From the July 2013 issue

Is Mars red because of iron corrosion? If so, what process caused it to occur?

Ron Chinchen, Berala, New South Wales, Australia
By | Published: July 22, 2013
Mars red surface
Mars’ rust-red surface results from iron oxidation. This image from the Curiosity rover shows how the colors would look to our eyes if we were standing on the martian surface. // NASA/JPL-Caltech/MSSS/ASU
“Corrosion” is a general term for the breakdown of bonds among molecules in a material — like the way battery acid can corrode the metal inside a flashlight. A more specific chemical alteration term — and one used to describe what’s happened on Mars — is “oxidation.” Oxidation occurs when atoms and molecules give up their electrons in the presence of free oxygen molecules (O2), which are strong electron acceptors.

One of the atoms most susceptible to oxidation is iron (Fe), which has a loosely bound outer shell electron that free oxygen easily can grab. When the iron is tied up in the typical kinds of volcanic rocks found in Hawaii, Iceland, or the dark regions of Mars (basalt), that outer electron stays bound in the structures of some of the minerals in those rocks — minerals like olivine and pyroxene, both of which are typical components of basalt.

However, when free oxygen comes in contact with those minerals — either from the breakdown of water molecules (H2O), carbon dioxide (CO2), or some other oxygen-containing source — the free oxygen steals iron’s outer electron. That process results in the formation of iron oxide minerals, usually at first as coatings on the outer surfaces of the original basalt. Most of these iron oxides, like the mineral hematite (Fe2O3), are bright brownish-red. This could be the origin of the Red Planet’s famous color.

As these coatings flake off and break up into smaller bits, many become small enough for wind to carry them. We think that these bits of oxidized iron minerals make up a significant component of the dust on Mars, although mineral measurements from martian rovers show that the dust contains a lot of unoxidized iron as well. Still, only a tiny fraction of oxidized material is needed, on the outer surface of a particle, to give it a reddish color.

Other important related questions are: When did that oxidation occur, and is it occurring today on Mars? We still don’t know the answers, but trying to find them out is a major focus of the missions exploring Mars today.

Jim Bell
Arizona State University, Tempe