An interesting thing happened for planetary science recently: two papers (Nature, vol 541 (Dauphas, pp 521 – 524; Fischer-Gödde and Kleine, pp 525 – 527) showed that much of how we think planets accreted is wrong. The papers showed that the Earth/Moon system has isotope distributions across a number of elements exactly the same as that found in enstatite chondrites, and that distribution applied over most of the accretion. The timing was based on the premise that different elements would be extracted into the core at different rates, and some not at all. Further, the isotope distributions of these elements are known to vary according to distance to the star, thus Earth is different from Mars, which in turn is clearly different from the asteroid belt. Exactly why they have this radial variation is an interesting question in itself, but for the moment, it is an established fact. If we assume this variation in isotope distribution follows a continuous function, then the variations we know about have sufficient magnitude that we can say that Earth accreted from material confined to a narrow zone.
Enstatite chondrites are highly reduced, their iron content tends to be as the metal or as a sulphide rather than as an oxide, and they may even contain small amounts of silicon as a silicide. They are also extremely dry, and it is assumed that they were formed at a very hot part of the accretion disk because they contain less forsterite and additionally you need very high temperatures to form silicides.
In my mind, the significance of these papers is two-fold. The first is, the standard explanation that Earth's water and biogenetic material came from carbonaceous chondrites must be wrong. The ruthenium isotope analysis falsifies the theory that so much water arrived from such chondrites. If they did, the ruthenium on our surface would be different. The second is the standard theory of planetary formation, in which dust accreted to planetesimals, these collided to form embryos, which in turn formed oligarchs or protoplanets (Mars sized objects) and these collided to form planets must be wrong. The reason is that if they did collide like that, they would do a lot of bouncing around and everything would get well-mixed. Standard computer simulations argue that Earth would have formed from a distribution of matter from further out than Mars to inside Mercury's orbit. The fact that the isotope ratios are so equivalent to enstatite chondrites shows the material that formed Earth came from a relatively narrow zone that at some stage had been very strongly heated. That, of course, is why Earth has such a large iron core, and Mars does not. At Mars, much of the iron remained as the oxide.
In my mind, this work shows that such oligarchic growth is wrong and that the alternative, monarchic growth, which has been largely abandoned, is in fact correct. But that raises the question, why are the planets where they are, and why are there such large gaps? My answer is simple: the initial accretion was chemically based, and certain temperature zones favoured specific reactions. It was only in these zones that accretion occurred at a sufficient rate to form large bodies. That, in turn, is why the various planets have different compositions, and why Earth has so much water and is the biggest rocky planet: it was in a zone that was favourable to the formation of a cement, and water from the disk gases set it. If anyone is interested, my ebook "Planetary Formation and Biogenesis" explains this in more detail, and a review of over 600 references explains why. As far as I am aware, the theory outlined there is the only one that requires the results of those papers. So, every now and again, something good happens! It feels good to know you could actually be correct where others are not.
So, will these two papers cause a change of thinking. In my opinion, it may not change anything because scientists not directly involved probably do not care, and scientists deeply involved are not going to change their beliefs. Why do I think that? Well, there was a more convincing paper back in 2002 (Drake and Righter, Nature 416: 39-44) that came to exactly the same conclusions. Instead of ruthenium isotopes, it used osmium isotopes, but you see the point. I doubt these two papers will be the straw that broke the camel's back, but I could be wrong. However, experience in this field shows that scientists prefer to ignore evidence that falsifies their cherished beliefs than change their minds. As a further example, neither of these papers cited the Drake and Righter paper. They did not want to admit they were confirming a previous conclusion, which is perhaps indicative they really do not wish to change people's minds, let alone acknowledge previous work that is directly relevant.
