The mystery of the magnetic field of the Moon

In Bighthink there was an interesting story telling about the strange finding related to the faces of the Moon. The finding is that the faces of the Moon are very different. Moon and Earth are in rotational resonance meaning that the we see always the same face of the Moon. In 1959 the first spacecraft flew around the Moon and it was found that the two sides of the Moon are very different.

The near side is heavily cratered and the lighter areas are in general more cratered that the dark areas known as maria. Craters have a fractal structure: craters within craters. Dark areas have different decomposition. At the far side there are relatively few dark maria and the dark side is thoroughly cratered and "rays" appear to radiate out from them.

The "obvious" explanation for the difference between the two sides is that there is a massive bombardment by heavy towards the far side whereas Earth has shielded the near side. This explanation fails quantitatively: the number of collisions at the near side should be only 1 per cent smaller at the far side. The far side is about 30 per cent more heavily cratered than the near side. There is no explanation for the size and abundance difference of the maria.

The article discusses the explanation in terms of Theia hypothesis stating that Moon was formed as a debris resulting from a collision of Mars size planet with Earth. If the Earth was very hot, certain elements would have been depleted from the surface of the Moon and chemical gradients would have changed its chemical decomposition. The very strong tidal forces when the Moon and Earth were near to each other would have led to a tidal locking. If the near side has thinner crust, Maria could be understood as resulting from molten lava flows into great basins and lowlands of the near side. If the maria solidified much later than the highlands one can understand why the number of craters is much lower. The impact did not leave any scars. The hot Earth near the Moon also explain the difference in crustal thickness.

TGD suggests a different explanation consistent with the Theia hypothesis. TGD predicts that cosmic expansion consists of a sequence of rapid expansions. This explains why the astrophysical objects participate in cosmic expansion but do not seem to expand themselves. The prediction is that astrophysical objects have experienced expansions. The latest expansion would have occurred .5 billion years ago and increased the radius of Earth by a factor 2. These epansion can be also explosions throwing away a layer of matter. Sun would created planets in this kind of explosions by the gravitational condensation of the resulting spherical layers to form the planet. Also Moon could have emerged in an explosion of Earth throwing out a thin expanding spherical layer. This would explains why the composition of Moon is similar to that of Earth.

The hypothesis resembles the Theia hypothesis. The hypothesis however suggests that the Moon should consist of a material originating from both Theia and Earth. The compositions of Earth and Moon are however similar. Why Theia and Earth would have had similar compositions?

This spherical layer was unstable against gravitational condensation to form the Moon. If the condensation was such that there was no radial mixing, the layer's inner side remained towards the Earth. This together with the tidal locking could allow to understand the differences between the near and far sides of the Moon. The chemical composition of the near side would correspond to that in the Earth's interior at certain depth h. One can estimate the thickness h of the layer as h= R_M^3/R_E² ≈ R_E/48 from R_M≈ R_E/4. This gives h≈ 130 km. The temperature of the recent Earth at this depth is around 1000 K (see this). At the time of the formation of Moon, the temperature could have been considerably higher, and it could have been in molten magma state.

Orbital locking would rely on the same mechanism as in Theia model. The half-molten state would have favored the development of the locking. The far side would represent the very early Earth affected by the meteoric bombardment or some other mechanism creating the craters.

Another mysterious observation is that Moon has apparentely turned itself inside out! The proposed mechanism indeed explains this. See the blog post.

See the article Moon is mysterious or the chapter Magnetic Bubbles in TGD Universe: Part I.