Martin Schiller and Martin Bizzarro of the University of Copenhagen, and Vera Assis Fernandes of the Natural History Museum in Berlin, have proposed a new model for the solar system to explain differences in the isotopic composition of planets. The authors reject the assumption that large and small objects grew simultaneously, but at different rates, and believe that the growth of small bodies ended earlier than large ones.
Almost all experts today believe that the Sun and the planets were formed from a single protoplanetary cloud. 99.9% of the mass of this disk fell on the luminary. When the sun burst out, the solar wind swept away light hydrogen and helium from the immediate vicinity of the star, so the gas giants are now beyond the orbit of Mars.
Under the rays of a young star, protoplanetary dust was sintered into granules called chondrules. Sticking together, these granules formed small stones - chondrites. By the way, it is this "construction waste" that accounts for 90% of the meteorites found on Earth.
Gradually, chondrites stuck together into larger and larger bodies - planetesimals. Gravity provided them with an influx of fresh material, and these "embryos" grew until the largest of them became planets, and the rest - asteroids. When the reserves of cosmic dust in the protoplanetary disk were depleted, the growth of bodies in the solar system ended.
The classical theory assumes that all bodies in the solar system grew at the same time, but at different rates. The more massive the body, the more powerful its gravity and the more surrounding matter it will collect, as a result, increasing its size even more. This is the snowball principle, or, scientifically, positive feedback. This law governs the growth of cities (people prefer to go to megacities, where there is more money and opportunities, which makes them grow even more), the prevalence of languages (the more people know a language, the more incentives to learn it), and so on.
Without questioning the rest of the theory, Schiller and his colleagues reject this growth model. In their opinion, the small bodies did not manage to grow, because they had finished the adhesion of material earlier (as experts say, accretion).
As the journal Nature reports in a review of the work, the authors were inspired by the difference in the isotopic composition of different bodies in the solar system. Namely, the authors studied the ratio of calcium isotopes 48Ca and 44Ca on Earth, Mars, Vesta and in samples of rare types of meteorites: ureilites and angrites.
If all planets and asteroids were formed in a single process from the same cosmic dust, why is the ratio of these isotopes different? This is usually associated with different distances to the Sun and, accordingly, different temperatures.
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However, the authors found that the ratio of calcium isotopes depends on the mass of the celestial body. The masses of the Earth, Mars and Vesta are known from astronomical observations, and the approximate masses of objects, the fragments of which are meteorites, were reconstructed by scientists based on the properties of the "heavenly guests".
The ratio of isotopes 48 Ca / 44 Ca second is measured by μ48Ca. It is calculated as follows: μ 48 Ca = (48 Ca / 44 Ca celestial body - 48 Ca / 44 Ca Earth) / (48 Ca / 44 Ca Earth). Due to the fact that differences in isotopic composition are small, μ 48 Ca is measured in parts per million (ppm). By definition, for the Earth μ 48 Ca = 0, and for other bodies this value can be both positive and negative.
Schiller and colleagues suggested that the inner part of the protoplanetary disk, located inside Jupiter's current orbit, had low μ 48 Ca values of about minus 150 ppm). This material was enough for planetesimals to grow to the size of the body - the homeland of the Ureilites (200 kilometers in diameter).
Then some of these bodies stopped growing. Those that continued to grow increased their mass already due to the outer part of the disk with μ 48 Ca about 200 ppm (a value typical for chondrites that formed beyond the orbit of Jupiter). Therefore, the longer the growth continued, the larger the final value of μ 48 Ca was. Vesta, stopped at a diameter of 530 kilometers, has minus 100 ppm, Mars - minus 20 ppm, and Earth, as already mentioned, is 0 ppm.
However, what was the force that caused some of these bodies to stop growing? This could be a complex gravitational interaction between the "embryos of the planets", changing their trajectories. The current planets, with their nearly circular orbits lying in the plane of the protoplanetary disk, roamed the richest regions of the nascent system and therefore continued to grow. Losers, however, pushed out to elongated, and perhaps lying in a different plane of the trajectory, remained on a starvation diet.
The conclusion about the different ages of the samples is also confirmed by dating by the content of radioactive isotopes.
However, it cannot be said that the new model has no problems. For example, there are difficult questions to her in connection with the formation of the moon. "Vesti. Nauka" (nauka.vesti.ru) told in detail about the collision of the Earth with Theia, which gave birth to our satellite. It is usually believed that Theia was significantly smaller than the Earth, but from the authors' theory it follows that two bodies of the same mass collided. This is not consistent with some known facts.
In addition, there are studies showing that the influx of matter from the outer part of the protoplanetary disk stopped already in the first millions of years of its existence due to the formation of Proto-Jupiter. It is not easy to explain the composition of chondrites within the framework of the authors' model.
Probably, the puzzle called "Formation of the Solar System" is still missing a few important pieces, without which a model that answers all questions cannot be built.
Anatoly Glyantsev