Scientists have taken the first colour images of the entire surface of the planet Mercury to reveal a mysterious world of complex geology and extraordinary temperature extremes.
The image is a composite of thousands of pictures taken from cameras with eight light filters on board Nasa’s Messenger space probe, which has been orbiting the innermost planet of the Solar System for the past two years.
Although the colours are enhanced with the aid of the light filters, they represent true differences in structure and composition of the rocks on the surface of Mercury, said David Blewitt, a planetary scientist at the Johns Hopkins University in Baltimore.
“Messenger has revealed Mercury to be a really fascinating, dynamic and complex world,” Dr Blewitt told the American Association for the Advancement of Science meeting in Boston.
“The colours represent real differences in the composition of the rocks on the surface. The orange areas represent volcanic plains and the areas of deep blue are reflections of blue light, but we’re not really sure what’s causing that,” he said.
Mercury is the nearest planet to the Sun where temperatures can fluctuate from minus 200C to 450C - the largest temperature variation on any planet. At the poles there are craters where sunlight never penetrates and water ice has collected, whereas at lower latitudes the heat is enough to vapourise rock minerals, Dr Blewitt said.
“Mercury is a planetary oddball. It is the smallest planet yet has the highest density. It is the only planet in a spin-orbit resonance, making exactly three turns on its axis for every two revolutions about the Sun,” he told the meeting.
“Although its appearance is superficially similar to that of the Moon, Messenger has demonstrated that Mercury is radically different from the Moon in all characteristics that have been measured,” he said.
The planet as a much bigger iron core for its size than any other rocky planet, including Earth, and its surface has surprisingly high levels of sulphur and potassium. Some of its craters are also pock-marked by mysterious depressions or “hollows” that have puzzled scientists because they are not seen on the crater-scarred surface of the Moon.
“The presence of sulphur compounds in surface rocks may be responsible for the formation of hollows, strange sublimation-like features that have no counterpart on the Moon but resemble a type of terrain found on the south polar ice cap of Mars,” Dr Blewitt said.
The only previous images of Mercury have been taken either by telescopes or the Mariner 10 space probe which made three planetary fly-bys in the mid 1970s. More than 30 years later, Messengers became the first and only probe to enter orbit around the planet and take a full 360-degree observation of its landscape.