Jack h. Marrion, age l0, of Duluth, Minn., for his question:

What is the surface of mercury like?

Mercury appears very close to the sun at dawn or dusk. Early telescope observations reveal permanent features on the surface of the little planet, with yellow, gray and pale patches that are almost white. Astronomers suspected its landscape was wrinkled with mountains and plains, perhaps somewhat like the rugged face of the moon. Certain extra bright spots, however, have been identified as active volcanoes.

The first of two spacecraft to visit the planet was Mariner 10, which mapped only about 45% of the planet’s surface from 1974 to 1975. The second is the MESSENGER spacecraft, which mapped another 30% during its flyby of January 14, 2008. MESSENGER will make one more pass by Mercury in 2009, followed by orbital insertion in 2011, and will then survey and map the entire planet.

Mercury is similar in appearance to the Moon: it is heavily cratered with regions of smooth plains, has no natural satellites and no substantial atmosphere. However, unlike the moon, it has a large iron core, which generates a magnetic field about 1% as strong as that of the Earth.[10] It is an exceptionally dense planet due to the large relative size of its core. Surface temperatures range from about 90 to 700 K (−183 °C to 427 °C, −297 °F to 801 °F),[11] with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest.

The average distance between mercury and the sun is only some 36 million miles. Its daylight side is hot enough to melt lead, and no planet is colder than the night side that faces away from the sun.