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.