ATMOSPHERE
| One surprising discovery was that Mercury has an atmosphere, albeit a very tenuous one. A deep atmosphere would not be expected because Mercury has a low surface gravity -- only about one-third that of Earth. Any atmosphere gases would long since have escaped into space. There are faint traces of gaseous helium which form 98 per cent of Mercury's 'atmosphere' with the remainder composed of hydrogen, minute traces of argon and neon. Scientists predict that the helium is probably captured from the sun because any gases that were emanated from the interior of the planet would have long ago dissipated into space. Because it has so little atmosphere, it means that Mercury's surface temperature varies widely. The mid-day temperature facing the sun is 620° F and the night temperature is 346° F. There is great variation in temperature because there is no atmosphere to hold the heat. Discovery of Mercury having a magnetic field also came as a surprise. But the strength of this field is only about 1% that of Earth. |