Without iron, some other material must be responsible for the color differences between old and new craters on Mercury, Solomon said.
"There is some difference in the detail, but it looks to be a space weathering process," he said.
Mission scientists say the most striking new observation so far is a large pattern of rays that appears to extend from a young crater in the north to regions south of a bright crater called Kuiper.
The Mariner 10 spacecraft first saw Kuiper when it flew by Mercury in 1974 and 1975, capturing images of about 45 percent of the planet's cratered surface.
The young, extensively rayed crater north of Kuiper, along with a prominent rayed crater to the southeast of Kuiper, were both seen in Earth-based radar images of Mercury but had not been imaged by spacecraft until now.
The rays are formed when material is ejected from the planet's surface during meteor impacts, Solomon said. On the moon, such features can last for hundreds of millions of years.
Although astronomers haven't teased out the geologic time scales for Mercury, "a comparable fraction of its craters have rays," he said, which suggests the planet developed along similar time scales as the moon.
MESSENGER, which is now about 61 million miles (99 million kilometers) from Earth, is expected to send back 1,200 pictures from its latest flyby.
Scientists are hoping that the data will continue to reveal more about geologic features on the largely unstudied planet.
Images from the first flyby, for example, have shown that tiny Mercury has a history of violent volcanism and is experiencing constant meteor bombardment.
The craft's next swing past Mercury is slated for September 2009, and it is expected to enter into orbit in March 2011.
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