Mean Radius of Mars

Mars is about half Earth's radius and, like Earth, is flattened by rotation — its day is only 40 minutes longer than ours. But the ellipsoid is a poor description of the real surface, because Mars has the most extreme topography of any rocky planet: Olympus Mons rises 22 km above the datum, and the Tharsis bulge lifts a continent-sized region several kilometres, while Hellas basin sinks 7 km below. There is no sea level to reference, so the datum is defined by a chosen mean-pressure surface and a gravity model.

The northern lowlands sit on average 5 km below the southern highlands — the Martian dichotomy, plausibly the scar of an enormous early impact. The mean radius still gives a useful surface area, about 145 million km², almost exactly the land area of Earth.