Why is there so little nitrogen in the Martian and Venusian atmospheres? Why don't our neighbors have much nitrogen?  You would think that, without 'nitrogen-fixing' organisms and such, there might be more.....
Nitrogen, with a molecular mass of 28 atomic mass units, is too light to have remained in Mars's atmosphere. Carbon dioxide, with a molecular mass of 44 amu, could (and does) exist on Mars, but it is rather sparse.
Venus's atmosphere appears to contain a small amount of nitrogen when viewed on a percentage basis. &quot;Only&quot; 3.5% of all of the gases in Venus's atmosphere is nitrogen. But that's not a particularly good metric. Venus has so very, very much carbon dioxide in its atmosphere that the CO2 dwarfs every other constituent. When viewed in terms of mass, Venus has a good deal of nitrogen in its atmosphere, four times more nitrogen than the Earth has in its atmosphere.