Fiction has the power to make us reflect on alternative realities, which sometimes come true. Just read “1984” or “Animal Farm”, by George Orwell, or even Isaac Asimov to draw immediate parallels with the present. Hollywood has the additional advantage of providing us with special effects that translate concepts and effects that scientists only sketch in their texts. This is also true for “Moonfall – Lunar Threat”, a movie recently released on streaming that, although I didn’t like it very much, makes me think: what keeps the Moon orbiting at exactly 384,400 kilometers from Earth?
Inspiring muse of “Lunik 9”, that 1967 song by Gilberto Gil, which says “Poetas, serenaders, boyfriends, correi”, the Moon holds mysteries of the first moments of the planet’s evolution. Since American space expeditions revealed that its composition is similar to that of the primitive Earth’s mantle, the studies that followed revealed its role in the stability of the planet’s axis of rotation, in controlling the tides and our climate.
Although we know a lot about the Moon, its origin was and still is a matter of controversy. The most accepted hypothesis is that it was formed by the terrestrial impact of a gigantic celestial body called Theia, at a time when our planet was still a large sphere of magma and was at the beginning of its differentiation.
In this process, the heavier elements sank towards the center of the Earth, forming an iron-nickel alloy core (whose rotation is responsible for our magnetic field) and leaving behind an outer layer enriched with lighter elements (mainly silicon). ), whose composition is similar to the magmas that are expelled today at the limits where the tectonic plates separate, forming the so-called mid-ocean ridges.
Our satellite is unique not only because of its influences on Earth’s dynamics, but also because of its size and for being rare in planets in systems neighboring ours, called extrasolar planets. Scientists estimate that only 5% to 10% of all known planetary systems have moons that are so large in size compared to the orbiting planet. This is because the formation of these bodies involves powerful impacts, which result in the irreversible scattering of the dust cloud generated in the collision.
Why then was Earth privileged with a satellite of such relevant size and importance? What are the conditions that favor the stabilization of dust clouds formed by large impacts? Answers to these questions begin to take shape from numerical simulations carried out by American and Japanese scientists in a work published this year in the journal “Nature Communications”.
Impact-derived moons originate when ejecta orbits the planets at an ideal distance known as the “hill radius”; above or below this distance, the ejected material is lost to space or collapses back to the home planet, respectively. The results of these simulations suggest that the planets most likely to form moons have radii that are at most 60% greater than Earth’s. Larger planets or those made of ice would form steam-rich disks or gas-rich proto-moons, and the loss of these gases to space would cause a decrease in angular momentum (the speed of movement of rotating bodies) and the consequent return of the ejected material to the planet. source.
An important implication of the study is that the search for moons derived from relatively large-sized impacts should be focused on planets whose radii are 1.6 times smaller than Earth’s radius, a requirement satisfied by only 57 of the more than 4,000 extrasolar planets already recognized. .
Interestingly, the work still paves the way for new models of the Earth’s compositional evolution, as scientists have never considered, for example, the potential effects of the return of ejected material at different stages of terrestrial differentiation.
If the Earth-Moon system seems to be unique in its interdependence relationships, and if part of this interdependence comes precisely from factors such as the relative size of our satellite and orbit distance, the study reveals bad news for scientists dedicated to the search for planets with external dynamics. and interior similar to those of Earth. It seems that the emergence of habitable conditions really came from the succession of events that were apparently independent, but that are intertwined and correlated to allow ideal climatic conditions for life as we know it.
*
Adriana Alves is a geologist and professor at USP.
Subscribe to the Instituto Serrapilheira newsletter to keep up with more news from the institute and from the Ciência Fundamental blog.
