1: There are measurable and observable meteor impact sites on our planet with the substance of what made the impact still being observable by the human eye (bits of meteorite are left)
2: Throughout history we have discovered that the earth has different layers by studying how earthquake waves travel through the planet and observing the changes in speed and behaviour as they encounter different densities.
3: We can date rocks by using radioactive dating to determine how old they are.
4: We know that in zero to very low gravity salt grains can clump together to form bigger "salt rocks" and that this works for particles up to 6mm.
