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)