Oxygen on Mars

Factual Questions
1

I was listening to a space podcast yesterday, and they made the claim that there used to be oxygen on Mars. But apparently solar winds blew away the atmosphere after Mars lost it’s magnetic field.

My understanding is that the oxygen on earth was formed as cyanobacteria algae conducted photosynthesis - life in other words.

I’ve also read that oxygen is a biosignature that scientists could look for when searching for life on exoplanets, so apparently oxygen is a strong indicator of life.

As far as I know there is no evidence that Mars ever had life so my question is, where would the oxygen have come from?

2

As I understand it, the oxygen came from solar radiation interacting with the water on Mars, converting it to hydrogen (which was lost quickly to space) and oxygen (which was lost more slowly). In addition, the oxygen produced also reacted with iron and manganese to form iron oxide and manganese oxide.

The rate of oxygen production went up tremendously once Mars lost its magnetic field about 4 billion years ago. Loss of the field increased the radiation that reached the surface of the planet.

Oxygen on Mars

3

Considering that the Mars atmosphere is CO2, there’s a lot of oxygen on Mars. (although not that much - it’s only 1/100th earth atmospheric pressure.) Plus as the link says, a lot is bound up in rocks.

So really what they meant was that unlike earth - here photosynthesis bacteria released so much oxygen that there was still some left over after all the exposed rocks and such were done oxidizing. Eventually it built up. They’re saying Mars, as solar winds (likely, not life) split the Martian water into H and O, the hydrogen more easily blew away because of the solar wind, and any oxygen left free combined with rocks or carbon… and blew away too, but more slowly.

4

There is plenty of oxygen for people to breathe, though. It’s bound up in the rocks, the water, and the C02 in the atmosphere and frozen up at the poles. The soil is also full of perchlorates. There are plenty of processes that can extract that oxygen, given enough energy.

5

So, just to be clear, there is plenty of oxygen on Mars and any other rocky planet. Nearly every mineral is some kind of oxide, and in terms of elemental mass oxygen forms 46.1% of Earth’s crust (CRC Handbook of Chemistry and Physics, 89th Ed), and probably something comparable for Mars. (It’s a little difficult to say exactly how much because of the limited surveys for Mars but of the the fourteen chemical compounds listed Chemistry of the Solar System (2011, Lodders and Fegley) as surveyed by the Viking I & II, Pathfinder, and MER-A and -B, thirteen are oxides with silicon dioxide (SiO2) being 42% to 46%, ferrous oxide (FeO) 15% to 20%, aluminum oxide (Al2O3) in the 7% to 10% range, manganese(II) oxide (MgO) in 6% to 9%, sulphur trioxide (S O3 at about 6% to 8%, calcium oxide (CaO) at 6% to 7%, and all other compounds in the single digit percentage range. The oxygen, like all ‘heavy’ elements, comes of course from stellar nucleosynthesis, predominately the CNO (carbon, nitrogen, oxygen) fusion cycle in heavy stars and is distributed by supernovae and stellar wind.

So, oxygen is highly abundant, and your query is about ‘free’ diatomic oxygen in the atmosphere. Although Mars does have an atmosphere (the only rocky body in our solar system other than Earth, Venus, and Titan to have a viscous fluid atmosphere) that has a high proportion of oxygen, the atmosphere is very thin (surface pressure is about 0.6 millibars, or just over half a percent of that of Earth). Mars did once have a much thicker atmosphere which we know because we can observe the residues and geological features of liquid water flowing on the surface, which means that it had to have had a pressure of at least a few hundred millibars to maintain steady concentrations of liquid water. We can only speculate about the actual climate during the first billion or so years of Mars but it was probably somewhat akin to Earth today. However, the loss of a rotating liquid ferrous core resulted in collapse of its magnetic field and the loss of virtually all of its atmosphere (not just oxygen but all species lighter than carbon dioxide) and vaporization or freezing of surface water. There are thick briny recurring slope lineae that allow ‘liquid water’ to flow today under peak surface temperatures of about 20 °C but these are more like a thick sludge.

Because oxygen tends to bind with nearly every other element (literally ‘oxidizing’ them into minerals), finding an atmosphere with ‘free’ diatomic oxygen would be indicative of either very energetic or highly organized chemical reactions which are continuously releasing oxygen from bound states. There are relatively few known or hypothesized natural phenomena that would continuously cause or catalyze the release of oxygen, and virtually none that could produce the concentration of oxygen seen in Earth’s atmosphere other than systems of living organisms, hence why the presence of diatomic oxygen in the absorption spectra of an exoplanet would be highly indicative of the presence of some kind of life-like system, which would no only release oxygen from the air (as photosynthetic plants do) but also from minerals in the ground (as certain archaea do). The same is true with other oxidizers like chlorine or fluorine, although any life-like systems based upon the latter would certainly be far more energetic than Earth life.

The curious thing about Mars (and all of the the other rocky bodies in the Solar System that have been surveyed) isn’t the lack of oxygen or an oxygen-rich atmosphere, but the diminutive amount of nitrogen that is found. While the chemical abundance of nitrogen in the universe is only about 1/10th that of oxygen, we actually only find trace amounts on Mars and other bodies, and yet it forms ~78% of our atmosphere and performs both critical climatological and biological functions without which Earth would not support life as we know it. There is virtually no nitrogen or nitrates to be found on Mars and only about 3.5% in the atmosphere of Venus, which is much less than a cursory assessment would expect, and nobody really knows why.

Stranger

6

Folks have come up with multiple mechanisms by which a planet might have some detectable amount of elemental oxygen in its atmosphere. But most of them don’t result in anywhere near as much oxygen as we have here due to photosynthetic life, and the ones that come closest require extremely specialized environments that wouldn’t ever have been found on Mars.

7

That’s more than Earth, though. Venus’s atmosphere weighs ~93x that of Earth’s, so the actual mass of nitrogen is 3.5/78*93 = ~4.2x. It’s just dilute because there’s so much CO2.