MARS: MORE LIKE THE MOON
Besides the earth and moon, Mars is the only planet for which scientists have seismic data. Both Viking landers carried instruments to record quakes on Mars. Unfortunately, only the one on Viking 2 worked, and on November 24,1976, it appears to have detected its first Martian quake. If real, it suggests that the Martian crust has an average thickness of about 40 kilometers, with a maximum thickness of about 75 kilometers under the Tharsis ridge and a minimum thickness in the Hellas basin of about 10 kilometers. By comparison the earth's average crust thickness is 30 kilometers or so, and it covers a planet with nearly twice the radius of Mars. Hence the crust of the earth is about 0.5 percent of its radius, while that of Mars is about 1.2 percent. The moon's crust is about 4 percent of its radius.
The seismic activity on Mars, although somewhat more extensive than that of the moon, is much less than that of the earth. This, as noted, suggests a lithosphere (crust and outer portion of the mantle) a couple of hundred kilometers thick on a chemically differentiated body, with silicate mantle and iron-rich core of about 1500 kilometers radius. Thus of the four terrestrial planets Mars has the smallest percentage of iron and the smallest core for its size.
