![]() ![]() The degree of broadening provides an exact measurement of the rotation rate of the planet. The resulting Doppler shift spreads or broadens the precise transmitted radar-wave frequency into a range of frequencies in the reflected signal (Figure 2). If a planet is turning, one side seems to be approaching Earth while the other is moving away from it. Radar observations of Mercury in the mid-1960s, however, showed conclusively that Mercury does not keep one side fixed toward the Sun. The effect is to cause both a redshift and a blueshift, widening the spread of frequencies in the radio beam. Doppler Radar Measures Rotation: When a radar beam is reflected from a rotating planet, the motion of one side of the planet’s disk toward us and the other side away from us causes Doppler shifts in the reflected signal. The existence of this field is consistent with the presence of a large metal core, and it suggests that at least part of the core must be liquid in order to generate the observed magnetic field.įigure 2. Unlike the Moon, Mercury does have a weak magnetic field. ![]() We could think of Mercury as a metal ball the size of the Moon surrounded by a rocky crust 700 kilometers thick (Figure 1). The core has a diameter of 3500 kilometers and extends out to within 700 kilometers of the surface. The most likely models for Mercury’s interior suggest a metallic iron-nickel core amounting to 60% of the total mass, with the rest of the planet made up primarily of silicates. Mercury’s high density tells us that it must be composed largely of heavier materials such as metals. Mercury’s composition is one of the most interesting things about it and makes it unique among the planets. Mercury’s density is 5.4 g/cm 3, much greater than the density of the Moon, indicating that the composition of those two objects differs substantially. Mercury is the smallest planet (except for the dwarf planets), having a diameter of 4878 kilometers, less than half that of Earth. Mercury’s mass is one-eighth that of Earth, making it the smallest terrestrial planet. Mercury’s Internal Structure: The interior of Mercury is dominated by a metallic core about the same size as our Moon. However, because its orbit has the high eccentricity of 0.206, Mercury’s actual distance from the Sun varies from 46 million kilometers at perihelion to 70 million kilometers at aphelion (the ideas and terms that describe orbits were introduced in Orbits and Gravity). The semimajor axis of Mercury’s orbit-that is, the planet’s average distance from the Sun-is 58 million kilometers, or 0.39 AU. As you might expect, it’s best seen when its eccentric orbit takes it as far from the Sun as possible. Because Mercury remains close to the Sun, it can be difficult to pick out in the sky. It is appropriately named for the fleet-footed messenger god of the Romans. Mercury is the nearest planet to the Sun, and, in accordance with Kepler’s third law, it has the shortest period of revolution about the Sun (88 of our days) and the highest average orbital speed (48 kilometers per second). As described later in this chapter, it also shares with the Moon the likelihood of a violent birth. Like the Moon, it has no atmosphere, and its surface is heavily cratered. The planet Mercury is similar to the Moon in many ways. Summarize our ideas about the origin and evolution of Mercury.Describe the topography and features of Mercury’s surface.Explain the relationship between Mercury’s orbit and rotation.Describe Mercury’s structure and composition.Characterize the orbit of Mercury around the Sun.By the end of this section, you will be able to: ![]()
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