A NASA probe that has circled Mercury for the past four years will make a dramatic death plunge into the planet's surface in late April when it runs out of fuel.

The MESSENGER spacecraft -- which stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging -- will end its run, as planned, on or around April 30, the US space agency said.

Its mission was initially only supposed to last one year, but since it was operating well and returning interesting data and discoveries, scientists extended its life as long as they could.

MESSENGER's key finding, in 2012, was a thick coat of ice in Mercury's polar regions, providing "compelling support for the hypothesis that Mercury harbors abundant frozen water and other volatile materials in its permanently shadowed polar craters," NASA said.

"For the first time, scientists began seeing clearly a chapter in the story of how the inner planets, including Earth, acquired water and some of the chemical building blocks for life," the agency said in a statement.

Scientists believe that the closest planet to the Sun likely got its water when comets and volatile-rich asteroids made impact, sometime in history.

MESSENGER was launched in 2004 and traveled for more than six years before it finally began orbiting Mercury on March 18, 2011.

Once the unmanned probe runs out of propellant, it will no longer be able to fight the downward push of the Sun's gravity and will fall, striking the planet at more than 8,750 miles per hour (3.91 kilometers per second) on the side of the planet facing away from Earth.

No images are expected from the impact.

"For the first time in history we now have real knowledge about the planet Mercury that shows it to be a fascinating world as part of our diverse solar system," said John Grunsfeld, associate administrator for the Science Mission Directorate at NASA.

Scientists will continue to analyze data from MESSENGER for years to come, he said.