J-M Souriau: On the dynamics of the solar system (p5)
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The theme of a lunar origin due to a collision between the Earth and another planet is not new. Very suggestive numerical simulations have recently brought this theme to the forefront of the scientific media scene, through beautiful artist's images. I suggest enriching this hypothetical scenario as follows:
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A "primitive Earth" would have initially followed a better balanced trajectory, relative to a general golden law. It may have already had an atmosphere. It may have also been carrying primitive life (if the collision occurred during the early stages of the solar system, less so if it was more recent).
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A collision, either the disturbance or one of its components, conjectured by Claire, would then occur with a very massive planet, very rich in metallic elements (iron).
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The collision would be accompanied by a huge "clump". The Earth's crust would be torn. Fragments of it and pieces of the Earth's magma would be ejected. These elements, these ejecta, would constitute the young Moon and possibly a set of asteroids and future satellites of other planets.
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Some objects, ejecta, could then be captured by large planets, forming "non-resonant" elements, the resonant satellites being the "original satellites".
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The elements constituting the disturbance would disrupt "Jupiter's ring", these elements forming future comets.
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Some elements would form the asteroid belt, joining a non-resonant orbit.
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Part of the mass of the impacting body would be incorporated into the Earth and would form its iron core.
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The Earth would then gain a "non-resonant" orbit. See
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corresponding to the golden law, above.
- During the collision with this X-body, to be defined, the Earth's magma would erupt and cover a large part of the initial Earth's crust:
Fig.13 (p020): The impact has just occurred. A mixture of surface magma and debris from the Earth's crust is projected. Since the collision is not perfectly frontal, an ejecta (the future Moon) is also projected. The magma falls back on the Earth in a corolla.
We cannot assume anything about the state of this primitive Earth before the impact, if it actually occurred. It may have already secreted its atmosphere, may have been carrying primitive life. And perhaps, why not, if the catastrophe had been late enough, of a highly organized life, even a civilization seriously damaged by this accident.
The kinetic energy from the impact heats its magma, then restarts intense volcanism, with the effect of reconstituting a "secondary primitive atmosphere".
Fig.14 (p021): After the collision. The Moon begins to form.
The Earth has regained its sphericity.
The magma would be exposed. While solidifying, it would form the ocean floor, of lesser thickness, the major scar being the Pacific Ocean.
Fig.15: The debris have been either scattered throughout the solar system or absorbed. The Moon has taken its spherical shape, keeping a bulge, a sign of its initial status as an ejecta. The Earth's magma solidifies. The Earth's primitive atmosphere (if it had one), has been either scattered or seriously disturbed. Intense volcanism, linked to the restart of convection, reconstitutes it through volcanism.
The heat from the impact must be evacuated. This energy input would restart convective currents. Life, if it existed before the collision and if it was not completely destroyed by this catastrophe, would resume its course, as best as possible. An evolution possibly accelerated by the survival of primitive organisms, despite the extremely severe conditions after the impact. The primitive continent, fossil of the primitive Earth's crust, would begin to break apart (continental drift, absent on the other solid planets of the solar system).
It is commonly assumed that the lunar seas are lava flows, linked to collisions with asteroids. Why, in these conditions, are they located facing the Earth? Why, at the time when the Moon was still rotating on itself, would these strong impacts not be evenly distributed on our satellite?
If these impacts had occurred after the Moon's orientation, facing the Earth, due to its bulge, they would have been weaker instead, the Earth protecting the Moon from such impacts and slowing down the bodies that, escaping its attraction, would have collided with its satellite. Conversely, the far side of the Moon is full of strong impacts, the incident objects benefiting from the Earth's gravitational assistance. Another interpretation: these seas could represent the "scar", on the Moon's side. It could have formed with two components: a fragment of the Earth's crust and some magma, denser. When this drop adopted a spherical shape, the dense part, facing the Earth, would have formed the bulge.
But there is another competing hypothesis, very strong, as I agree, mentioned by Souriau: Initially, the Moon would have rotated on itself. Then it would have suffered a formidable impact, corresponding to a single dense object, which, by tidal effect, like the comet Shoemaker-Levy, would have broken into fragments before impact. These fragments, falling at very high speed, would have then penetrated the ground, piercing the crust of the young Moon, whose magma would have then flowed out. Hence the seas. All these seas would have formed at the same time, during a single multiple impact. But these fragments would not have been able to freely flow to the center of the Moon, already too viscous at that time, and would have then been embedded under the crust, forming "mascons" (mass concentrations), subcrustal, also detected. Thus weighted, the Moon would have acquired its bulge and would have stopped rotating.
End of this article