https://web.archive.org/web/20210530171304/https://tharsis.gsfc.nasa.gov/mola.summary.pdf

Zero elevation on Mars from MOLA is defined as the equipotential surface (gravitational plus rotational) whose average value at the equator is equal to the mean radius as de- termined by MOLA (cf. Table 4). The planetary radius and a gravity model derived from MGS Doppler tracking data [Lemoine et al., this issue] with the IAU91 coordinate sys- tem parameters for Mars [Davies et al., 1992a] collectively provided the geopotential of Mars’ mean equatorial radius. This equipotential surface was then extended to all latitudes as the zero-level reference for topography.