Mercury whips round the sun in only 88 Earth days but spins on its own axis at such a leisurely pace that it takes 176 Earth days to complete one Mercury day. As the sun rises over the bleak and blistered surface, it appears 3 times larger than on Earth and 11 times brighter, contrasting cruelly with a totally black sky which can’t scatter any light in the absence of an atmosphere. As the long day advances, temperatures rise to over 450°C, enough to melt lead and obliterate any delicate life forms. When the long night sets it, temperatures drop to a glacial -200°C.
Mercury is named after the Roman God of commerce, cunning and theft who acted as the fleet-footed messenger of the Gods, probably because of the planet’s habit of appearing briefly in the twilight of the morning or evening and then making off with the last of the light.
The planet has been visited only once before by NASA when a Mariner probe flew by in the 1970’s and photographed 30% of its surface. Now it has been targeted by a new probe known as the MErcury Surface, Space ENvironment, GEochemistry, and Ranging probe, MESSENGER for short, which just shows you how far NASA will go for a witty acronym. It was shot out from Cape Canaveral in 2004 and flung round Earth once, Venus twice, and Mercury three times, using gravitational sling shots to control its speed and avoid the gravitational catch of the Sun. It finally flew into orbit around Mercury in March of this year.
Instruments on the probe will seek information from the surface to settle many questions that have been perplexing scientists for decades. Although in appearance it’s like the moon, in composition it’s completely different. It is so dense that over 60% of its mass must arise from a metal core of iron and nickel, that’s twice as much as the Earth’s. One theory is that the planet was once larger with a mantle and crust like the Earth which was shorn off by collision with an asteroid. It also has a magnetic field like the Earth, but unlike Venus and Mars. This suggests that some of the iron core is molten and mobile, but it’s unclear where the heat to sustain this might come from. The planet’s surface is not only pock-marked by craters, including one of the largest in the solar system, the Caloris basin, but also puckered and wrinkled, which suggests that the planet has been shrinking over time, though the mechanism is unknown.
As our knowledge of the planet deepens so does our knowledge of the solar system and the origin of our own planet – a bargain at only half a billion dollars!
This blog first appeared as an article in the Cairo BCA magazine of April 2011 to promote star gazing in the Sinai