What's sea level on the moon?

[dracoi]

@SlowMindThinking

Slight nit. I doubt it is referred to as a geoid, since “geo” refers to the earth. But, yes, altitudes should be given with respect to a standard, determinable reference shape.

Words with “geo” in them are often applied to other planet-like bodies. You have lunar geography and lunar geology, for example.

[Bytegeist]

@Polycarp

Going back to Bonzer’s quote from Wikipedia, it sounds like Mars has an actual “sea level” – that is, the zero reference is the point below which (due vto higher atmospheric pressure at lower altitudes) liquid water could exist if Mars had any. Am I reading that correctly.

It sounds like a standard for calibrating a barometer on the surface of Mars. Maybe it’s been used by some of the lander probes?

The USGS has this to say about an elevation standard. (Warning: PDF again.) Bolding is mine.

The rotational ellipsoid with best-fit dimensions A = 3396.19 km, B = 3376.20 km is being adopted by the USGS as the reference surface for map projections for a variety of Mars image mosaics and maps currently in production and will be used in products generated in the foreseeable future. [ … ]

It is important to note that the ellipsoid will be used only as the reference surface for defining map projections. Other surfaces can be adopted for other purposes, with no inconsistency. In particular, the detailed MOLA topographic model will be used for accurate projection of images and other remote-sensing data onto the planet, and a spherical-harmonic representation of an equipotential surface will be used as the reference for elevations.

(MOLA is the altimeter instrument on the Mars Global Surveyor.) The IAU appear to use the same standard:

Recommended topographic reference surface for Mars. As cited in the current report, the topographic reference surface of Mars is that specified in the final MOLA Mission Experiment Gridded Data Record (MEGDR) Products. [ … ] http://pds-geosciences.wustl.edu/missions/mgs/mola.html

From that page:

The MOLA Mission Experiment Gridded Data Records (MEGDRs) are global topographic maps of Mars created by binning altimetry values from the MOLA PEDR products acquired over the entire MGS mission. MEGDRs have been produced at resolutions of 4, 16, 32, 64, and 128 pixels per degree. [ … ] A MEGDR product consists of a set of three or four maps:
[ol]
[li]The planetary radius as recorded by the MOLA instrument[/li][li]The areoid, a model for an equipotential surface of Mars, analogous to “sea level” on Earth (this map is omitted for the 32, 64 and 128 pixel/degree and polar MEGDRs)[/li][li] The topography computed as the difference between the planetary radius and the areoid[/li][li] …[/li][/ol]