First Planet from the Sun Mercury, the planet nearest the sun, is difficult to observe from the Earth because it rises and sets within two hours of the sun. ______【46】 Planetary scientists can estimates the age of a planet's surface by the number of impact craters on it. In general, the older the surface, the more craters it has. ______【47】. Between these regions are areas of gently rolling plains that may have been smoothed by volcanic lava flows or by accumulated deposits of fine material ejected during impacts. These plains are also old enough to have accumulated a large number of impact craters. Elsewhere on the planet are smooth, flat plains that are probably younger and volcanic in origin. ______【48】. Sometime between the formation of the intercrater plains and the formation of the smooth plains, the whole planet may have shrunk as it cooled, causing the crust to buckle and form. the long, steep cliffs called scarps. The largest impact basin on Mercury has a diameter of about 800 miles (1, 300 kilometers) and is surrounded by mountains that rise to heights of about 1. 2 miles (2 kilometers). ______【49】. On the opposite side of the planet is an area of hilly, linear terrain that probably resulted from seismic waves caused by the same impact. Like other airless, solid bodies in the solar system, the entire surface of Mercury is covered with a layer of rubble called regolith, which is composed of material, ranging from dust to boulders, that was scattered when impact craters were formed. This debris was in turn broken up and redistributed by subsequent impacts. Mercury is very dense and has a magnetic field that is about 1 percent as strong as earth's. This suggests the existence of a planetary core composed of iron and nickel and constituting about 40 percent of the planet's volume. ______【50】. Radar images taken of Mercury in 1991 show what are considered to be large ice patches at the planet's North Pole.