水星（英文）mercury.doc

/subjects/astronomy/planets/ Mercury Mercury (0.4 AU) is the closest planet to the Sun and the smallest planet (0.055 Earth masses). Mercury has no natural satellites, and its only known geological features besides impact craters are wrinkle-ridges, probably produced by a period of contraction early in its history.Mercurys almost negligible atmosphere consists of atoms blasted off its surface by the solar wind.Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, and that it was prevented from fully accreting by the young Suns energy. GENERAL INFORMATION ON MERCURY  Mercury is the planet closest to the Sun in our Solar System. This small, rocky planet has almost no atmosphere. Mercury has a very elliptical orbit and a huge range in temperature. During the long daytime (which lasts 58.65 Earth days or almost an entire Mercurian year, which is 88 days long), the temperature is hotter than an oven; during the long night (the same length), the temperature is colder than a freezer. Mercury is so close to the Sun that you can only see it near sunrise or sunset.  Craters on the surface of Mercury. Mercury is a heavily cratered planet; its surface is similar to the surface of our Moon. Cratering on Mercury triggered volcanic eruptions that filled much of the surrounding area. Mercury does have a magnetic field (probably generated by a partly-liquid iron core). SIZE Mercury is about 3,031 miles (4,878 km) in diameter. It is the smallest planet in our solar system (it used to be considered the second-smallest planet, when Pluto was still considered to be a planet). Mercury is a bit over one third of the diameter of the Earth. Mercury is only slightly larger than the Earths moon. MASS AND GRAVITY Mercurys mass is about 3.3 x 1023 kg. This is about 1/20th of the mass of the Earth. The gravity on Mercury is 38% of the gravity on Earth. A 100 pound person