Nasa probe Messenger sends images of Mercury's polar ice

US space agency Nasa's Messenger spacecraft has sent back its first visible-light images of water ice on Mercury, the tiny charbroiled planet that orbits closest to the sun.

The findings, described in the journal Geology, reveal that the ice deposits look surprisingly "fresh" - and hint that water could have been very recently delivered to rocky little Mercury.

Even though Mercury sits less than 58 million kilometres from the sun - less than two-fifths of the earth's comfortable distance from the sun - some ice still manages to cling to the planet's surface. That is because the ice lies at the poles, in permanently shadowed regions inside craters.

It is tough to actually see these shadowed regions with the spacecraft's visible-light camera. But recently, the team was able to refine the images of the ice-deposit surfaces with the help of what little light was reflecting off the crater walls.

The scientists examined Prokofiev, which at roughly 100km in diameter is the largest crater at Mercury's north pole thought to have water-ice deposits. There, the surface ice had a "cratered" texture - showing that it was placed there more recently than the smaller underlying craters.

And in other spots, such as Berlioz Crater, the researchers found that the boundaries of those icy regions were surprisingly sharp - they had not been in place long enough to get smoothed out.