A giant crater, 430 miles wide, has been discovered on the planet Mercury. The crash site, blasted out by a devastating asteroid impact, has come as a surprise to space scientists.

It was spotted by a NASA spaceprobe called Messenger as it flew low over the closest planet to the sun. The robotic craft photographed nearly a third of its surface that had never previously been observed.

Details of the flyby, made on October 6 last year, were revealed this week by NASA. It was the second close approach to Mercury by the probe which will settle into a permanent orbit around the rocky world in 2011.

Messenger captured more than 1,200 snapshots of the planet’s surface in the flyby. The new crater – termed an impact basin – has been named Rembrandt. Unusually, it has not been buried by flows of volcanic lava and scientists say the impact happened relatively recently.

Thomas Watters, of the Smithsonian Institution in Washington, said: “This basin formed about 3.9 billion years ago, near the end of the period of heavy bombardment of the inner solar system. Although ancient, the Rembrandt basin is younger than most other known impact basins on Mercury.”

Half of Mercury’s surface was unknown until a little more than a year ago. But spacecraft images have now allowed scientists to see 90 per cent of the planet’s surface at high resolution. The results are helping them to understand how the planet’s surface crust was formed, and the process has been surprisingly dynamic.

Brett Denevi of Arizona State University said: “After mapping the surface, we see that approximately 40 per cent is covered by smooth plains. Many of these smooth plains are interpreted to be of volcanic origin, and they are globally distributed. Much of Mercury’s crust may have formed through repeated volcanic eruptions in a manner more similar to the crust of Mars than to that of the moon.”

Messenger, which made some interesting discoveries including a peculiar spider formation on Mercury in its first flyby, is preparing to gather more information from a third flyby of the planet on 29 September.

Principal investigator Sean Solomon said that another big surprise was how the interaction of the planet’s magnetic field with the solar wind had changed since the first flyby. Messenger also made the first detection of magnesium in Mercury’s thin atmosphere.

A European mission to Mercury, BepiColombo, is due for launch in 2013.

Picture: Newly discovered impact basin Rembrandt, imaged by Messenger. (Photo: NASA).

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