Mercury is the innermost planet in our Solar System. Because it is so close to the Sun, it can be observed from Earth for only about two hours before sunrise and two hours after sunset, and then only if the ecliptic is very steeply inclined to the horizon. Within 217 years, observers on Earth may watch Mercury passing as a black dot before the bright disk of the Sun on twenty occasions in November and on nine occasions in May. The next transit will be on 9 May 2016.
As Mercury‘s orbit is highly elliptical, its distance to the Sun differs greatly between aphelion and perihelion. At perihelion, the planet approaches the Sun to 46 million kilometers, counting from the Sun‘s center, while its distance grows to 70 million kilometers at aphelion. Because of its relative closeness to the Sun, it is fairly easy to demonstrate that the rotation of Mercury‘s perihelion is partly due to relativistic influences: it is the gravity pull of the Sun and, to a lesser extent, that of other planets that causes Mercury‘s perihelion to rotate slowly to the right around the center of gravity it shares with the Sun, so that in the long run the shape of its path around the Sun resembles a rosetta.
Mercury‘s rotation and orbital periods are linked, for it circles the Sun twice while it rotates three times around its axis. Consequently, a day/night period on Mercury extends over 176 terrestrial days. During that cycle, surface temperatures fluctuate between -180 and + 430 degrees centigrade.
The smallest planet in the Solar System, Mercury is smaller even than the Jovian moon Ganymed and the Saturnian moon Titan. Nevertheless, its mean density is comparable to that of Earth, which leads scientists to assume that Mercury must have a relatively extensive and heavy core of iron and nickel. Measurements show that the strength of Mercury‘s magnetic field is about one percent of Earth‘s.
| Mass | 3.302 x 1023 kg |
| Radius | 2,439.7 km |
| Density | 5.427 g/cm3 |
| Rotation period | 58.65 days |
| Orbital period | 88 days |
| Mean Distance from the Sun | 57.91 x 106 km |
Like the Moon, Mercury‘s surface is riddled with impact craters of all sizes, the most impressive surface structure being the Caloris basin with a diameter of 1,300 kilometers. In this case, the body that struck the planet appears to have landed with so much force that the effects of the shock waves focused in the interior are apparent on the opposite side of the planet. Roughly speaking, Mercury‘s interior resembles a little that of Earth and its surface that of the Moon. Unlike the Moon, however, Mercury shows marked scarps which suggest that its core shrank during the cooling phase.
There is much that we still do not understand about the composition of Mercury‘s surface and its inner structure. Being so close to the Sun, Mercury is difficult to observe from Earth with telescopes and equally difficult to reach with space probes: in any approach to Mercury, the immense attraction of our central star must be taken into account as well as its enormously intense radiation. At the moment, the Messenger space probe is following a complicated path which, having already led it past Mercury on two occasions, is supposed to end in an orbit around Mercury in 2011. At least, 25 percent of Mercury‘s surface that had not been mapped before could be photographed during the flybys. The new images show many features which indicate that volcanism must have once been active on Mercury.
Like the Moon, Mercury is unable to retain an atmosphere, and yet Mariner 10‘s spectrometer revealed a paper-thin gaseous shell composed of hydrogen, helium and oxygen and containing traces of sodium and potassium. However, the total mass of these volatile elements amounts to no more than a thousand kilograms. It is likely that a large proportion of these particles was provided directly by the Sun and only a little by outgassings from the planet‘s interior. Moreover, ice may be hiding in some deep craters located at the poles of the planet that are permanently cold because they never receive any solar radiation; radar observations made from Earth indicate as much.
Last update: 31/05/2010 15:26
