It isn't just surface area, but also the elemental composition. When the earth was molten, every siderophilic[0] element sank into the earth's core, leaving th

It isn't just surface area, but also the elemental composition. When the earth was molten, every siderophilic[0] element sank into the earth's core, leaving the crust with relatively low concentrations of precious metals[1]. All the precious metals we can mine are from later asteroid impacts, not the initial material that formed the earth. Mining asteroids would potentially give us more access to those precious metals. [0] https://en.wikipedia.org/wiki/Goldschmidt_classification#Siderophile_elements https://en.wikipedia.org/wiki/Goldschmidt_classification#Sid... [1] https://www.sciencedaily.com/releases/2011/09/110907132044.htm https://www.sciencedaily.com/releases/2011/09/110907132044.h...