The Surprising Ways Mars is Hostile to Life

4:10 this part doesn’t really make sense to me. She says that they know mars’ atmosphere was thin because they have impact craters of relatively small bodies, which, if mars had a thick atmosphere, would burn up. But couldn’t those impact craters have been created by larger bodies that were reduced to small ones when burning up in the atmosphere? How do we know that those small impact craters were caused by full extramartian bodies and not the remnants of larger ones? Do these foreign bodies typically display signs that indicate burning? I’d so, are these marks/traces present in the impact craters? I’d really like to know this.