For other bodies a fixed surface feature is usually referenced, which for Mars is the meridian passing through the crater Airy-0.
Common European Framework of Reference for Languages | Reference | Self-reference | reference | Frame of reference | Vouch by Reference | The bureaucrat's offices are laid out in reference to a Rubik's Cube | Terrestrial reference frame | Standard Color Reference of America | self-reference | Role and reference grammar | Reference work | Reference question | Reference interview | Reference Daily Intake | Reference.com | Programming language reference | Magnetic Reference Laboratory | James J. Hill Reference Library | IPv4 subnetting reference | International Earth Rotation and Reference Systems Service | Inertial frame of reference | Guide to Reference | Frick Art Reference Library | frame of reference | Dietary Reference Intake | CIDOC Conceptual Reference Model | centrifugal force (rotating reference frame) | causal theory of reference | Brokaw bandgap reference |
Undulation of the geoid, the separation between the geoid and the reference ellipsoid of the Earth