[meteorite-list] MESSENGER: Craters with Dark Halos on Mercury

[Ron Baalke]

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.