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In the following gravity map of Mars the changes in gravity are shown according to mass distribution:

Map removed

However, since no moon or planet other than the Earth with 9.80665 m/s² has a surface gravity defined by standard, I wonder what counts as 0 mGal there. We see the differences from an obviously average gravity that isn't given. The average surface gravity of Mars is 3.72076 m/s². Do you know whether the value was used on that map?

I have a same question for the following gravity map of the Moon (was 1.625 m/s² taken as average?):

Map removed

I wonder why they don't mention which gravity they use as standard. Gravity maps like these are far more informative:

Map removed

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  • $\begingroup$ I have a hunch that the maps show deviations form a spherically symmetric potential on a spherical surface with some reference radius, but I cant say that with surety. You may find this answer and others there helpful, as well as the discussion of Bouguer gravitational anonaly in this question. You might also find discussions on this and this page interesting. $\endgroup$
    – uhoh
    Apr 6 '20 at 11:49
  • $\begingroup$ @uhoh The "0 mGal" value should be mentioned in Bouguer anomaly maps. $\endgroup$
    – user35272
    Apr 6 '20 at 12:08
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    $\begingroup$ There is no "0 mGal" value. It's mostly differences to what you would expect from a perfectly homogeneous body. All the effects of height, oblateness, latitude, terrain shape ... have been subtracted. It's not a "gravity map" that tells you which gravity you feel at any given point. $\endgroup$
    – asdfex
    Apr 6 '20 at 14:30
  • $\begingroup$ @asdfex Thank you. That's an answer too. I ask you to transform your comments into official answers. $\endgroup$
    – user35272
    Apr 6 '20 at 15:09
  • $\begingroup$ I will when I have time. Writing an answer takes a bit more time than commenting. Btw, you might want to comply with the licenses of the images you copied here. In wikipedia they are licensed cc-by so that you are required to name the author. $\endgroup$
    – asdfex
    Apr 7 '20 at 10:50
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The Gravity Anomaly or Bouguer Anomaly Maps are no actual gravity maps. They are rather a map of ground density, expressed as an excess or deficit of gravity caused by differences in density.

The maps are the result of measuring the gravity and subtracting all effects of oblateness, terrain shape, terrain height and so on. The result is a map of differences in gravity due to the structure of the material under the surface.

The image below compares the elevation, surface gravity and gravity anomaly maps of the Moon. The difference of the plots can be seen rather nicely around the two large craters close to the border of the maps:

Maps of elevation, surface gravity and gravity anomaly of the Moon

From the elevation map we can see that both crates have a somewhat three-level structure: an deep inner part (blueish), an outer part with intermediate depth (greenish) and surrounding mountains (reddish).

If the underground was homogeneous, we might expect to see the same structure in the surface gravity plot: The deeper the terrain, the higher the gravity due to being closer to the more dense core of the Moon. But, there is only a two-fold structure: High gravity in the inner part of the crater, and then a somewhat constant gravity despite the elevation change.

This is reflected in the Gravity Anomaly plot: The ring with intermediate elevation has a negative anomaly (dark blue) while the inner part and the surroundings have a positive anomaly (red). That means that the outer ring of the craters is made up of less dense material. Not being a geologist I'd guess that this is due to material kicked up during the impact and settling in a more loose way when falling back down.

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