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Page 150 of 851

  • 20 - The Terrestrial Planets

  • from III - THE SOLAR SYSTEM
  • Bradley W. Carroll, Weber State University, Utah, Dale A. Ostlie, Weber State University, Utah
  • Book:
    An Introduction to Modern Astrophysics
    Published online:
    16 August 2019
    Print publication:
    07 September 2017, pp 737-774

  • Summary

    MERCURY

    The four terrestrial planets have a number of characteristics in common, such as being small, rocky, and slowly rotating (see Table 19.1). Our own Moon and several of the moons of the giant planets also share many of those same characteristics. In this chapter we shall focus our attention on the terrestrial planets and their moons, saving our discussion of the giant planets and their systems for Chapter 21.

    The 3-to-2 Spin–Orbit Coupling of Mercury

    As we learned in Section 17.1, the innermost planet, Mercury (Fig. 20.1), orbits so close to the Sun (0.39 AU) that Kepler's laws begin to break down. The reason is that spacetime in the vicinity of massive objects is affected in such a way that Newton's familiar inversesquare law (Eq. 2.11) is no longer a completely adequate description of gravity. It was the slow advance of the perihelion point of Mercury's rather eccentric orbit (e = 0.2056) that presented one of the first tests of Einstein's general theory of relativity.

    The first hint that Mercury's orbit also exhibits another curious feature came in 1965 when Rolf B. Dyce and Gordon H. Pettengill successfully bounced radar signals off the planet using the Arecibo radio telescope. The reflected signals had a spread of wavelengths that revealed Mercury's rotation speed; because of the Doppler effect, radio waves that hit the approaching limb were blueshifted and those that struck the receding limb were redshifted. These observations indicated that Mercury's rotation period was approximately 59 days. More precise measurements made by the Mariner 10 spacecraft during its repeated flybys of the planet in 1974 and 1975 showed that the rotation period was actually 58.6462 days, exactly two-thirds the length of its sidereal orbital period of 87.95 days.

    How this peculiar 3-to-2 relationship between rotation and orbital periods developed can be understood in light of the process of tidal evolution discussed in Section 19.2. At perihelion, Mercury experiences the strongest tidal force, causing the planet to try to align its bulge axis along the line connecting the planet's center of mass to the center of mass of the Sun.

Page 150 of 851