"Figuring out an internal structure that was consistent with Mercury's gravity field was very challenging," planetary scientist Maria Zuber, with the Massachusetts Institute of Technology, told Discovery News.
"Our geochemistry colleagues kept sending us back to the showers saying 'Your gravity field can't be right because none of the internal structure models are fitting.' But we do now know that we got the gravity field right. It was very difficult."
And unprecedented. Scientists know of no other planet put together like Mercury, with a solid iron core surrounded by liquid iron topped by solid iron-sulfide. Together, the core now accounts for a whopping 85 percent of the planet's structure, leaving a relatively puny mantle and crust.
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"One of the things this has told us is that there are some unusual dynamics in the interior of Mercury going on that we haven't thought about before and that we don't understand," Zuber said.
"Mercury is showing us that there are other processes at work that are changing the topographic heights by kilometers in some instances," added Carnegie Institution planetary scientist Sean Solomon, lead researcher of the ongoing NASA MESSENGER mission.