As MESSENGER flew by Mercury, the Narrow Angle Camera (NAC) of the Mercury
Dual Imaging System (MDIS) captured this view on January 14, 2008. Two of
the larger craters in this image appear to have darkened crater rims and
partial "halos" of dark material immediately surrounding the craters. Both
craters appear to have nearly complete rims and interior terraced walls,
suggesting that they formed more recently than the other nearby shallower
craters of similar size. There are two possible explanations for their
dark halos: (1) Darker subsurface material may have been excavated during
the explosions from the asteroid or comet impacts that produced the
craters. (2) Large cratering explosions may have melted a fraction of the
rocky surface material involved in the explosions, splashing so-called
"impact melts" across the surface; such melted rock is often darker (lower
albedo) than the pre-impact target material. In either case, the
association of the dark material with relatively recently formed craters
suggests that the processes that gradually homogenize Mercury's surface
materials have not yet had time to reduce the contrast of these dark
halos. The crater with associated dark material in the lower-left part of
this image is about 100 km (60 miles) in diameter, and the crater with
patches of dark material in the upper right is about 70 km (40 miles)
across. These dark-halo craters, located near Mercury's south pole, are
also visible in the previously released false-color image created from
three Wide Angle Camera (WAC) frames (see PIA10398).
Information from images taken in the 11 different color filters of the WAC
will help MESSENGER scientists to understand the nature of the dark
material associated with the craters shown in this image and will
determine whether they reveal the presence of subsurface material of a
different composition, are examples of impact melt, or perhaps have some
other explanation.
Image Mission Elapsed Time (MET): 108828161
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.