NASA's MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) probe is is set to enter orbit around Mercury in March 2011 and carry out much needed observations of the planet for one year. It is well equipped with miniaturised space instruments and communications facilities and will cost around $286 million. MESSENGER will be the first mission to Mercury for more than 35 years – the last probe there was Mariner 10 which ended its work in 1975.

Firstly, MESSENGER aims to explain why Mercury is so dense. It has a density of 5.3g/cm3 which, for its size, is much greater than would be expected! The large density implies a 65% metal-rich constituent (twice the proportion on Earth). By examining the composition of the surface, it will also be possible to discover if it is an actual crust layer, as on Earth, or is in fact an exposed mantle.

There are very distinguishable terrestrial (rocky) features on the surface of Mercury which may have been formed through the movement of tectonic plates or as a result of volcanic eruptions. MESSENGER's camera will map the remaining 55% of the planet's surface not photographed by the earlier Mariner 10 mission and a laser altimeter will be used to measure the topography of Mercury.

The Earth's magnetic field is produced by the currents in the molten iron outer core. However Mercury is so small that its core should have cooled and solidified billions of years ago. The study of such a field around Mercury will hopefully enable scientists to understand global fields different to that of our own, as well as applying it to our existing knowledge of the Earth.

Mercury's axis is almost perpendicular to the plane of its orbit around the Sun. This means that the Sun's rays always strike at a very shallow angle to the surface at the poles and the floors of the deepest craters are never exposed to sunlight. Earth-based radar has detected an extremely reflective material in these regions which may be sublimed sulphur or even water ice.

MESSENGER will use gamma-ray and neutron spectrometers to find any hydrogen present proving the 'ice theory’ whilst ultraviolet and energy particle spectrometers will be able to detect sulphur.

Finally MESSENGER will discover the composition of Mercury's outer atmosphere (exosphere) using an ultraviolet detector. Once compared to the surface composition it will be possible to determine which molecules entered the atmosphere from the surface and whether others entered from elsewhere.