Why Does The Earth Pull On One Side Of The Moon More? Is The Moon Lopsided?

Why does the Earth pull on one side of the Moon more than the other side? Is it because the mass of the Moon is not even, and one side has more mass than the other?
This image shows the variations in the lunar gravity field as measured by NASA’s Gravity Recovery and Interior Laboratory (GRAIL) during the primary mapping mission from March to May 2012. Very precise microwave measurements between two spacecraft, named Ebb and Flow, were used to map gravity with high precision and high spatial resolution. The field shown resolves blocks on the surface of about 12 miles (20 kilometers) and measurements are three to five orders of magnitude improved over previous data. Red corresponds to mass excesses and blue corresponds to mass deficiencies. The map shows more small-scale detail on the far side of the moon compared to the nearside because the far side has many more small craters. Image credit: NASA/JPL-Caltech/MIT/GSFC

This image shows the variations in the lunar gravity field as measured by NASA’s Gravity Recovery and Interior Laboratory (GRAIL) during the primary mapping mission from March to May 2012. Very precise microwave measurements between two spacecraft, named Ebb and Flow, were used to map gravity with high precision and high spatial resolution. The field shown resolves blocks on the surface of about 12 miles (20 kilometers) and measurements are three to five orders of magnitude improved over previous data. Red corresponds to mass excesses and blue corresponds to mass deficiencies. The map shows more small-scale detail on the far side of the moon compared to the nearside because the far side has many more small craters. Image credit: NASA/JPL-Caltech/MIT/GSFC

Originally posted on Forbes!

The Moon is pretty round; it’s set up much like the Earth – with a core of iron, surrounded by a mantle of other rocks, and topped off with a crust between 21 and 27 miles thick, at the surface. As far as we can tell, each of those components is pretty perfectly centered within the sphere that makes up our Moon.

Could we tell if the mass of the moon were offset somehow? Let’s say the near side of the Moon were made of denser rock (more mass per unit area)  than the far side for some reason. What measurements would that change? 

It wouldn’t necessarily change the elevation mapping; after all, you can just as easily have a large pile of dense rocks as you can have a large pile of less dense rock. However, what would change is the gravitational field mapping. In order to better understand exactly this sort of question - are the rocks over here roughly the same density as the rocks over there - we have mapped out very detailed measurements of the gravitational pull surrounding a number of other worlds  - not to mention our own.

The final result looks like the image below (when unfurled from the sphere of the Moon), when viewed through the eyes of the twin Gravity Recovery and Interior Laboratory (GRAIL) satellites. These two satellites, dubbed Ebb and Flow, orbited the moon for nine months in 2012.

This map shows the gravity field of the moon as measured by NASA's GRAIL mission. The viewing perspective, known as a Mercator projection, shows the far side of the moon in the center and the nearside (as viewed from Earth) at either side. Units are milliGalileos where 1 Galileo is 1 centimeter per second squared. Reds correspond to mass excesses which create areas of higher local gravity, and blues correspond to mass deficits which create areas of lower local gravity. Image credit: NASA/JPL-Caltech/GSFC/MIT

This map shows the gravity field of the moon as measured by NASA's GRAIL mission. The viewing perspective, known as a Mercator projection, shows the far side of the moon in the center and the nearside (as viewed from Earth) at either side. Units are milliGalileos where 1 Galileo is 1 centimeter per second squared. Reds correspond to mass excesses which create areas of higher local gravity, and blues correspond to mass deficits which create areas of lower local gravity. Image credit: NASA/JPL-Caltech/GSFC/MIT

These maps allow us to rule out the Moon’s mass being completely lopsided, because there's no asymmetric gravitational pull being measured. If the Moon's mass were lopsided, you'd expect to see a huge swath of high gravitational pull along the left and right edges of the map, which we don't see here. These maps show us the gravitational acceleration above each of these locations, and typically the punches you see in the surface - the red circles scattered across the Moon - coincide with impact craters we know about.

This graphic depicting the bulk density of the lunar highlands on the near and far sides of the moon was generated using gravity data from NASA's GRAIL mission and topography data from NASA's Lunar Reconnaissance Orbiter. This graphic depicting the bulk density of the lunar highlands on the near and far sides of the moon was generated using gravity data from NASA's GRAIL mission and topography data from NASA's Lunar Reconnaissance Orbiter. Red corresponds to higher than average densities and blue corresponds to lower than average densities. The average bulk density of the lunar highlands crust is 2,550 kilograms per meter cubed, which is 12 percent lower than generally assumed. White denotes regions that contain mare basalts (thin lines) and that were not analyzed. Solid circles correspond to prominent impact basins. The largest basin on the moon's far side hemisphere, the South Pole-Aitken basin, has a higher than average density that reflects its atypical iron-rich surface composition. Image credit: NASA/JPL-Caltech/IPGP

This graphic depicting the bulk density of the lunar highlands on the near and far sides of the moon was generated using gravity data from NASA's GRAIL mission and topography data from NASA's Lunar Reconnaissance Orbiter. This graphic depicting the bulk density of the lunar highlands on the near and far sides of the moon was generated using gravity data from NASA's GRAIL mission and topography data from NASA's Lunar Reconnaissance Orbiter. Red corresponds to higher than average densities and blue corresponds to lower than average densities. The average bulk density of the lunar highlands crust is 2,550 kilograms per meter cubed, which is 12 percent lower than generally assumed. White denotes regions that contain mare basalts (thin lines) and that were not analyzed. Solid circles correspond to prominent impact basins. The largest basin on the moon's far side hemisphere, the South Pole-Aitken basin, has a higher than average density that reflects its atypical iron-rich surface composition. Image credit: NASA/JPL-Caltech/IPGP

So if the Moon isn’t lopsided inherently, what is it that causes the Earth’s gravitational pull on one side to be significantly stronger than the pull on the far side? This question works in both ways, as it’s the Moon’s reciprocal pull on the Earth which causes high tide. The answer is simply distance.

Gravitational pull weakens with distance; in fact, if you drift twice as far away from an object, the gravitational pull weakens by a factor of four. Drift four times further, and gravity loosens by a factor of 16. Now, the near side of the Moon and the far side of the Moon are only 2,159 miles apart from each other. If you want to get a general handle on the gravitational pull between the earth and the Moon, you can do the calculation assuming the Moon is a mass collected into an infinitesimally small point. This will get you close to describing the orbit of the Moon around the Earth, but it glosses over a few details.

This image approximates the look of the Nov. 14, 2016, full moon with data from NASA's Lunar Reconnaissance Orbiter. Credit: NASA Goddard's Scientific Visualization Studio

This image approximates the look of the Nov. 14, 2016, full moon with data from NASA's Lunar Reconnaissance Orbiter. Credit: NASA Goddard's Scientific Visualization Studio

One of those details is the difference in gravitational force on the near and far surfaces, knowing that the moon isn't flat. Because the Moon’s front side is 2,159 miles closer to the Earth than the far side, the gravitational force on the near side is just a tad stronger than the gravitational force on the far side. This is not a strong effect, but it is measurable, and it is this differential across the planet and our moon which causes both the liquid of our planet's oceans to pull towards the moon, and the same side of the moon to always face us.

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