When Mariner 10 flew past Mercury three times in 1974 and 1975, the same
hemisphere was in sunlight during each encounter. As a consequence,
Mariner 10 was able to image less than half the planet. Planetary
scientists have wondered for more than 30 years about what spacecraft
images might reveal about the hemisphere of Mercury that Mariner 10 never
viewed. (See Mariner 10 images.)
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, part of the Mercury Dual Imaging System (MDIS) instrument, about
80 minutes after MESSENGER's closest approach to Mercury (2:04 pm EST),
when the spacecraft was at a distance of about 27,000 kilometers (about
17,000 miles). The image shows features as small as 10 kilometers (6
miles) in size. This image was taken through a filter sensitive to light
near the red end of the visible spectrum (750 nm), one of a sequence of
images taken through each of MDIS's 11 filters.
Like the previously mapped portion of Mercury, this hemisphere appears
heavily cratered. 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.
Mission Elapsed Time (MET) of image: 108829708
These images are from MESSENGER, a NASA Discovery mission to conduct the
first orbital study of the innermost planet, Mercury. For information
regarding the use of images, see the MESSENGER image use policy.
