Credit: NASA

On 14 January 2008, NASA's MESSENGER spacecraft made its closest approach yet to Mercury. After a three and a half year, two billion mile journey to the planet closest to the sun, the spacecraft beamed back high-resolution images that call into question the assumption that Mercury is geologically similar to the Earth's moon.

Planetary scans reveal extensive cliff formations left behind from fault activity early in Mercury's history. Impact structures also show distinct morphological differences from their lunar counterparts. The impactderived Caloris basin, estimated to be 1,550 kilometres in diameter, has highly reflective interior plains — the opposite of what is seen in lunar craters. The team also discovered a unique feature in the middle of the basin, which they have nicknamed 'The Spider'. This formation consists of over one hundred narrow troughs radiating from a central region, which is marked by a potentially related crater. No similar features have been recorded before, either on the Moon or on Mercury.

MESSENGER is also armed with a complex array of sensors. An onboard magnetometer recorded variations in Mercury's magnetic field, and spectrometers were used to investigate the sodium and hydrogen tails that trail the planet for over 40,000 kilometres. The spectrometers are also expected to provide data about the mineralogical make-up of Mercury's surface.

Project scientists are eagerly awaiting results from two more scheduled flybys and an intensive orbital mission, which are expected to bring even more surprising findings.