flood of findings. Volcanic plains far more extensive than expected suggested Mercury had been geologically active for far longer than models predicted. Its magnetic field, generated by a still mysterious internal dynamo, was found to be significantly offset toward the planet's north, an asymmetry that puzzled scientists trying to model the planet's molten core.
MESSENGER's data eventually included the discovery of water ice in permanently shadowed polar craters, remarkable given daytime temperatures elsewhere on the planet exceed 800 degrees Fahrenheit, showing Mercury to be stranger and more dynamic than its reputation as a dead, scorched rock had suggested.