and transects two craters 35 and 55 kilometres in diameter. It is up to 2 km high in some places and it appears to be a fault produced by compression of the crust. (NASA/JPL/Northwestern University)

Mercury's relatively small size and proximity to the Sun, resulting in low gravity and high temperature, are the reasons for its lack of atmosphere. There is a layer made up of atoms blasted off the surface by the solar wind. The tenuous "atmosphere" is composed mainly of oxygen, sodium, and helium. The atoms quickly escape into space and are constantly replenished.

Due to the absence of an atmosphere, the temperature on Mercury drops very rapidly after sunset. The rotational axis is almost perpendicular to the orbital plane; therefore it is possible that, close to the poles, there are areas where the temperature is permanently below the freezing point. Radar echos from the surface of Mercury show several anomalously reflective and highly depo-

larized features at the north and south poles. Some of these areas can be addressed to the craters, the bottoms of which are permanently in shadow. One candidate of the radar-bright features is water ice that has survived in the permanent shadow.

The only relevant data concerning the interior of Mercury were obtained during the Mariner 10 fly-by when the gravity field was measured. Since Mercury has no satellites, the mass (and mass distribution) and density could not be determined before the force exerted by the gravitational field on a spacecraft was measured.

It has been said that Mercury looks like the Moon from the outside but is terrestrial from the inside. According to theoretical models, the internal structure is similar to that of the Earth but the core is substantially larger. The density of the planet is about the same as that of the Earth, indicating that the size of the Fe-Ni core is roughly about 75% of the planet's radius.

Due to the vicinity of the Sun, the temperature of the primeval nebula at the distance of Mercury was quite high during planetary formation. Thus the relative abundances of the volatile elements are smaller than on any other terrestrial planet.

Mercury has a weak magnetic field, about 1% as strong as that of the Earth. The presence of the magnetic field is unexpected because Mercury is much smaller than the Earth and it rotates slowly. According to the dynamo theory, a magnetic field is generated by flows in a liquid, electrically conducting core. The magnetic field cannot be a remnant from ancient times, since the internal temperature of the planet must have exceeded the critical Curie point. Therefore, it must be assumed that a part of the core is molten.

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