Mercury MESSENGER's Final Plunge

Mercury - Visual Solar System Tour
Mercury, the planet closest to the Sun. USGS
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Mercury Messenger Takes its Final Plunge

Traveling at 3.91 kilometers per second (more than 8,700 miles per hour), the MESSENGER spacecraft slammed into the surface of Mercury in this region. It created a crater about 156 meters across. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

When NASA's MESSENGER spacecraft plunged to the surface of Mercury, the world it was sent to study for more than four years, it had just relayed back the last of several years of mapping data of the surface. It was an incredible accomplisment and taught planetary scientists a great deal about this tiny world.

Relatively little was known about Mercury, despite a visit by the Mariner 10 spacecraft in the 1970s. This is because Mercury is notoriously hard to study due to its closeness to the Sun and the harsh environment in which it orbits. 

Over its time in orbit around Mercury, MESSENGER's cameras and other instruments took thousands of images of the surface. It measured the planet's mass, magnetic fields, and sampled its extremely thin (almost nonexistent) atmosphere. Eventually, the spacecraft ran out of maneuvering fuel, leaving controllers unable to steer it into a higher orbit. Its final resting place is its own self-made crater in the Shakespeare impact basin on Mercury.  

MESSENGER went into orbit around Mercury on March 18, 2011, the first spacecraft to do so. It took 289,265 high-resolution images, traveled nearly 13 billion kilometers, flew as close as 90 kilometers to the surface (before its final orbit), and made 4,100 orbits of the planet. Its data comprise a library of more than 10 terabytes of science. 

The spacecraft was originally planned to orbit Mercury for one year. However, it performed so well, exceeding all expectations and returning incredible data; it lasted for more than four years.

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What did Planetary Scientists Learn about Mercury from MESSENGER?

Images of Mercury's surface from 2011 and 2015.
The first and last images sent from Mercury by the MESSENGER mission. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

The "news" from Mercury delivered via MESSENGER was fascinating and some of it quite surprising.

  • MESSENGER discovered water ice at the poles of the planet. Although most of Mercury's surface is alternately plunged into sunlight or hidden in shadow during its orbit, it turns out that water could exist there. Where? Shadowed craters are cold enough to maintain the frozen ice for long periods of time. The water ice was very likely delivered by cometary impacts and asteroids rich in what are called "volatiles" (frozen gases). 
  • the surface of Mercury appears very dark, likely due to the action of the same comets that delivered water.
  • Mercury's magnetic fields and magnetosphere (the region of space bounded by its magnetic fields), although not strong, are very active.They appear to be offset by 484 kilometers from the planet's core.  That is, they are not formed in the core, but in a nearby region. No one is sure why. Scientists also studied how the solar wind affected the Mercury magnetic field. 
  • Mercury was a slightly larger world when it first formed. As it cooled, the planet shrank in on itself, creating cracks and valleys. Over time, Mercury lost seven kilometers of its diameter. 
  • At one time, Mercury was a volcanically active world, flooding its surface with thick layers of lava. MESSENGER sent back images of ancient lava valleys. Volcanic activity also eroded the surface, covering up ancient impact craters and creating smooth plains and basins. Mercury, like the other terrestrial (rocky) planets, was bombarded early it its history by objects left over from the formation of the planets.
  • The planet has mysterious "hollows" that scientists are still trying to understand. One big questions is: how and why do they form? 

MESSENGER launched on August 3, 2004 and made one flyby past Earth, two trips past Venus, and three past Mercury before settling into orbit. It carried an imaging system, a gamma-ray and neutron spectrometer as well as an atmospheric and surface composition spectrometer, an x-ray spectrometer (to study the mineralogy of the planet), a magnetometer (to measure magnetic fields), a laser altimeter (used as a sort of "radar" to measure the heights of surface features), a plasma and particle experiment (to measure the energetic particle environment around Mercury), and a radio science instrument (used to measure the spacecraft's speed and distance from Earth).  

Mission scientists continue to pore over their data and build up a more complete picture of this small, but fascinating planet and its place in the solar system. What they learn will help fill in the gaps of our knowledge about how Mercury and the other rocky planets formed and evolved. 

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Petersen, Carolyn Collins. "Mercury MESSENGER's Final Plunge." ThoughtCo, Mar. 2, 2017, Petersen, Carolyn Collins. (2017, March 2). Mercury MESSENGER's Final Plunge. Retrieved from Petersen, Carolyn Collins. "Mercury MESSENGER's Final Plunge." ThoughtCo. (accessed January 20, 2018).