Science Questions

What caused the red sands of Mars?

Sun, 15th Nov 2009

Part of the show Producing Planets


Blaine asked:

Were the red sands of Mars caused by biological activities because when the levels of oxygen rose on Earth billions of years ago, large quantities of iron rusted out of the atmosphere, and that left iron oxide deposits in rocks. So could the same thing have happened to Mars?


We put this to Dr Matt Balme,

Possibly, I think it's probably not the right reason though.  I think the reason sands on Mars are red is really just due to a magnetite composition or ferrous iron mineral composition.  You know, Mars may or may not have had an ocean.  If it did have an ocean, it probably didnít exist for a very long period of time, relatively speaking, not like the Earth, which has been around for billions of years.  So you know, weíre loathe to rule anything out until we actually go there.  I mean, thatís one of the wonders of planetary science.  Thereís an alternative answer to almost everything.


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Blane asked the Naked Scientists: When levels of oxygen began to rise on earth billions of years ago (as a result of biological activity), great quantities of iron "rusted" out of the atmosphere, leaving iron oxide deposits in rocks. So could the large amount of iron oxide on the surface of Mars (which gives it the red colour) be the result of a similar phenomenon? Without plate tectonics, the deposits would not have been buried as they were on earth and have presumably been blowing around for a few billion years. What do you think? Blane, Fri, 7th Aug 2009

Interesting question.

Mars has certainly experienced a fair bit of geological activity in the past, and when its core was still molten, might had also had tectonic plate activity.  If so, the abundant rust on the surface would seem to have arrived after that period.  If the rust was formed out of the atmosphere then, this had to occur after Mar's core had solidified and not before, but which would have probably been quite a long time after the surface had solidified.  This being the case, what could have happened to cause the iron to rust out of the atmosphere after the crust had solidified, but not until the core solidified?  If it was a purely atmospheric affect, I would have expected it to start as soon as the surface solidified, not just once the core solidified.  This is all just speculation, of course.

Another possible alternative is that Mars, being smaller than the Earth, had its iron more widely diffused through the planet and was less densely packed into a core, as it is on Earth.  If a substantial proportion of it were nearer the surface than on Earth then the lava produced by the many volcanoes on Mars could have been relatively rich in iron and perhaps after weathering, that's where most of the rust has come from.  Just more speculation again.

Hopefully, one of the forum's geologists will take a peek at this thread and offer some informed comments. LeeE, Fri, 7th Aug 2009

I guess I should clarify my question a bit.  I was not under the impression that the actual iron was in the atmosphere, but that the biological activity provided the free oxygen responsible for oxidation of reduced iron present on the surface or in bodies of water (presumably the state it would be in in an atmosphere with little free oxygen).  Otherwise, where would all the free oxygen needed to oxidize that much iron come from? As for the timing of the cessation of plate tectonics, which would leave all that iron oxide laying around on the surface, couldn't it have crapped out shortly (geologically speaking) after the great rusting event? 

The scenario I am imagining for Mars is the same as for early earth progressing through the development of something like blue-green bacteria that release oxygen, oxygenating the atmosphere, reduced iron "rusts out" but plate tectonics stops, kind of freezing things in place, the atmosphere is blown away by the solar wind or whatever, the water freezes and or sublimes, etc. etc. Blane, Mon, 10th Aug 2009

The question really is where did Mar's oxygen come from; was it produced biologically?

I don't know.  I think a biological origin is plausible, but we're still trying to find proof of any life on Mars. LeeE, Mon, 10th Aug 2009

Yep, the free oxygen is the question, the rust is just the proof that it, or some other compound capable of oxidizing iron was present in large amounts.  On earth, biological activity produced the free oxygen, which resulted in large quantities of reduced iron being oxidized and rusted out, leaving behind the banded iron deposits.  If it can happen on Earth, why not Mars too? Perhaps the boldest evidence for life on Mars is the characteristic that has defined it for most of human history, its red color.

Blane, Tue, 11th Aug 2009

True a lot of the earth's oxidation was poduced by biological activity but free oxygen can also be produced by the breakdown of carbon dioxide by ultraviolet light and carbon dioxide is the heaviest of the commonly available molecular gases and most likely to stick around on Mars Soul Surfer, Tue, 11th Aug 2009

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