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 MERCURY'S COMET-LIKE TAIL

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PostSubject: MERCURY'S COMET-LIKE TAIL   Wed Nov 04, 2009 9:07 pm

from spaceweather.com:

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The ultrathin atmosphere of Mercury is blown back by solar radiation pressure, forming an enormous comet-like tail. NASA's MESSENGER spacecraft flew through that tail on Sept. 29th and found it less enormous than it used to be. The following diagram compares the situation in Oct. 2008 vs. Sept. 2009:



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Red traces the distribution of sodium atoms detected by a spectrometer onboard MESSENGER. "The neutral sodium tail, so prominent in our first two flybys of Mercury, is now significantly reduced in extent," announced planetary scientist Ron Vervack at a NASA press conference yesterday.

The material in Mercury's tail comes from the surface of the planet itself, which is blasted by solar wind and micrometeorites. During MESSENGER's recent flyby of Mercury, the net effect of solar radiation pressure was small, and the sodium atoms were not accelerated away from the sun as they were during the earlier flybys, resulting in a diminished planetary tail. That's space weather. Get the full story from Science@NASA.
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PostSubject: Re: MERCURY'S COMET-LIKE TAIL   Wed Nov 04, 2009 9:11 pm

follow up with Space@NASA

Hidden Territory on Mercury Revealed

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Nov. 3, 2009: The MESSENGER spacecraft's third flyby of the planet Mercury has given scientists, for the first time, an almost complete view of the planet's surface and revealed some dramatic changes in Mercury's comet-like tail.

"The new images remind us that Mercury continues to hold surprises," says Sean Solomon, principal investigator for the mission and director of the Department of Terrestrial Magnetism at the Carnegie Institution of Washington.

The probe flew by Mercury on Sept. 29th, executing a critical gravity assist maneuver designed to help MESSENGER enter Mercury-orbit in 2011. Despite shutting down temporarily because of a power system switchover during a solar eclipse, the spacecraft's cameras and instruments revealed 6 percent of the planet's surface never before seen at close range, including this picturesque region pocked by impact craters and molded by volcanic activity:



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The bright region in the upper-right corner of the image surrounds a suspected explosive volcanic vent. The 290-km-diameter double-ring basin near the bottom of the image has a smooth interior that may be the result of effusive volcanism.

"This double-ring basin, seen in detail for the first time, is remarkably well preserved," notes Brett Denevi, a member of the probe's imaging team and a postdoctoral researcher at Arizona State University. "The inner floor of this basin is even younger than the basin itself and differs in color from its surroundings. We may have found the youngest volcanic material on Mercury."

One of the spacecraft's instruments conducted its most extensive observations to date of Mercury's ultrathin atmosphere or "exosphere." Material in the exosphere comes mainly from the surface of Mercury, knocked aloft by solar radiation, solar wind bombardment and meteoroid vaporization: diagram. This wispy gaseous envelope is stretched by solar radiation pressure into a long, comet-like tail, which seems to be changing as Mercury moves around the sun.

"A striking illustration of what we call 'seasonal' effects in Mercury's exosphere is that the neutral sodium tail, so prominent in the first two flybys, is now significantly reduced in extent," says participating scientist Ron Vervack of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.

article continues here:

http://science.nasa.gov/headlines/y2009/03nov_hiddenterritory.htm
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PostSubject: Re: MERCURY'S COMET-LIKE TAIL   Wed Nov 04, 2009 9:12 pm

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