http://messenger.jhuapl.edu/news_room/details.php?id=92
MESSENGER Mission News
February 21, 2008
Craters with Dark Halos on Mercury
As MESSENGER flew by Mercury on January 14, 2008, the Narrow Angle Camera (NAC) of the Mercury Dual Imaging System (MDIS) captured this view <http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php? gallery_id=2&image_id=166>. 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 kilometers (60 miles) in diameter, and the crater with patches of dark material in the upper right is about 70 kilometers (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 <http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php? page=1&gallery_id=2&image_id=143>.
Information from images taken in the 11 different color filters of the WAC will help MESSENGER scientists 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.
Additional information and features from MESSENGER’s first flyby of Mercury are online at http://messenger.jhuapl.edu/mer_flyby1.html.
MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is a NASA-sponsored scientific investigation of the planet Mercury and the first space mission designed to orbit the planet closest to the Sun. The MESSENGER spacecraft launched on August 3, 2004, and after flybys of Earth, Venus, and Mercury will start a yearlong study of its target planet in March 2011. Dr. Sean C. Solomon, of the Carnegie Institution of Washington, leads the mission as principal investigator. The Johns Hopkins University Applied Physics Laboratory built and operates the MESSENGER spacecraft and manages this Discovery -class mission for NASA.