Talk:Mars/@comment-29003152-20160711211028/@comment-90.200.46.38-20181113140415

I guess they made the atmosphere intentionally thick during the terraforming to keep it there. Mars has no magnetic field to speak of and an atmosphere like Earth's would be quickly blown away.

As for the 7.2% O2, that would be fairly likely for a planet where algae form the only photosynthetic life - as it would, as Mars is clearly still very cold. Earth's is only 21% because of its vast continental shelf and the phytoplankton, not to mention its continental forests.