Although it is the smallest of the terrestrial planets, Mercury is the densest. If the planet has the cosmic abundance of heavy elements, then its large density requires that Mercury is 60% to 70% Fe by mass. With the iron concentrated in a central core, Mercury could best be described as a ball of iron surrounded by a thin silicate shell. In photographs obtained by the Mariner 10 spacecraft during 1974 and 1975 (Figure 1–63), portions of Mercury’s surface strongly resemble the heavily cratered lunar highlands. In addition, there are large areas of relatively smooth terrain and a number of ringed basins believed to be impact structures. The largest of these is the 1300-km-diameter Caloris basin, which is similar to the Imbrium and Orientale basins on the Moon. The Caloris basin is covered with a relatively smooth plains type of material, perhaps similar to the lunar maria, having many fewer craters than the heavily cratered terrain. Areas of relatively smooth terrain known as intercrater plains are also found interspaced between the basins and craters. Lobate scarps, probably curved fault scarps, which are several kilometers high and extend for hundreds of kilometers across Mercury’s surface, have no lunar counterpart. These scarps are suggestive of thrust faults resulting from crustal shortening and compression. Several hypotheses have been advanced to explain the compressional surface features on Mercury. The first hypothesis concerns tidal despining. Early in its evolution Mercury may have had a rapid rotation. If the planet was hot it would have had a near hydrostatic shape with considerable polar flattening and an equatorial bulge. As the planet cooled, a global lithosphere developed with considerable rigidity and ellipticity. However, tidal interactions with the sun gradually slowed the rotation of the planet. The rigidity of the lithosphere preserved a fossil ellipticity associated with the early rapid rotation but as a result large lithospheric stresses developed. The resultant compressional stresses in the equatorial region are one explanation for the observed compressional features. An alterative explanation is that they were caused by the formation and/or solidification of the large iron core on Mercury. Mercury’s high mean density of 5440 kg m−3, almost equal to the Earth’s, is attributed to a large iron core with a 500 to 600 km thick cover of silicate rocks. One explanation for the high mean density is that a massive collision blasted off a large fraction of an early mantle of larger size. Magnetic field measurements by Mariner 10 showed that Mercury has an intrinsic global magnetic field. Because of the limited amount of data, there are large uncertainties in the inferred value of Mercury’s magnetic dipole moment. Most estimates lie in the range of 2 to 5 × 1019 A m2, or about 1.18 Mars 99 5 × 10−4 of the Earth’s magnetic field strength. Although a magnetized crust cannot be ruled out as a source of this field, it seems more likely that it originates by dynamo action in a liquid part of Mercury’s core. Because of the similarities in the surfaces of Mercury and the Moon, their evolutions must have been similar in several respects. Separation of the iron and silicates in Mercury and crustal differentiation must have occurred very early in its history because the planet’s surface preserves an ancient record of heavy bombardment similar to the lunar highlands. The filling of the Caloris basin must have occurred subsequent to the termination of this severe cratering phase because the basin material is relatively free of craters. The lobate scarps must also have formed at the end of or subsequent to the early phase of severe bombardment because they sometimes pass through and deform old craters (Figure 1–64). The scarps may be a consequence of the cooling and contraction of the core, and if so, they are the only surface features that distinguish Mercury with its large core from the Moon with only a very small core or none at all.