Data on the body of Mars are as yet insufficient to allow computation of meaningful models of the internal structure or density stratification within the planet. The flattening of the solid surface appears to be greater than the flattening of the equipotential surface by about a factor of two. The resultant possibility that the equatorial regions are high compared to the polar areas has many geological and biological implications.
Most of the inner planets—Mercury, Venus, Earth, Moon, and Mars—have bulk compositions which differ from one another as well as from that of the stony meteorites. The thermal histories of the planets have therefore differed because of different original compositions as well as different sizes and distances from the Sun.
More data on the surface topography are required in order to make geological interpretations which are not largely subjective. The dark areas could be either highlands or lowlands relative to surrounding bright areas. Although sharp relief of several thousand feet is probably absent or nearly so, substantial mountain ranges, plateaus, or basins could exist if the slopes on their flanks are gradual. Liquid water cannot exist on the surface of Mars, and aqueous erosion features, if they once existed, have become subdued by aeolian activity.
Winds on Mars may move more and coarser material than winds on Earth. Deep accumulations of aeolian deposits have formed, at least locally. Mariner IV pictures show an impact-crater distribution much like the highland areas of the Moon. Many craters with original depths of less than about 8000–10,000 feet have become obliterated on Mars by windblown dust.