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New images of filled-in craters — one the size of the Baltimore-Washington area and filled in with more than a mile deep of cooled lava — show that 3.8 to 4 billion years ago, Mercury was more of a volcanic hotspot than the moon ever was, Zuber said.
But it isn't just filled-in craters. Using special cameras, the probe showed what one scientist called "the mysterious dark blue material."
It was all over the planet.
That led Arizona State University geologist Mark Robinson to speculate that the mineral is important but still unknown stuff ejected from Mercury's large core in the volcanic eruptions.
That material was seen with NASA's first partial view of Mercury by Mariner 10 in the 1970s. It was spotted again in Messenger's first images of Mercury's unseen side earlier this year.
The latest Messenger images, added to earlier photos show about 95 percent of the planet, and the blue stuff was in many places, more than astronomers had anticipated.
Although Robinson described the material as "dark blue," it only looks that way to special infrared cameras.
In normal visible light, it would have "a soft blue tinge and it would be less red" than the rest of Mercury, he said.
It's too early to tell what that material is, but it may have iron in it, Robinson said.
That would be a surprise because Mariner 10 didn't find much iron, he said.