Fire and Ice: A MESSENGER Recap   
       
   April 30, 2015:  The planet closest to the Sun is, ironically, one of the   
   coldest.   
       
   That's just one of many mind-bending discoveries about Mercury that NASA's   
   MESSENGER spacecraft beamed back to Earth over the past 7 years.  Earlier   
   today, the mission ended with a crash as spectacular as some of its findings.   
       
   http://apod.nasa.gov/apod/ap130301.html   
       
   The colors of the solar system's innermost planet are enhanced in this   
   tantalizing view, based on global image data from the Mercury-orbiting   
   MESSENGER spacecraft. More information   
       
   Mission controllers at the Johns Hopkins University Applied Physics Laboratory   
   in Laurel, Maryland, have confirmed that MESSENGER slammed into the surface of   
   Mercury on April 30th at 3:26 p.m. EDT. It had used the last of its propellant   
   on April 24th and could no longer maintain a stable orbit. Traveling some   
   8,750 mph, the plummeting spacecraft made an unseen crater on the side of the   
   planet facing away from Earth.   
       
   "Going out with a bang as it impacts the surface of Mercury, we are   
   celebrating MESSENGER as more than a successful mission," says John Grunsfeld,   
   associate administrator for the Science Mission Directorate at NASA   
   Headquarters in Washington. "Now, we begin the next phase of this   
   mission--analyzing the exciting data already in the archives, and unravelling   
   the mysteries of Mercury."   
       
   Here are some of MESSENGER's most important findings so far:   
       
   The hidden face of Mercury: In the mid-1970s when Mariner 10 flew past Mercury   
   three times, the probe imaged less than half the planet.  Until MESSENGER   
   arrived, the rest of Mercury was a land of mystery.  MESSENGER was the first   
   spacecraft to view the entirety of the mighty Caloris basin-one of the biggest   
   and youngest impact features in the solar system.  Moreover, MESSENGER spotted   
   volcanic vents around the rim of the basin, proving that volcanism-and not   
   only impacts-have shaped the surface of the innermost planet.   
       
   The irony of Mercury's poles: Mercury would seem to be an unlikely place to   
   find ice. But the tilt of Mercury's rotational axis is almost zero - less than   
   one degree - so the floors of craters at the planet's poles never see   
   sunlight. Scientists suggested decades ago that there might be frozen water   
   trapped there.  The idea received a boost in 1991 when the Arecibo radio   
   telescope in Puerto Rico and the Goldstone antenna in California detected   
   unusually bright radar reflections from Mercury's poles-the kind of   
   reflections that ice would make.  From Mercury orbit, MESSENGER was able to   
   look down on Mercury's poles like no other spacecraft or telescope, and it   
   confirmed the unlikely:  Permanently shadowed craters near Mercury's poles   
   have temperatures less than -280F (-173C), and water ice is stable on their   
   dark inner surfaces.  Some of the polar ice is covered by a mysterious dark   
   organic material that researchers still do not understand.   
       
   http://tinyurl.com/mg2tgbe   
       
   These graphics show the predicted location and time of MESSENGER's impact on   
   Mercury's surface.  [details] The incredible shrinking planet: The dominant   
   tectonic landforms on Mercury are huge cliffs called "lobate scarps."  Even   
   before MESSENGER, researchers thought these scarps were signs of global   
   shrinkage, like wrinkles on a raisin.  Why would Mercury shrink? The planet's   
   core makes up a whopping 60-70% of its mass. Cooling of this oversized core   
   has led to a remarkable contraction of the planet. MESSENGER's images of   
   lobate scarps show that the total contraction is two to seven times greater   
   than researchers previously thought.   
       
   Magnetically speaking, Mercury is alive: Until Mariner 10 discovered Mercury's   
   magnetic field in the 1970s, Earth was the only other terrestrial planet known   
   to have a global magnetic field. Earth's magnetism is generated by the   
   planet's churning hot, liquid-iron core via a mechanism called a magnetic   
   dynamo. Researchers have been puzzled by Mercury's field because its iron core   
   was supposed to have finished cooling long ago and stopped generating   
   magnetism. Some researchers thought that the field may have been a relic of   
   the past, frozen in the outer crust. MESSENGER data show otherwise: Mercury's   
   field appears to be generated by an active dynamo in the planet's core. It is   
   not a relic.   
       
   A planet with a tail: Orbiting Mercury, MESSENGER made the first in situ   
   observations of Mercury's unique exosphere. The exosphere is an ultrathin   
   atmosphere where atoms and molecules are so far apart they are more likely to   
   collide with the surface than with each other. This material is derived mainly   
   from the surface of Mercury itself, knocked aloft by solar radiation, solar   
   wind bombardment and meteoroid vaporization. MESSENGER was able to determine   
   the chemical composition of the exosphere (hydrogen, helium, sodium,   
   potassium, and calcium) and monitor the material as it was stretched out into   
   a comet-like tail as long as 2 million km by the action of the solar wind.    
   This tail, as well as Mercury's magnetic field, was often buffeted by solar   
   activity during MESSENGER's long mission, giving the spacecraft a point-blank   
   view of the roughest space weather in the solar system.   
       
   In addition to science discoveries, the mission provided many technological   
   firsts, including the development of a ceramic cloth sunshade that protected   
   the spacecraft's instruments and electronics from fierce solar radiation.   
       
   "The front side of the sunshade routinely experienced temperatures in excess   
   of 300ø Celsius (570ø Fahrenheit), whereas the majority of components in its   
   shadow routinely operated near room temperature (20øC or 68øF)," said Helene   
   Winters, mission project manager at the Johns Hopkins University Applied   
   Physics Laboratory (APL). "This technology to protect the spacecraft's   
   instruments was a key to mission success during its prime and extended   
   operations."   
       
   Goodbye, MESSENGER, and thanks!   
       
   Credits:   
   Author: Dr. Tony Phillips | Production editor: Dr. Tony Phillips | Credit:   
   Science@NASA   
       
   More information:   
       
   The spacecraft was designed and built by APL. The lab manages and operates the   
   mission for NASA's Science Mission Directorate. The mission is part of NASA's   
   Discovery Program, managed for the directorate by the agency's Marshall Space   
   Flight Center in Huntsville, Alabama.   
       
   For a complete listing of science findings and technological achievements of   
   the mission visit: http://www.nasa.gov/messenger   
       
       
   Regards,   
       
   Roger   
      
   --- D'Bridge 3.99   
    * Origin: NCS BBS - Houma, LoUiSiAna (1:3828/7)