NASA's Messenger probe beamed images of Mercury's craters and nearly mile-high cliffs back to Earth, expanding astronomers' understanding of the planet nearest the sun and furthering the U.S. goal of mapping its surface.

The car-sized Messenger flew as close as 125 miles above Mercury's scarred, rocky surface, NASA said yesterday. It was the probe's second pass since January and revealed about one-third of Mercury, an area never seen by scientists.

"There are some fantastic features, including prominent lines that emanate from crater impacts that run along the entire face of the planet," said Jeff McNutt, a mission project scientist at Johns Hopkins University's Applied Physics Laboratory in Laurel, Md. "And some cliffs stretch almost a mile high."

Mercury's surface, pockmarked with craters, is the solar system's oldest and least disturbed since the planets were formed four billion years ago. The images may offer clues about their formation.

Mercury has the most extreme temperature range of any planet in the system, with days as hot as 800 degrees Fahrenheit and nights as cold as 300 degrees below zero.

The sun has long since baked off any atmosphere the planet had, which means Mercury lacks the weathering that Earth and other planets have endured, McNutt said. The planet has also had few recent tectonic upheavals.

"In one sense, Mercury is the Rosetta Stone of planets in the inner solar system," McNutt said, referring to the stone tablet that helped archaeologists unlock the meaning of ancient Egyptian writing.

Messenger, launched by NASA four years ago, is the first probe to visit Mercury since 1975, when Mariner 10 photographed less than half of the planet.

About 95 percent of the surface has now been imaged. The final portion will be photographed in 2011, when Messenger returns for an extended orbit.

The spacecraft is more than halfway through a journey of almost five billion miles, including 15 trips around the sun. Messenger flew past Venus in 2006 and 2007.