Probe plunge to end Mercury mission
An American spacecraft is due to smash into the planet Mercury tomorrow after running out of fuel, bringing a dramatic end to its four year mission.
The Messenger probe launched in 2004 has gathered a wealth of scientific data since it began orbiting the planet on March 18, 2011.
It has also beamed back stunning images of the heavily cratered scorched world, the smallest and closest planet to the sun where daytime temperatures can reach 427C.
The 500 kilogram spacecraft is set to plunge onto the planet's surface at 8,750 miles per hours, creating a sizeable crater of its own.
But the impact will be an anticlimax for observers from Earth, since it will occur on the far side of the planet.
The American space agency Nasa will only be able to confirm that the crash has taken place when the probe fails to appear from behind the planet hours later.
John Grunsfeld, associate administrator for the science mission directorate at Nasa headquarters in Washington, said: "While spacecraft operations will end, we are celebrating Messenger as more than a successful mission. It's the beginning of a longer journey to analyse the data that reveals all the scientific mysteries of Mercury."
One key finding that has already emerged suggests that Mercury harbours abundant frozen water in its permanently shadowed polar craters.
Data received in 2012 indicated enough ice in the planet's polar regions to create a layer two miles thick if spread over an area the size of Washington DC.
Dark deposits on the ice are believed to be composed of organic compounds.
The discovery supports the theory that both water and the building blocks of life were delivered from the outer solar system to the inner planets, including the Earth.
Planetary geoscientist Professor David Rothery, from The Open University, said: "Messenger has been an amazing mission. The only previous Mercury mission, Mariner 10, flew past the planet three times in 1974-5, giving us only an incomplete view.
"Messenger revealed the whole globe in detail, especially its northern hemisphere to which its deliberately eccentric orbit took it closest before soaring upwards to escape the furnace-like conditions near the 400C surface.
"It is now apparent that Mercury is a misfit planet that seems not to belong where we now find it. It is dense even for a rocky planet, showing that its iron-rich core occupies more than 80% of Mercury's radius. The outer part of the core must still be molten, because this is where Mercury's magnetic field is generated - a characteristic shared with the Earth, but not with Venus, Mars or the Moon."
Messenger had also revealed evidence of recent explosive volcanic eruptions punching through Mercury's vast ancient lava fields, he said.
Another major find was Mercury's mysterious "hollows" - steep, flat-bottomed depressions where the top 10 metres or so of the planet's surface had "simply vanished".
Prof Rothery added: "Airless Mercury has no wind to blow it away, and there are no signs of collapse into underground cavities, so we are forced to conclude that something in the ground has been turned to vapour and lost to space.
"What this stuff is, and whether it turns from solid to vapour through heating or because chemical bonds are broken by radiation of some sort, we don't know - but it is another piece in the jigsaw puzzle that must one day be fitted together if we are to understand where and how the sun's smallest and closest planet formed."
Messenger stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging mission.