Among the images was a view of the second-largest crater on Mercury. Called the Rembrandt basin, the crater is surprising more for the geological features within its rim than for its size, planetary scientists said last week.

Much of the terrain inside Rembrandt remains pristine from the time of impact, about 3.9 billion years ago, near the end of the “heavy bombardment” period of the early solar system. The basin is young compared with most of the giant ancient impact craters in the solar system.

“In most large impact basins on Mercury, the moon and the other inner planets, this terrain is completely buried by volcanic flows erupted after the basin formed,” Thomas R. Watters, a senior scientist at the Smithsonian Institution’s Center for Earth and Planetary Studies, said. In Rembrandt’s central region where lava did flow, the terrain has deformed into ridges and troughs that radiate outward in a wheel-and-spoke pattern—“unlike any we have seen in any other impact basin in the solar system,” Watters said.

Ridges form when planetary crust is pressed together, troughs when the crust is pulled apart. Scientists will have to devise an explanation of how the crust within the basin could have been both pressed together and pulled apart to form the ridges and troughs next to one another.

Another remarkable feature is a line of cliffs 620 miles long that cuts into the Rembrandt crater. Mercury’s surface cracked as its interior cooled and shrank, and this fault—perhaps the longest of these cracks—shows that Mercury was still shrinking at the time of the impact.

Messenger’s images indicate that smooth plains, most probably consisting of hardened lava flows, make up 40 per cent of the surface, compared with 20 per cent on the Earth’s moon.

Scientists also reported the first detection of magnesium in Mercury’s tenuous atmosphere by a Messenger instrument that samples its surroundings. Because much of Mercury’s “air” consists of molecules knocked off the surface, this discovery helps confirm the presence of magnesium in the crust, which is not a surprise.

“What is surprising is the distribution of magnesium,” said William McClintock of the University of Colorado’s Laboratory for Atmospheric and Space Physics. Calcium and magnesium are similar chemically, and the distribution of the two elements was expected to be similar. Instead, calcium peaked near the equator, while the distribution of magnesium was more uniform. Messenger will make one more flyby of Mercury, on September 29, before entering orbit around the planet in March 2011.