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.

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.

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 300° Celsius (570° Fahrenheit), whereas the majority of components in its shadow routinely operated near room temperature (20°C or 68°F),” 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