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

[Mark]

Almost looks like fresh material doesn't it?
when you look around you have a whitish surface, but on the larger crater 
(bottom left direction) it looks dark (the mentioned halo) almost as this is 
the freshly ejected material.



----- Original Message ----- 
From: "Jerry" <grf2 at verizon.net>
To: "Ron Baalke" <baalke at zagami.jpl.nasa.gov>; "Meteorite Mailing List" 
<meteorite-list at meteoritecentral.com>
Sent: Friday, February 22, 2008 7:12 PM
Subject: Re: [meteorite-list] MESSENGER: Craters with Dark Halos on Mercury


> Definitely a different optical appearance from Martian craters where 
> lighter material is seen to have been ejected.
> Perhaps the "metallic" nature of he subsurface of Mercury is being 
> validated in these images.
> 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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