Tau.Neutrino said:
New theory to explain why planets in our solar system have different compositions
A team of researchers with the University of Copenhagen and the Museum für Naturkunde, Leibniz-Institut für Evolutions has come up with a new explanation regarding the difference in composition of the planets in our solar system. In their paper published in the journal Nature, they describe their study of the calcium-isotope composition of certain meteorites, Earth itself, and Mars, and use what they learned to explain how the planets could be so different. Alessandro Morbidelli with Observatoire de la Côte d’Azur in France offers a News & Views piece on the work done by the team in the same journal issue.
Read more at: https://phys.org/news/2018-03-theory-planets-solar-compositions.html#jCp
I thought this was already known.
> The protoplanets all grew at the same rate, the group suggests, but stopped growing at different times.
OK, that’s new.
> The researchers developed their theory after studying the calcium-isotope composition of several meteorites called angrites and ureilites, as well as that of Mars and Earth, and also from the asteroid Vesta. Calcium isotopes, they note, are involved in the formation of rock, and because of that, offer clues about their origins. The researchers found that isotopic ratios in samples correlated with the masses of their parent planets and asteroids, which they claim provides a proxy for their accretion timeline. And that, they further claim, provides evidence of the different compositions of the planets, as the smaller ones ceased accreting material while the larger ones continued to add material that was different from what had come before.
Hmm, I’ll have to think about that one. I can confirm that calcium isotopes are an excellent guide to the formation of the first rocks. I can also confirm that the inner solar nebula in which the rocky planets formed was probably well stirred.
But I’m far from sure about their overall conclusion that accretion on different rocky planets stopped at different times. My limited knowledge suggests that two accretion methods – slow gravitational accretion of gasses and planetesimal impact of solids – should have stopped at all the rocky planets at close to the same time.
But if so then why would calcium isotope ratios be correlated with planetary mass? Fractionation due to gravitational influences?