Until now, the innermost and smallest planet, Mercury, had been relatively unexplored apart from the brief flyby by Mariner 10 in the 1970s. But the orbiting MESSENGER spacecraft is changing that, revealing new details about this little world.
New images now surface at an average of 250 metres per pixel and a global base colour map at about 1.2 kilometres per pixel. The extensive volcanic plains near the north pole may be several kilometres deep. Some of the most interesting features are clusters of irregular, rimless pits which vary in size from hundreds of metres to several kilometres in size, and photographed with a resolution down to only 10 metres per pixel. They are often surrounded by some sort of higher-reflectance material.
Despite Mercury’s accurate reputation of being a searingly hot world so close to the sun, MESSENGER is also providing new data indicating that there may be water ice on the floors of craters which are permanently in shadow, which have also been observed by radar with the Arecibo Observatory on Earth. Since there is no atmosphere to speak of to distribute heat, temperatures in daylight can be hundreds of degrees, yet far below zero in the shade, similar to the Moon.
An additional good overview of the results so far is here.