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

[Jerry]

I'd add as well, "a not very Lunar landscape". Quite distinctive as one 
might expect from current theoretical models.
Jerry Flaherty
----- Original Message ----- 
From: "Ron Baalke" <baalke at zagami.jpl.nasa.gov>
To: "Meteorite Mailing List" <meteorite-list at meteoritecentral.com>
Sent: Friday, February 22, 2008 6:53 PM
Subject: [meteorite-list] MESSENGER: Craters with Dark Halos on Mercury


>
> 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.
>
>
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