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Chondrules – the origin of planets?


Collisions between planetary embryos, the “seeds” of the primordial planets of the solar system, would be the process that gave birth to the mysterious chondrules – drops of molten rock before they solidified – present in meteorites. The research is published in Nature.
If confirmed, it is one of those discoveries that will force to update Wikipedia. Where the item “chondrules” (consulted in the English edition), we read that they are present in chondrites round balls, and so far no objection, which would turn in bricks – the building blocks – our planetary system. Well, it could be gone just the opposite: as reconstructed by a team of planetary scientists at MIT and Purdue University, coordinated by an authority in the industry, Jay Melosh, would be in fact the chondrules that are at the origin of the planets, and not vice versa.

David Minton (left) and Jay Melosh (right) with a fragment of a meteorite containing condrule.

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Credit: Purdue University/John Underwood

The origin of chondrules is a long-standing puzzle. For over a century that scientists analyze these millimeter-sized grains, these meteorites, and never been able to come to a convincing and shared conclusion about their training. Among the most recent hypotheses, was one that to generate the high temperatures needed to liquefy the rock has been the magnetic fields present in the primordial protoplanetary disk.
Well, according to the model developed by the team of Melosh, described in the latest issue of Nature, there is no need to go that far: to explain their formation and their abundance would be sufficient impacts between protoplanets occurred during the first five millions of years of planetary accretion. If the impact occurs at speeds of at least 2.5 kilometers per second, the article states, signed by Brandon Johnson and colleagues, that the rock material reaches, in the region surrounding the collision surface, temperatures are high enough to melt rock and expel it in the form of liquid jet. Jet which in turn gives birth to droplets of millimeter scale, which would cool then at a speed of between ten and one thousand degrees per hour, consistent with what is required to produce the chondrules as those actually observed in meteorites.
“Understanding the process of underlying the formation of chondrules is a bit like looking through the keyhole: although we cannot see everything that happens behind the door, it gives us a clear view of a portion of the other room, which in our case is equivalent to a look at the dawn of the solar system”, says Melosh. “What we found is that the model based on the impacts, describes very well what we know of this unique material and the early solar system. So, contrary to what most of the experts in meteorites believe, asteroids are the remains of the material from which the planets have taken shape, and clots of chondrules are not in turn prerequisites for a planet. ”
This conclusion would not affect just the role until now covered by meteorites in the study of planetary formation. “Chondrites have been voted long a material similar to that which gave birth to the planets. What emerges from our study is that instead chondrules may be nothing more than a byproduct of impacts between objects of a previous generation, “observes David Minton, of Purdue University, one of the co-authors of the article,” and meteorites could therefore not be as representative of the material from which the planets form. ”
The next step, says Minton, now will be to study how this process of formation of chondrules could possibly find a place in a new model of planet formation: one known as pebble accretion, in which a crucial role is played by the gas removed from the protoplanetary nebula.

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