30-04-09 Huge impact crater discovered on Mercury

Until recently,scientists say the closest planet to the sun remained the least understood of the four terrestrial planets — Mercury,Venus,Earth and Mars.For a long time it was thought to be very similar to Earth's moon in composition,since both worlds have a similar gray,pockmarked appearance.More like Mars.The new observations reveal that Mercury's crust was largely created through volcanism,with past volcanoes spitting out lava that spread and dried on the surface.In contrast,volcanism is thought to have played less of a role in forming the moon's crust."Volcanism was a really important process on Mercury,which is pretty exciting because before MESSENGER's flybys of Mercury we were really not even sure that volcanism existed on Mercury," said Brett Denevi,a postdoctoral researcher in the School of Earth and Space Exploration at Arizona State University.Denevi is lead author of a paper in the May 1 issue of the journal Science describing MESSENGER's new global map of Mercury.In fact,Mercury's surface now appears to be more similar to that of Mars than of the moon.The newly-discovered gigantic crater,called Rembrandt,stretches more than 700 kilometers (430 miles) in diameter.The bowl-like indentation in Mercury's surface was likely formed about 3.9 billion years ago by an impacting space rock.It has managed to survive with parts of its original floor still intact,without being filled in by later flows of volcanic lava,as most craters have been