equipotential

For one thing, they don’t seem to quite follow a so-called equipotential surface.

But on a spinning planet, centripetal acceleration counters gravitational acceleration, so the equipotential surfaces depend to a large degree on where the spin axis of a planet is in relation to its geographic features.

However, these shorelines fail to follow an equipotential surface, and this has been used to challenge the notion that they formed via an early ocean9 and hence to question the existence of such an ocean.

However, surface loading from the oceans can drive polar wander only if Tharsis formed far from the equator10, and most evidence indicates that Tharsis formed near the equator11,12,13,14,15, meaning that there is no current explanation for the shorelines’ deviation from an equipotential that is consistent with our geophysical understanding of Mars.

The result of all these factors is that the surface may look perfectly "flat" to the naked eye, but it's really a slightly rough, irregular and undulating surface, which approximately follows an equipotential gravity surface on the surface of the Earth.