A scenario in the far future -- The moon has been successfully terraformed. All problems have been worked-out, from flinging volatiles into collision course with the moon, up to creating and retaining a stable and breathable atmosphere. With state-of-the art technology, an artificial magnetic field shields the atmosphere from solar winds, and additional shielding retains it despite the moon's low gravity.

While deploying lunar orbiters, astronomers had to take mascons into account. The mascons create fluctuations in the surface gravity. Along with the moon's low gravity, the changes are sufficient to disturb lunar orbiters and kick them out of orbit, if the right orbit was not chosen. Here is a map of gravitational distribution affected by mascons. (View source here, from the GRAIL mission). I did not find information on how noticeable the anomaly would be to an astronaut standing on the surface.

The question -- One (among many) of those anomalies happens to be under the ocean's floor. Given the relatively high anomalies (did not find the exact figure in percentage compared to average), would the anomaly cause some water to "pile-up" and create a "hill" noticeable from some distance? Would that be a significant "curve" of the water's surface to make a tourist attractions for colonists and visitors alike?

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    $\begingroup$ it would be way too small to see. $\endgroup$ – Fattie Dec 14 '18 at 20:47
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    $\begingroup$ a great idea though! :) $\endgroup$ – Fattie Dec 14 '18 at 20:47

This article gives us some useful numbers:

The mascons' gravitational anomaly is so great—half a percent—that it actually would be measurable to astronauts on the lunar surface. "If you were standing at the edge of one of the maria, a plumb bob would hang about a third of a degree off vertical, pointing toward the mascon," Konopliv says.

So the "water hill" would essentially be a plateau, hundreds of kilometers in diameter, with a slope of a third of a degree at the edges. It wouldn't be prominent enough to be noticeable, or distinguishable from the normal curvature of the Moon (where 1/3 degree is equivalent to 10 km).

Kicking satellites out of orbit might seem impressive, but keep in mind that this happens over time, after the satellites passes over the mascons many times and is nudged a tiny bit each time. And an orbit is a rather delicate thing. The local effects of a mascon are not really that impressive.

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    $\begingroup$ I'm struggling to think of a way to define "vertical" that's not "straight up and down" so that we could measure when gravity is no longer "vertical".... $\endgroup$ – Wildcard Dec 15 '18 at 2:49
  • $\begingroup$ @Wildcard Me too. What we perceive as vertical is exactly what the plumb bomb indicates. All you'd perceive is, that the terrain is slightly slanted, and by an amount that's negligible compared to the slanted terrains in mountainous regions, crater outskirts, etc. I even doubt that you would be able to realize that the horizon is not a straight line... $\endgroup$ – cmaster - reinstate monica Dec 15 '18 at 11:03
  • $\begingroup$ @Wildcard "vertical" here means a straight line between you and the center of the Moon, I suppose, but that's indeed quite hard to measure. $\endgroup$ – Aetol Dec 17 '18 at 9:39

What you ask already happens on Earth, where gravitational anomalies influence the ocean level.

These hills and valleys are not visible to the naked eyes, but can be detected by satellites.

The first attempts of measuring G were actually done measuring gravitational anomalies close to large mountains.

I guess the same would happen on the Moon. There would be hills where gravity is lower than average and valleys where the gravity is higher than average, but they would hardly be noticeable to the naked eyes. Or even be perceived by the unaided human body.

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    $\begingroup$ Other way round. The hills (in the equipotential surface, relative to the reference ellipsoid) are where the gravity is higher than average; the valleys are where the gravity is lower than average. $\endgroup$ – ecatmur Dec 14 '18 at 15:17
  • $\begingroup$ @ecatmur Not necessarily this way round. Have a look at the wikipedia article - mascons can and do sometimes actually happen in a depression. $\endgroup$ – hitchhiker Dec 14 '18 at 22:33
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    $\begingroup$ @hitchhiker But once this depression is filled with an ocean, the water will concentrate at the location of the mascon. $\endgroup$ – cmaster - reinstate monica Dec 15 '18 at 11:05

Here's a similar question which has an answer talking about the relative field strengths of Mascons compared to the regular surface gravity of the moon.

The short answer is that mascons produce tiny fractions of a percent of difference and you wouldn't notice the difference as you walked through them.

I would not expect that a Mascon would produce enough effect over a small enough area to be noticeable. Gravity as a force has a pretty shallow dropoff curve compared to the other fundamental forces, the International Space Station for example still receives about 89% of normal surface gravity at 250 miles up.


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