The scientists at NASA analysed a set of photos taken by the Messenger probe in January, and reached the conclusion that Solar System’s smallest planet, Mercury, is continuously shriking.

According to their estimates, the planet is now smaller by more than a mile in diameter and the process is very likely to continue, as a result of the fact that the planet’s core is slowly cooling down. Even though its metal core is still active, the fact that it is cooling leads to the solidification of the upper layers and to their intrusion over the molten layers below.

"Cooling of the planet's core not only fuelled the magnetic dynamo, it also led to contraction of the entire planet," said Principal Investigator Sean Solomon of the Carnegie Institution of Washington, US, as quoted by BCC News.

The whole study with a detailed analysis was presented on July 3 in the journal Science. Other information presented by the scientists refer to Mercury’s core, which is filled with sodium, sulfur and silicon.

The Messenger was lauched on August 3, 2004 with the mission to determine the compounds of Mercury’s surface, its geologic history and the nature of the planet’s exosphere and magnetosphere. Its mission involves a long-term effort, but so far the results of its investigations and tests have proved that it is indeed a worthy and useful investment.

Earlier this year, the Messenger spacecraft passed Mercury at a distance of about 200 km and managed to take some close up pictures of the planet, the first set of photos since Mariner 10’s near-by visit in March 1975.

The scientists involved in the mission are preparing Messenger’s Mercury orbit inseration, which is scheduled for March 11, 2011 and is expected to provide huge amounts of valuable data.