[Iwonda (OP)]

I read that Mars gets its red color from all of the iron oxide that covers it. I have also read that when iron oxide lacks oxygen it shows up as a greenish color and that it only turns red when in the present of oxygen. Is the iron oxide on Mars red because it was once rich with oxygen or is there another process that can make iron oxide red without the presents of oxygen?

[Halc]

I read that Mars gets its red color from all of the iron oxide that covers it. I have also read that when iron oxide lacks oxygen it shows up as a greenish color and that it only turns red when in the present of oxygen. Is the iron oxide on Mars red because it was once rich with oxygen or is there another process that can make iron oxide red without the presents of oxygen?

If there's anything-oxide on Mars, then it still is an oxygen rich planet.
If you're asking if free oxygen (like O2) was ever prevalent on Mars, the answer is no. That is only a biproduct of photosynthesis and does not naturally occur in the absence of the sort of biological chemistry such as Earth has. Earth has had free O2 for less than 2.5 billion years. If you time travelled back even a billion years, the levels of oxygen would still be too low to allow breathing without a space suit.

[alancalverd]

That doesn't seem entirely logical. However Mars was formed, it seems unlikely that it spontaneously generated or collected iron oxide. Iron meteorites are mostly pure metal and as far as we know the interior of the earth  consists of molten iron. All our creation models involve the initial formation of elements, not compounds.

It is true that there very little free oxygen will remain on the surface of a planet that is belching volcanic iron into a fairly thin   atmosphere, but intuitively it must have been there at some time to form the oxide from elemental iron, however that arrived.

[Eternal Student]

Hi.

I read that Mars gets its red color from all of the iron oxide that covers it.

   Yes, seems reasonable.  Plenty of references exist for that.

I have also read that when iron oxide lacks oxygen it shows up as a greenish color and that it only turns red when in the present of oxygen.

   Also OK.    We want Iron in a valency state of III  (+3 written in Roman numerals) to give us a red colour, meanwhile Iron(II) would tend to give us greenish compounds.  Exactly as you have stated, Ion(II) does tend to oxidise further and produce Iron(III) compounds when there is a surplus of Oxygen but the phrase "surplus" just means that there is some additional Oxygen available and not that there is loads of Oxygen.  Given enough time, Iron(II) Oxide will become Iron(III) Oxide even if the Oxygen concentration is very low.
   Additionally, Oxygen is not the only thing that Iron can react with to form reddish iron(III) compounds.  However, on Mars we do think that it is mainly Iron(III) Oxide that is giving the surface it's reddish colour.

Is the iron oxide on Mars red because it was once rich with oxygen or is there another process that can make iron oxide red without the presents of oxygen?

     It's not really that there was some Iron(II) oxide already there and later it was further oxidised to Iron(III) Oxide.   We tend to look upon the overall change from elemental Iron to a compound that would be Iron(III) Oxide.  Alancalverd mentioned this in his earlier post.  We think a lot of elemental Iron would have started on the planet of Mars.  Now there probably was a little iron(II) oxide there as well but that doesn't matter too much.  All we need to focus on is that we want some oxygen.  Regardless of whether the Iron was present as elemental Iron or Iron(II) Oxide all of this will tend to form Iron(III) Oxide (and give us the red colour we observe) in the presence of Oxygen.
    Obviously, the Oxygen atoms had to come from somewhere.  However, it isn't necessary for there to be a high concentration of free oxygen  (O₂ ) in the atmosphere.   We think there was Water  (H₂O)  on the surface of Mars about 4 billion years ago.   One reasonable source of the oxygen atoms we need would be from this water.  There are mechanisms suggested where energetic particles and radiation from the sun would rip water molecules apart.  Some free Oxygen and free Hydrogen probably would get into the atmosphere.  The Hydrogen is light and would tend to drift away from the planet but the Oxygen is heavier and the gravity of Mars and would tend to keep it in the atmosphere for a short time.  Although this would only provide a low concentration of Oxygen, there is plenty of time and over that time the Iron on the surface of Mars will react with the Oxygen and form Iron(III) Oxide.
    Anyway, the atmosphere of Mars never needed to have a high concentration of Oxygen, it's more likely there was just a little oxygen and plenty of time for the Iron to react.

Best Wishes.

[Iwonda (OP)]

Thanks for all the great answers, have a very good picture of what may have taken place on Mars now.

[emelymorris]

The Martian atmosphere is mainly composed of carbon dioxide (about 96%), argon (about 2%), and nitrogen (about 2%). It contains only 0.146% oxygen.
Given the exploration of this planet, there may have been oxygen, but not on such a scale.
The first installation that can extract oxygen from the atmosphere of Mars is installed aboard the Perseverance rover. The device is called MOXIE and is comparable in size to a toaster. To produce oxygen, the device breaks down carbon dioxide (CO2) by taking one oxygen atom from it. A byproduct of the reaction, carbon monoxide, is released back into the planet's atmosphere by MOXIE. The device is made of heat-resistant materials since MOXIE needs a temperature of about 800 ° C to operate.