The Earth's and other celestial bodies' atmospheres become thinner the higher you go. Imagine you were able to see atoms/molecules. At what altitude above the Earth's and Mars' surface would you see few enough molecules per cubic foot so you'd be able to count them? If you were, say, 500 miles (805 km) above the Earth's surface, how many air molecules per cubic foot (30 cm³) would you see?
Or, one can ask vice versa: at what altitude above the Earth would you see, say, a 100 air molecules per ft³? Which gas doesn't matter, I'm asking for all atmosphere molecules within the certain volume. And since even interplanetary space isn't perfect vacuum, what would it look like there, e.g. halfway between the Earth and Mars orbit?