Ridges form when planetary crust is pressed together, troughs when the crust is pulled apart. Therefore, scientists will have to devise an explanation of how the crust within the Rembrandt 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.  More globally, Messenger’s images indicate that smooth plains, most probably consisting of hardened lava flows, make up 40 percent of the surface, compared with 20 percent on the Earth’s Moon.  “This indicates that volcanism was a really important process on Mercury,” said Brett Denevi, a postdoctoral researcher at Arizona State University. “Which is pretty exciting, because before Messenger’s flybys of Mercury, just a year and a half or so ago, we were really not even sure that volcanism existed on Mercury.”  She said that comparison of different terrains suggested that although Mercury was similar to the Moon in appearance, its geological history may have been more like that of Mars.  In one of the articles, scientists reported the first detection of magnesium in Mercury’s tenuous atmosphere by a Messenger instrument that samples its surroundings. Since 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 and a co-investigator of the mission.  Hydrogen, helium, sodium, calcium and potassium had been detected before. 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 Sept. 29, before entering orbit around the planet in March 2011.  A version of this article appears in print on , on Page D4 of the New York edition with the headline: In Mercury Images, Remarkable Features in a Crater. Order Reprints| Today's Paper|Subscribe
Last October, NASA’s Messenger spacecraft beamed back a view of the second-largest crater on Mercury, as wide as the distance from Boston to Washington.