Free Astronomy Magazine November-December 2017

NOVEMBER-DECEMBER 2017 more complicated and profound. It is more complex because there are also other classes and subclasses (although less prominent than S and C), as well as a slight mixing of the classes in terms of their orbits. It is more profound because the predominance of certain surface elements indicates a dif- ferent internal breakdown of minerals and thus a different location at the time they formed. The classic hypothesis, that the as- teroids in the main belt had largely formed where they are still in orbit, substantially confirms that their differences in composi- tion reflect the distribution of the ele- ments that made up the gases and dust present in the protoplanetary disk at those distances from the protoSun. But accord- ing to Raymond and Izidoro, there are a couple of valid reasons to believe that the macroscopic differentiation between the S-type and C-type asteroids, as we see it today, cannot be a direct consequence of the original distribution of the matter in that area. The first reason is the current absence of dust in the main belt: if a planet had been about to be formed, in addition to what remains of the planetes- imals (that is, the asteroids more or less shattered by mutual impacts), there should still be a significant amount of dust there, which is instead absent. The second rea- son is the fact that the rocky planets, the ones that in their mineralogical composi- tion are the closest to the S-type asteroids, T he collisional evolution of the asteroids, represented by the background illustration, is the mechanism by which the main belt was transformed be- ginning from its formation. This mechanism is still valid in Raymond and Izidoro’s sce- nario. [NASA/ JPL-Caltech]