Although both Viking landers carried a seismometer on board, there were
no seismic data collected. Due to instrument packaging difficulties, the
two seismometers were put on the top of the landers. As a result, they
became wind gauges instead of instruments to detect Martian quakes. Like
all other celestial bodies, therefore, the Martian interior can only be
deduced at present from geophysical observations such as mean density
and mean radius. It is estimated that Mars has a low density crust about
25-70 km thick.