Strange Hollows Discovered on Mercury   
       
   Oct 24, 2011: NASA's MESSENGER spacecraft has discovered strange hollows on   
   the surface of Mercury. Images taken from orbit reveal thousands of peculiar   
   depressions at a variety of longitudes and latitudes, ranging in size from 60   
   feet to over a mile across and 60 to 120 feet deep. No one knows how they got   
   there.   
       
   "These hollows were a major surprise," says David Blewett, science team member   
   from the Johns Hopkins University Applied Physics Laboratory. "We've been   
   thinking of Mercury as a relic - a place that's really not changing much   
   anymore, except by impact cratering. But the hollows appear to be younger than   
   the craters in which they are found, and that means Mercury's surface is still   
   evolving in a surprising way."   
       
   http://messenger.jhuapl.edu/news_room/telecon7/Blewett04B_lg.jpg   
       
   Hollows inside the Raditladi impact basin. Credit: NASA/Johns Hopkins   
   University Applied Physics Laboratory/Carnegie Institution of Washington   
   [larger image]   
   Mars Reconnaissance Orbiter spotted similar depressions in the carbon dioxide   
   ice at Mars' south pole, giving that surface a "swiss cheese" appearance. But   
   on Mercury they're found in rock and often have bright interiors and halos.   
       
   "We've never seen anything quite like this on a rocky surface."   
       
   If you could stand in one of these "sleepy" hollows on Mercury's surface,   
   you'd find yourself, like Ichabod Crane, in a quiet, still, haunting place,   
   with a black sky above your head.   
       
   http://messenger.jhuapl.edu/news_room/telecon7/Blewett05_lg.jpg   
       
   Another example of hollows in crater Tyagaraja. Courtesy Science/AAAS [larger   
   image] "There's essentially no atmosphere on Mercury," explains Blewett. "And   
   with no atmosphere, wind doesn't blow and rain doesn't fall. So the hollows   
   weren't carved by wind or water. Other forces must be at work."   
       
   As the planet closest to the Sun, Mercury is exposed to fierce heat and   
   extreme space weather. Blewett believes these factors play a role.   
       
   A key clue, he says, is that many of the hollows are associated with central   
   mounds or mountains inside Mercury's impact craters. These so-called "peak   
   rings" are thought to be made of material forced up from the depths by the   
   impact that formed the crater. Excavated material could be unstable when it   
   finds itself suddenly exposed at Mercury's surface.   
       
   "Certain minerals, for example those that contain sulfur and other volatiles,   
   would be easily vaporized by the onslaught of heat, solar wind, and   
   micrometeoroids that Mercury experiences on a daily basis," he says. "Perhaps   
   sulfur is vaporizing, leaving just the other minerals, and therefore weakening   
   the rock and making it spongier. Then the rock would crumble and erode more   
   readily, forming these depressions."   
       
   http://messenger.jhuapl.edu/news_room/telecon7/Blewett06_lg.jpg   
       
   A fresh impact crater. Hollows are present on a section of the crater wall   
   that has slid partway down toward the floor. Courtesy  Science/AAAS [larger   
   image] MESSENGER has indeed proven Mercury unexpectedly rich in sulfur. That   
   in itself is a surprise that's forcing scientists to rethink how Mercury was   
   formed. The prevailing models suggest that either (1) very early in Solar   
   System history, during the final sweep-up of the large planetesimals that   
   formed the planets, a colossal impact tore off much of Mercury's rocky outer   
   layering; or (2) a hot phase of the early Sun heated up the surface enough to   
   scorch off the outer layers. In either case, the elements with a low boiling   
   point - volatiles like sulfur and potassium - would have been driven off.   
       
   But they're still there.   
       
   "The old models just don't fit with the new data, so we'll have to look at   
   other hypotheses."   
       
   To figure out how the planets and Solar System came to be, scientists must   
   understand Mercury.   
       
   "It's the anchor at one end of the Solar System. Learning how Mercury formed   
   will have major implications for the rest of the planets. And MESSENGER is   
   showing that, up to now, we've been completely wrong about this little world   
   in so many ways!"   
       
   What other surprises does Mercury hold? The sleepy hollows of the innermost   
   planet may be just the beginning.   
       
       
   Author: Dauna Coulter | Editor: Dr. Tony Phillips | Credit: Science@NASA   
       
   More Information   
   MESSENGER -- the Mercury Surface, Space Environment, Geochemistry and Ranging   
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   Regards,   
       
   Roger   
      
   --- D'Bridge 3.64   
    * Origin: NCS BBS (1:3828/7)