On its second flyby of the planet Mercury last October, NASA’s Messenger spacecraft beamed back 1,200 pictures, revealing 30 per cent of the planet’s surface that had never been seen up close before.

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.

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