Probe returns first image of Mercury's unseen side

15:21 16 January 2008
NewScientist.com news service
Stuart Clark

NASA's Messenger spacecraft has taken its first look at the unseen side of Mercury, the planet closest to the Sun. It has revealed the full extent of Mercury's gigantic Caloris Basin, one of the largest impact craters in the solar system and discovered its first Mercury mystery: unusual dark-rimmed craters.

Messenger flew past Mercury on Monday, making its closest approach at 1904 GMT. About 80 minutes afterwards, the spacecraft captured this image. Showing about half of the unseen side, the spacecraft was 27,000 kilometres away when it took the picture. Nevertheless, the image reveals surface details as small as 10 km across.

The Caloris Basin can be seen at the top right of the image as the large, lightly coloured region. It stretches around 1300 km in diameter and was created between 3 and 3.5 billion years ago by the impact of a large asteroid or comet. This is the first time that the whole structure of Caloris has been seen – back in the mid-1970s, NASA's Mariner 10 only managed to glimpse its eastern half, with the Sun low on Mercury's horizon.

"Messenger's is a great image. Our whole impression of Caloris is different now," says David Rothery, a planetary scientist at the Open University in the UK. That is because Messenger's image is illuminated with the Sun directly overhead.

Inside the basin, the Messenger image shows some peculiar, smaller craters. They have dark rings around them, which NASA describes as 'unusual'. George Fraser, director of the Space Research Centre at Leicester University in the UK agrees.
Different composition"The material forming these rings must have been excavated during the impacts that formed each crater. I assume it means that the composition of Mercury is very different just below the surface," Fraser says.

Fraser is building an X-ray instrument to measure Mercury's surface composition for launch in 2013 on a European-Japanese Mercury mission called Bepi-Colombo. He says Messenger will help researchers develop Bepi. "I'm interested in the particle environment that Messenger reveals, as that will help determine how sensitive our instrument will be," he says. On Friday, the European Space Agency will sign the contract with EADS Astrium to build Bepi Colombo.

For the immediate future, Messenger is the star of the show. It captured many more images and data during the flyby, including around 100 close-up views of the planet. The science phase lasted for 55 hours and recorded some 700GB of data that is currently being radioed back to Earth for analysis.

The NASA spacecraft will return for its second flyby on 6 October 2008, and then again on 29 September 2009. The flybys are part of an elaborate celestial manoeuvre that will drop the spacecraft into its mapping orbit around the planet on 18 March 2011.