The Moon's average density, about 3300 kg/m3, is similar to the measured density of lunar surface rock, virtually eliminating any chance that the Moon has a large, massive, and very dense nickel—iron core like that of Earth. In fact, the low density implies that the entire Moon is actually deficient in iron and other heavy metals relative to our planet. In addition, there is no evidence for any lunar magnetic field. As we saw in Chapter 7, researchers believe that planetary magnetism requires a rapidly rotating liquid metal core, like Earth's. (Sec. 7.4) Thus, the absence of a lunar magnetic field could be a consequence of the Moon's slow rotation, the absence of a liquid core, or both.
Our knowledge of the details of the Moon's interior is very limited. Models based on the available data indicate that the Moon's interior is of rather uniform density. As depicted schematically in Figure 8.24, the models suggest a core of perhaps 200-km radius surrounded by a roughly 500-km-thick inner mantle of semisolid rock having properties similar to Earth's asthenosphere. The core is probably somewhat more iron-rich than the rest of the Moon (although it is still iron-poor compared with Earth's core). Near the center, the temperature may be as low as 1500 K, too cool to melt rock. However, seismic data collected by sensitive equipment left on the surface by Apollo astronauts (see Interlude 8-1) suggest that the inner parts of the core may be at least partially molten, implying a somewhat higher temperature.
Figure 8.24 Cross-sectional diagram of the Moon. Unlike Earth's rocky lithosphere, the Moon's is very thickabout 1000 km. Below the lithosphere is the inner mantle, or lunar aesthenosphere, similar in properties to that of Earth. At the center lies the core, which may be partly molten.
Above these regions lies an outer mantle of solid rock, some 900950; km thick, topped by a 60150;-km crust (considerably thicker than that of Earth). Together these layers constitute the Moon's lithosphere. The crust material, which forms the lunar highlands, is lighter than the mantle, which is similar in chemical composition to the lunar maria.
The crust on the lunar far side is thicker than that on the side facing Earth. If we assume that lava takes the line of least resistance in getting to the surface, then we can readily understand why the far side of the Moon has no large mariavolcanic activity did not occur on the far side simply because the crust was too thick to allow it to occur there. But why is the far-side crust thicker? The answer is probably related to Earth's gravitational pull. Just as heavier material tends to sink to the center of Earth, the denser lunar mantle tended to sink below the lighter crust in Earth's gravitational field. The effect of this was that the crust and the mantle became slightly off center with respect to each other. The mantle was pulled a little closer to Earth, while the crust moved slightly away. Thus, the crust became thinner on the near side and thicker on the far side.
Mercury's magnetic field, discovered by Mariner 10, is about 1/100 that of Earth. Actually, the discovery that Mercury has any magnetic field at all came as a surprise to planetary scientists. Having detected no magnetic field in the Moon (and, in fact, none in Venus or Mars, either), they had expected Mercury to have no measurable magnetism. Mercury certainly does not rotate rapidly, and it may lack a liquid metal core, yet a magnetic field undeniably surrounds it. Although weak, the field is strong enough to deflect the solar wind and create a small magnetosphere around the planet.
Scientists have no clear understanding of the origin of Mercury's magnetic field. If it is produced by ongoing dynamo action, as in Earth, Mercury's core must be at least partially molten. Yet the absence of any recent surface geological activity suggests that the outer layers are solid to a considerable depth, as on the Moon. It is difficult to reconcile these two considerations in a single theoretical model of Mercury's interior. If the field is being generated dynamically, Mercury's slow rotation may at least account for the field's weakness. Alternatively, Mercury's current weak magnetism may simply be the remnant of an extinct dynamothe planet's iron core may have solidified long ago but still bears a permanent magnetic imprint of the past. The models are inconclusive on this issue, and no spacecraft is scheduled to revisit Mercury in the foreseeable future.
Mercury's magnetic field and large average density together imply that the planet is differentiated. Even without the luxury of seismographs on the surface, we can infer that most of its interior must be dominated by a large, heavy, iron-rich core with a radius of perhaps 1800 km. Whether that core is solid or liquid remains to be determined. Probably a less dense, lunarlike mantle lies above this core, to a depth of about 500 to 600 km. Thus, about 40 percent of the volume of Mercury, or 60 percent of its mass, is contained in its iron core. The ratio of core volume to total planet volume is greater for Mercury than for any other object in the solar system. Figure 8.25 illustrates the relative sizes and internal structures of Earth, the Moon, and Mercury.
Figure 8.25 The internal structures of Earth, the Moon, and Mercury, drawn to the same scale. Note how large a fraction of Mercury's interior is core.
Terrestrial Planets Part II