As for where this Martian free oxygen came from? Well, there are several possibilities, the first of which is the thought-provoking idea that it came about thanks to microbes, in a Martian version of the GOE on Earth.
However, without any evidence for microbial life on Mars – past or present – this has to remain an unlikely, but not impossible, scenario. It’s more likely that it has something to do with the Sun’s destruction of Mars’ atmosphere.
“One potential way that oxygen could have gotten into the Martian atmosphere is from the breakdown of water when Mars was losing its magnetic field,” Nina Lanza, a planetary scientist at Los Alamos National Laboratory in New Mexico, and lead author of the Geophysical Research Letters study, said in a statement.
By “breakdown,” Lanza is referring to the fact that, when our Sun began to strip away Mars’ thicker atmosphere, ionizing radiation from the Sun could hit the water at the surface and split it into hydrogen and oxygen. The weak gravitational field strength of Mars meant that the lighter hydrogen atoms escaped into space, but the denser oxygen atoms managed to stay behind. This oxygen then reacted with the surface, forming manganese oxides and iron oxides, the latter of which turned the surface of the world red.
Either way, the oxygen levels of Mars today are incredibly low, save for some lingering in the upper atmosphere, which suggests that it either all reacted to form oxide compounds, or much of it left when the atmosphere was nearly completely washed away by powerful, persistent solar winds.
The Sun stripping away Mars' ancient atmosphere. NASA/GSFC