London, Jan 17 : NASA's Messenger spacecraft has sent back the first image of the unseen region of Mercury - one of the largest impact craters in the solar system, which has mysterious smaller dark-rimmed craters.
According to a report in New Scientist, the spacecraft was 27,000 kilometres away when it took the picture of the gigantic "Caloris Basin", which is the name given to the massive crater.
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.
Inside the basin, the Messenger image shows some peculiar, smaller craters. They have dark rings around them, which NASA described as 'unusual'.
"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," said George Fraser, director of the Space Research Centre at Leicester University in the UK.
The report states that this is the first time that the whole structure of Caloris has been seen, as 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 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 October 6, 2008, and then again on September 29, 2009. These flybys are part of an elaborate celestial manoeuvre that will drop the spacecraft into its mapping orbit around the planet on 18 March 2011.