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The intenal structures of the moon, Mars, Venus, and Mercury are examined in the light of what is known about the constitution of the earth. A review of the seismic determination of the elastic constitution of the earth's mantie, using results on the stability of silicates at high pressures, leads to the following interpretation: the rapid increase of velocity beginning at 200 km depends on the olivine-spinel transition and the breakdown of silicates to oxides. Preliminary calculations of the stability field of periclase (MgO) and stishovite (high-density SiO/sub 2/) relative to olivine (MgSiO/sub 4/) indicate that the oxides are stable at pressures greater than 1.0 x 10/sup 5/ 1.5 x 10/sup 5/ bars. The oxide transition produces a change in volume of about 20 per cent. The gravitational figure of the earth, as obtained from satellite orbits, is used to estimate the possible deviations from hydrostatic equilibrium on other planets. The near coincidence of the present rate of heat production of a chondritic earth and the present surface heat flow is discussed as a limiting condition of the internal thermal structure of the earth. Observations of the orbital and rotational motion of the moon give its gravitational figure. Calculations ofmore » its thermal structure show that a model moon with uniform radioactivity and chondritic composition is inconsistent with the calculated gravitational figure. The inferred strength of the moon requires either that the radioactivity be substantially less than that of chondrites or that the heat sources be concentrated in the outer layers. The problem of differentiation without melting is noted. The average lunar material has a radioactivity perhaps one- half, or less, that of chondritic materials. The astronomical data on the mean density and gravitational figure of Mars are examined. If the mean radius of Mars is taken to be 3310 km, the planet must have a surface density of about 3.8 to 3.9 and must be nearly homogeneous. If Mars were homogeneous and had a radioactivity equal to that of chondrites, the interior would be molten and large- scale gravitational differentiation would be expected. Since this differentiation is not apparent in the gravitational data, it is concluded that the radioactive composition of Mars differs from that of chondrites. The internal structures of Mercury and Venus are examined in terms of their inferred rotational history. In conclusion, the inner planets differ both in the abundances of the heavy elements and in the abundances of potassium, uranium, and thorium. Chondrites may provide a satisfactory chemical model for the earth but not for the other inner planets; Venus is a possible exception, only on the grounds of ignorance of its intenal constitution. (auth)« less