Mercury

The composition of the motions of rotation and revolution makes that Mercury is the only planet in which the day is around the double of the year. This peculiarity, added to the proximity of the Sun and the almost total absence of atmosphere (traces of helium), is cause of the strong thermal excursion in the day and the night: from 400°C on the face illuminated to -170°C on the face in shade.

The aspect of the surface was revealed with abundance of details by the images gathered by the probe Mariner 10, that it flew for three times over the planet, passing at only 800 km from the surface, between 1974 and 1975. The most numerous structures are the impact craters, due to the collision with meteoritic bodies. There are besides ample zones less craterous which were called planitiae: the greatest one is Planitia caloris, originated probably by a big meteorite that has dug a breadth basin then filled by lave and deposits.

Mercury has also a small but well defined magnetic field: it is possible therefore that the planet, for some time inactive in surface, still possesses some residual of inside activity.