In some of the craters, volcanic material apparently oozed out to form a smooth floor.
"There's very little doubt that there have been widespread volcanoes on Mercury's surface," said Louise Prockter, a mission scientist from Johns Hopkins University in Maryland.
This volcanism, she said, was one of Mercury's biggest surprises.
Prockter and her colleagues are also intrigued by a unique feature they're calling "the Spider."
The odd formation consists of more than a hundred raised, narrow troughs radiating from a complex central region.
The Spider lies in the middle of a large, 3.8-billion-year-old impact crater called the Caloris Basin, about half of which sits on the side of Mercury that had gone unseen until the MESSENGER mission.
The scientists aren't sure whether a smaller crater near the center of the Spider has anything to do with its odd, raised shape.
In general, Mercury's extensive cratering record is "going to give us a lot of clues about the geologic history of Mercury and evolution of terrestrial planets," said Robert Strom, a science team member from the University of Arizona in Tucson.
"Mercury has been a big hole in that."
The astronomers stress that their announced findings from MESSENGER's first flyby are very preliminary.
Prockter, from Johns Hopkins, declined to elaborate on initial results showing possible sources for Mercury's volcanic history, noting only that she and her colleagues are studying several "intriguing features" on the surface.
Still to come are geologic maps of Mercury's surface and an analysis of its superthin atmosphere, where molecules of sodium and hydrogen barely hang together.
There's also the mystery of why a planet with temperatures reaching higher than 1,292 degrees Fahrenheit (700 degrees Celsius) seems to harbor ice.
Future data from MESSENGER could explain why Earth-based radar imaging of Mercury has revealed areas of high reflectivity—an indicator of ice—near the north and south poles.
And future data could add to our understanding of Mercury's puzzling magnetosphere, which appears to have poles like Earth's magnetic field.
(Read "Liquid Mercury: Tiny Planet Has Molten Core" [May 3, 2007].)
The Carnegie Institution's Solomon said he's most curious to know where Mercury's short-lived atmosphere comes from, how the atmosphere interacts with the magnetosphere, and how the magnetosphere interacts with solar wind.
"It's a very complex planet," he said.
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