Something besides volcanic eruptions and asteroid and comet impacts has sculpted the surface of Mercury -- an unknown process, possibly still going on today, that causes the ground to swell from the inside out.
The evidence, collected by a NASA spacecraft orbiting the innermost planet, is scattered all over Mercury, including a dramatic finding that half of the floor of the biggest crater on the planet has been raised above the walls.
The topographical mishmash is a visible manifestation of a much deeper mystery. New studies show Mercury's massive iron core is even bigger than expected, with a previously unknown solid layer of iron-sulfide buried beneath the mantle.
"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.
"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.
"Directions that used to be level or downhill are far from level or are no longer downhill. Those changes have to be coming from inside Mercury," Solomon told Discovery News.
"The magnitude and the widespread nature of these changes is a big surprise. If we can figure out how to read these changes, we will learn a lot more about how the interior of Mercury operated," he said.
The research was presented Wednesday at the Lunar and Planetary Science Conference in Houston and will be published in this week's Science.
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