# Why is it said that the far side of the moon's crust is thicker?

This video states that NASA has determined that the far side of the moon's crust is thicker than the near side's crust. How did they determine this?

Please note I'm not asking for theories or hypotheses on how this came to be, but rather if the difference in thickness indeed is measurable scientifically today.

[So off-topic: If it is in fact true, I favour the theory that the centrifugal force of the earth's gravity along with the fact that the far side is always pointing outwards, is responsible for this and not some other moon smashing into our moon etc. as is shown in that video.]

• Regarding the final off-topic, italicized paragraph: Given the significant difference between the two hemispheres of the Moon, there are two stable configurations for the Moon's tidally locked orientation. One is with a line segment drawn from the Moon's center of figure to the Moon's center of mass pointing more or less toward the Earth (i.e., how we see the Moon today). The other stable configuration is to have that line segment pointing more or less away from the Earth. – David Hammen May 31 '17 at 9:36

The near side and far side have very different topography. Compared to the near side, the far side is far more rugged and has far fewer maria. The near side in places has very thin crust (the maria).

Other than these surface features, the biggest indicator of a disparity in crustal thickness is the two kilometer offset between the Moon's center of figure and it's center of mass. This is typically attributed to a thicker crust on the far side of the Moon, consistent with the observations of the different surfaces of the near side and far side.

NASA discovered that the center of figure (geometrical center) and the center of mass are offset from one another in the late 1960s. The simple act of observing how a probe orbits an object gives clues regarding the interior of that object. Scientists know, for example, that Mars has a partially molten core from observing the orbits of the many probes that have been sent to Mars.

In the case of the Moon, NASA sent 16 unmanned probes to the Moon in the 1960s prior to the Apollo 8 mission. The first four of these, Ranger 6, 7, 8, and 9, were impactors. Their impacts the Moon indicated the Moon's center of mass was displaced from its center of figure by 2-3 kilometers. Later missions confirmed this, and also confirmed that the Moon's near side has five large mass concentrations ("mascons"). The Apollo 15, 16, and 17 missions carried laser altimeters that gave very precise measurements of the distance to the Moon's surface. These further improved the estimate of the Moon's center of figure / center of mass offset.

(Aside: Read Bizarre Lunar Orbits for a description of how those lunar mascons change how things orbit the Moon.)

You may find this article by Phil Plait or this one on EarthSky.org interesting.

Basically, a new hypothesis proposes that, after the impact that created the moon, it got tidally locked to the Earth in only a few (~100) days. Because the Earth was still hot (2,500°C) from the residual heat of impact, it cooked the near side but let the far side cool more rapidly, thus forming a thicker crust than the near side.

I like this hypothesis better than the secondary splat hypothesis, because it's neater and seems to follow what would have been a more natural sequence of events.

I know this specific question was not asked but thought this hypothesis was germane to the discussion.