Mariner 10 flew past Mercury three times in 1974 and 1975 but because the same hemisphere was in sunlight during each encounter it was able to image less than half the planet. After  more than 30 years we have new pictures of the planet missed out by Mariner 10.

On January 14, 2008, the MESSENGER spacecraft observed about half of the hemisphere missed by Mariner 10. This image was snapped by the Wide Angle Camera, about 80 minutes after MESSENGER's closest approach to Mercury, when the spacecraft was at a distance of about 27,000 kilometers. The image shows features as small as 10 kilometers in size.

The newly pictured hemisphere appears heavily cratered just like the part that was pictured by Mariner 10. It also reveals some unique and distinctive features. On the upper right is the giant Caloris basin, including its western portions never before seen by spacecraft. Formed by the impact of a large asteroid or comet, Caloris is one of the largest, and perhaps one of the youngest, basins in the Solar System. The new image shows the complete basin interior and reveals that it is brighter than the surrounding regions and may therefore have a different composition. Darker smooth plains completely surround Caloris, and many unusual dark-rimmed craters are observed inside the basin. Several other multi-ringed basins are seen in this image for the first time. Prominent fault scarps (large ridges) lace the newly viewed region.

Other images obtained during the flyby will reveal surface features in color and in much more detail. Collectively, these images and measurements made by other MESSENGER instruments will soon provide a detailed global view of the surface of Mercury, yielding key information for understanding the formation and geologic history of the innermost planet.

Note to stargazers: Mercury is now visible in the West after Sunsets. Consult your skymaps.

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