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According to one theory, the region between Mars and Jupiter was once populated by several primordial planets with a combined mass of roughly the earth. These planets provided the material that made up the original asteroid belt. Since then, 99.9% of this material was lost. The current composition of the asteroid belt is about 10% metallic and 90% silicate/carbonaceous. Could we assume that the original asteroid belt had the same composition? Or, at least a very similar composition?

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    $\begingroup$ Do you have a reference for that theory? Or at least an epoch it was supposed to happen in? I guess it'd predate en.wikipedia.org/wiki/Grand_tack_hypothesis $\endgroup$ – PM 2Ring May 27 '19 at 18:09
  • $\begingroup$ There was a planet there that exploded. That's the most plausible theory and it's supported by math (based on current planetary orbit distances). $\endgroup$ – Overmind May 28 '19 at 7:50
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    $\begingroup$ @Overmind no, that’s a myth. Regarding the math, Bode’s Law was discredited long ago. $\endgroup$ – Chappo Says SE Dudded Monica May 29 '19 at 1:28
  • $\begingroup$ If you consider Neptune to be the 'first' Kuiper belt object, then the law is correct. You must take SBOs into account, not only planets. $\endgroup$ – Overmind May 29 '19 at 9:43
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We can be pretty sure that the asteroid belt once held one or more sizable planetesimals,as there is evidence that it or they were large enough for gravity & internal heating to drag the heavy metals (mainly iron & nickel) to the centre,forming a metallic core. Bodes Law decrees that there should be a planet where the asteroid belt is now,but it seems that as this planet was forming it was shattered by an impact & the remnants scattered. Some reached Earth,which is how we know that their materials had differentiated into metallic core & rocky mantle. It is likely that some of the huge asteroids which have struck the Earth in the distant past were fragments thrown out of the asteroid belt & that some hit Mars & Venus as well. There may be more material in the asteroid belt than we think,because some fragments are so small & widely dispersed that they don't register in our telescopes. I think we can assume that the primordial planetesimals had a similar though not identical composition to the asteroid belt of today.

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