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September 3

gravity on Phobos

The article Phobos once contained this sentence: Because of its shape alone, the gravity on its surface varies by about 210%; the tidal forces raised by Mars more than double this variation (to about 450%) .... I asked for clarification (% of what?) but never got any. Does it mean that the apparent gravity at the near/far poles is negative? Can someone tell us what local apparent gravity is at each of the three pairs of poles (in/out, north/south, leading/trailing)? —Tamfang (talk) 04:21, 3 September 2023 (UTC)[reply]

Possibly, it was supposed to mean that if the minimum value of apparent gravity over the surface is

x

ms⁻², the maximum value is 450% higher, that is, 1.45

x

ms⁻². In another interpretation it refers to the ratio, giving a maximum value of 4.5

x

ms⁻². --Lambiam 09:52, 3 September 2023 (UTC)[reply]

This source gives the surface gravity of Phobos as ranging from 0.3 cm/s² to 0.6 cm/s², which fits with a ratio interpretation of 210% = 2.1. Using, for example, an average surface gravity of 0.44 cm/s² and a tidal acceleration of 0.28 cm/s², we get a ratio of (0.44+0.28)/(0.44−0.28) = 4.5. So there is no need for negativity. --Lambiam 10:13, 3 September 2023 (UTC)[reply]

This appears to be a reliable source that gives detailed data. It is very technical, though (and also behind a paywall). Determining the minimum and maximum values for real and apparent gravity from the given data is not an elementary exercise. --Lambiam 10:38, 3 September 2023 (UTC)[reply]