Numerical simulations fed by gravimetric data from the Grail mission suggest an explanation for the asymmetry of the faces of the Moon. The hidden side would be covered with debris resulting from a collision with a dwarf planet the size of Ceres.
60 years ago, Lunik 1 became the first artifact of humanity to fly over the Moon. In about 6 months, it will also be the anniversary of another Russian probe, Lunik 3, which, in 1959, revealed the first pictures of the hidden side of the Moon. It was not until Lunik 9 in 1966 that the Russians managed the first smooth landing and this time, the first picture of the surface of our satellite taken in was revealed.
Other lunar missions would follow, including of course the Apollo missions, but they will only make the selenologists more and more perplexed by confirming that the dark side of the Moon does not look like the face that humanity has been looking for since she is standing up and using tools.
Gravimetry, a probe from the inside of planets
From the 1970s, after the Apollo missions and the beginning of the Voyager missions, the planetology and the cosmogony of the Solar System will make spectacular progress thanks to space missions and the rise of computers. These two last conjugates will allow simulations of the birth of the planets as well as the data processing of the sensors of the space probes, that these data concern the field of gravity or the magnetosphere or the spectra of the atmospheres and planetary surfaces.
Among the results provided by the machines of the 21st century, there is particular gravity data from the mission Gravity Recovery and Interior Laboratory (GRAIL) that was used to map more accurately the Moon’s gravitational field. As in the case of what has been done on Earth for decades with satellites in orbit, it is possible to reverse the data – as geophysicists say – to go back from a signal to the characteristics of the source of that signal. In this case, it is possible to have the equivalent of a scanner showing the distribution of masses and densities inside the Moon.
A team of researchers has published its findings about the analysis of Grail’s data in an article in the Journal of Geophysical Research: Planets, shedding new light on the enigma of the difference between the two faces of the Moon. This work reinforces a hypothesis already presented in the previous article below that Futura had devoted to this enigma, namely that this difference should be the product of a collision between the Moon and a celestial body a little smaller there is more 4 billion years old. But it would not be a twin moon appeared at the same time as our satellite after the collision of the Earth with a small planet the size of Mars, the famous Theia.
Will there be a collision with a planet the size of Ceres?
No. According to the researchers, this other little planet would come from another region of the Solar System where it was formed. Indeed, it is well known that according to the distance to the young Sun, the chemical composition of the dust and gases in the protoplanetary disk where the planets were born was not the same, despite the influence of turbulence and mixtures of matter that it implies.
For this new scenario of impact with the young Moon, the cosmogonists arrived there by doing 360 simulations on a computer, with impactors of masses and speeds different. The idea behind all this was to account for an extra thickness of 5 to 10 km from the hidden surface of the Moon in relation to its visible face that would come from the debris deposit of the impact that would have covered the frozen surface of the old ocean of lunar magma.
Among the simulations, those that best account gravimetric data generated by this layer of debris involve an object whose size would have been about 720 to 780 km in diameter, so the order of magnitude of Ceres, arriving with speeds of 22,000 to 24,000 km/h.
If this discovery were to be confirmed, it would not only be an important result in understanding the origin of the Moon. Other celestial bodies in the Solar System have similar asymmetries between two hemispheres, which would then suggest that the scenario presented today has in fact occurred several times over 4 billion years ago.