Eleven years after it launched, Messenger will end its mission on 30 April by crashing down, more than 4,100 orbits and four years after it reached its perch above and around the planet.

Among the mission's biggest discoveries was that both of Mercury's polar regions contain water ice despite the planet's close proximity to the sun and daytime temperatures that can reach 800F (430F) elsewhere.

Data from Messenger also revealed that "most of these deposits don't consist of water ice directly at the surface, but rather water-ice covered by a dark layer", said Sean Solomon, Nasa's principal investigator for the mission. That dark layer, roughly 30cm thick, is "much colder than the average mercury material".

Solomon added that some hypotheses hold that "this dark material is in fact organic, carbonaceous material delivered to Mercury" from the outer solar system – meaning that the "the building blocks for organic chemistry and life" may be resting on the planet closest to the sun.

"We don't see anything in the geological features that indicate running water as we see on Mars," said the planetary science director, James Green. "It's not likely on a regular basis that there's liquid water on Mercury."

The mysterious dark material may have come to the planet by way of debris from space crashing on to Mercury's surface, in parallel to the idea that a comet or meteorites may have brought basic organic material to Earth.