So, I must address some inaccuracies in the author’s letter regarding gravity. First, all mass has gravity – from a microscopic piece of space dust to super-massive black holes, including all planets. Actually, gravity can be measured. To keep it simple, Earth’s gravity measures in at .98 Newtons (as in Sir Isaac) and Mars gravity is only .3711N. Advertisement Scientists don’t know yet what gravity is, how it works, or what it is made of. Even the concept of dark matter has yet to be proven.