Because Mars has no oceans and hence no "sea level", a zero-elevation surface had to be selected as a reference level; this is called the areoid124 of Mars, analogous to the terrestrial geoid. Zero altitude was defined by the height at which there is 610.5 Pa (6.105 mbar) of atmospheric pressure.125 This pressure corresponds to the triple point of water, and it is about 0.6% of the sea level surface pressure on Earth (0.006 atm).126 In practice, today this surface is defined directly from satellite gravity measurements.
125Zeitler, W.; Ohlhof, T.; Ebner, H. (2000). "Recomputation of the global Mars control-point network" (PDF). Photogrammetric Engineering & Remote Sensing. 66 (2): 155–161. Archived from the original (PDF) on November 13, 2011. Retrieved December 26, 2009.
126Lunine, Cynthia J. (1999). Earth: evolution of a habitable world. Cambridge University Press. p. 183. ISBN 978-0-521-64423-5.
Question: For Mars' areoid and geoid (if there is one) or gravitational zero elevation datum:
- How were they defined? For the areoid, how were atmospheric models near the equator and pole measured and averaged (over a day and and over a year) to establish the 610.5 Pa surface. For the gravitational zero elevation datum, how was gravity data process to choose or decide a zero altitude surface?
- What are their shapes? Is either one spherical, or are they prolate spheroids. If so, did they have the same eccentricity or oblateness factor?